September 2009

Publication No. FHWA-NHI-09-111

Hydraulic Engineering Circular No. 23
Bridge Scour and Stream Instability
Countermeasures: Experience,
Selection, and Design Guidance-Third
Edition

Volume 1



Technical Report Documentation Page
1. Report No. FHWA NHI
HEC-23

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle

BRIDGE SCOUR AND STREAM INSTABILITY
COUNTERMEASURES
Experience, Selection and Design Guidance
Volumes 1 and 2
Third Edition
5. Report Date

September 2009

6. Performing Organization Code

7. Author(s)
P.F. Lagasse, P.E. Clopper, J.E. Pagn-Ortiz, L.W. Zevenbergen,
L.A. Arneson, J.D. Schall, L.G. Girard
8. Performing Organization Report No.
9. Performing Organization Name and Address

Ayres Associates
3665 JFK Parkway
Building 2, Suite 200
Fort Collins, Colorado 80525
10. Work Unit No. (TRAIS)




11. Contract or Grant No.
DTFH61-06-D-00010/T-06-001
12. Sponsoring Agency Name and Address

Office of Bridge Technology National Highway Institute
FHWA, HIBT-20 4600 North Fairfax Dr., Suite 800
1200 New Jersey Ave., SE Arlington, Virginia 22203
Washington, D.C. 20590
13. Type of Report and Period Covered



14. Sponsoring Agency Code

15. Supplementary Notes
Project Managers: Dr. Larry A. Arneson and Mr. Jorge E. Pagn-Ortiz, FHWA
Technical Assistants: Scott Anderson, Kornel Kerenyi, Joe Krolak, Barry Siel, FHWA; S. Mishra, Ayres Associates;
B. Hunt, STV Inc.
1 16. This document identifies and provides design guidelines for bridge scour and stream instability countermeasures that have
been implemented by various State departments of transportation (DOTs) in the United States. Countermeasure experience,
selection, and design guidance are consolidated from other FHWA publications in this document to support a comprehensive
analysis of scour and stream instability problems and provide a range of solutions to those problems. Selected innovative
countermeasure concepts and guidance derived from practice outside the United States are introduced. Management
strategies and guidance for developing a Plan of Action for scour critical bridges are outlined, and guidance is provided for
scour monitoring using portable and fixed instrumentation.
The results of recently completed National Cooperative Highway Research Program (NCHRP) projects are incorporated in the
design guidance, including: countermeasures to protect bridge piers and abutments from scour; riprap design criteria,
specifications, and quality control; and environmentally sensitive channel and bank protection measures. This additional
material required expanding HEC-23 to two volumes. Volume 1 now contains a complete chapter on riprap design,
specifications, and quality control as well as an expanded chapter on biotechnical countermeasures. The guidance on scour
monitoring instrumentation has been updated and now includes additional installation case studies. Volume 2 contains 19
detailed design guidelines grouped into six categories, including countermeasures for: (1) stream instability (2) streambank
and roadway embankment protection, (3) bridge pier protection, (4) abutment protection, (5) filter design, and (6) special
applications.
17. Key Words
stream stability, scour, countermeasures, plan of action,
bendway weirs, soil cement, wire enclosed riprap, articulating
concrete block systems, concrete armor units, gabion
mattresses, grout filled mattresses, grout bags, rock riprap,
partially grouted riprap, spurs, guide banks, check dams,
revetments, scour monitoring instrumentation
18. Distribution Statement
This document is available to the public
through the National Technical Information
Service, Springfield, VA 22161 (703) 487-4650
19. Security Classif. (of this report)

Unclassified
20. Security Classif. (of this page)

Unclassified
21. No. of Pages

256
22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized



i
Bridge Scour and Stream Instability Countermeasures
Experience, Selection, and Design Guidance
Third Edition
Volume 1


TABLE OF CONTENTS


LIST OF FIGURES.............................................................................................................. vii
LIST OF TABLES................................................................................................................. xi
DESIGN GUIDELINES (Volume 2)..................................................................................... xiii
ACKNOWLEDGMENTS AND DISCLAIMER....................................................................... xv
GLOSSARY ...................................................................................................................... xvii

CHAPTER 1. INTRODUCTION.........................................................................................1.1

1.1 PURPOSE..................................................................................................................1.1
1.2 BACKGROUND..........................................................................................................1.1
1.3 MANUAL ORGANIZATION ........................................................................................1.2
1.4 COMPREHENSIVE ANALYSIS..................................................................................1.3
1.5 PLAN OF ACTION .....................................................................................................1.5
1.6 DUAL SYSTEM OF UNITS.........................................................................................1.6

CHAPTER 2. PLAN OF ACTION AND THE COUNTERMEASURES MATRIX..................2.1

2.1 STRATEGIES FOR PROTECTING SCOUR CRITICAL BRIDGES............................2.1

2.1.1 Technical Advisories................................................................................................2.1
2.1.2 Additional Guidance and Requirements ..................................................................2.2
2.1.3 Management Strategies for a Plan of Action ...........................................................2.2
2.1.4 Inspection Strategies in a Plan of Action .................................................................2.3
2.1.5 Closure Instructions.................................................................................................2.4
2.1.6 Countermeasure Alternatives and Schedule............................................................2.5
2.1.7 Other Information Necessary in a Plan of Action.....................................................2.5
2.1.8 Development and Implementation of a POA............................................................2.5

2.2 STANDARD TEMPLATE FOR A PLAN OF ACTION..................................................2.6

2.2.1 Overview.................................................................................................................2.6
2.2.2 Executive Summary.................................................................................................2.7

2.3 THE COUNTERMEASURE MATRIX..........................................................................2.7
2.4 COUNTERMEASURE GROUPS................................................................................2.8

2.4.1 Group 1. Hydraulic Countermeasures ....................................................................2.8
2.4.2 Group 2. Structural Countermeasures..................................................................2.13
2.4.3 Group 3. Biotechnical Countermeasures..............................................................2.13
2.4.4 Group 4. Monitoring..............................................................................................2.14

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2.5 COUNTERMEASURE CHARACTERISTICS............................................................2.15

2.5.1 Functional Applications..........................................................................................2.15
2.5.2 Suitable River Environment ...................................................................................2.16
2.5.3 Maintenance..........................................................................................................2.16
2.5.4 Installation/Experience by State Departments of Transportation ...........................2.17
2.5.5 Design Guideline Reference..................................................................................2.17
2.5.6 Summary...............................................................................................................2.17

CHAPTER 3. CONSIDERATIONS FOR SELECTING COUNTERMEASURES.................3.1

3.1 INTRODUCTION........................................................................................................3.1
3.2 CRITERIA FOR THE SELECTION OF COUNTERMEASURES.................................3.2

3.2.1 Erosion Mechanism.................................................................................................3.2
3.2.2 Stream Characteristics ............................................................................................3.2
3.2.3 Construction and Maintenance Requirements.........................................................3.4
3.2.4 Vandalism ...............................................................................................................3.4
3.2.5 Countermeasure Selection Based on Cost ..............................................................3.4
3.2.6 Countermeasure Selection Based on Risk ..............................................................3.8

3.3 COUNTERMEASURES FOR MEANDER MIGRATION..............................................3.9
3.4 COUNTERMEASURES FOR CHANNEL BRAIDING AND ANABRANCHING..........3.11
3.5 COUNTERMEASURES FOR DEGRADATION AND AGGRADATION.....................3.12

3.5.1 Countermeasures to Control Degradation .............................................................3.12
3.5.2 Countermeasures to Control Aggradation .............................................................3.13

3.6 SELECTION OF COUNTERMEASURES FOR SCOUR AT BRIDGES....................3.14

3.6.1 Countermeasures for Contraction Scour ...............................................................3.15
3.6.2 Countermeasures for Local Scour .........................................................................3.16
3.6.3 Monitoring .............................................................................................................3.18

CHAPTER 4. COUNTERMEASURE DESIGN CONCEPTS..............................................4.1

4.1 COUNTERMEASURE DESIGN APPROACH.............................................................4.1

4.1.1 Investment in Countermeasures..............................................................................4.1
4.1.2 Service Life and Safety............................................................................................4.1
4.1.3 Design Approach.....................................................................................................4.2

4.2 ENVIRONMENTAL PERMITTING..............................................................................4.3
4.3 HYDRAULIC ANALYSIS ............................................................................................4.4

4.3.1 Overview.................................................................................................................4.4
4.3.2 Physical Models.......................................................................................................4.4
4.3.3 Scour at Transverse Structures...............................................................................4.6
4.3.4 Scour at Longitudinal Structures..............................................................................4.7
4.3.5 Scour at Protected Bendways ...............................................................................4.10
4.3.6 Hydraulic Stress on a Bendway.............................................................................4.11

iii
CHAPTER 5. RIPRAP DESIGN, FILTERS, FAILURE MODES, AND
ALTERNATIVES .........................................................................................5.1

5.1 OVERVIEW................................................................................................................5.1
5.2 RIPRAP DESIGN.......................................................................................................5.2

5.2.1 Introduction .............................................................................................................5.2
5.2.2 Riprap Revetment ...................................................................................................5.2
5.2.3 Riprap for Bridge Piers ............................................................................................5.3
5.2.4 Riprap for Bridge Abutments ...................................................................................5.4
5.2.5 Riprap Protection for Countermeasures ..................................................................5.4
5.2.6 Riprap for Special Applications................................................................................5.4
5.2.7 Termination Details .................................................................................................5.5
5.2.8 Riprap Size, Shape, and Gradation .........................................................................5.5

5.3 FILTER REQUIREMENTS .........................................................................................5.7

5.3.1 Overview.................................................................................................................5.7
5.3.2 Placing Geotextiles Under Water ............................................................................5.8

5.4 RIPRAP FAILURE MODES......................................................................................5.10

5.4.1 Riprap Revetment Failure Modes ..........................................................................5.12
5.4.2 Pier Riprap Failure Modes.....................................................................................5.17
5.4.3 Pier Riprap Failure Modes Schoharie Creek Case Study ...................................5.18

5.5 RIPRAP INSPECTION GUIDANCE..........................................................................5.21

5.5.1 General .................................................................................................................5.21
5.5.2 Guidance for Recording Riprap Condition .............................................................5.22
5.5.3 Performance Evaluation ........................................................................................5.22

5.6 GROUTED AND PARTIALLY GROUTED RIPRAP..................................................5.22
5.7 CONCRETE ARMOR UNITS ...................................................................................5.25

CHAPTER 6. BIOTECHNICAL ENGINEERING................................................................6.1

6.1 OVERVIEW................................................................................................................6.1
6.2 CURRENT PRACTICE...............................................................................................6.1
6.3 GENERAL CONCEPTS .............................................................................................6.2
6.4 ADVANTAGES AND LIMITATIONS OF BIOTECHNICAL ENGINEERING ................6.3
6.5 DESIGN CONSIDERATIONS FOR BIOTECHNICAL COUNTERMEASURES...........6.4
6.6 COMMONLY USED VEGETATIVE METHODS..........................................................6.6
6.7 ENVIRONMENTAL CONSIDERATIONS AND BENEFITS.........................................6.6
6.8 APPLICATION GUIDANCE FOR BIOTECHNICAL COUNTERMEASURES............6.10

6.8.1 Streambank Zones................................................................................................6.10
6.8.2 Biotechnical Engineering Treatments ....................................................................6.11

6.9 SUMMARY...............................................................................................................6.13

iv
CHAPTER 7. COUNTERMEASURE DESIGN GUIDELINES ............................................7.1

7.1 INTRODUCTION........................................................................................................7.1
7.2 DESIGN GUIDELINES...............................................................................................7.2

7.2.1 Countermeasures for Stream Instability...................................................................7.2
7.2.2 Countermeasures for Streambank and Roadway Embankment Protection .............7.2
7.2.3 Countermeasures for Bridge Pier Protection ...........................................................7.3
7.2.4 Countermeasures for Abutment Protection..............................................................7.4
7.2.5 Filter Design............................................................................................................7.4
7.2.6 Special Applications ................................................................................................7.4

CHAPTER 8. OTHER COUNTERMEASURES AND CASE HISTORIES OF
PERFORMANCE ........................................................................................8.1

8.1 INTRODUCTION........................................................................................................8.1
8.2 HARDPOINTS............................................................................................................8.1
8.3 RETARDER STRUCTURES ......................................................................................8.1

8.3.1 Jacks and Tetrahedrons..........................................................................................8.2
8.3.2 Fence Retarder Structures ......................................................................................8.4
8.3.3 Timber Pile..............................................................................................................8.4
8.3.4 Wood Fence............................................................................................................8.4

8.4 LONGITUDINAL DIKES.............................................................................................8.5

8.4.1 Earth or Rock Embankments...................................................................................8.5
8.4.2 Rock Toe-Dikes.......................................................................................................8.7
8.4.3 Crib Dikes................................................................................................................8.8
8.4.4 Bulkheads ...............................................................................................................8.8

8.5 CHANNEL RELOCATION........................................................................................8.11
8.6 CASE HISTORIES OF COUNTERMEASURE PERFORMANCE.............................8.13

8.6.1 Flexible Revetment................................................................................................8.13
8.6.2 Rigid Revetments ..................................................................................................8.15
8.6.3 Bulkheads .............................................................................................................8.16
8.6.4 Spurs.....................................................................................................................8.16
8.6.5 Retardance Structures...........................................................................................8.17
8.6.6 Dikes .....................................................................................................................8.17
8.6.7 Guide Banks..........................................................................................................8.17
8.6.8 Check Dams..........................................................................................................8.18
8.6.9 Jack or Tetrahedron Fields....................................................................................8.19
8.6.10 Special Devices for Protection of Piers..................................................................8.19
8.6.11 Channel Alterations ...............................................................................................8.20
8.6.12 Modification of Bridge Length and Relief Structures..............................................8.20
8.6.13 Investment in Countermeasures............................................................................8.20

CHAPTER 9. SCOUR MONITORING AND INSTRUMENTATION....................................9.1

9.1 INTRODUCTION........................................................................................................9.1
9.2 PORTABLE INSTRUMENTATION.............................................................................9.2
v
9.2.1 Components of a Portable Instrument System........................................................9.2
9.2.2 Instrument for Making the Measurement .................................................................9.2
9.2.3 System for Deploying the Instrument.......................................................................9.7
9.2.4 Positioning Information..........................................................................................9.10
9.2.5 Data Storage Devices............................................................................................9.11

9.3 FIXED INSTRUMENTATION....................................................................................9.11

9.3.1 NCHRP Project 21-3 .............................................................................................9.11
9.3.2 Scour Measurement ..............................................................................................9.12
9.3.3 Summary of NCHRP Project 21-3 Results ............................................................9.13
9.3.4 Operational Fixed Instrument Systems..................................................................9.14
9.3.5 NCHRP Project 20-5 .............................................................................................9.22
9.3.6 Application Guidelines ...........................................................................................9.23

9.4 SELECTING INSTRUMENTATION..........................................................................9.24

9.4.1 Portable Instruments .............................................................................................9.25
9.4.2 Fixed Instruments..................................................................................................9.26

9.5 FIXED INSTRUMENT CASE HISTORIES................................................................9.30

9.5.1 Introduction ...........................................................................................................9.30
9.5.2 Typical Field Installations.......................................................................................9.30

CHAPTER 10. REFERENCES........................................................................................10.1

APPENDIX A Metric System, Conversion Factors, and Water Properties ...................... A.1
APPENDIX B Standard Template for a Plan of Action.................................................... B.1
APPENDIX C Pier Scour Countermeasure Selection Methodology................................ C.1
APPENDIX D Riprap Inspection Recording Guidance.................................................... D.1

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vii
LIST OF FIGURES

Figure 1.1 Flow chart for scour and stream stability analysis and evaluation..................1.4

Figure 3.1 Risk-based approach using NBI data ............................................................3.8

Figure 3.2 Comparison of channel bend cross sections for natural conditions
and for stabilized bend ................................................................................3.10

Figure 3.3 Meander migration in a natural channel and a channel with stabilized
bend............................................................................................................3.11

Figure 3.4 Typical guide bank layout ............................................................................3.17

Figure 4.1 BAW laboratory, Karlsruhe, Germany, pier scour model of railway
bridge over Rhine River near Mannheim.......................................................4.6

Figure 4.2 Scour along a vertical wall as a function of unconstrained valley width .........4.9

Figure 4.3 Definition sketch of width and mean water depth at the crossing
upstream of the bend and maximum water depth in the bend .....................4.11

Figure 4.4 Shear stress multiplier K
b
for bends ............................................................4.12

Figure 5.1 Riprap shape described by three axes ..........................................................5.5

Figure 5.2 Close-up photo of SandMat
TM
geocomposite blanket ....................................5.9

Figure 5.3 SandMat
TM
geocomposite blanket being unrolled .........................................5.9

Figure 5.4 Sand-filled geotextile containers..................................................................5.10

Figure 5.5 Filling 1.0 metric tonne geotextile container with sand.................................5.11

Figure 5.6 Handling a 1.0 metric tonne sand-filled geotextile container........................5.11

Figure 5.7 Two hundred lb (91 kg) sand-filled geotextile containers.............................5.12

Figure 5.8 Riprap failure by particle erosion .................................................................5.12

Figure 5.9 Damaged riprap on left bank of Pinole Creek at Pinole, CA ........................5.13

Figure 5.10 Riprap failure by translational slide..............................................................5.14

Figure 5.11 Riprap on Cosumnes River at Site 2 near Sloughhouse, CA.......................5.14

Figure 5.12 Riprap failure by modified slump .................................................................5.15

Figure 5.13 Riprap on Cosumnes River at Site 3 near Sloughhouse, CA.......................5.15

viii
Figure 5.14 Riprap failure due to slump..........................................................................5.16

Figure 5.15 Riprap on left bank Consumnes River at Site 1 near Sloughhouse, CA.......5.16

Figure 5.16 South elevation Schoharie Creek Bridge showing key structural
features and a schematic geological section...............................................5.18

Figure 5.17 Pier scour holes at Schoharie Creek Bridge in 1987 ...................................5.19

Figure 5.18 Photograph of riprap at pier 2, October 1956 ..............................................5.20

Figure 5.19 Photograph of riprap at pier 2, August 1977................................................5.20

Figure 5.20 Close-up view of partially grouted riprap......................................................5.24

Figure 5.21 "Conglomerate" of partially grouted riprap...................................................5.24

Figure 5.22 Concrete armor units...................................................................................5.25

Figure 5.23 Laboratory study of Toskanes for pier scour protection...............................5.28

Figure 5.24 Installation of A-Jacks modular units installed by Kentucky DOT..............5.28

Figure 6.1 Details of brush mattress technique with stone toe protection.......................6.5

Figure 6.2 Details of root wad and boulder revetment technique....................................6.5

Figure 6.3 Vegetated riprap willow bundle method......................................................6.7

Figure 6.4 Vegetated riprap bent pole method ............................................................6.7

Figure 6.5 Vegetated riprap brush layering with pole planting .....................................6.8

Figure 6.6 Vegetated riprap construction techniques ..................................................6.8

Figure 6.7 Vegetated riprap with joint planting................................................................6.9

Figure 6.8 Bank zones defined for slope protection......................................................6.11

Figure 8.1 Perspective view of hardpoint installation with section detail .........................8.2

Figure 8.2 Typical tetrahedron design ............................................................................8.3

Figure 8.3 Typical jack unit.............................................................................................8.3

Figure 8.4 Retarder field schematic................................................................................8.4

Figure 8.5 Timber pile bent retarder structure ................................................................8.5

Figure 8.6 Typical wood fence retarder structure ...........................................................8.6

Figure 8.7 Light double row wire fence retarder structure...............................................8.6
ix
Figure 8.8 Heavy timber-pile and wire fence retarder structures ....................................8.7

Figure 8.9 Typical longitudinal rock toe-dike geometries................................................8.9

Figure 8.10 Longitudinal rock toe-dike tiebacks................................................................8.9

Figure 8.11 Timber pile, wire mesh crib dike with tiebacks..............................................8.10

Figure 8.12 Anchorage schemes for a sheetpile bulkhead.............................................8.10

Figure 8.13 Encroachments on meandering streams .....................................................8.11

Figure 9.1 Sounding pole measurement.........................................................................9.4

Figure 9.2 Lead-line sounding weight .............................................................................9.4

Figure 9.3 Portable sonar in use ....................................................................................9.5

Figure 9.4 Kneeboard float .............................................................................................9.5

Figure 9.5 Pontoon float .................................................................................................9.6

Figure 9.6 AIDI system...................................................................................................9.6

Figure 9.7 FHWA articulated arm in use ........................................................................9.8

Figure 9.8 NCHRP 21-3 articulated arm truck ................................................................9.8

Figure 9.9 Sonar instrument deployed from articulated arm truck ..................................9.9

Figure 9.10 Unmanned, remote control boat ..................................................................9.10

Figure 9.11 Master station with data logger for use with any of the fixed instruments ....9.14

Figure 9.12 Above-water serviceable low-cost fathometer system.................................9.15

Figure 9.13 Sonar scour monitor installation including remote station............................9.16

Figure 9.14 Manual read out magnetic sliding collar device ...........................................9.17

Figure 9.15 Automated read out magnetic sliding collar system.....................................9.18

Figure 9.16 Detail of magnetic sliding collar on the streambed.......................................9.19

Figure 9.17 Float-out devices prior to installation ...........................................................9.19

Figure 9.18 Tilt meter on California bridge .....................................................................9.20

Figure 9.19 Detail of tilt meter ........................................................................................9.20

Figure 9.20 Time domain reflectometry probe................................................................9.21
x
Figure 9.21 Brisco Monitor sounding rods installed at a bridge pier in New York............9.21

Figure 9.22 Active streambed scour monitoring locations in Alaska ...............................9.31

Figure 9.23 Oblique aerial photograph of the Knik River Old Glenn Highway.................9.32

Figure 9.24 Conduit to sonar scour monitor at Wantagh Parkway over Goose Creek....9.33

Figure 9.25 Solar panel, remote station and (inset) conduit to sonar monitor .................9.34

Figure 9.26 Adjustable stainless steel sonar mounting bracket .....................................9.34

Figure 9.27 Installation of a sonar scour monitor on Salinas River Bridge......................9.35

Figure 9.28 Close-up of sonar scour monitor on Salinas River Bridge............................9.36

Figure 9.29 CALTRANS drilling with hollow stem auger for installation of
float out devices at Salinas River Bridge .....................................................9.37

Figure 9.30 Installation of float out device on Salinas River Bridge ................................9.37

Figure 9.31 Typical instrument shelter with data logger, cell-phone telemetry................9.38

Figure 9.32 Installation of a float out device by Nevada DOT to monitor riprap ..............9.38








xi
LIST OF TABLES


Table 1.1 Commonly Used Engineering Terms in SI and English Units .........................1.6

Table 2.1 Stream Instability and Bridge Scour Countermeasures Matrix .......................2.9

Table 5.1 Minimum and Maximum Allowable Particle Size in Inches..............................5.7

Table 5.2 Minimum and Maximum Allowable Particle Weight in Pounds........................5.7

Table 9.1 Instrumentation Summary by Category ........................................................9.25

Table 9.2 Portable Instrumentation Summary ..............................................................9.25

Table 9.3 Positioning System Summary.......................................................................9.25

Table 9.4 Estimated Cost Information for Portable Instruments ...................................9.26

Table 9.5 Fixed Instrumentation Summary...................................................................9.26

Table 9.6 Fixed Instrumentation Selection Matrix.........................................................9.28

Table 9.7 Estimated Cost Information for Fixed Instruments........................................9.29

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xiii
Bridge Scour and Stream Instability Countermeasures
Experience, Selection, and Design Guidance
Third Edition
Volume 2

DESIGN GUIDELINES

SECTION 1 - COUNTERMEASURES FOR STREAM INSTABILITY

Design Guideline 1 Bendway Weirs/Stream Barbs.................................................... DG1.1
Design Guideline 2 Spurs.......................................................................................... DG2.1
Design Guideline 3 Check Dams/Drop Structures..................................................... DG3.1

SECTION 2 - COUNTERMEASURES FOR STREAMBANK AND ROADWAY
EMBANKMENT PROTECTION

Design Guideline 4 Riprap Revetment....................................................................... DG4.1
Design Guideline 5 Riprap Design for Embankment Overtopping ............................. DG5.1
Design Guideline 6 Wire Enclosed Riprap Mattress .................................................. DG6.1
Design Guideline 7 Soil Cement................................................................................ DG7.1
Design Guideline 8 Articulating Concrete Block Systems.......................................... DG8.1
Design Guideline 9 Grout-Filled Mattresses .............................................................. DG9.1
Design Guideline 10 Gabion Mattresses ................................................................. DG10.1

SECTION 3 - COUNTERMEASURES FOR BRIDGE PIER PROTECTION

Design Guideline 8 Articulating Concrete Block Systems at Bridge Piers................ DG8.21
Design Guideline 9 Grout-Filled Mattresses at Bridge Piers .................................... DG9.14
Design Guideline 10 Gabion Mattresses at Bridge Piers ....................................... DG10.13
Design Guideline 11 Rock Riprap at Bridge Piers ................................................... DG11.1
Design Guideline 12 Partially Grouted Riprap at Bridge Piers ................................. DG12.1

SECTION 4 - COUNTERMEASURES FOR ABUTMENT PROTECTION

Design Guideline 13 Grout/Cement Filled Bags ...................................................... DG13.1
Design Guideline 14 Rock Riprap at Bridge Abutments .......................................... DG14.1
Design Guideline 15 Guide Banks........................................................................... DG15.1

SECTION 5 - FILTER DESIGN

Design Guideline 16 Filter Design ........................................................................... DG16.1

SECTION 6 SPECIAL APPLICATIONS

Design Guideline 17 Riprap Design for Wave Attack .............................................. DG17.1
Design Guideline 18 Riprap Protection for Bottomless Culverts .............................. DG18.1
Design Guideline 19 Concrete Armor Units............................................................. DG19.1

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xv
ACKNOWLEDGMENTS


This manual is a major revision of the second edition of HEC-23 which was published in
2001. The writers wish to acknowledge the contributions made by Morgan S. Byars (formerly
Ayres Associates) as a co-author of the first edition (1997). Technical assistance for the
second edition was provided by J. Sterling Jones (FHWA) and A. Firenzi, J.L. Morris, E.V.
Richardson, W.J. Spitz, and A. Waddoups (Ayres Associates).



DISCLAIMER


Mention of a manufacturer, registered or trade name does not constitute a guarantee or
warranty of the product by the U.S. Department of Transportation or the Federal Highway
Administration and does not imply their approval and/or endorsement to the exclusion of
other products and/or manufacturers that may also be suitable.


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xvii
GLOSSARY

abrasion: Removal of streambank material due to entrained sediment, ice,
or debris rubbing against the bank.

aggradation: General and progressive buildup of the longitudinal profile of a
channel bed due to sediment deposition.

alluvial channel: Channel wholly in alluvium; no bedrock is exposed in channel at
low flow or likely to be exposed by erosion.

alluvial fan: Fan-shaped deposit of material at the place where a stream
issues from a narrow valley of high slope onto a plain or broad
valley of low slope. An alluvial cone is made up of the finer
materials suspended in flow while a debris cone is a mixture of all
sizes and kinds of materials.

alluvial stream: Stream which has formed its channel in cohesive or noncohesive
materials that have been and can be transported by the stream.

alluvium: Unconsolidated material deposited by a stream in a channel,
floodplain, alluvial fan, or delta.

alternating bars: Elongated deposits found alternately near the right and left banks
of a channel.

ambient bed elevation: Initial (unscoured) bed elevation.

anabranch: Individual channel of an anabranched stream.

anabranched stream: Stream whose flow is divided at normal and lower stages by large
islands or, more rarely, by large bars; individual islands or bars
are wider than about three times water width; channels are more
widely and distinctly separated than in a braided stream.

anastomosing stream: An anabranched stream.

angle of repose: Maximum angle (as measured from the horizontal) at which
gravel or sand particles can stand.

annual flood: Maximum flow in 1 year (may be daily or instantaneous).

apron: Protective material placed on a streambed to resist scour.

apron, launching: Apron designed to settle and protect the side slopes of a scour
hole after settlement.

armor (armoring): Surfacing of channel bed, banks, or embankment slope to resist
erosion and scour. (A) Natural process whereby an erosion-
resistant layer of relatively large particles is formed on a
streambed due to the removal of finer particles by streamflow; (B)
placement of a covering to resist erosion.
xviii

articulated concrete
mattress:
Rigid concrete slabs which can move without separating as scour
occurs; usually hinged together with corrosion-resistant cable
fasteners; primarily placed for lower bank protection.

average velocity: Velocity at a given cross section determined by dividing discharge
by cross sectional area.

avulsion: Sudden change in the channel course that usually occurs when a
stream breaks through its banks; usually associated with a flood
or a catastrophic event.

backfill: Material used to refill a ditch or other excavation, or the process
of doing so.

backwater: Increase in water surface elevation relative to elevation occurring
under natural channel and floodplain conditions. It is induced by
a bridge or other structure that obstructs or constricts the free
flow of water in a channel.

backwater area: Low-lying lands adjacent to a stream that may become flooded
due to backwater.

bank: Sides of a channel between which the flow is normally confined.

bank, left (right): Sides of a channel as viewed in a downstream direction.

bankfull discharge: Discharge that, on average, fills a channel to the point of
overflowing.

bank protection: Engineering works for the purpose of protecting streambanks
from erosion.

bank revetment: Erosion-resistant materials placed directly on a streambank to
protect the bank from erosion.

bar: Elongated deposit of alluvium within a channel, not permanently
vegetated.

base floodplain: Floodplain associated with the flood with a 100-year recurrence
interval.

bed: Bottom of a channel bounded by banks.

bed form: Recognizable relief feature on the bed of a channel, such as a
ripple, dune, plane bed, antidune, or bar. Bed forms are a
consequence of the interaction between hydraulic forces
(boundary shear stress) and the bed sediment.

bed layer: Flow layer, several grain diameters thick (usually two)
immediately above the bed.

xix

bed load: Sediment that is transported in a stream by rolling, sliding, or
skipping along the bed or very close to it; considered to be within
the bed layer (contact load).

bed load discharge
(or bed load):
Quantity of bed load passing a cross section of a stream in a unit
of time.

bed material: Material found in and on the bed of a stream (May be transported
as bed load or in suspension).

bedrock: Solid rock exposed at the surface of the earth or overlain by soils
and unconsolidated material.

bed sediment discharge: Part of the total sediment discharge that is composed of grain
sizes found in the bed and is equal to the transport capability of
the flow.

bed shear stationary
(tractive force):
Force per unit area exerted by a fluid flowing past a boundary.

bed slope: Inclination of the channel bottom.

biotechnical engineering: Countermeasure techniques that combine the use of vegetation
with structural (hard) elements.

blanket: Material covering all or a portion of a streambank to prevent
erosion.

boulder: Rock fragment whose diameter is greater than 250 mm.

braid: Subordinate channel of a braided stream.

braided stream: Stream whose flow is divided at normal stage by small
mid-channel bars or small islands; the individual width of bars and
islands is less than about three times water width; a braided
stream has the aspect of a single large channel within which are
subordinate channels.

bridge opening: Cross-sectional area beneath a bridge that is available for
conveyance of water.

bridge owner: Any Federal, State, Local agency, or other entity responsible for a
structure defined as a highway bridge by the National Bridge
Inspection Standards (NBIS).

bridge waterway: Area of a bridge opening available for flow, as measured below a
specified stage and normal to the principal direction of flow.

bulk density: Density of the water sediment mixture (mass per unit volume),
including both water and sediment.

xx

bulkhead: Vertical, or near vertical, wall that supports a bank or an
embankment; also may serve to protect against erosion.

bulking: Increasing the water discharge to account for high concentrations
of sediment in the flow.

catchment: See drainage basin.

causeway: Rock or earth embankment carrying a roadway across water.

caving: Collapse of a bank caused by undermining due to the action of
flowing water.

cellular-block mattress: Interconnected concrete blocks with regular cavities placed
directly on a streambank or filter to resist erosion. The cavities
can permit bank drainage and the growth of vegetation where
synthetic filter fabric is not used between the bank and mattress.

channel: Bed and banks that confine surface flow of a stream.

channelization: Straightening or deepening of a natural channel by artificial
cutoffs, grading, flow-control measures, or diversion of flow into
an engineered channel.

channel diversion: Removal of flows by natural or artificial means from a natural
length of channel.

channel pattern: Aspect of a stream channel in plan view, with particular reference
to the degree of sinuosity, braiding, and anabranching.

channel process: Behavior of a channel with respect to shifting, erosion and
sedimentation.

check dam: Low dam or weir across a channel used to control stage or
degradation.

choking (of flow): Excessive constriction of flow which may cause severe backwater
effect.

clay (mineral): Particle whose diameter is in the range of 0.00024 to 0.004 mm.

clay plug: Cutoff meander bend filled with fine grained cohesive sediments.

clear-water scour: Scour at a pier or abutment (or contraction scour) when there is
no movement of the bed material upstream of the bridge crossing
at the flow causing bridge scour.

cobble: Fragment of rock whose diameter is in the range of 64 to 250
mm.

xxi

concrete revetment: Unreinforced or reinforced concrete slabs placed on the channel
bed or banks to protect it from erosion.
confluence: Junction of two or more streams.

constriction: Natural or artificial control section, such as a bridge crossing,
channel reach or dam, with limited flow capacity in which the
upstream water surface elevation is related to discharge.

contact load: Sediment particles that roll or slide along in almost continuous
contact with the streambed (bed load).

contraction: Effect of channel or bridge constriction on flow streamlines.

contraction scour: Contraction scour, in a natural channel or at a bridge crossing,
involves the removal of material from the bed and banks across
all or most of the channel width. This component of scour results
from a contraction of the flow area at the bridge which causes an
increase in velocity and shear stress on the bed at the bridge.
The contraction can be caused by the bridge or from a natural
narrowing of the stream channel.

Coriolis force: Inertial force caused by the Earth's rotation that deflects a moving
body to the right in the Northern Hemisphere.

countermeasure: Measure intended to prevent, delay or reduce the severity of
hydraulic problems.

crib: Frame structure filled with earth or stone ballast, designed to
reduce energy and to deflect streamflow away from a bank or
embankment.

critical shear stress: Minimum amount of shear stress required to initiate soil particle
motion.

crossing: Relatively short and shallow reach of a stream between bends;
also crossover or riffle.

cross section: Section normal to the trend of a channel or flow.

current: Water flowing through a channel.

current meter: Instrument used to measure flow velocity.

cut bank: Concave wall of a meandering stream.

cutoff: (A) Direct channel, either natural or artificial, connecting two
points on a stream, thereby shortening the original length of the
channel and increasing its slope; (B) natural or artificial channel
which develops across the neck of a meander loop (neck cutoff)
or across a point bar (chute cutoff).

xxii

cutoff wall: Wall, usually of sheet piling or concrete, that extends down to
scour-resistant material or below the expected scour depth.

daily discharge: Discharge averaged over 1 day (24 hours).

debris: Floating or submerged material, such as logs, vegetation, or
trash, transported by a stream.

degradation (bed): General and progressive (long-term) lowering of the channel bed
due to erosion, over a relatively long channel length.

depth of scour: Vertical distance a streambed is lowered by scour below a
reference elevation.

design flow
(design flood):
Discharge that is selected as the basis for the design or
evaluation of a hydraulic structure.

dike: An impermeable linear structure for the control or containment of
overbank flow. A dike-trending parallel with a streambank differs
from a levee in that it extends for a much shorter distance along
the bank, and it may be surrounded by water during floods.

dike (groin, spur, jetty): Structure extending from a bank into a channel that is designed
to: (A) reduce the stream velocity as the current passes through
the dike, thus encouraging sediment deposition along the bank
(permeable dike); or (B) deflect erosive current away from the
streambank (impermeable dike).

discharge: Volume of water passing through a channel during a given time.

dominant discharge: (A) Discharge of water which is of sufficient magnitude and
frequency to have a dominating effect in determining the
characteristics and size of the stream course, channel, and bed;
(B) discharge which determines the principal dimensions and
characteristics of a natural channel. Dominant formative
discharge depends on the maximum and mean discharge,
duration of flow, and flood frequency. For hydraulic geometry
relationships, it is taken to be the bankfull discharge which has a
return period of approximately 1.5 years in many natural
channels.

drainage basin: Area confined by drainage divides, often having only one outlet
for discharge (catchment, watershed).

drift: Alternate term for vegetative "debris."

eddy current: Vortex-type motion of a fluid flowing contrary to the main current,
such as the circular water movement that occurs when the main
flow becomes separated from the bank.

entrenched stream: Stream cut into bedrock or consolidated deposits.
xxiii

ephemeral stream: Stream or reach of stream that does not flow for parts of the year.
As used here, the term includes intermittent streams with flow
less than perennial.
equilibrium scour: Scour depth in sand-bed stream with dune bed about which live
bed pier scour level fluctuates due to variability in bed material
transport in the approach flow.

erosion: Displacement of soil particles due to water or wind action.

erosion control matting: Fibrous matting (e.g., jute, paper, etc.) placed or sprayed on a
streambank for the purpose of resisting erosion or providing
temporary stabilization until vegetation is established.

fabric mattress: Grout-filled mattress used for streambank protection.

fall velocity: Velocity at which a sediment particle falls through a column of still
water.

fascine: Matrix of willow or other natural material woven in bundles and
used as a filter. Also, a streambank protection technique
consisting of wire mesh or timber attached to a series of posts,
sometimes in double rows; the space between the rows may be
filled with rock, brush, or other materials.

fetch: Area in which waves are generated by wind having a rather
constant direction and speed; sometimes used synonymously
with fetch length.

fetch length: Horizontal distance (in the direction of the wind) over which wind
generates waves and wind setup.

fill slope: Side or end slope of an earth-fill embankment. Where a fill-slope
forms the streamward face of a spill-through abutment, it is
regarded as part of the abutment.

filter: Layer of fabric (geotextile) or granular material (sand, gravel, or
graded rock) placed between bank revetment (or bed protection)
and soil for the following purposes: (1) to prevent the soil from
moving through the revetment by piping, extrusion, or erosion; (2)
to prevent the revetment from sinking into the soil; and (3) to
permit natural seepage from the streambank, thus preventing the
buildup of excessive hydrostatic pressure.

filter blanket: Layer of graded sand and gravel laid between fine-grained
material and riprap to serve as a filter.

filter fabric (cloth): Geosynthetic fabric that serves the same purpose as a granular
filter blanket.

xxiv

fine sediment load: That part of the total sediment load that is composed of particle
sizes finer than those represented in the bed (wash load).
Normally, the fine-sediment load is finer than 0.062 mm for
sand-bed channels. Silts, clays and sand could be considered
wash load in coarse gravel and cobble-bed channels.

flanking: Erosion around the landward end of a stream stabilization
countermeasure.

flashy stream: Stream characterized by rapidly rising and falling stages, as
indicated by a sharply peaked hydrograph. Typically associated
with mountain streams or highly disturbed urbanized catchments.
Most flashy streams are ephemeral, but some are perennial.

flood-frequency curve: Graph indicating the probability that the annual flood discharge
will exceed a given magnitude, or the recurrence interval
corresponding to a given magnitude.

floodplain: Nearly flat, alluvial lowland bordering a stream, that is subject to
frequent inundation by floods.

flow-control structure: Structure either within or outside a channel that acts as a
countermeasure by controlling the direction, depth, or velocity of
flowing water.

flow hazard: Flow characteristics (discharge, stage, velocity, or duration) that
are associated with a hydraulic problem or that can reasonably be
considered of sufficient magnitude to cause a hydraulic problem
or to test the effectiveness of a countermeasure.

flow slide: Saturated soil materials which behave more like a liquid than a
solid. A flow slide on a channel bank can result in a bank failure.

fluvial geomorphology: Science dealing with morphology (form) and dynamics of streams
and rivers.

fluvial system: Natural river system consisting of (1) the drainage basin,
watershed, or sediment source area, (2) tributary and mainstem
river channels or sediment transfer zone, and (3) alluvial fans,
valley fills and deltas, or the sediment deposition zone.

freeboard: Vertical distance above a design stage that is allowed for waves,
surges, drift, and other contingencies.

Froude Number: Dimensionless number that represents the ratio of inertial to
gravitational forces in open channel flow.

gabion: Basket or compartmented rectangular container made of wire
mesh. When filled with cobbles or other rock of suitable size, the
gabion becomes a flexible and permeable unit with which flow-
and erosion-control structures can be built.
xxv

general scour: General scour is a lowering of the streambed across the stream
or waterway at the bridge. This lowering may be uniform across
the bed or non-uniform. That is, the depth of scour may be
deeper in some parts of the cross section. General scour may
result from contraction of the flow or other general scour
conditions such as flow around a bend.

geomorphology/
morphology:
That science that deals with the form of the Earth, the general
configuration of its surface, and the changes that take place due
to erosion and deposition.

grade-control structure
(sill, check dam):
Structure placed bank to bank across a stream channel (usually
with its central axis perpendicular to flow) for the purpose of
controlling bed slope and preventing scour or headcutting.

graded stream: Geomorphic term used for streams that have apparently achieved
a state of equilibrium between the rate of sediment transport and
the rate of sediment supply throughout long reaches.

gravel: Rock fragment whose diameter ranges from 2 to 64 mm.

groin: Structure built from the bank of a stream in a direction transverse
to the current to redirect the flow or reduce flow velocity. Many
names are given to this structure, the most common being "spur,"
"spur dike," "transverse dike," "jetty," etc. Groins may be
permeable, semi-permeable, or impermeable.

grout: Fluid mixture of cement and water or of cement, sand, and water
used to fill joints and voids.

guide bank: Dike extending upstream from the approach embankment at
either or both sides of the bridge opening to direct the flow
through the opening. Some guide banks extend downstream from
the bridge (also spur dike).

hardpoint: Streambank protection structure whereby "soft" or erodible
materials are removed from a bank and replaced by stone or
compacted clay. Some hard points protrude a short distance into
the channel to direct erosive currents away from the bank. Hard
points also occur naturally along streambanks as passing
currents remove erodible materials leaving nonerodible materials
exposed.

headcutting: Channel degradation associated with abrupt changes in the bed
elevation (headcut) that generally migrates in an upstream
direction.

helical flow: Three-dimensional movement of water particles along a spiral
path in the general direction of flow. These secondary-type
currents are of most significance as flow passes through a bend;
their net effect is to remove soil particles from the cut bank and
deposit this material on a point bar.
xxvi

hydraulics: Applied science concerned with behavior and flow of liquids,
especially in pipes, channels, structures, and the ground.

hydraulic model: Small-scale physical or mathematical representation of a flow
situation.
hydraulic problem: An effect of streamflow, tidal flow, or wave action such that the
integrity of the highway facility is destroyed, damaged, or
endangered.

hydraulic radius: Cross-sectional area of a stream divided by its wetted perimeter.

hydraulic structures: Facilities used to impound, accommodate, convey or control the
flow of water, such as dams, weirs, intakes, culverts, channels,
and bridges.

hydrograph: The graph of stage or discharge against time.

hydrology: Science concerned with the occurrence, distribution, and
circulation of water on the earth.

imbricated: Reference to stream bed sediment particles, having an
overlapping or shingled pattern.

icing: Masses or sheets of ice formed on the frozen surface of a river or
floodplain. When shoals in the river are frozen to the bottom or
otherwise dammed, water under hydrostatic pressure is forced to
the surface where it freezes.

incised reach: A stretch of stream with an incised channel that only rarely
overflows its banks.

incised stream: Stream which has deepened its channel through the bed of the
valley floor, so that the floodplain is a terrace.

invert: Lowest point in the channel cross section or at flow control
devices such as weirs, culverts, or dams.

island: A permanently vegetated area, emergent at normal stage, that
divides the flow of a stream. Islands originate by establishment of
vegetation on a bar, by channel avulsion, or at the junction of
minor tributary with a larger stream.

jack: Device for flow control and protection of banks against lateral
erosion consisting of three mutually perpendicular arms rigidly
fixed at the center. Kellner jacks are made of steel struts strung
with wire, and concrete jacks are made of reinforced concrete
beams.
xxvii

jack field: Rows of jacks tied together with cables, some rows generally
parallel with the banks and some perpendicular thereto or at an
angle. Jack fields may be placed outside or within a channel.

jetty: (A) Obstruction built of piles, rock, or other material extending
from a bank into a stream, so placed as to induce bank building,
or to protect against erosion; (B) similar obstruction to influence
stream, lake, or tidal currents, or to protect a harbor (also spur).

lateral erosion: Erosion in which the removal of material is extended horizontally
as contrasted with degradation and scour in a vertical direction.

launching: Release of undercut material (stone riprap, rubble, slag, etc.)
downslope or into a scoured area.

levee: Embankment, generally landward of top bank, that confines flow
during high-water periods, thus preventing overflow into lowlands.

live-bed scour: Scour at a pier or abutment (or contraction scour) when the bed
material in the channel upstream of the bridge is moving at the
flow causing bridge scour.

load (or sediment load): Amount of sediment being moved by a stream.

local scour: Removal of material from around piers, abutments, spurs, and
embankments caused by an acceleration of flow and resulting
vortices induced by obstructions to the flow.

longitudinal profile: Profile of a stream or channel drawn along the length of its
centerline. In drawing the profile, elevations of the water surface
or the thalweg are plotted against distance as measured from the
mouth or from an arbitrary initial point.

lower bank: That portion of a streambank having an elevation less than the
mean water level of the stream.

mathematical model: Numerical representation of a flow situation using mathematical
equations (also computer model).

mattress: Blanket or revetment of materials interwoven or otherwise lashed
together and placed to cover an area subject to scour.

meander or full meander: Meander in a river consists of two consecutive loops, one flowing
clockwise and the other counter-clockwise.

meander amplitude: Distance between points of maximum curvature of successive
meanders of opposite phase in a direction normal to the general
course of the meander belt, measured between center lines of
channels.

xxviii

meander belt: Distance between lines drawn tangent to the extreme limits of
successive fully developed meanders.

meander length: Distance along a stream between corresponding points of
successive meanders.

meander loop: Individual loop of a meandering or sinuous stream lying between
inflection points with adjoining loops.

meander ratio: Ratio of meander width to meander length.


meander radius of
curvature:
Radius of a circle inscribed on the centerline of a meander loop.

meander scrolls: Low, concentric ridges and swales on a floodplain, marking the
successive positions of former meander loops.

meander width: Amplitude of a fully developed meander measured from
midstream to midstream.

meandering stream: Stream having a sinuosity greater than some arbitrary value. The
term also implies a moderate degree of pattern symmetry,
imparted by regularity of size and repetition of meander loops.
The channel generally exhibits a characteristic process of bank
erosion and point bar deposition associated with systematically
shifting meanders.

median diameter: Particle diameter of the 50th percentile point on a size distribution
curve such that half of the particles (by weight, number, or
volume) are larger and half are smaller (D
50
).

mid-channel bar: Bar lacking permanent vegetal cover that divides the flow in a
channel at normal stage.

middle bank: Portion of a streambank having an elevation approximately the
same as that of the mean water level of the stream.

migration: Change in position of a channel by lateral erosion of one bank
and simultaneous accretion of the opposite bank.

mud: A soft, saturated mixture mainly of silt and clay.

natural levee: Low ridge that slopes gently away from the channel banks that is
formed along streambanks during floods by deposition.

nominal diameter: Equivalent spherical diameter of a hypothetical sphere of the
same volume as a given sediment particle.

nonalluvial channel: Channel whose boundary is in bedrock or non-erodible material.

xxix

normal stage: Water stage prevailing during the greater part of the year.


overbank flow: Water movement that overtops the bank either due to stream
stage or to overland surface water runoff.

oxbow: Abandoned former meander loop that remains after a stream cuts
a new, shorter channel across the narrow neck of a meander.
Often bow-shaped or horseshoe-shaped.

pavement: Streambank surface covering, usually impermeable, designed to
serve as protection against erosion. Common pavements used on
streambanks are concrete, compacted asphalt, and soil-cement.

paving: Covering of stones on a channel bed or bank (used with
reference to natural covering).

peaked stone dike: Riprap placed parallel to the toe of a streambank (at the natural
angle of repose of the stone) to prevent erosion of the toe and
induce sediment deposition behind the dike.

perennial stream: Stream or reach of a stream that flows continuously for all or most
of the year.

phreatic line: Upper boundary of the seepage water surface landward of a
streambank.

pile: Elongated member, usually made of timber, concrete, or steel,
that serves as a structural component of a river-training structure.

pile dike: Type of permeable structure for the protection of banks against
caving; consists of a cluster of piles driven into the stream, braced
and lashed together.

piping: Removal of soil material through subsurface flow of seepage
water that develops channels or "pipes" within the soil bank.

point bar: Alluvial deposit of sand or gravel lacking permanent vegetal cover
occurring in a channel at the inside of a meander loop, usually
somewhat downstream from the apex of the loop.

poised stream: Stream which, as a whole, maintains its slope, depths, and
channel dimensions without any noticeable raising or lowering of
its bed (stable stream). Such condition may be temporary from a
geological point of view, but for practical engineering purposes,
the stream may be considered stable.

probable maximum flood: Very rare flood discharge value computed by hydro-
meteorological methods, usually in connection with major
hydraulic structures.

xxx

quarry-run stone: Stone as received from a quarry without regard to gradation
requirements.

railbank protection: Type of countermeasure composed of rock-filled wire fabric
supported by steel rails or posts driven into streambed.

rapid drawdown: Lowering the water against a bank more quickly than the bank
can drain without becoming unstable.

reach: Segment of stream length that is arbitrarily bounded for purposes
of study.

recurrence interval: Reciprocal of the annual probability of exceedance of a hydrologic
event (also return period, exceedance interval).

regime: Condition of a stream or its channel with regard to stability. A
stream is in regime if its channel has reached an equilibrium form
as a result of its flow characteristics. Also, the general pattern of
variation around a mean condition, as in flow regime, tidal regime,
channel regime, sediment regime, etc. (used also to mean a set
of physical characteristics of a river).

regime change: Change in channel characteristics resulting from such things as
changes in imposed flows, sediment loads, or slope.

regime channel: Alluvial channel that has attained, more or less, a state of
equilibrium with respect to erosion and deposition.

regime formula: Formula relating stable alluvial channel dimensions or slope to
discharge and sediment characteristics.

reinforced-earth
bulkhead:
Retaining structure consisting of vertical panels and attached to
reinforcing elements embedded in compacted backfill for
supporting a streambank.

reinforced revetment: Streambank protection method consisting of a continuous stone
toe-fill along the base of a bank slope with intermittent fillets of
stone placed perpendicular to the toe and extending back into the
natural bank.

relief bridge: An opening in an embankment on a floodplain to permit passage
of overbank flow.

retard (retarder
structure):
Permeable or impermeable linear structure in a channel parallel
with the bank and usually at the toe of the bank, intended to
reduce flow velocity, induce deposition, or deflect flow from the
bank.

revetment: Rigid or flexible armor placed to inhibit scour and lateral erosion.
(See bank revetment).

xxxi

riffle: Natural, shallow flow area extending across a streambed in which
the surface of flowing water is broken by waves or ripples.
Typically, riffles alternate with pools along the length of a stream
channel.

riparian: Pertaining to anything connected with or adjacent to the banks of
a stream (corridor, vegetation, zone, etc.).

riprap: Layer or facing of rock or broken concrete dumped or placed to
protect a structure or embankment from erosion; also the rock or
broken concrete suitable for such use. Riprap has also been
applied to almost all kinds of armor, including wire-enclosed
riprap, grouted riprap, partially grouted riprap, sacked concrete,
and concrete slabs.

river training: Engineering works with or without the construction of
embankment, built along a stream or reach of stream to direct or
to lead the flow into a prescribed channel. Also, any structure
configuration constructed in a stream or placed on, adjacent to, or
in the vicinity of a streambank that is intended to deflect currents,
induce sediment deposition, induce scour, or in some other way
alter the flow and sediment regimes of the stream.

rock-and-wire mattress: Flat wire cage or basket filled with stone or other suitable
material and placed as protection against erosion.

roughness coefficient: Numerical measure of the frictional resistance to flow in a
channel, as in the Manning's or Chezy's formulas.

rubble: Rough, irregular fragments of materials of random size used to
retard erosion. The fragments may consist of broken concrete
slabs, masonry, or other suitable refuse.

runoff: That part of precipitation which appears in surface streams of
either perennial or intermittent form.

sack revetment: Sacks (e.g., burlap, paper, or nylon) filled with mortar, concrete,
sand, stone or other available material used as protection against
erosion.

saltation load: Sediment bounced along the streambed by energy and
turbulence of flow, and by other moving particles.

sand: Rock fragment whose diameter is in the range of 0.062 to 2.0
mm.

scour: Erosion of streambed or bank material due to flowing water; often
considered as being localized (see local scour, contraction scour,
total scour).

xxxii

sediment or fluvial
sediment:
Fragmental material transported, suspended, or deposited by
water.

sediment concentration: Weight or volume of sediment relative to the quantity of
transporting (or suspending) fluid.

sediment discharge: Quantity of sediment that is carried past any cross section of a
stream in a unit of time. Discharge may be limited to certain sizes
of sediment or to a specific part of the cross section.

sediment load: Amount of sediment being moved by a stream.

sediment yield: Total sediment outflow from a watershed or a drainage area at a
point of reference and in a specified time period. This outflow is
equal to the sediment discharge from the drainage area.

seepage: Slow movement of water through small cracks and pores of the
bank material.
shear stress: See unit shear force.

shoal: A relatively shallow submerged bank or bar in a body of water.

sill: (A) Structure built under water, across the deep pools of a stream
with the aim of changing the depth of the stream; (B) low
structure built across an effluent stream, diversion channel or
outlet to reduce flow or prevent flow until the main stream stage
reaches the crest of the structure.

silt: Particle whose diameter is in the range of 0.004 to 0.062 mm.

sinuosity: Ratio between the thalweg length and the valley length of a
stream.

slope (of channel or
stream):
Fall per unit length along the channel centerline or thalweg.

slope protection: Any measure such as riprap, paving, vegetation, revetment, brush
or other material intended to protect a slope from erosion, slipping
or caving, or to withstand external hydraulic pressure.

sloughing: Sliding or collapse of overlying material; same ultimate effect as
caving, but usually occurs when a bank or an underlying stratum
is saturated.

slope-area method: Method of estimating unmeasured flood discharges in a uniform
channel reach using observed high-water levels.

slump: Sudden slip or collapse of a bank, generally in the vertical
direction and confined to a short distance, probably due to the
substratum being washed out or having become unable to bear
the weight above it.
xxxiii

soil-cement: A designed mixture of soil and Portland cement compacted at a
proper water content to form a blanket or structure that can resist
erosion.

sorting: Progressive reduction of size (or weight) of particles of the
sediment load carried down a stream.

spill-through abutment: Bridge abutment having a fill slope on the streamward side. The
term originally referred to the "spill-through" of fill at an open
abutment but is now applied to any abutment having such a
slope.

spread footing: Pier or abutment footing that transfers load directly to the earth.

spur: Permeable or impermeable linear structure that projects into a
channel from the bank to alter flow direction, induce deposition, or
reduce flow velocity along the bank.

spur dike: See guide bank.

stability: Condition of a channel when, though it may change slightly at
different times of the year as the result of varying conditions of
flow and sediment charge, there is no appreciable change from
year to year; that is, accretion balances erosion over the years.

stable channel: Condition that exists when a stream has a bed slope and cross
section which allows its channel to transport the water and
sediment delivered from the upstream watershed without
aggradation, degradation, or bank erosion (a graded stream).

stage: Water-surface elevation of a stream with respect to a reference
elevation.

stone riprap: Natural cobbles, boulders, or rock dumped or placed as
protection against erosion.

stream: Body of water that may range in size from a large river to a small
rill flowing in a channel. By extension, the term is sometimes
applied to a natural channel or drainage course formed by flowing
water whether it is occupied by water or not.

streambank erosion: Removal of soil particles or a mass of particles from a bank
surface due primarily to water action. Other factors such as
weathering, ice and debris abrasion, chemical reactions, and land
use changes may also directly or indirectly lead to bank erosion.

streambank failure: Sudden collapse of a bank due to an unstable condition such as
removal of material at the toe of the bank by scour.

streambank protection: Any technique used to prevent erosion or failure of a streambank.

xxxiv

suspended sediment
discharge:
Quantity of sediment passing through a stream cross section
above the bed layer in a unit of time suspended by the turbulence
of flow (suspended load).

sub-bed material: Material underlying that portion of the streambed which is subject
to direct action of the flow. Also, substrate.

subcritical, supercritical
flow:
Open channel flow conditions with Froude Number less than and
greater than unity, respectively.

tetrahedron: Component of river-training works made of six steel or concrete
struts fabricated in the shape of a pyramid.



tetrapod: Bank protection component of precast concrete consisting of four
legs joined at a central joint, with each leg making an angle of
109.5E with the other three.

thalweg Line extending down a channel that follows the lowest elevation of
the bed.

tieback: Structure placed between revetment and bank to prevent flanking.

timber or brush mattress: Revetment made of brush, poles, logs, or lumber interwoven or
otherwise lashed together. The completed mattress is then placed
on the bank of a stream and weighted with ballast.

toe of bank: That portion of a stream cross section where the lower bank
terminates and the channel bottom or the opposite lower bank
begins.

toe protection: Loose stones laid or dumped at the toe of an embankment, groin,
etc., or masonry or concrete wall built at the junction of the bank
and the bed in channels or at extremities of hydraulic structures to
counteract erosion.

total scour: Sum of long-term degradation, general (contraction) scour, and
local scour.

total sediment load: Sum of suspended load and bed load or the sum of bed material
load and wash load of a stream (total load).

tractive force: Drag or shear on a streambed or bank caused by passing water
which tends to move soil particles along with the streamflow.

trench-fill revetment: Stone, concrete, or masonry material placed in a trench dug
behind and parallel to an eroding streambank. When the erosive
action of the stream reaches the trench, the material placed in the
trench armors the bank and thus retards further erosion.

xxxv

turbulence: Motion of fluids in which local velocities and pressures fluctuate
irregularly in a random manner as opposed to laminar flow where
all particles of the fluid move in distinct and separate lines.

ultimate scour: Maximum depth of scour attained for a given flow condition. May
require multiple flow events and in cemented or cohesive soils
may be achieved over a long time period.

uniform flow: Flow of constant cross section and velocity through a reach of
channel at a given time. Both the energy slope and the water
slope are equal to the bed slope under conditions of uniform flow.

unit discharge: Discharge per unit width (may be average over a cross section, or
local at a point).


unit shear force
(shear stress):
Force or drag developed at the channel bed by flowing water. For
uniform flow, this force is equal to a component of the gravity
force acting in a direction parallel to the channel bed on a unit
wetted area. Usually in units of stress, Pa (N/m
2
) or (lb/ft
2
).

unsteady flow: Flow of variable discharge and velocity through a cross section
with respect to time.

upper bank: Portion of a streambank having an elevation greater than the
average water level of the stream.

velocity: Time rate of flow usually expressed in m/s (ft/sec). Average
velocity is the velocity at a given cross section determined by
dividing discharge by cross-sectional area.

vertical abutment: An abutment, usually with wingwalls, that has no fill slope on its
streamward side.

vortex: Turbulent eddy in the flow generally caused by an obstruction
such as a bridge pier or abutment (e.g., horseshoe vortex).

wandering channel: Channel exhibiting a more or less non-systematic process of
channel shifting, erosion and deposition, with no definite
meanders or braided pattern.

wandering thalweg: Thalweg whose position in the channel shifts during floods and
typically serves as an inset channel that conveys all or most of the
stream flow at normal or lower stages.

wash load: Suspended material of very small size (generally clays and
colloids) originating primarily from erosion on the land slopes of
the drainage area and present to a negligible degree in the bed
itself.

xxxvi

watershed: See drainage basin.

waterway opening width
(area):
Width (area) of bridge opening at (below) a specified stage,
measured normal to the principal direction of flow.

weephole: A hole in an impermeable wall or revetment to relieve the neutral
stress or pore pressure in the soil.

windrow revetment: Row of stone placed landward of the top of an eroding
streambank. As the windrow is undercut, the stone is launched
downslope, thus armoring the bank.

wire mesh: Wire woven to form a mesh; where used as an integral part of a
countermeasure, openings are of suitable size and shape to
enclose rock or broken concrete or to function on fence-like spurs
and retards.
1.1
CHAPTER 1

INTRODUCTION


1.1 PURPOSE

The purpose of this document is to identify and provide design guidelines for bridge scour
and stream instability countermeasures that have been implemented by various State
departments of transportation (DOTs) in the United States. Countermeasure experience,
selection, and design guidance are consolidated from other FHWA publications in this
document to support a comprehensive analysis of scour and stream instability problems and
provide a range of solutions to those problems. The results of recently completed National
Cooperative Highway Research Program (NCHRP) projects are incorporated in the design
guidance, including: countermeasures to protect bridge piers and abutments from scour;
riprap design criteria, specifications, and quality control, and environmentally sensitive
channel and bank protection measures. Selected innovative countermeasure concepts and
guidance derived from practice outside the United States are introduced. In addition,
guidance for the preparation of Plans of Action (POA) for scour critical bridges has been
expanded to include a standard template for POA and instructions for its use.

1.2 BACKGROUND

Scour and stream instability problems have always threatened the safety of our nation's
highway bridges. Countermeasures for these problems are defined as measures
incorporated into a highway-stream crossing system to monitor, control, inhibit, change,
delay, or minimize stream instability and bridge scour problems. A plan of action, which can
include timely installation of stream instability and scour countermeasures, must be
developed for each scour critical bridge. Monitoring structures during and/or after flood
events as a part of a plan of action, can also be considered an appropriate countermeasure.

Numerous measures are available to counteract the actions of humans and nature which
contribute to the instability of alluvial streams. These include measures installed in or near
the stream to protect highways and bridges by stabilizing a local reach of the stream, and
measures which can be incorporated into the highway design to ensure the structural
integrity of the highway in an unstable stream environment. Countermeasures include river
stabilizing works over a reach of the river up- and downstream of the crossing.
Countermeasures may be installed at the time of highway construction or retrofitted to
resolve scour and instability problems as they develop at existing crossings. The selection,
location, and design of countermeasures are dependent on hydraulic and geomorphic
factors that contribute to stream instability, as well as costs and construction and
maintenance considerations.

While considerable research has been dedicated to design of countermeasures for scour
and stream instability, many countermeasures have evolved through a trial and error
process. In addition, some countermeasures have been applied successfully in one locale,
state or region, but have failed when installations were attempted under different geomorphic
or hydraulic conditions. In some cases, a countermeasure that has been used with success
in one state or region is virtually unknown to highway design and maintenance personnel in
another state or region. Thus, there is a significant need for information transfer regarding
stream instability and bridge scour countermeasure design, installation, and maintenance.

1.2
1.3 MANUAL ORGANIZATION

This manual is presented in two volumes. Volume 1 is organized to:

Provide strategies and general guidance for developing a Plan of Action for a scour
critical bridge (Chapter 2)

Highlight the various groups of countermeasures and identify their individual
characteristics (Chapter 2)

For a wide-range of countermeasures, list information on their functional applicability to a
particular problem, their suitability to specific river environments, the general level of
maintenance resources required, and which DOTs have experience with specific
countermeasures (Chapter 2 and the Countermeasures Matrix).

Provide general criteria for selection of countermeasures for bridge scour and stream
instability problems (Chapter 3)

Discuss countermeasure design concepts including design approach, hydraulic analysis,
and environmental permitting (Chapter 4).

Provide an overview of design considerations related to riprap: including filters; riprap
specifications, testing and quality control; riprap failure modes, and inspection guidance
(Chapter 5).

Discuss biotechnical engineering approaches and provide conceptual guidelines
(Chapter 6).

Provide detailed design guidelines for specific bridge scour and stream instability
countermeasures (Chapter 7 and Volume 2, Design Guidelines 1 through 19).

Summarize general guidance for other countermeasures and case histories of
countermeasure performance (Chapter 8).

Provide criteria for selecting portable and fixed instrumentation for monitoring scour at
bridges (Chapter 9).

Volume 2 presents detailed Design Guidelines for 16 stream instability and bridge scour
countermeasures. Design Guidelines are presented for the following functional applications:

Countermeasures for stream instability
Countermeasures for streambank and roadway embankment protection
Countermeasures for bridge pier protection
Countermeasures for abutment protection
Granular and geotextile filter requirements
Special Applications

1.3
1.4 COMPREHENSIVE ANALYSIS

This manual is part of a set of Hydraulic Engineering Circulars (HEC) issued to provide
guidance for bridge scour and stream stability analyses. The three manuals in this set are:

HEC-18 Evaluating Scour at Bridges
HEC-20 Stream Stability at Highway Structures
HEC-23 Bridge Scour and Stream Instability Countermeasures

The Flow Chart shown in Figure 1.1 illustrates the interrelationship between these three
documents and emphasizes that they should be used as a set. A comprehensive scour
analysis or stability evaluation must be based on information presented in all three
documents.

While the flow chart does not attempt to present every detail of a complete stream stability
and scour evaluation, it has sufficient detail to show the major elements in a complete
analysis, the logical flow of a typical analysis or evaluation, and the most common decision
points and feedback loops. It clearly shows how the three documents tie together, and
recognizes the differences between design of a new bridge and evaluation of an existing
bridge.

The HEC-20 (Lagasse et al. 2001a) block of the flow chart outlines initial data collection and
site reconnaissance activities leading to an understanding of the problem, evaluation of river
system stability and potential future response. The HEC-20 procedures include both
qualitative and quantitative geomorphic and engineering analysis techniques which help
establish the level of analysis necessary to solve the stream instability and scour problem for
design of a new bridge, or for the evaluation of an existing bridge that may require
rehabilitation or countermeasures. The "Classify Stream," "Evaluate Stream Stability," and
"Assess Stream Response" portions of the HEC-20 block are expanded in HEC-20 into a
six-step Level 1 and an eight-step Level 2 analysis procedure. In some cases, the HEC-20
analysis may be sufficient to determine that stream instability and/or scour problems do not
exist, i.e., the bridge has a "low risk of failure" regarding scour susceptibility.

In most cases, the analysis or evaluation will progress to the HEC-18 (Richardson and Davis
2001) block of the flow chart. Here more detailed hydrologic and hydraulic data are
developed, with the specific approach determined by the level of complexity of the problem
and waterway characteristics (e.g., tidal or riverine). The "Scour Analysis" portion of the
HEC-18 block encompasses a seven-step specific design approach which includes
evaluation of the components of total scour.

Since bridge scour evaluation requires multidisciplinary inputs, it is often advisable for the
hydraulic engineer to involve structural and geotechnical engineers at this stage of the
analysis. Once the total scour prism is plotted, then all three disciplines must be
involved in a determination of the structural stability of the bridge foundation.

For a new bridge design, if the structure is stable the design process can proceed to
consideration of environmental impacts, cost, constructability, and maintainability or if the
bridge is unstable, revise the design and repeat the analysis. For an existing bridge, a
finding of structural stability at this stage will result in a "low risk" evaluation, with no further
action required. However, a Plan of Action must be developed for an unstable existing
bridge (scour critical) to correct the problem as outlined in Sections 1.5 and 2.1.
1.4
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Figure 1.1. Flow chart for scour and stream stability analysis and evaluation.

1.5
The scour problem may be so serious that installing countermeasures would not provide a
viable solution and a replacement or substantial bridge rehabilitation would be required. If
countermeasures would correct the stream instability or scour problem at a reasonable cost
and with acceptable environmental impacts, the analysis would progress to the HEC-23
block of the flow chart.

Hydraulic Engineering Circular 23 provides a range of resources to support bridge scour and
stream instability countermeasure selection and design. A countermeasure matrix (Chapter
2) presents a variety of countermeasures that have been used to control scour and stream
instability at bridges.

HEC-23 also includes specific Design Guidelines for the most common (and some
uncommon) countermeasures used by DOTs, or references to sources of design guidance.
Inherent in the design of any countermeasure are an evaluation of potential environmental
impacts, permitting for countermeasure installation, and redesign, if necessary, to meet
environmental requirements. As shown in the flow chart, to be effective most
countermeasures will require a monitoring plan, inspection, and maintenance.

1.5 PLAN OF ACTION

Each bridge identified as scour critical in Item 113 of the National Bridge Inventory must
have a plan of action as required by the National Bridge Inspection Standards (NBIS)
regulation 23 CFR 650.313(e) describing what will be done to address the scour problem.
The plan of action should include a monitoring program and a schedule for the timely design
and construction of hydraulic or structural countermeasures, if any are warranted. The
purpose of the plan of action is to provide for the safety of the traveling public, and to
minimize the potential for bridge failure, by prescribing site-specific actions that will be taken
at the bridge to correct the scour problem. The actions (or countermeasures) taken can be
categorized as hydraulic countermeasures, structural countermeasures, biotechnical
countermeasures, or a monitoring program (see Chapter 2).

Hydraulic countermeasures are primarily designed to modify the stream flow or resist erosive
forces. Examples of hydraulic countermeasures include the installation of river training
structures and the placement of riprap at piers or abutments. Structural countermeasures
usually involve modification of the bridge substructure to increase bridge stability. Typical
structural countermeasures are underpinning and pier modification.

A scour monitoring program as part of the plan of action includes two primary components:

1. The frequency and type of measurements to facilitate early identification of potential
scour problems, and
2. Specific instructions describing precisely what must be done if a bridge is at risk due to
scour.

Note that a monitoring program involves more than just instrumentation. It must describe
specific actions to be taken once a scour problem has been identified. In some cases, a
properly designed scour monitoring program can be an acceptable countermeasure by itself.
However, monitoring does not fix the scour problem, and therefore, does not allow changing
the Item 113 coding on a scour-critical bridge. In other cases, a monitoring program allows
time to implement hydraulic or structural countermeasures. Information in Section 2.1
outlines how to develop a plan of action for a scour critical bridge, and provides specific
strategies for deciding when and how to implement a monitoring program.
1.6
1.6 DUAL SYSTEM OF UNITS

This edition of HEC-23 uses dual units (English and SI metric). The "English" system of
units as used throughout this manual refers to U.S. Customary units. In Appendix A, the
metric (SI) unit of measurement is explained. The conversion factors, physical
properties of water in the SI and English systems of units, sediment particle size
grade scale, and some common equivalent hydraulic units are also given. This edition
uses for the unit of length the meter (m) or foot (ft); of mass the kilogram (kg) or slug; of
weight/force the newton (N) or pound (lb); of pressure the Pascal (Pa, N/m
2
) or (lb/ft
2
); and
of temperature the degree Centigrade (EC) or Fahrenheit (EF). The unit of time is the same
in SI as in English system (seconds, s). Sediment particle size is given in millimeters (mm),
but in calculations the decimal equivalent of millimeters in meters is used (1 mm = 0.001 m)
or for the English system feet (ft). The values of some hydraulic engineering terms used in
the text in SI units and their equivalent English units are given in Table 1.1.

Table 1.1. Commonly Used Engineering Terms in SI and English Units.
Term English Units SI Units
Length 3.28 ft 1 m
Volume 35.31 ft
3
1 m
3

Discharge 35.31 ft
3
/s 1 m
3
/s
Acceleration of Gravity 32.2 ft/s
2
9.81 m/s
2

Unit Weight of Water 62.4 lb/ft
3
9800 N/m
3

Density of Water 1.94 slugs/ft
3
1000 kg/m
3

Density of Quartz 5.14 slugs/ft
3
2647 kg/m
3

Specific Gravity of Quartz 2.65 2.65
Specific Gravity of Water 1 1
Temperature EF EC = 5/9 (EF - 32)



2.1
CHAPTER 2

PLAN OF ACTION AND THE COUNTERMEASURES MATRIX


2.1 STRATEGIES FOR PROTECTING SCOUR CRITICAL BRIDGES

2.1.1 Technical Advisories

The National Bridge Inspection Standards (23 CFR 650, Subpart C) requires bridge owners
to maintain a bridge inspection program that includes procedures for underwater inspection.
A national scour evaluation program as an integral part of the National Bridge Inspection
standards was established in 1988 by Technical Advisory T5140.20 (USDOT 1988).

Technical Advisory T5140.20 was superseded in 1991 by Technical Advisory T5140.23, to
provide more guidance on the development and implementation of procedures for evaluating
bridge scour to meet the requirements of 23 CFR 650, Subpart C). Specifically, Technical
Advisory T5140.23 provides guidance on:

1. Developing and implementing a scour evaluation for designing new bridges
2. Evaluating existing bridges for scour vulnerability
3. Using scour countermeasures
4. Improving the state-of-practice for estimating scour at bridges

The Technical Advisory suggests that scour evaluations of both new and existing bridges
should be conducted by an interdisciplinary team comprised of hydraulic, geotechnical and
structural engineers. The recommendation for new bridges is to design the bridge foundation
for potential scour by assuming that all streambed material in the computed scour prism has
been removed and is not available for bearing or lateral support. Bridge foundations should
be designed to withstand scour during floods equal to or less than the 100-year flood, and
should be checked to ensure they will not fail during a superflood (on the order of the 500-
year event). The procedures for computing the scour prism, which represents calculated
scour conditions, are detailed in HEC-18 (Richardson and Davis 2001).

The recommendation for existing bridges is to evaluate every bridge over a waterway for
scour to determine if it is scour critical or low risk. For a scour critical bridge, prudent
measures should be taken for its protection. A scour critical bridge is one with abutment or
pier foundations that are coded as unstable due to (1) observed scour at the bridge site, or
(2) scour potential as determined from a scour evaluation study. A bridge that is not scour
critical was defined as low risk, generally considered to have little potential for scour or
stream instability problems. Results of the scour evaluation study for existing bridges are
coded in Item 113 of the 1995 Recording and Coding Guide for the Structure Inventory and
Appraisal of the Nations Bridges - FHWA Report PD-96-001 (more commonly known as the
Coding Guide) (FHWA 1995, 2001).

Technical Advisory T5140.23 specifies that a plan of action should be developed for each
existing bridge found to be scour critical. The two primary components of the plan of
action are instructions regarding the type and frequency of inspections to be made at
the bridge, and a schedule for the timely design and construction of scour
countermeasures. The Technical Advisory further recommends appropriate training and
instruction for bridge inspectors in scour issues. These include issues such as collection
2.2
and comparison of cross section data, identification of conditions indicative of potential scour
problems, and effective notification procedures when an actual or potential problem is
identified at or in the vicinity of the bridge.

Information in this chapter provides direction for developing a plan of action. A "standard"
template for preparing a plan of action is referenced and discussed. Issues related to the
type and frequency of inspections are described, followed by the range of scour
countermeasures available that could be incorporated in the plan of action. Finally, the
countermeasures matrix is introduced, which provides a concise summary of the available
countermeasures in categories classified as hydraulic, structural, biotechnical, and
monitoring. The remainder of HEC-23 details the various scour countermeasures available in
each category, which might be implemented through a plan of action.

2.1.2 Additional Guidance and Requirements

Revisions to the Coding Guide for Item 113 (FHWA 2001) provide new guidance for coding
bridges for observed and assessed scour conditions. HEC-23 is referenced for guidance
on countermeasures for the protection of bridge foundations. These revisions to Item 113
codes allow bridge owners to consider countermeasures when coding (or re-coding) a bridge
as stable, low risk, or scour critical based on the results of a bridge inspection and/or a scour
evaluation.

As noted, responsibility for the National Bridge Inspection Standards (NBIS) is established in
the Code of Federal Regulations (23 CFR 650, Sub-part C). Updates to 23 CFR 650, Sub-
part C, underscore actions required for bridges that are determined to be scour
critical. These include preparation of a plan of action to monitor known and potential
deficiencies and to address critical findings, and monitoring of bridges in accordance with the
plan for bridges that are scour critical (USDOT 2004, effective January 2005).


Inspection procedures for bridge owners are delineated in the CFR (650.313). Regarding
follow-up on critical findings, the regulation requires that: (1) a state- or federal agency-wide
procedure be established to assure that critical findings are addressed in a timely manner,
and (2) FHWA be notified periodically of the actions taken to resolve or monitor critical
findings.

2.1.3 Management Strategies for a Plan of Action

As described above, when a bridge is found to be scour critical, either by inspection or by
evaluation (i.e., assessed or calculated), a plan of action must be developed and
implemented for that bridge. While many bridges may be found to be scour critical, the
severity of the problem and the risk involved to the traveling public can vary dramatically. As
a result, the management strategy for the plan of action, including factors such as the
urgency of the response, the type and frequency of the inspection work, the redundancy in
the plan, and amount of money and resources allocated to countermeasures (including
monitoring), can vary from one scour critical bridge to the next.

For example, a bridge found to be scour critical by inspection, such as during an underwater
inspection that finds a substantial scour hole undermining the foundation, would obviously be
a greater concern than a bridge that is currently stable, but rated scour critical based on
calculations of conditions that might develop during the 100-year flood. In the first case, the
bridge has already experienced scour and is at risk of failure, whereas in the second case
2.3
the bridge is not presently at risk, but might develop a scour problem in the future when it is
subjected to the 100-year flood. The resulting management strategy for developing and
implementing the plan of action would be much more urgent in the first case.

The management strategy may also vary according to the importance of the roadway to the
transportation network and may require a risk-based analysis. For example, a bridge with
high average daily traffic (ADT), or one that provides the only access in and out of a given
area would be a greater concern than a low ADT bridge, or one for which alternate routes or
detours were available. Similarly, a bridge that provides access for a hospital or fire station
would be very important and might justify more resources or concern in developing and
implementing a plan of action. A bridge that is along an evacuation route or provides access
to an airport might also require a different level of response in developing a plan of action.

The management strategy might vary as a result of other repair or replacement plans. For
example, a bridge found to be scour critical but already programmed for replacement in the
near future might be treated differently from another bridge that was newer, or not
considered for replacement for many years. In the first case, the use of monitoring as a
countermeasure until replacement can occur might be reasonable, whereas in the second
case, a structural countermeasure, at substantially greater cost, would probably be
necessary.

As noted in Section 2.1.2, updates to Item 113 codes allow bridge owners to consider
countermeasures when coding a bridge as stable, low risk, or scour critical, based on the
results of a bridge inspection and/or a scour evaluation.

Hydraulic or structural
countermeasures that have been selected and designed by the interdisciplinary team, and
properly installed can change a scour critical coding under Item 113 to a low risk code. Also,
the updates allow bridge owners to consider mitigation measures installed during and/or
immediately after a flood event in determining the appropriate Item 113 code. For this case,
a plan of action must include specific instructions for monitoring the countermeasures to
reduce the risk to the public users from a bridge failure.

2.1.4 Inspection Strategies in a Plan of Action

The type and frequency of inspection work called for in the plan of action can also vary
dramatically based on the management strategy. Bridges that are more important, or at
higher risk, may justify more intense inspection efforts. Factors such as when to begin the
inspection work, how often to visit the bridge during a flood, and when monitoring is no
longer necessary must be addressed in the plan of action.

If a bridge foundation is determined to be unstable for the assessed or calculated stream
stability or scour condition, and field inspection shows no evidence of a scour problem, the
inspection requirements may not be any more than those required by the NBIS. For
example, a bridge that is rated scour critical by calculations, but has a relatively deep piles in
an erosion-resistant material and has been in place for many years with no sign of scour,
might adequately be addressed through the regular inspection cycle and after major flood
events.

If more frequent inspections are required, the plan of action should to describe when to
begin monitoring efforts. Initiation of inspection work can be based on discharge or stage
measurements. While discharge is used to define or analyze scour conditions (e.g. scour
during the 100-year flood), it is typically not the best criteria for triggering flood monitoring
and inspection work. The primary limitation of a discharge based criteria is that the inspector
2.4
often does not have a way of determining the discharge in the river, such as gaging station
or flood forecasting results.

A more viable approach to define when to begin scour measurements has been to use the
stage data corresponding to a critical discharge condition. However, even stage data must
be specified in a manner that is easily understood and measurable by bridge inspection
crews. For example, defining the initiation of scour measurements based on flood stage is
only practical if stage information is readily available, and/or a gaging station is located at or
near the bridge. Alternatively, if the critical water surface elevation is defined based on the
distance from the guard rail or curb line of the bridge, the inspector could measure that
distance and know when to begin data collection. An even more direct approach is to mark
a line on a pier or abutment that defines when data collection or monitoring should be
initiated.

On a basin wide basis, it may be possible to define flood watch requirements based on flood
forecasting information. A simplistic approach is to implement monitoring after a given
amount of rain has occurred. For example, the criteria might be to begin monitoring after a
cumulative rainfall of 10 inches (25 cm) in 24 hours. A general criteria such as this might
require the bridge inspection crews to immediately begin monitoring all scour critical bridges
in that basin. Alternatively, in a more instrumented watershed with extensive flood warning
systems, the use of GIS data and flood forecasting models could define in advance which
bridges will need to be monitored at what times during the flood.

Once the flood inspection program is underway, the inspector needs to know exactly what
constitutes a critical scour condition, and what to do when this condition has been detected.
Specifically, a scour critical water surface elevation should be defined in the plan of action
for each pier or abutment to be monitored. Information on who to call and what action to
take once that elevation has been reached should also be detailed in the plan of action. This
could extend as far as discussion of emergency repair measures and/or bridge closure
instructions.

2.1.5 Closure Instructions

Closure instructions can range from load restrictions, lane closures or complete bridge
closure, again depending on the severity of the problem and the risk involved. The method
of closure should also be described. In some cases barricades may be adequate, while in
other cases it may require, or justify based on the risk involved, the posting of a law
enforcement officer at the bridge to insure that no one attempts to cross the structure. The
availability and description of detour routes should be included in the plan of action, so when
a bridge is closed an alternative route has already been defined to minimize traffic
disruption. The scour vulnerability of bridges along the detour route should be known and
evaluated when developing detour alternatives.

Instructions on the criteria for re-opening the bridge or traffic lane, or removing the load
restriction, should also be provided. In many cases, the act of closing is easier than re-
opening. Virtually anyone who detects a problem, such as an inspector, law enforcement
officer, or bridge owner could make the decision to close a bridge, but the decision about
when it is safe to re-open may require more information and engineering analysis by the
interdisciplinary team. The person authorized to make the decision to re-open should be
identified in the plan of action.


2.5
2.1.6 Countermeasure Alternatives and Schedule

The two primary components of the plan of action are instructions regarding the type and
frequency of inspections to be made at the bridge, and a schedule for the timely design and
construction of scour countermeasures. Developing a schedule for the timely construction of
countermeasures first requires defining the preferred countermeasure alternative. It is typical
that several different alternatives might be appropriate countermeasures for a given scour or
stream stability problem at a bridge. A comprehensive plan of action should provide enough
information that an independent reviewer could arrive at the same conclusion regarding the
preferred alternative.

In order to evaluate alternatives a conceptual design should be developed for various
alternatives. This facilitates evaluation of the engineering feasibility of the alternative, and
allows developing preliminary cost estimates. The various alternatives developed should be
presented in the plan of action, and a narrative provided describing why the preferred
alternative was chosen.

Once the preferred alternative is selected, a schedule should be developed for the timely
design and construction of the preferred alternative. It may be that a more intense
monitoring alternative is recommended as a measure to reduce the risk from scour, prior to
design and construction of countermeasures to make the bridge safe from scour.

2.1.7 Other Information Necessary in a Plan of Action

The plan of action can include other information to the inspector, including special conditions
to watch for such as debris build-up and associated problems. It might include instructions
on communications with the media, such as who is authorized to make statements and what
information should be provided. Actions such as bridge closures and/or bridge failures
generate a lot of interest and concern from both the media and the public. Developing a
communications plan ahead of time can minimize confusion and mis-communication. The
plan of action might also describe emergency action countermeasures, such as what type of
riprap is adequate, local sources, and installation methods during a flood situation.

2.1.8 Development and Implementation of a POA

The development of a POA means that bridge owners have held meetings involving the
appropriate personnel from internal units within their corresponding agency (design,
construction, inspection and maintenance, districts, and others as applicable) and with
external entities (local authorities such as a commissioner, police department, fire
department, and others as needed) to identify and document:

General information about the bridge responsibility for POA
Scour vulnerability
Recommended countermeasure(s) or alternatives
NBI coding information
Countermeasure selection(s) including priority ranking and cost
Bridge closure plan
Detour route
Any other supportive information

Additional guidance on developing a POA is presented in Section 2.2 and Appendix B.
2.6
The implementation of a POA means that bridge owners have completed disseminating
POAs to the appropriate personnel within their internal offices/units and external entities and
have met with these offices/units and with external entities to communicate:

General information and instructions contained in each POA
Individuals responsible for the POA implementation
Detour routes
When to close/open a bridge
Countermeasure selection
Design and installation schedules

Bridge owners should make sure that responsible parties identified in the POA understand
their roles and responsibilities that they are provided with periodic training on the
implementation of selected components of a POA such as bridge closure/opening
procedures. Implementation also includes establishing the frequency to conduct periodic
reviews and updates of the information presented in a POA.

2.2 STANDARD TEMPLATE FOR A PLAN OF ACTION

2.2.1 Overview
In order to facilitate the development of a POA, the FHWA has created a "standard"
template for bridges that are scour critical. The standard template and instructions for
completing a POA are provided in Appendix B. This template includes the minimum
information recommended by FHWA for a POA.

The template is intended to be a guide and tool for bridge owners to use in developing their
POAs. The template provides the program manager with a summary of the type of
information that should be part of a plan of action for a bridge identified as scour critical.
Bridge owners may also want to consider using the POA standard template for bridges which
have unknown foundations (i.e., foundation characteristics such as width, depth, and length
may be unknown).

All the fields in the template may be modified so that local terminology is employed, unique
information may be added regarding local and site-specific scour and stream stability
concerns, and local sources of information may be included. The electronic Microsoft Word
document template can be downloaded from the FHWA website:

http://www.fhwa.dot.gov/engineering/hydraulics/bridgehyd/poa.cfm

All blocks in this template will expand automatically to allow additional space. Where check
boxes are provided, they can be checked by double-clicking on the box and selecting the
"checked" option.

To provide guidance and training on preparation of POA for scour critical bridges, the FHWA
developed an on-line module which includes the POA standard template and illustrates its
application to field case studies. This training module which is referenced as NHI Course
No. 135085 is offered by NHI and can be accessed at:

http://fhwa.acrobat.com/n135085 seminar

2.7
A state's Bridge Management System is a useful source of data for developing a POA.
Many DOT's are now using information technology (IT) systems that provide immediate
access via the bridge engineer's desktop computer to an integrated system of bridge
management information and data bases. Much of the information outlined in the template
may be obtained from these systems.

2.2.2 Executive Summary

The standard template contains ten sections. Sections 1 through 4 are intended as an
executive summary for the busy reviewer/manager who may not need the details of Sections
5 through 10, and show:

Section 1: General information
Section 2: Who prepared the POA
Section 3: The source of the problem
Section 4: What actions are recommended and their status

To assist in completing a POA using the template, Appendix B contains general guidance for
each section of the template. An abbreviated set of instructions is also appended to the
template.

2.3 THE COUNTERMEASURE MATRIX

Selecting the countermeasures to be included in the plan of action requires evaluating a
number of alternatives. These alternatives could include hydraulic countermeasures,
structural countermeasures or monitoring, either individually or in some combination. To
facilitate selection of alternatives to be considered in the plan of action, a matrix
describing the various countermeasures and their attributes has been developed.
This countermeasure matrix is introduced and described in this section.

A wide variety of countermeasures have been used to control channel instability and scour at
bridge foundations. The countermeasure matrix, presented in Table 2.1, is organized to
highlight the various groups of countermeasures and to identify their individual
characteristics. The left column of the matrix lists types of countermeasures in groups. In
each row of the matrix, distinctive characteristics of a particular countermeasure are
identified. The matrix identifies most countermeasures used by DOTs and lists information
on their functional applicability to a particular problem, their suitability to specific river
environments, the general level of maintenance resources required, and which states have
experience with specific countermeasures. Finally, a reference source for design guidelines
is noted, where available.

Countermeasures have been organized into groups based on their functionality with respect
to scour and stream instability. The four main groups of countermeasures are: hydraulic
countermeasures, structural countermeasures, biotechnical countermeasures, and
monitoring. The following outline identifies the countermeasure groups in the matrix:


2.8
Group 1. Hydraulic Countermeasures

Group 1.A: River training structures

- Transverse structures
- Longitudinal structures
- Areal structures

Group 1.B: Armoring countermeasures

- Revetments and bed armor
- Rigid
- Flexible/articulating
- Local scour armoring

Group 2. Structural Countermeasures

Foundation strengthening
Pier geometry modification

Group 3. Biotechnical Countermeasures

Group 4. Monitoring

Fixed instrumentation
Portable instrumentation
Visual monitoring

2.4 COUNTERMEASURE GROUPS

2.4.1 Group 1. Hydraulic Countermeasures

Hydraulic countermeasures are those which are primarily designed either to modify the flow
or resist erosive forces caused by the flow. Hydraulic countermeasures are organized into
two groups: river training structures and armoring countermeasures. The performance
of hydraulic countermeasures is dependent on design considerations such as edge
treatment and filter requirements, which are discussed in Sections 5.2 and 5.4, respectively.

Group 1.A River Training Structures. River training structures are those which modify the
flow. River training structures are distinctive in that they alter hydraulics to mitigate
undesirable erosional and/or depositional conditions at a particular location or in a river
reach. River training structures can be constructed of various material types and are not
distinguished by their construction material, but rather, by their orientation to flow. River
training structures are described as transverse, longitudinal or areal depending on their
orientation to the stream flow.

Transverse river training structures are countermeasures which project into the flow
field at an angle or perpendicular to the direction of flow.
Longitudinal river training structures are countermeasures which are oriented parallel
to the flow field or along a bankline.
Areal river training structures are countermeasures which cannot be described as
transverse or longitudinal when acting as a system. This group also includes
countermeasure "treatments" which have areal characteristics such as channelization,
flow relief, and sediment detention.






2.13
Group 1.B Armoring Countermeasures. Armoring countermeasures are distinctive because
they resist the erosive forces caused by a hydraulic condition. Armoring countermeasures
do not necessarily alter the hydraulics of a reach, but act as a resistant layer to hydraulic
shear stresses providing protection to the more erodible materials underneath. Armoring
countermeasures generally do not vary by function, but vary more in material type. Armoring
countermeasures are classified by two functional groups: revetments and bed armoring or
local scour armoring.

Revetments and bed armoring are used to protect the channel bank and/or bed from
erosive/hydraulic forces. They are usually applied in a blanket type fashion for areal
coverage. Revetments and bed armoring can be classified as either rigid or
flexible/articulating. Rigid revetments and bed armoring are typically impermeable and
do not have the ability to conform to changes in the supporting surface. These are
subject to failure due to undermining. Flexible/ articulating revetments and bed
armoring can conform to changes in the supporting surface and adjust to settlement;
however, these countermeasures can fail by removal and displacement of the armor
material.
Local scour armoring is used specifically to protect individual substructure elements of
a bridge from local scour. Generally, the same material used for revetments and bed
armoring is used for local scour armoring, but these countermeasures are designed and
placed to resist local vortices created by obstructions to the flow.

2.4.2 Group 2. Structural Countermeasures

Structural countermeasures involve modification of the bridge structure (foundation) to
prevent failure from scour. Typically, the substructure is modified to increase bridge stability
after scour has occurred or when a bridge is assessed as scour critical. These modifications
are classified as either foundation strengthening or pier geometry modifications.

Foundation strengthening includes additions to the original structure which will
reinforce and/or extend the foundations of the bridge. These countermeasures are
designed to prevent failure when the channel bed is lowered to an expected scour
elevation, or to restore structural integrity after scour has occurred. Design and
construction of bridges with continuous spans provide redundancy against catastrophic
failure due to substructure displacement as a result of scour. Retrofitting a simple span
bridge with continuous spans could also serve as a countermeasure after scour has
occurred or when a bridge is assessed as scour critical.
Pier geometry modifications are used to either reduce local scour at bridge piers or to
transfer scour to another location. These modifications are used primarily to minimize
local scour.

2.4.3 Group 3. Biotechnical Countermeasures

Vegetation has been used increasingly over the past few decades to control streambank
erosion or as a bank stabilizer. It has been used primarily in stream restoration and
rehabilitation projects and can be applied independently or in combination with structural
countermeasures. There are several terms that describe vegetative streambank stabilization
and countermeasures. The use of 'soft' revetments (consisting solely of living plant materials
or plant products) is often referred to as bioengineering. The techniques that combine the
use of vegetation with structural (hard) elements include biotechnical engineering and
biotechnical slope protection. Where riprap constitutes the "hard" component of biotechnical
slope protection, the term vegetated riprap is also used (see Chapter 6).

2.14
The matrix considers representative categories for biotechnically engineered counter-
measures (which incorporate rock) including:

Vegetated geosynthetic products
Facines/woody mats
Vegetated riprap
Root wads
Live staking

Biotechnical engineering can be a useful and cost-effective tool in controlling bank or
channel erosion, while increasing the aesthetics and habitat diversity of the site. However,
where failure of the countermeasure could lead to failure of a bridge or highway structure,
the only acceptable solution in the immediate vicinity of a structure is a traditional, "hard"
engineering approach.

2.4.4 Group 4. Monitoring

Monitoring describes activities used to facilitate early identification of potential scour
problems. Monitoring could also serve as a continuous survey of the scour progress around
the bridge foundations. While monitoring does not fix the scour problem at a scour critical
bridge, it allows for action to be taken before the safety of the public is threatened by the
potential failure of the bridge. Monitoring can be accomplished with instrumentation or visual
inspection. A well designed monitoring program can be a very cost-effective
countermeasure. Two types of instrumentation are used to monitor bridge scour: fixed
instruments and portable instruments (see Chapter 9).

Fixed instrumentation describes monitoring devices which are attached to the bridge
structure to detect scour at a particular location. Typically, fixed monitors are located at
piers and abutments. The number and location of piers to be instrumented should be
defined, as it may be impractical to place a fixed instrument at every pier and abutment
on a bridge. Instruments such as sonar monitors can be used to provide a timeline of
scour, whereas instruments such as magnetic sliding collars can only be used to monitor
the maximum scour depth. Data from fixed instruments can be downloaded manually at
the site or it can be telemetered to another location.
Portable instrumentation describes monitoring devices that can be manually carried
and used along a bridge and transported from one bridge to another. Portable
instruments are more cost effective in monitoring an entire bridge than fixed instruments;
however, they do not offer a continuous watch over the structure. The allowable level of
risk will affect the frequency of data collection using portable instruments.
Visual inspection describes standard monitoring practices of inspecting the bridge on a
regular interval and increasing monitoring efforts during high flow events (flood watch).
Typically, bridges are inspected on a biennial schedule where channel bed elevations at
each pier location are taken. The channel bed elevations should be compared with
historical cross sections to identify changes due to scour. Channel elevations should also
be taken during and after high flow events. If measurements cannot be safely collected
during a high flow event, the bridge owner should determine if the bridge is at risk and if
closure is necessary. Underwater inspections of the foundations could be used as part of
the visual inspection after a flood.

2.15
2.5 COUNTERMEASURE CHARACTERISTICS

The countermeasure matrix (Table 2.1) was developed to identify distinctive characteristics
for each type of countermeasure. Five categories of countermeasure characteristics were
defined to aid in the selection and implementation of countermeasures:

Functional Applications
Suitable River Environment
Maintenance
Installation/Experience by State
Design Guidelines Reference

These categories were used to answer the following questions:

For what type of problem is the countermeasure applicable?
In what type of river environment is the countermeasure best suited or, are there river
environments where the countermeasure will not perform well?
What level of resources will need to be allocated for maintenance of the
countermeasure?
What states or regions in the U.S. have experience with this countermeasure?
Where do I obtain design guidance reference material?

2.5.1 Functional Applications

The functional applications category describes the type of scour or stream instability problem
for which the countermeasure is prescribed. The six main categories of functional
applications are local scour at abutments and piers, contraction scour, vertical and lateral
instability, and overtopping flow on approach embankments. Vertical instability implies the
long-term processes of aggradation or degradation over relatively long river reaches, and
lateral instability involves a long-term process of channel migration and bankline erosion
problems. While overtopping flow considers, primarily, roadway approach embankments,
the functional application could also include overtopping of countermeasures such as spurs
or guide banks. To associate the appropriate countermeasure type with a particular problem,
filled circles, half circles and open circle are used in the matrix as described below:

well suited/primary use - the countermeasure is well suited for the application; the
countermeasure has a good record of success for the application; the
countermeasure was implemented primarily for this application.
possible application/secondary use - the countermeasure can be used for the
application; the countermeasure has been used with limited success for the
application; the countermeasure was implemented primarily for another application
but also can be designed to function for this application.
In addition, this symbol can identify an application for which the countermeasure has
performed successfully and was implemented primarily for that application, but there
is only a limited amount of data on its performance and therefore the application
cannot be rated as well suited.
unsuitable/rarely used - the countermeasure is not well suited for the application;
the countermeasure has a poor record of success for the application; the
countermeasure was not intended for this application.
N/A not applicable - the countermeasure is not applicable to this functional application.
2.16
2.5.2 Suitable River Environment

This category describes the characteristics of the river environment for which a given
countermeasure is best suited or under which there would be a reasonable expectation of
success. Conversely, this category could indicate conditions under which experience has
shown a countermeasure may not perform well. The river environment characteristics that
can have a significant effect on countermeasure selection or performance are:

River type
Stream size (width)
Bend radius
Flow velocity
Bed material
Ice/debris load
Bank condition
Floodplain (width)

For each environmental characteristic, a qualitative range is established (e.g., stream size:
Wide, Moderate, or Small) to serve as a suitability discriminator. While most characteristics
are self explanatory, both HEC-20 ("Stream Stability at Highway Structures") and HDS 6
("River Engineering for Highway Encroachments") provide guidance on the range and
definitions of these characteristics of the river environment (Lagasse et al. 2001a;
Richardson et al. 2001). In the context of this matrix, the bank condition (slope)
characteristic (Very Steep, Steep or Mild) considers the effectiveness of a given
countermeasure to protect a bank with that configuration, or in some cases, the suitability
for the countermeasure to armor a bank with that configuration.

T Where a block is checked for a given countermeasure under an environmental
characteristic, the countermeasure is considered suitable or has been applied
successfully for the full range of that environmental characteristic.

The checked block means that the characteristic does not influence the selection of
the countermeasure, i.e., the countermeasure is suitable for the full range of that
characteristic. For example, guide banks have been applied successfully in braided,
meandering, and straight streams; however, bendway weirs/stream barbs are most
suitable for installation on meandering streams.

For further discussion of these and other river environment characteristics, see Chapter 3.

2.5.3 Maintenance

The maintenance category identifies the estimated level of maintenance that may need to be
allocated to service the countermeasure. The ratings in this category range from "Low" to
"High" and are subjective. The ratings represent the relative amount of resources required
for maintenance with respect to other countermeasures within the matrix shown in Table 2.1.
A low rating indicates that the countermeasure is relatively maintenance free, a moderate
rating indicates that some maintenance is required, and a high rating indicates that the
countermeasure requires more maintenance than most of the countermeasures in the
matrix.

2.17
2.5.4 Installation/Experience by State Departments of Transportation

This category identifies DOTs for which information on the use of a particular
countermeasures was available. These listings may not include all of the states which have
used a particular countermeasure. Information on state use was obtained from three
sources: a National Cooperative Highway Program questionnaire (University of Minnesota
survey for NCHRP Project 24-07(1); Brice and Blodgett, "Countermeasures for Hydraulic
Problems at Bridges, Volumes 1 and 2," (1978); and correspondence with DOT staff.
Certain countermeasures are used by many states. These countermeasures have a listing of
"Widely Used" in this category. Both successful, and unsuccessful experiences are reflected
by the listing.

2.5.5 Design Guideline Reference

Reference manuals which provide guidance in countermeasure design have been developed
by government agencies through research programs. The FHWA has produced a wealth of
information through the federally coordinated program of highway research and development
(see Chapter 10, References). In addition, each Design Guideline in Volume 2 identifies
reference manuals where additional guidance on design can be obtained. Countermeasures
for which design guidelines are provided within this document are referenced using DG#,
where # represents the number assigned to the design guideline (see Chapter 7,
Countermeasure Design Guidelines and Volume 2). Where additional design guidance can
be found in Volume 1, the appropriate Chapter (CH) is referenced in the matrix.

2.5.6 Summary

The countermeasures matrix is a convenient reference guide on a wide range of
countermeasures applicable to scour and stream stability problems. A comprehensive plan
of action should provide conceptual design and cost information on several alternative
countermeasures, with a recommended alternative based on a variety of engineering,
environmental and cost factors. The countermeasures matrix is a good way to begin
identifying and prioritizing possible alternatives. The information provided in the matrix
related to functional applications, suitable river environment, and maintenance issues should
facilitate preliminary selection of feasible alternatives prior to more detailed investigation.
2.18



























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3.1
CHAPTER 3

CONSIDERATIONS FOR SELECTING COUNTERMEASURES


3.1 INTRODUCTION

As previously noted, a countermeasure is defined as a measure incorporated into a
highway-stream crossing system to monitor, control, inhibit, change, delay, or minimize
stream and bridge stability problems. A monitoring program at structures during and/or after
flood events and river stabilizing works over a reach of the river up and downstream of the
crossing can also be considered countermeasures.

Countermeasures may be installed at the time of highway construction or retrofitted to
resolve stability problems at existing crossings. Retrofitting is good economics and good
engineering practice in many locations because the magnitude, location, and nature of
potential stability problems are not always discernible at the design stage, and indeed, may
take a period of several years to develop.

A countermeasure does not need to be a separate structure, but may be an integral part of
the highway. For example, relief bridges on floodplains are countermeasures which alleviate
scour from flow contraction at the bridge over the main stream channel. Some features that
are integral to the highway design serve as countermeasures to minimize stream stability
problems. Abutments and piers properly aligned with the flow reduce local scour and
contraction scour. Also, reducing the number of piers and/or setting back the abutments
reduces contraction scour.

Countermeasures which are not integral to the highway may serve one function at one
location and a different function at another. For example, bank revetment may be installed
to control bank erosion from meander migration, or it may be used to stabilize streambanks
in the contracted area at a bridge. Other countermeasures are useful for one function only.
This category of countermeasures includes spurs constructed in the stream channel to
control meander migration.

In selecting a countermeasure it is necessary to evaluate how the stream might respond to
the countermeasure, and also how the stream may respond as the result of other activities
upstream and downstream.

A countermeasure for scour critical bridges and unknown foundations could also be
monitoring a bridge during and/or after a flood event. If monitoring is selected and if the risk
of scour failure is high, protection to reduce the risk such as riprap or instrumentation should
be provided. Even if riprap is placed around piers or abutments, the high risk bridge should
be monitored during and inspected after floods. If monitoring is selected, a plan of action
must be implemented which includes a notification process, flood watch procedures, a
highway closure process, documentation of available detours, inspection procedures,
assessment procedures, and a repair notification process.

The next section provides some general criteria for the selection of countermeasures for
stream instability and scour. Then, the selection of countermeasures for specific stream
instability and bridge scour problems is discussed.
3.2
3.2 CRITERIA FOR THE SELECTION OF COUNTERMEASURES

The selection of an appropriate countermeasure for a specific erosion or scour problem is
dependent on factors such as the erosion or scour mechanism, stream characteristics,
construction and maintenance requirements, potential for vandalism, and costs. Perhaps
more important, however, is the effectiveness of the measure selected in performing the
required function.

For example, protection of an existing bank line may be accomplished with revetments,
spurs, retards, longitudinal dikes, or bulkheads (see Chapter 2 and Table 2.1). Spurs,
longitudinal dikes, and area retardance structures can be used to establish a new flow path
and channel alignment, or to constrict flow in a channel. Because of their high cost,
bulkheads may be appropriate for use only where space is at a premium. Channel relocation
may be used separately or in conjunction with other countermeasures to change the flow
path and flow orientation.

3.2.1 Erosion Mechanism

Bank erosion mechanisms include surface erosion and/or mass wasting. Surface erosion is
the removal of soil particles by the velocity and turbulence of the flowing water. Mass
wasting is by slides, rotational slip, piping and block failure. In general slides, rotational slip
and block failure result from the bank being undercut by the flow. Also, seepage force of the
pore water in the bank is another factor that can cause surface erosion or mass wasting.
The type of mechanism is determined by the magnitude of the erosive forces of the water,
type of bed and bank material, vegetation, and bed elevation stability of the stream. These
mechanisms are described in HDS 6 (Richardson et al. 2001) and HEC-20 (Lagasse et al.
2001a).

3.2.2 Stream Characteristics

Stream characteristics that influence the selection of countermeasures include (see also
Table 2.1):

Channel width
Bank height
Channel configuration
Channel material
Vegetative cover
Sediment transport condition
Bend radius
Channel velocities and flow depth
Ice and debris
Floodplain characteristics

Channel Width. Channel width influences the use of bendway weirs and other spur-type
countermeasures. On smaller streams (250 ft (<75 m) wide), flow constriction resulting from
the use of spurs may cause erosion of the opposite bank. However, spurs can be used on
small channels where the purpose is to shift the location of the channel. Changes in channel
width can influence contraction scour and impact bridge pier and abutment
countermeasures.

3.3
Bank Height. Low banks (<10 ft (3 m)) may be protected by any of the countermeasures,
including bulkheads. Medium height banks (from 10 to 20 ft (3 to 6 m)) may be protected by
revetment, retardance structures, spurs, and longitudinal dikes. High banks (>20 ft (6 m))
generally require revetments used alone or in conjunction with other measures.

Channel Configuration. Spurs and jack fields have been successfully used as a
countermeasure to control the location of the channel in meandering and braided streams.
Also, bulkheads, revetments, and riprap have been used to control bank erosion resulting
from stream migration. On anabranching streams, revetments, riprap, and spurs have been
used to control bank erosion and channel shifting. Also, secondary or side channels that do
not carry large flows can and have been closed off. In one case, HDS 6 reports that a large
channel was closed off and revetment and riprap used to control erosion in the other channel
(Richardson et al. 2001).

Channel Material. Spurs, revetments, riprap, jack fields, or check dams can be used in any
type of channel material if they are designed correctly. However, jack fields should only be
placed on streams that carry appreciable debris and sediment in order for the jacks to cause
deposition and eventually be buried. For channels in the sand size range the movement of
bedforms (ripples, dunes, and antidunes) will influence the design of bridge pier and
abutment scour countermeasures (see HEC-20 and HDS 6). Some countermeasures may
be subject to abraision and consequent failure from coarse bed load sediment.

Bank Vegetation. Vegetation such as willows can enhance the performance of structural
countermeasures for stream instability and may, in some cases, reduce the level of
structural protection needed. Meander migration and other bank erosion mechanisms are
accelerated on many streams in reaches where vegetation has been cleared.

Sediment Transport. Sediment transport conditions can be described as regime, threshold,
or rigid. Regime channel beds are those which are in motion under most flow conditions,
generally in sand or silt-size noncohesive materials. Threshold channel beds have no bed
material transport at normal flows, but become mobile at higher flows. They may be cut
through cohesive or noncohesive materials, and an armor layer of coarse-grained material
can develop on the channel bed. Rigid channel beds are cut through rock or boulders and
rarely or never become mobile. In general, permeable structures will cause deposition of
bed material in transport and are better suited for use in regime and some threshold
channels than in rigid channel conditions. Impermeable structures are more effective than
permeable structures in channels with little or no bed load, but impermeable structures can
also be very effective in mobile bed conditions. Revetments can be effectively used with
mobile or immobile channel beds. Live-bed and clear-water conditions must be considered
when estimating contraction scour and will influence the design of pier scour
countermeasures (Richardson and Davis 2001).

Bend Radius. Bend radius affects the design of stream instability countermeasures, because
some countermeasures will only function properly in long or moderate radius bends. Thus,
the cost per meter (foot) of bank protection provided by a specific countermeasure may differ
considerably between short-radius and longer radius bends. Impinging flow and increased
shear stress in short radius bends also affect the stability of scour countermeasures.

Channel Velocities and Flow Depth. Channel hydraulics affect the selection and design of all
countermeasures. For stream instability countermeasures, structural stability and induced
scour must be considered. Some of the permeable flow retardance measures may not be
structurally stable and countermeasures which utilize piles may be susceptible to scour
3.4
failure in high velocity environments. Velocity and depth have a direct influence on the
design of bridge pier and abutment scour countermeasures (see Chapter 4).

Ice and Debris. Ice and debris can damage or destroy any countermeasure and should
always be considered during the selection process. On the other hand, the performance of
some permeable spurs and area retardance structures is enhanced by debris where debris
accumulation induces additional sediment deposition.

Floodplains. In selecting countermeasures for stream stability and scour, the amount of flow
on the floodplain is an important factor. For example, if there is appreciable overbank flow,
then the use of guide banks to protect abutments should be considered. Also, spurs
perpendicular to the roadway approach embankment may be required to control erosion.

3.2.3 Construction and Maintenance Requirements

Standard requirements regarding construction or maintenance such as the availability of
materials, construction equipment requirements, site accessibility, time of construction,
contractor familiarity with construction methods, and a program of regular maintenance,
inspection, and repair are applicable to the selection of appropriate countermeasures.
Additional considerations for countermeasures located in stream channels include:
constructing and maintaining a structure that may be partially submerged at all times, the
extent of bank or channel bed disturbance which may be necessary, and the desirability of
preserving streambank vegetative cover to the extent practicable.

3.2.4 Vandalism

Vandalism is always a maintenance concern since effective countermeasures can be made
ineffective by vandals. Documented vandalism includes dismantling of devices, burning, and
cutting or chopping with knives, wire cutters, and axes. Countermeasure selection or
material selection for construction may be affected by concerns of vandalism. For example,
rock-filled baskets (gabions) may not be appropriate in some urban environments.

3.2.5 Countermeasure Selection Based on Cost

Cost comparisons should be used to study alternative countermeasures with an
understanding that the measures were installed under widely varying stream conditions, that
the conservatism (or lack thereof) of the designer is not accounted for, that the relative
effectiveness of the measures cannot be quantitatively evaluated, and that some measures
included in the cost data may not have been fully tested by floods.

Life-cycle cost information is difficult to quantify. Initial construction costs are relatively easy
to develop; however, even for a specific countermeasure, these costs can vary widely
depending on regional availability of materials, site conditions, and access constraints.
Therefore, a countermeasure type can be very cost effective in one locale and prohibitively
expensive in another. Extending these issues to life-cycle maintenance requirements
requires an even broader set of assumptions. Riprap, for example, is a standard
countermeasure type in many states; however, alternatives to riprap may need to be
investigated because of cost and availability limitations. The risks and consequences of
failure at any given site further complicate the issue. Life-cycle costs were , however, the
focus of a countermeasure selection methodology for a range of bridge pier scour
countermeasures.
3.5
A selection methodology was developed under NCHRP Project 24-07(2) (Lagasse et al.
2007) to provide a quantitative assessment of the suitability of six armoring-type pier scour
countermeasures based on selection factors that consider river environment, construction
considerations, maintenance, performance, and estimated life-cycle cost. With the
exception of life-cycle costs, the methodology analyzes the design factors by stepping the
user through a series of decision branches, ultimately resulting in a site-specific numerical
rating for each selection factor. Countermeasures evaluated by this methodology are:

Standard (loose) riprap
Partially grouted riprap
Articulating concrete blocks
Gabion mattresses
Grout filled mattresses
Grout filled bags

Five factors are used to compute a Selection Index (SI) for each countermeasure:

S1: Bed material size and transport
S2: Severity of debris or ice loading
S3: Constructability constraints
S4: Inspection and maintenance requirements
LCC: Life-cycle costs

The Selection Index is calculated as:

SI = (S1 x S2 x S3 x S4)/LCC (3.1)

The countermeasure that has the highest value of SI is considered to be most appropriate
for a given site, based not only on its suitability to the specific riverine and project site
conditions, but also in consideration of its economy. The approach is sensitive to
assumptions regarding initial construction cost, remaining service life, assumed frequency of
maintenance events, and extent of maintenance required. Each of these factors requires
experience and engineering judgment, as well as site- or region-specific information on the
cost of materials and delivery, construction practices, and prevailing labor rates. It should
be noted that the methodology can be used simply to rank the countermeasures in
terms of suitability alone by assuming that the life cycle costs are the same for all
countermeasures.

The following sections describe the five factors that comprise the methodology. Flow charts
illustrating selection factors S1 S4 are included in Appendix C.

Bed Material. Bed material is included as a selection factor for two reasons. Abrasion
caused by the transport of coarse bed sediments will cause the wire mesh on a gabion
mattress to weaken and break, whereas other countermeasure types are relatively resistant
to degradation by abrasion. For this reason, when bed material is greater than 2 mm,
gabion mattresses are eliminated from the selection process. Bed material size also assists
in distinguishing whether dune-type bedforms are anticipated. Grout filled mats are
susceptible to failure in the presence of bedforms because the mats do not articulate as well
as other countermeasures. When the bed material is less than 2 mm and bedforms are not
anticipated, all countermeasures included in the selection process are deemed equally
viable.
3.6
Ice and/or Debris Loading. Debris in this context is considered floating material such as
logs, other woody materials, man-made materials that are typically transported during floods,
or ice. The intent of this selection factor is to recognize that high debris loads can be
detrimental to gabion mattresses, as indicated in the countermeasure selection matrix in
Chapter 2. When a user indicates that anticipated debris loading is high, gabion mattresses
receive a low rating of "1" but are not eliminated from the selection process. When debris
loading is not anticipated, all countermeasures included in the selection process are deemed
equally viable.

Construction Constraints. Construction constraints take into account the different needs and
challenges required for placing a countermeasure in the dry versus installation underwater
or, in the extreme case, in flowing water. All ratings that consider construction constraints
are divided into two categories: piers that have shallow footings versus piers that are more
deeply embedded. This results from the fact that riprap-based countermeasures are typically
thicker than alternative countermeasures, and they require pre-excavation that may
undermine the footer.

In addition, the requirements for specialized equipment are addressed. For example, the
equipment requirements, placement techniques, and construction QA/QC requirements for
partially grouted riprap are straightforward for working in the dry; however, placement
underwater requires construction equipment and placement technologies that are much
more sophisticated. Subgrade preparation requirements and placement tolerances also vary
among countermeasure types. For example, a relatively thin veneer of articulating concrete
blocks requires finer grading techniques than an equivalent, and much thicker, riprap layer.

Working beneath a bridge deck that affords little headroom will dictate the type of equipment
that can be used for countermeasure installation. Lastly, alternative placement techniques,
particularly for rock riprap, typically dictate the strength requirements for geotextiles in order
to meet criteria for geotextile survivability during installation.

The decision box for flow velocity is intended to reflect the relative difficulty in placing a
mattress system, such as ACBs, gabion mattresses, or grout mattresses under fast flowing
water V > 4 ft/s (1.2 m/s). When the countermeasure does not need to be placed under
water and access for construction equipment of all types is good, all countermeasures
included in the selection process are deemed equally viable.

Inspection and Maintenance. Inspection and maintenance guidelines vary greatly among
countermeasure types. Underwater or buried installations require different considerations to
ensure that the countermeasure can be adequately inspected, compared to surficial
treatments in ephemeral or intermittent stream environments. The numerical values
assigned to this selection factor reflect the relative difficulty of repairing and/or replacing
"manufactured" countermeasures, such as ACBs, gabion mattresses, and grout mattresses,
versus the relative ease of adding more riprap stone.

The maintenance required for gabion mattresses as presented may be somewhat higher
than for other forms of revetment because the wire mesh used to construct the gabion is
susceptible to vandalism. When the countermeasure can be inspected and maintenance
performed in the dry, all countermeasures included in the selection process are deemed
equally viable.

3.7
Life-Cycle Costs. The Selection Index calculation is similar to the "Risk Priority Number"
method suggested by Johnson and Niezgoda (2004). Johnson and Niezgoda use the
concept of "risk categories" in contrast to this selection methodology concept of "suitability
categories" to relate various factors. Both methods represent relatively simple techniques
for selecting pier scour countermeasures. However, due to the complexity of determining
costs associated with countermeasure design and implementation, Johnson and Niezgoda
discussed life cycle costs but did not include those costs in the scope of their procedure.

Without consideration of life-cycle cost, the suitability of a countermeasure is dictated solely
by the environment of the river and its interaction with the bridge structure, combined with
the strengths and vulnerabilities of the countermeasure. This selection methodology
attempts to simplify the life-cycle cost estimation process through a series of spreadsheets
that assist the user in evaluating regional availability of materials, installation expenses, and
an estimation of maintenance based on experience and engineering judgment (see
Appendix C).

Life-cycle cost information can be difficult to quantify. Initial construction costs are relatively
easy to develop; however, even for a specific countermeasure, these costs can vary widely
depending on regional availability of materials, site conditions, and access or constructability
constraints. Therefore, a particular countermeasure might be very cost effective in one locale
and prohibitively expensive in another. Extending these issues to life-cycle maintenance
requires an even broader set of assumptions. This portion of the assessment attempts to
ease this process for the practitioner by providing templates for cost estimation.

Estimating life-cycle costs for pier scour countermeasures requires consideration of three
major components:

1. Initial construction materials and delivery costs
2. Initial construction installation costs associated with labor and equipment
3. Periodic maintenance during the life of the installation

Each of the above components is comprised of multiple elements, which differ among the
various countermeasure types. For example, quantities and unit costs of alternative
materials will vary depending on the specific project conditions, as well as local and regional
factors. Experience with these factors, as well as project-specific knowledge of the bridge
site, are required in order to be as accurate as practicable when using this selection
methodology.

The following issues should be considered when developing life-cycle cost estimates:

Availability of materials of the required size and weight
Haul distance
Site access
Equipment requirements for the various countermeasures being considered
Construction underwater vs. placement in the dry
Environmental and water quality issues and permitting requirements
Habitat and/or migration issues for threatened and endangered species
Traffic control during construction and/or maintenance activities
Local labor rates
Construction using in-house resources vs. outside contract
Design life of the installation
Anticipated frequency and extent of periodic maintenance and repair activities
3.8
Quantifying each of these factors requires experience and engineering judgment. For this
reason, these variables are user inputs in the life cycle cost worksheets referenced in
Appendix C. The default values that are provided in the Excel spreadsheet program
can and should be changed by the user to reflect both site-specific and state or
regional conditions. For access to the Excel spreadsheet, see Forward to Lagasse et al.
2007.

3.2.6 Countermeasure Selection Based on Risk

As suggested by the various scenarios described in Chapter 2, a risk-based analysis may be
necessary to develop the plan of action for multiple bridges with scour critical ratings. The
level of response and the actions taken will be different from one scour critical bridge to the
next. Given limited resources and multiple options, it is up to the interdisciplinary team to
formulate the best alternative for any given plan of action considering all available
information.

The need to evaluate countermeasure alternatives considering economics and risk led to the
development of a systematic, risk-based method to determine the level of resources
appropriate for protection of a bridge that is scour critical but has a limited life before
scheduled replacement (Pearson et al. 2000). This method determines the optimum level of
protection for the bridge and the maximum expenditures that should be accepted to increase
the level of protection. An overview of the concepts involved in this approach is provided in
this section. Equations to apply the method and an application example are provided in
Pearson et al. (2000).

The objectives of the risk-based approach to countermeasure selection are to:

Identify bridges at risk from scour using data from the National Bridge Inventory (NBI)
Examine the economic feasibility of scour countermeasure alternatives

For convenience, NBI data is used to estimate the relative risk of bridge failure due to scour,
where risk is defined as the costs associated with bridge failure multiplied by the probability
of failure. The NBI codes can be used to estimate both the cost of failure and the probability
of failure as shown in Figure 3.1.


Figure 3.1. Risk-based approach using NBI data.
3.9
The failure probability can be derived from the NBI Item 113 coding or, alternatively, from a
combination of route class, substructure condition, channel protection, and waterway
adequacy. The failure probability can be adjusted based on bridge age (NBI Item 27).
Economic factors that influence costs associated with bridge failure include: bridge length,
width, and classification, detour length, and average daily traffic (ADT).

Thus, the risk (expected loss) calculated by this method is the product of the probability of
scour failure (or heavy damage) and the economic losses associated with such an event.
The year-to-year risk (expected loss) of scour failure associated with a bridge installation
over water is determined by Equation 3.2.

Risk ($) = KP [(rebuild cost) + (running cost) + time cost)] (3.2)

where:

Risk ($) = Risk of scour failure (annualized for 1 year given current physical
condition)
K = Risk adjustment factor based on foundation type and type of span (NBI
Item 43), and
P = Probability of failure each year (NBI Item 113 or Items 26, 60, 61, and
71)

Rebuild cost, running cost, and time cost can be developed from equations or graphs
provided in Pearson et al. (2000).

With the risk of bridge failure established in terms of cost, the risk can be compared with the
costs and benefits associated with a range of countermeasure alternatives. This can be
done by balancing costs and risks by using a simple benefit/cost ratio, or by a net benefit
analysis for candidate countermeasures. As noted by Pearson et al. (2000), balancing the
costs and risks represents a traditional approach to risk managementfinding the
countermeasure cost that corresponds as closely as possible to the risk cost. This
approach, however, hides the economic benefits offered by available countermeasures.
Maximizing the net benefit illuminates the economic benefits of countermeasures but may
result in countermeasure costs greater than necessary to achieve a particular protection
goal. Maximizing the benefit/cost ratio offers the soundest economic approach to
countermeasure selection since it results in the greatest countermeasure benefit per dollar
spent. For an application of risk-based concepts, see Pearson et al. (2000).

3.3 COUNTERMEASURES FOR MEANDER MIGRATION

The best countermeasure against meander migration is to locate the bridge crossing on a
relatively straight reach of stream between bends. At many such locations, countermeasures
may not be required for several years because of the time required for the bend to move to a
location where it becomes a threat to the highway facility. However, bend migration rates on
other streams may be such that countermeasures will be required after a few years or a few
flood events and, therefore, should be installed during initial construction [see HEC-20
(Lagasse et al. 2001a)] for further discussion of lateral channel instability, and NCHRP
Report 533 (Lagasse et al. 2004) for a methodology for estimating rates of meander
migration).

3.10
Stabilizing channel banks at a highway stream crossing can cause a change in the channel
cross section and an increase in stream sinuosity upstream of the stabilized banks. Figure
3.2a illustrates a natural channel section in a bend with the deeper section at the outside of
the bend and a gentle slope toward the inside bank resulting from point bar growth. Figure
3.2b illustrates the scour which results from stabilizing the outside bank of the channel and
the resulting steeper slope of the point bar on the inside of the bend. This effect must be
considered in the design of the countermeasure and the bridge (see Section 4.3.5). It should
also be recognized that the thalweg location and flow direction can change as sinuosity
upstream increases.

Figure 3.3a illustrates meander migration in a natural stream and Figure 3.3b, the effects of
bend stabilization on upstream sinuosity. As sinuosity increases, meander amplitude may
increase, meander radii will become smaller, deposition may occur because of reduced
slopes, and the channel width-depth ratio may increase as a result of bank erosion and
deposition, as at the bridge location shown in Figure 3.3b. Ultimately, cutoffs can occur.
These changes can also result in hydraulic problems downstream of the stabilized bend.



Figure 3.2. Comparison of channel bend cross sections (a) for natural conditions, and (b) for
stabilized bend (after Brown 1985).

Countermeasures for meander migration include those that:

Protect an existing bank line
Establish a new flow line or alignment
Control and constrict channel flow

The classes of countermeasures identified for bank stabilization and bend control are bank
revetments, spurs, bendway weirs, longitudinal dikes, vane dikes, bulkheads, and channel
relocations. Also, a carefully planned cutoff may be an effective way to counter problems
created by meander migration. These measures may be used individually or in combination
to combat meander migration at a site. Some of these countermeasures are also applicable
to bank erosion from causes other than bend migration.

3.11

Figure 3.3. Meander migration in (a) a natural channel, and (b) a channel with stabilized
bend (after Brown 1985).

3.4 COUNTERMEASURES FOR CHANNEL BRAIDING AND ANABRANCHING

Channel braiding occurs in streams with an overload of sediment, causing deposition and
aggradation. As aggradation occurs, the slope of the channel increases, velocities increase,
and multiple, interconnected channels develop. The overall channel system becomes wider
and multiple channels are formed as bars of sediment are deposited in the main channel.

Braiding can also occur where banks are easily eroded and there is a large range in
discharge. The channel becomes wider at high flows, and low-flow forms multiple
interconnected channels. In an anabranched stream, flow is divided by islands rather than
bars, and the anabranched channels are more permanent than braided channels and
generally convey more flow.

A meandering stream may change to a braided stream if the slope is increased by channel
straightening or the dominant discharge is increased. Lane's relation may be used to
determine if there can be a shift from a meandering channel to a braided one. If, after a
change in discharge or slope the stream still plots in the meandering zone, then it will likely
remain a meandering stream. However, if it moves closer to or into the braided zone, then
the stream may become braided (see HEC-20).

Braided channels change alignment rapidly, and are very wide and shallow even at flood
flow. They present problems at bridge sites because of the high cost of bridging the
complete channel system, unpredictable channel locations and flow directions, difficulties
with eroding channel banks, and in maintaining bridge openings unobstructed by bars and
islands.

3.12
Countermeasures used on braided and anabranched streams are usually intended to
confine the multiple channels to one channel. This tends to increase the sediment transport
capacity in the principal channel and encourage deposition in secondary channels. These
measures usually consist of dikes constructed from the margins of the braided zone to the
channel over which the bridge is constructed. Guide banks at bridge abutments (Design
Guideline 15) in combination with revetment on highway fill slopes (Design Guideline 4),
riprap on highway fill slopes only, and spurs (Design Guideline 2) arranged in the stream
channels to constrict flow to one channel have also been used successfully.

Since anabranches are permanent channels that may convey substantial flow, diversion and
confinement of an anabranched stream is likely to be more difficult than for a braided
stream. The designer may be faced with a choice of either building more than one bridge,
building a long bridge, or diverting anabranches into a single channel.

3.5 COUNTERMEASURES FOR DEGRADATION AND AGGRADATION

Bed elevation instability problems are common on alluvial streams. Degradation in streams
can cause the loss of bridge piers in stream channels and can contribute to the loss of piers
and abutments located on caving banks. Aggradation causes the loss of waterway opening
in bridges and, where channels become wider because of aggrading streambeds, overbank
piers and abutments can be undermined. At its worst, aggradation may cause streams to
abandon their original channels and establish new flow paths which could isolate the existing
bridge (see HEC-20 for further discussion of vertical channel instability).

3.5.1 Countermeasures to Control Degradation

Countermeasures used to control bed degradation include check dams and channel linings.
Check-dams and structures which perform functions similar to check-dams include drop
structures, cutoff walls, and drop flumes. A check-dam is a low dam or weir constructed
across a channel to prevent upstream degradation (Design Guideline 3).

Channel linings of concrete and riprap have proved unsuccessful at stopping degradation.
To protect the lining, a check-dam may have to be placed at the downstream end to key it to
the channel bed. Such a scheme would provide no more protection than would a check dam
alone, in which case the channel lining would be redundant.

Bank erosion is a common hydraulic hazard in degrading streams. As the channel bed
degrades, bank slopes become steeper and bank caving failures occur. The USACE found
that longitudinal stone dikes, or rock toe-dikes (see Chapter 8), provided the most effective
toe protection of all bank stabilization measures studied for very dynamic and/or actively
degrading channels.

The following is a condensed list of recommendations and guidelines for the application of
countermeasures at bridge crossings experiencing degradation:

Check-dams or drop structures are the most successful technique for halting degradation
on small to medium streams.
Channel lining alone may not be a successful countermeasure against degradation
problems.
Riprap on channel banks and spill slopes will fail if unanticipated channel degradation
occurs.
3.13
Successful pier protection involves providing deeper foundations at piers and pile bents.
Jacketing piers with steel casings or sheet piles has also been successful where
expected degradation extends only to the top of the original foundation.
The most economical solution to degradation problems at new crossing sites on small to
medium size streams is to provide adequate foundation depths. Adequate setback of
abutments from slumping banks is also necessary.
Rock-and-wire mattresses are recommended for use only on small (100 ft [<30 m])
channels experiencing lateral instability and little or no vertical instability.
Longitudinal stone dikes placed at the toe of channel banks are effective
countermeasures for bank caving in degrading streams. Precautions to prevent
outflanking, such as tiebacks to the banks, may be necessary where installations are
limited to the vicinity of the highway stream crossing.

3.5.2 Countermeasures to Control Aggradation

Currently, measures used in attempts to alleviate aggradation problems at highways include
channelization, debris basins, bridge modification, and/or continued maintenance, or
combinations of these. Channelization may include dredging and clearing channels,
constructing small dams to form debris basins, constructing cutoffs to increase the local
slope, constructing flow control structures to reduce and control the local channel width, and
constructing relief channels to improve flow capacity at the crossing. Except for debris
basins and relief channels, these measures are intended to increase the sediment transport
capacity of the channel, thus reducing or eliminating problems with aggradation. Cutoffs
must be designed with considerable study as they can cause erosion and degradation
upstream and deposition downstream. These studies would involve the use of sediment
transport relations given in HDS 6 (Richardson et al. 2001) or the use of sediment transport
models such as BRI-STARS (Molinas 1990) or HEC-6 (USACE 1993). The most common
bridge modifications are increasing the bridge length by adding spans and increasing the
effective flow area beneath the structure by raising the bridge deck.

A program of continuing maintenance has been successfully used to control problems at
bridges on aggrading streams. In such a program, a monitoring system is set up to survey
the affected crossing at regular intervals. When some pre-established deposition depth is
reached, the bridge opening is dredged or cleared of the deposited material. In some cases,
this requires clearing after every major flood. This solution requires surveillance and
dedication to the continued maintenance of an adequate waterway under the bridge.
Otherwise, it is only a temporary solution. A debris basin or a deeper channel upstream of
the bridge may be easier to maintain. Continuing maintenance is not recommended if
analysis shows that other countermeasures are practicable.

Over the short term, maintenance programs prove to be very cost effective when compared
with the high cost of channelization, bridge alterations, or relocations. When costs over the
entire life of the structure are considered, however, maintenance programs may cost more
than some of the initially more expensive measures. Also, the reliability of maintenance
programs is generally low because the programs are often abandoned for budgetary or
priority reasons. However, a program of regular maintenance could prove to be the most
cost efficient solution if analysis of the transport characteristics and sediment supply in a
stream system reveals that the aggradation problem is only temporary (perhaps the excess
sediment supply is coming from a transient land use activity such as logging) or will have
only minor effects over a relatively long period of time.
3.14
An alternative similar to a maintenance program which could be used on streams with
persistent aggradation problems, such as those on alluvial fans, is the use of controlled sand
and gravel mining from a debris basin constructed upstream of the bridge site. Use of this
alternative would require careful analysis to ensure that the gravel mining did not upset the
balance of sediment and water discharges downstream of the debris basin. Excessive
mining could induce degradation downstream, potentially impacting the bridge or other
structures.

The following is a list of guidelines regarding aggradation countermeasures:
Extensive channelization projects have generally proven unsuccessful in alleviating
general aggradation problems, although some successful cases have been documented.
A sufficient increase in the sediment carrying capacity of the channel is usually not
achieved to significantly reduce or eliminate the problem. Channelization should be
considered only if analysis shows that the desired results will be achieved.
Alteration or replacement of a bridge is often required to accommodate maximum
aggradation depths.
Maintenance programs have been unreliable, but they provide the most cost-effective
solution where aggradation is from a temporary source or on small channels where the
problem is limited in magnitude.
At aggrading sites on wide, shallow streams, spurs or dikes with flexible revetment have
been successful in several cases in confining the flow to narrower, deeper sections.
A debris basin and controlled sand and gravel mining might be the best solution on
alluvial fans (see HEC-20) and at other crossings with severe problems.

3.6 SELECTION OF COUNTERMEASURES FOR SCOUR AT BRIDGES

The selection of an appropriate countermeasure for scour at a bridge requires an
understanding of the erosion mechanism producing the specific scour problem. For example,
contraction scour results from a sediment imbalance across most or all of the channel, while
local scour at a pier or abutment results from the action of vortices at an obstruction to the
flow. Degradation is a component of total scour, but is considered a channel instability
problem (see Section 3.5). Since the selection of a countermeasure depends on the type of
scour involved, this section provides a brief overview of the principal scour components.

Scour is the result of the erosive action of running water, excavating and carrying away
material from the bed and banks of streams. Different materials scour at different rates.
Loose granular soils are rapidly eroded under water action while cohesive or cemented soils
are more scour-resistant. However, ultimate scour in cohesive or cemented soils can be as
deep as scour in sand-bed streams. Scour will reach its maximum depth in sand and gravel
bed materials in hours; cohesive bed materials in days; glacial tills, poorly cemented sand
stones and shales in months; hard, dense and cemented sandstone or shales in years; and
granites in centuries. Massive rock formations with few discontinuities can be highly resistant
to scour and erosion during the lifetime of a typical bridge [see HEC-18 (Richardson and
Davis 2001) for detailed discussion and equations for calculating all bridge scour
components].

Designers and inspectors need to carefully study site-specific subsurface information in
determining scour potential at bridges, giving particular attention to foundations on rock.

3.15
Total Scour. Total scour at a highway crossing is comprised of three components. These
components are:

Aggradation and Degradation. These are long-term streambed elevation changes due to
natural or human-induced causes within the reach of the river on which the bridge is
located (see Section 3.5).
Contraction Scour. This type of scour involves the removal of material from the bed and
banks across all or most of the width of a channel. Most commonly, this scour is the
result of a contraction of the flow by the approach embankments to the bridge
encroaching onto the floodplain and/or into the main channel which causes an increase
in transport of the bed material in the bridge cross section.
Local Scour. This scour occurs around piers, abutments, spurs, and embankments and
is caused by the acceleration of the flow and the development of vortex systems induced
by these obstructions to the flow.

In addition to the types of scour mentioned above, lateral migration of the stream may also
erode the approach roadway to the bridge or change the total scour by changing the angle of
the flow in the waterway at the bridge crossing. Factors that affect lateral migration and the
stability of a bridge are the geomorphology of the stream, location of the crossing on the
stream, flood characteristics, and the characteristics of the bed and bank materials (see
HEC-20).

3.6.1 Countermeasures for Contraction Scour

Severe contraction of flow at highway stream crossings has resulted in numerous bridge
failures at abutments, approach fills, and piers from contraction scour. Design alternatives
to decrease contraction scour include longer bridges, relief bridges on the floodplain,
superstructures at elevations above flood stages of extreme events, and a crest vertical
profile on approach roadways to provide for overtopping during floods exceeding the design
flood event (see HEC-20). These design alternatives are integral features of the highway
facility which reduce the contraction at bridges and, therefore, reduce the magnitude of
contraction scour.

The elevation of bridge superstructures is recognized as important to the integrity of the
bridge because of hydraulic forces that may damage the superstructure. These include
buoyancy and impact forces from ice and other floating debris (see HEC-18). Contraction
scour is another consideration in setting the superstructure elevation. When the
superstructure of a bridge becomes submerged or when ice or debris lodged on the
superstructure causes the flow to contract, flow may be accelerated and more severe scour
can occur. For this reason, where contraction scour is of concern, bridge superstructures
should be located with clearance for debris, and, if practicable, above the stage of floods
larger than the design flood.

Another design feature which should be considered relative to contraction scour is the
effective depth of the superstructure. Present day superstructures often include bridge
railings which are solid parapets. These increase the effective depth of the superstructure
and, thus, the importance of locating the bridge superstructure above high water with
clearance for debris passage. It also increases the importance of alternate provisions for the
passage of flood waters in the event of debris blockage of the waterway or superstructure
submergence. Possible alternate provisions include relief bridges on the floodplain and a
highway profile which provides for overtopping before the bridge superstructure begins to
become submerged.
3.16
Similarly, pier design, span length, and pier location become more important contributors to
contraction scour where debris can lodge on the piers and further contract flow in the
waterway. In streams which carry heavy loads of debris, longer, higher spans and solid piers
will help to reduce the collection of debris. Where practicable, piers should be located out of
the main current in the stream, i.e., outside the thalweg at high flow. There are numerous
locations where piers occupy a significant area in the stream channel and contribute to
contraction scour, especially where devices to protect piers from ship traffic are provided.

The principal countermeasure used for reducing the effects of contraction is revetment on
channel banks and fill slopes at bridge abutments (Design Guidelines 4 and 14). Additional
countermeasures used to reduce flow contraction include measures which retard flow along
highway embankments on floodplains. Flow along highway fills usually intersects with flow
within bridge openings at large angles. This causes additional contraction of the flow,
vortices, and turbulence which produce local scour. The contraction of flow can be reduced
by using spurs on the upstream side of the highway embankment to retard flow parallel with
the highway (Bradley 1978).

Similarly, guide banks (also referred to as spur dikes) at bridge abutments can improve the
alignment of the flow in the bridge opening. They reduce contraction scour because they
increase the efficiency of the bridge opening. The primary purpose of guide banks,
however, is to reduce local scour at abutments (Design Guideline 15).

The potential for undesired effects from stabilizing all or any portion of the channel perimeter
at a contraction should be considered. Stabilization of the banks may only result in
exaggerated scour in the streambed near the banks or, in a relatively narrow channel,
across the entire channel. Stabilization of the streambed may also result in exaggerated
lateral scour in any size stream. Stabilization of the entire stream perimeter may result in
downstream scour or failure of some portion of the countermeasures used on either the
streambed or banks.

3.6.2 Countermeasures for Local Scour

Local scour occurs at bridge piers and abutments. In general, design alternatives against
structural failure from local scour consist of measures which reduce scour depth, such as
pier shape and orientation, and measures which retain their structural integrity after scour
reaches its maximum depth, such as placing foundations in sound rock and using deep
piling. Countermeasures which can reduce the risk from local scour include placing armor
(e.g., riprap) at the structure or installing monitoring devices.

Abutments. Countermeasures for local scour at abutments consist of measures which
improve flow orientation at the bridge end and move local scour away from the abutment, as
well as revetments and riprap placed on spill slopes to resist erosion.

Guide banks are earth or rock embankments placed at abutments. Flow disturbances, such
as eddies and cross-flow, will be eliminated where a properly designed and constructed
guide bank is placed at a bridge abutment. Guide banks also protect the highway
embankment, reduce local scour at the abutment and adjacent piers, and move local scour
to the upstream end of the guide bank (Design Guideline 15).

Local scour also occurs at abutments as a result of expanding flow downstream of the
bridge, especially for bridges on wide, wooded floodplains that have been cleared for
construction of the highway. Short guide banks extending downstream of the abutment to
the tree line will move this scour away from the abutment, and the trees will retard velocities
so that flow redistribution can occur with minimal scour.

3.17
The effectiveness of guide banks is a function of stream geometry, the quantity of flow on
the floodplain, and the size of bridge opening. A typical guide bank at a bridge opening is
shown in Figure 3.4.

QUARTER
ELLIPSE
FLOW
L
s
0.4 L
s
Area of scour
REVETMENT ARMOR
(ROCK RIPRAP OR EQUAL)
VEGETATIVE
COVER
QUARTER
ELLIPSE
FLOW
L
s
0.4 L
s
Area of scour
REVETMENT ARMOR
(ROCK RIPRAP OR EQUAL)
VEGETATIVE
COVER

Figure 3.4. Typical guide bank layout (after Bradley 1978).

Other countermeasures have been successfully used to inhibit scour at abutments where the
abutment is located at the streambank or within the stream channel. These measures
include dikes to constrict the width of braided streams and retards to reduce velocities near
the streambank.

Piers. Three basic methods may be used to prevent damage from local scour at piers. The
first method is to place the foundation of the structure at such a depth that the structural
stability will not be at risk with maximum scour. This must be done on all new or replacement
bridges (Richardson and Davis 2001). The second method (for existing bridges) is to provide
protection at or below the streambed to inhibit the development of a scour hole. The third
measure is to prevent erosive vortices from forming or to reduce their strength and intensity.

Streamlining the pier nose decreases flow separation at the face of the pier, reducing the
strength of the horseshoe vortices which form at piers. Practical application of this principle
involves the use of rounded or circular shapes at the upstream and downstream faces of
piers in order to reduce the flow separation. However, flow direction can and does change
with time and with stage on some streams. Piers oriented with flow direction at one stage or
at one point in time may be skewed with flow direction at another. Also, flow direction
changes with the passage of bed forms. In general, piers should be aligned with the main
channel design flow direction and skew angles greater than 5 degrees should be avoided.
Where this is not possible, a single cylindrical pier or a row of cylindrical columns will
produce a lesser depth of local scour.

3.18
The tendency of a row of columns to collect debris should be considered. Debris can greatly
increase scour depths. Webwalls have been used between columns to add to structural
strength and to reduce the tendency to collect debris. Webwalls should be constructed at
the elevation of stream flood stages which carry floating debris and extended to the elevation
of the streambed. When installing a webwall as a countermeasure against debris, the
potential for significantly increased scour depths should be considered if the approach flow
might impinge on the wall at a high angle of attack.

Riprap is commonly used to inhibit local scour at piers at existing bridges. This practice is
not recommended as an adequate substitute for foundations or piling located below
expected scour depths for new or replacement bridges. It is recommended as a retrofit or a
measure to reduce the risk where scour threatens the integrity of a pier (Design Guideline
11). The use of partially grouted riprap offers the opportunity to use smaller rock for pier
scour protection (Design Guideline 12), and geotextile sand containers can provide a
methodology to install an appropriate filter in a pier scour hole in flowing water (see Section
5.4.2 and Design Guideline 11). A comprehensive selection methodology for pier scour
countermeasures is introduced in Section 3.2.6.

The practice of heaping stones around a pier is not recommended because experience has
shown that continual replacement is usually required. Success rates have been better with
alluvial bed materials where the top of the riprap was placed at or below the elevation of the
streambed.

Piles (sheet, H beams or concrete) have been successfully used as a retrofit measure to
lower the effective foundation elevation of structures where footings or pile caps have been
exposed by scour. The piling is placed around the pile footings and anchored to the pile cap
or seal to retain or restore the bearing capacity of the foundation. The increased mass of
the retrofit pile will, however, produce a greater depth of scour.

Where sheet pile cofferdams are used during construction, the sheet piling should be
removed or cut off below the level of expected contraction scour in order to avoid
contributing to local scour. Cofferdams should not be much wider than the pier itself since
the effect may be to greatly increase local scour depth. Leaving or removing cofferdams
must be carefully evaluated because leaving a cofferdam that is higher than the contraction
scour elevation may increase local scour depth. A study by Jones (1989) gives a method to
evaluate the expected scour depths for cofferdams.

3.6.3 Monitoring

Monitoring or closing a bridge during high flows and inspection after the flood may be an
effective countermeasure to reduce the risk from scour. However, monitoring of bridges
during high flow may not reveal that they are about to collapse from scour. It also may not
be practical to close the bridge during high flow because of traffic volume, no (or poor)
alternate routes, the need for emergency vehicles to use the bridge, etc. Under these
circumstances, scour countermeasures such as riprap could be installed. A countermeasure
installed at a bridge to reduce the risk from scour along with monitoring during and
inspection after high flows could provide for the safety of the public without closing the bridge
(see Chapter 9).

4.1
CHAPTER 4

COUNTERMEASURE DESIGN CONCEPTS


4.1 COUNTERMEASURE DESIGN APPROACH

4.1.1 Investment in Countermeasures

At stream crossings, the objective of DOTs is to protect highway users and the investment in
the highway facility, and to avoid causing damage to other properties, to the extent
practicable. Countermeasures should be designed and installed to stabilize only a limited
reach of stream and to ensure the structural integrity of highway components in an unstable
stream environment. Countermeasures are often damaged or destroyed by the stream, and
streambanks and beds often erode at locations where no countermeasure was installed.
However, as long as the primary objectives are achieved in the short-term as a result of
countermeasure installation, the countermeasure installation can be deemed a success.
Therefore, the DOTs' interest in stream stability often entails long-term protection of costly
structures by committing to maintenance, reconstruction, and installation of additional
countermeasures as the responses of streams and rivers to natural and man-induced
changes are identified.

While it is sometimes possible to predict that bank erosion will occur at or near a given
location in an alluvial stream, one can frequently be in error about the exact location or
magnitude of potential erosion. At some locations, unexpected lateral erosion occurs
because of a large flood, a shifting thalweg, or from other actions of the stream or human
activities. Where the investment in a highway crossing is not in imminent danger of being
lost, it is often prudent to delay the installation of countermeasures until the magnitude and
location of the problem becomes obvious.

Thus, for stream instability countermeasures, a "wait and see" attitude may constitute the
most economical approach. Retrofitting can be considered sound engineering practice in
many locations because the magnitude, location, and nature of potential instability problems
are not always discernible at the design stage, and indeed, may take a period of several
years to develop.

4.1.2 Service Life and Safety

A life-cycle concept, as applied to an erosion or scour countermeasure incorporates a host
of factors into a framework for decision making considering initial design, construction, and
long-term maintenance. These factors could include engineering judgment applied to design
alternatives, materials availability and cost, installation equipment and practices, and
maintenance assumptions (see Section 3.2.5).

When selecting a service life criterion for various types of countermeasures for
transportation facilities, safety must be a primary consideration. For DOTs safety of the
traveling public is the first priority when setting service-life standards for countermeasures.
Concurrent goals are protection of public and private property, protection of fish and wildlife
resources, and enhancement of environmental attributes (Lagasse et al. 2006).

4.2
Thus, service-life for a countermeasure installation should be based on the importance of the
facility to the public, that is, the risk of losing the facility and how that loss may directly or
indirectly affect the traveling public, as well as the difficulty and cost of future repair or
replacement. The conditions that constitute an "end of service life" for a countermeasure
installation are largely dependent on the confidence one has that a degraded condition will
be detected and corrected in a timely manner (e.g., during a post-flood inspection).
Generally, for facilities that are rarely checked or inspected a very conservative (i.e., shorter)
service life would be appropriate, while a less conservative standard could be used for
facilities that are inspected regularly.

Service life for a countermeasure installation can be considered a measure of the durability
of the total, integrated bank, pier or abutment protection system. The durability of system
components and how they function in the context of the overall design will determine the
service life of an installation. The response of the system over time to typical stresses such
as flow conditions (floods and droughts) or normal deterioration of system components must
also be considered. Response to less typical (but plausible) stresses such as fire, vandalism,
seismic activity or accidents may also affect service life. Finally, there may be opportunities
for maintenance during the life cycle of the installation and, where such work does not
constitute total reconstruction or replacement, maintenance should not be considered as the
end of service life for the system. In fact, a life-cycle approach to maintenance may extend
the service life of a countermeasure installation and reduce the total cost over the life of the
project.

4.1.3 Design Approach

The design of any countermeasure for the protection of highway crossings requires the
designer to be cognizant of the factors which affect stream stability and the morphology of
the stream. In most cases, the installation of any countermeasure will cause the bed and
banks to respond to the change in hydraulic conditions imposed by the countermeasure.

Thus, the analysis procedures outlined in HEC-20 (Lagasse et al. 2001a) are a necessary
prerequisite to the detailed design of specific countermeasures. The goal in any
countermeasure design is to achieve a response which is beneficial to the protection of the
highway crossing and to minimize adverse effects either upstream or downstream of the
highway crossing. An appropriate level of hydraulic analysis must support the design of any
stream instability or bridge scour countermeasure. In general, a countermeasure should be
designed to provide a level of protection commensurate with the design event for the
structure involved (see Section 4.3).

The bridge scour and stream instability countermeasures matrix (Table 2.1) helps define the
set of specific countermeasures that are best suited to specific site conditions. The
countermeasures matrix is intended, primarily, to assist with the selection of an appropriate
countermeasure. Consideration of potential environmental impacts, maintenance,
construction-related activities, and legal aspects can be used to refine the selection. The
final selection criteria, and perhaps the most important, are the initial and long-term costs.
The countermeasure that provides the desired level of protection at the lowest total cost may
be the "best" for a particular application.

The following principles should be followed in designing and constructing stream instability
and bridge scour countermeasures:

Initial and long-term cost should not exceed the benefits to be derived.
Countermeasures to make the bridge safe from scour and stream instability should be
used for important bridges on main roads and where the results of failure would be
intolerable. Expendable works may be used where traffic volumes are light, alternative
routes are available, and the risk of failure is acceptable.
4.3
Designs should be based on studies of channel trends and processes and on experience
with comparable situations. The environmental effects of the countermeasures on the
channel both up- and downstream should be considered.
Field reconnaissance by the designer is highly desirable and should include the
watershed and river system up- and downstream from the bridge.
Evaluation of time-sequenced aerial photography is a useful tool to detect long-term
trends in river stability [see for example NCHRP Report 533 (Lagasse et al. 2004)].
Soil and geotechnical characteristics of the site and their influence on countermeasure
and filter design must be considered. This may include a geotechnical slope stability
assessment, possibly under rapid drawdown conditions. This will require input from a
licensed geotechnical engineer.
The possibility of using physical model studies as a design aid should receive
consideration at an early stage.
Countermeasures must be inspected periodically after floods to check performance and
modify the design, if necessary. The first design may require modification. Continuity in
treatment, as opposed to sporadic attention, is advisable. The condition of the
countermeasure should be documented with photographs to enable comparison of its
condition from one inspection to another.
In most cases, the countermeasure does not "cure" the instability or scour problem, and
planning (funding) for continued maintenance of the countermeasure will be required.

In some cases, a combination of two or more countermeasures could be required due to
site-specific problems or as a result of changing conditions after the initial installation. The
great number of possible countermeasure combinations makes it impractical to suggest
design procedures for combined countermeasures. However, combined countermeasures
should complement each other. That is to say, the design of one countermeasure must not
adversely impact on another or the overall protection of the highway crossing. The principles
of river mechanics, as discussed in HDS 6 (Richardson et al. 2001) and HEC-20, coupled
with sound engineering judgment should be used to design countermeasure strategies
involving two or more countermeasures.

4.2 ENVIRONMENTAL PERMITTING

The environmental permitting process can have a significant effect on the planning, design
and implementation of river engineering works. Often, permitting can become a lengthy
process for the implementation of bridge scour and stream instability countermeasures. To
expedite this process, a memorandum dated February 11, 1997, was prepared jointly by the
U.S. Army Corps of Engineers (USACE) Directorate of Civil Works and the Federal Highway
Administration (FHWA). The purpose of the memorandum is to facilitate timely decisions on
permit applications for work associated with measures to protect bridges determined to be at
risk as the result of scouring around their foundations. The USACE and FHWA consider this
agreement essential to assure the safety of the traveling public while protecting the
environment.

Recognizing the importance of protecting the foundations of our Nation's scour critical
bridges with properly designed scour countermeasures and the need for environmentally
sound projects, the FHWA and the USACE agree to work together with the bridge owners, in
a cooperative effort, to plan ahead for managing projects that will need a USACE permit. A
strong cooperative effort will aid in advanced planning to avoid and minimize environmental
4.4
impacts, and in identifying locations where mitigation may be appropriate. If the bridge
foundation has been determined to be scour critical as part of the bridge owner's scour
evaluation program, the USACE will give priority to the bridge owner's request for
authorization for the installation of scour countermeasures. Bridge owners must provide the
FHWA and USACE Districts advance notice of the proposed countermeasure design and
construction schedule. The notice must include an evaluation of the environmental impacts
of the proposed scour countermeasure and appropriate mitigation of unavoidable impacts to
aquatic resources, including fisheries and wetlands. This will allow appropriate and timely
cooperation on project reviews. The USACE will make the maximum use possible of forms
of expedited authorization, such as nationwide permits and regional permits, and Letters of
Permission and the use of FHWA's Categorical Exclusion when the condition of the bridge
foundation meets the criteria for codes 0 through 4 for Item 113 (FHWA 1995).

4.3 HYDRAULIC ANALYSIS

4.3.1 Overview

To be successful, the design of any countermeasure must incorporate an appropriate level
of hydraulic analysis. The hydraulic principles of open channel flow and fundamentals of
alluvial channel flow are summarized in HDS 6 and hydraulic factors that influence stream
stability are presented in HEC-20. In addition, HEC-20 provides a general solution approach
which includes hydrologic and hydraulic analysis steps in a multi-level analysis procedure
(see Figure 1.1). Finally, HEC-18 (Richardson and Davis 2001) provides references and
discussion of the standard 1- and 2-dimensional hydraulic computer models used for riverine
and tidal analyses.

Both physical hydraulic modeling in a laboratory and numerical computer modeling are
among the standard techniques available to analyze the scour problem and design
countermeasures. This section introduces the use of physical modeling for the design of
scour and stream instability countermeasures. Guidance is also provided for the analysis of
several complex hydraulic conditions applicable to countermeasure design: scour at both
transverse structures (spurs, jetties, dikes, and guide banks), and longitudinal structures
(bendway revetment and vertical walls). The concept of radial stress on a bend is introduced
as a method to evaluate (or predict) countermeasure performance in an eroding bendway.

4.3.2 Physical Models

The use of physical models as a tool in hydraulic design is commonly accepted. Many
hydraulic phenomena which occur in nature are too complex to be described by rigorous
mathematical techniques and models are used as an alternative means of obtaining the
information necessary to complete an efficient and satisfactory design. Even in relatively
simple situations, such as the design of spillways or water diversion structures, it is often
impossible to predict the exact nature of the flow patterns without conducting a model study
(Sharp 1981).

A summary of the principles of physical modeling for both rigid-boundary and movable-bed
river models can be found in Shen (1979) who notes that hydraulic modeling has contributed
significantly to design of hydraulic structures, training of rivers, and basic hydraulic research.
It is a common practice to conduct hydraulic model tests to verify or modify the design of
prototype structures. Hydraulic model tests are particularly useful in the study of complex
flow phenomena for which no completely satisfactory theoretical analysis is available.
4.5

Some hydraulic tests are rather routine; many others are complex. For simple situations,
hydraulic model tests provide accurate information that can be applied directly to prototype
situations. However, for complex situations, hydraulic modeling is still more of an art than a
science (Shen 1979). The following comments summarize important considerations for
applying a physical model to the design of hydraulic structures, including countermeasures
for bridge scour and stream instability.

A physical model is very useful in the study of characteristics of complex flow
phenomena involving significant flow variations in all three dimensions where no
theoretical analysis is available.
Dimensional analysis and physical reasoning are essential approaches to the selection of
the governing similarity criteria. If more than one similarity criterion are needed,
extensive knowledge of the basic process under investigation is necessary to deal with
the situation.
If the prototype is large, a distorted model may be necessary. In a distorted model the
vertical model scale is usually smaller than the horizontal scale.
A movable bed model may be necessary if a significant 3-dimensional variation of
sediment movement occurs in the prototype. Since movable bed model results are
difficult to interpret, it can be advantageous to first investigate general flow variations in a
fixed bed model.
Verification of model results is absolutely necessary. Model results are usually verified
with at least three flow conditions: high, medium, and low flow.
In order to design a river model study correctly, one must decide the purpose of the
model tests, know the principles of modeling thoroughly, and also have a thorough
technical knowledge of hydraulics and river mechanics.

FHWA's "River Engineering for Highway Encroachments" (HDS 6) provides additional
discussion of similitude for rigid-boundary and mobile-bed models.

A 1998 investigation of European practice for bridge scour and stream instability
countermeasures (TRB 1999) concluded that in Europe it is much more likely that physical
modeling, often in conjunction with computer modeling, will be used as an integral part of the
hydraulic design process for bridge foundations and countermeasures than we are
accustomed to in the United States. Government research agencies and private sector
laboratories (e.g., Delft Hydraulics in the Netherlands) maintain extensive physical modeling
capabilities for the following reasons: validation of computer modeling, fundamental research
with respect to physical processes, and solving problems for which computers cannot
presently be applied.

In a report on testing the effectiveness of scour countermeasures by physical modeling, the
Federal Waterways Engineering and Research Institute (BAW) in Karlsruhe, Germany notes
that the physical modeling of the scouring process at bridge piers is a proven method to get
information about the size of the scour and the flow velocities generating the scour. On the
basis of this information, appropriate countermeasures can be designed. The advantage of
the physical model is its application on even the most complex pier geometries (Eisenhauer
and Rossbach 1999).
4.6
At BAW, model tests were conducted for the piers of a new bridge over the Rhine River near
Mannheim, Germany (Figure 4.1). Soon after the driving of sheet pile as a formwork for the
lower part of the pier (pier width 36 ft (11 m) below mean water level and 18 ft (5.5 m) above
mean water level) severe scouring of the river bed (d
50
= 8 mm) occurred. As a
consequence, the stability of the sheet pile formwork was endangered. An emergency
countermeasure of placing riprap of 6-8 inches (15-20 cm) diameter into the scour hole did
not stop local scouring; however, an additional cover layer of coarser stones (diameter 8
24 inches [20-60 cm]) was placed on top of the previous layer, stopping the erosion process
at mean flow. A series of model tests were conducted in order to estimate the durability and
stability of the emergency countermeasure for flood events. The tests proved the riprap to
be stable even at flood stage while the scour was shifted away from the pier to the margin
between the riprap and the sand of the natural river bed.


Figure 4.1. BAW laboratory, Karlsruhe, Germany, pier scour model of railway bridge over
Rhine River near Mannheim (Eisenhauer and Rossbach 1999).

4.3.3 Scour at Transverse Structures

Several commonly used countermeasures for channel instability or scour protection project
transversely into the flow (e.g., spurs, dikes, and jetties) or intercept overbank flow as it
returns to the main channel (e.g., guide banks). Estimating scour at the nose of these
structures is critical to successful design. Equation 4.1 is presented in HEC-18 as an
alternative abutment scour equation when the projecting embankment/abutment length is
large in relation to flow depth (a/y
1
> 25).

y
y
F
s
r
1
0 33
4 =
.
(4.1)

where:

y
s
= Equilibrium depth of scour (measured from the mean bed level to the
bottom of the scour hole), ft (m)
y
1
= Average upstream flow depth in the main channel or on the overbank
outside the influence of the structure, ft (m)
a = Structure length projecting normal to the flow, ft (m)
F
r
= Upstream Froude Number outside the influence of the structure
4.7
This equation is based on field data on scour at the nose of rock spurs in the Mississippi
River (obtained by the USACE) and is suggested here for estimating local scour at the nose
of any transverse structure projecting into the flow.

For cases where the transverse structure length is small in comparison to flow depth (a/y
1

#25) HDS 6 (see "Highways in the River Environment," 1990 Edition) presents the following
equation for local live-bed scour in sand at a stable spill slope when the flow is subcritical:

y
y
a
y
F
s
r
1 1
0 4
0 33
11 =

.
.
.
(4.2)

Where the variables are defined as for Equation 4.1. This equation is suggested here for
estimating local scour at the nose of a transverse structure projecting into the flow when the
conditions for Equation 4.1 are not met.

4.3.4 Scour at Longitudinal Structures

Variations in bed elevation during flow events or after bank hardening can result in the
undermining of bank protection structures including longitudinal structures. Therefore,
methods are needed for estimating maximum scour in order to design stable bank
protection. The following sections provide methods for estimating scour along longitudinal
countermeasures such as bulkheads and vertical walls.

Scour with Flow Parallel to a Vertical Wall. The probable mechanism causing scour along a
vertical wall when the flow is parallel to the wall is an increase in boundary shear stress
produced by locally increased velocity gradients that result from the reduced roughness of
the vertical wall, as compared to the natural channel. It is reasonable to conclude that this
scour will continue until the local flow area has increased enough to reduce the local velocity,
and hence the local boundary shear stress, to values typical of the rest of the channel cross
section (RCE 1994).

The magnitude of boundary shear stress around the perimeter of a channel is not constant.
In channels of uniform roughness, the boundary shear stress has a maximum value near the
channel centerline, and a secondary peak about one-third of the way up the sideslope. On
average, the maximum on the bottom is about 0.97 times the average boundary shear stress
(e.g., as defined by (RS) for the cross section and the maximum on the side is about 0.76
times the average boundary shear stress. However, experimental data indicate a range of
values, with maximum shear stresses as much as 1.6 times the average. In general, the
boundary shear stress distribution is more uniform as the width to depth ratio increases.

Similar information is not available for channel cross sections of nonuniform roughness;
however, reasonable conclusions can be drawn from intuitive arguments. For a straight
channel with a vertical wall with smoother roughness than the rest of the channel along one
side, the boundary shear stress distribution would be skewed towards the wall side of the
channel. The sideslope peak value would be larger and could possibly be greater than the
peak along the channel bed, which would also be shifted off the centerline location. These
effects would be more pronounced in narrow channels and/or channels with steep
sideslopes. As the channel gets wider, or the sideslope flattens, these effects would be
diminished.

Insight on the magnitude of these effects can be obtained by considering local velocity
conditions as determined by conveyance weighting concepts (see HEC-18 and HEC-20).
The analysis assumes that the boundary roughness within the channel can be divided into
4.8
two distinct regions: one region defining the roughness of the channel banks and the other
defining the roughness of the channel bottom (note that this division of roughness, while
logical, is not always analytically useful as it can create numerical problems leading to errors
in the computation of conveyance for the entire cross section).

For purposes of illustration, a wide, shallow natural channel has a uniform roughness with a
Manning's n value of 0.03, but with a concrete vertical wall the n value of the bank region
is reduced by a factor of two, to 0.015. Evaluation of the distribution of discharge by
conveyance weighting shows that this reduction of "n" nearly doubles the conveyance,
discharge, and velocity adjacent to the bank (i.e., next to the wall). Recognizing that
boundary shear stress is proportional to velocity squared, this increase in velocity increases
the boundary shear stress by a factor of 4.

Based on the experimental results for a uniform roughness channel, the maximum boundary
shear stress along the vertical wall could be as much as 3 times the average boundary shear
stress. However, this is not totally accurate given the simplistic assumptions made and the
likely changes in the distribution pattern that would result under conditions produced by a
vertical wall. Nonetheless, this simplified analysis suggests that significant increases in the
boundary shear stress are possible adjacent to the wall.

To apply this concept, it is appropriate to define a shear stress multiplier that can be applied
to the average boundary shear stress to define the locally increased boundary shear stress
adjacent to a vertical wall. Based on the above argument, a shear stress factor of 3 is
suggested. Recognizing that boundary shear stress is proportional to velocity squared, the
reduction in velocity necessary to lower the shear stress to an acceptable value is defined by
the inverse of the square root of the shear stress multiplier (0.577) for the shear stress factor
of 3. For the reduction in velocity to occur, the flow area must then be increased by the
inverse of this factor (1/0.577 = 1.73). For a vertical wall, this calculation simplifies to a unit
width basis and the scour depth is a multiplier of the average flow depth (0.73 y
1
).

It is important to understand that this provides a first approximation of the potential scour
along a vertical wall due to flow parallel to the wall. Using this relation, the total scour along
the wall due to parallel flow can be approximated as the sum of the above relation, which
results from a differential in shear stress, plus scour associated with the passage of
antidunes (see HDS 6). This results in the following relationship:

y
y
F
s
r
1
2
0 73 014 = + . .
(4.3)

where:

y
s
= Equilibrium depth of scour (measured from the mean bed level to the
bottom of the scour hole), ft (m)
y
1
= Average upstream flow depth in the main channel, ft (m)
F
r
= Upstream Froude Number

This equation is applicable only where parallel flow can be assured (e.g., vertical walls along
both banks).

Scour with Flow Impinging at an Angle on a Vertical Wall. When an obstruction such as an
abutment or vertical wall projects into the flow, the depth of scour at the nose or face of the
obstruction can be estimated from Equation 4.1. Considering the physical configuration of
the channels for which the data on which this relation is based, this can reasonably be
4.9
assumed to be the upper limit of the scour that could be expected for flow along a vertical
wall when the flow impinges on the wall at an approximately 90E angle. The total scour along
a vertical wall, thus, will vary as a proportion of that given by Equations 4.1 and 4.3.
Assuming that the relative significance of the two scour mechanisms is related to the change
in momentum associated with the change in flow direction from some angle 2 relative to the
wall, the two relations can be combined using a weighting factor based on the sine or cosine,
respectively, of the angle of the flow to the wall (0E to 90E). The resulting relationship is
given by (RCE 1994):

y
y
F Cos F Sin
s
r r
1
2 0 33
0 73 014 4 = + + ( . . )
.

(4.4)

where:

2

= Angle between the impinging flow direction and the vertical wall

Scour Along a Vertical Wall Relative to Unconstrained Valley Width. The potential scour that
could occur along a vertical wall due to changes in planform as the channel evolves can be
estimated by combining Equation 4.4 with the relationships for ideal meander geometry (see
HEC-20). Using these relationships, it can be shown that the maximum angle will vary from
zero, when the width of the valley is constrained to the width of the channel, to approximately
71E, when the unconstrained valley width is approximately 3.5 times the width of the
channel. These values are based on the assumption that the meander wavelength is 14
times the channel width. The resulting dimensionless scour depth as a function of the
unconstrained valley width is plotted in Figure 4.2 for a range of Froude Numbers (F
r
).

It is possible for the channel to impinge perpendicular to the wall due to local flow deflection
or other local factors. For this case, the angle of impingement is no longer related to the
valley width, and the maximum scour depth can best be estimated based strictly on Equation
4.1.


Figure 4.2. Scour along a vertical wall as a function of unconstrained valley width (RCE 1994).
4.10
In using Figure 4.2, it is important to recognize that the relationships are based on an
assumed ideal meander geometry and scour relationships that, while they are the best
available, are very approximate. Considering the extreme local variability that can occur in a
given stream and the approximate nature of the relationships upon which these results are
based, engineering judgment is critical in evaluating the reasonableness of the results for a
specific problem. In particular, the potential for flow deflection and its effect on the angle of
impingement on the wall should be considered and a conservatively large angle applied in
Equation 4.4. If there is any reasonable possibility of flow perpendicular to the wall, an angle
of 90E (thus, Equation 4.1) is recommended. When the results of this analysis are used to
design the burial depth for a vertical wall, a safety factor of at least 1 ft (0.3 m) should be
added to the predicted scour depth.

4.3.5 Scour at Protected Bendways

Deep sections at the toe of the outer bank of a bendway are the result of scour. High velocity
along the outer bank is caused by secondary currents and greater outer-bank depths, and
together with the resultant shear stress, produce scour and cause a difference between the
sediment load entering and exiting the outer-bank zone. Since secondary currents transport
sediment supplied, in large part, from outer bank erosion toward the inner bank of a bend,
hardening of the outer bank by longitudinal bank protection structures may cause the
channel cross section to narrow and deepen by preventing the recruitment of eroded outer
bank sediments.

Experience is usually the most reliable means of estimating scour depth when designing a
bank protection project for a particular stream. Lacking experience on a particular stream,
scour depths may be estimated using physically based analytical models or empirical
methods. Although scour-depth can be estimated analytically or empirically, empirical
methods were generally found to provide better agreement with observed data.

Maynord (1996) provides an empirical method for determining scour depths on a typical
bendway bank protection project. Although his studies are restricted to sand bed streams,
the Maynord method agrees reasonably well with the limited number of gravel-bed data
points obtained by Thorne and Abt (1993). Nonetheless, the techniques presented by
Maynord are restricted to meandering channels having naturally developed widths and
depths, and cannot be applied to channels that have been confined to widths significantly
less than a natural system.

Maynord's method of estimating scour depth is based on a regression analysis of 215 data
points. The scour data used in developing his equation were measured at high discharges
that were within the channel banks and had return intervals of 1-5 years. Maximum depth as
defined in his best-fit equation for scour depth estimation is a function of R
c
/W, width to
depth ratio, and mean depth as follows:

D
D
R
W
W
D
mxb
mnc
c
mnc
=

18 0 051 0 0084 . . .
(4.5)

where:

R
c
= Centerline radius of the bend, ft (m)
W = Width of the bend, ft (m)
D
mxb
= Maximum water depth in the bend, ft (m)
D
mnc
= Average water depth in the crossing upstream of the bend, ft (m)
4.11
The terms D
mxb
and D
mnc
are defined in Figure 4.3.


Figure 4.3. Definition sketch of width (W) and mean water depth (D
mnc
) at the crossing
upstream of the bend and maximum water depth in the bend (D
mxb
).

The applicability of Maynord's equation is limited to streams with R
c
/W from 1.5 to 10 and
W/D
mnc
from 20 to 125 because of the lack of data outside these ranges. He recommends
that for channels with R
c
/W <1.5 or width to depth ratios less than 20, the scour depth for
R
c
/W = 1.5 and W/D
mnc
= 20, respectively, be used.

In addition, Thorne and Abt (1993) suggest these methods are valid until there is significant
interaction between the main channel flow and overbank flow. Therefore, Maynord (1996)
recommends that application of these empirical methods to overbank flow conditions should
be limited to overbank depth less than 20% of main channel depth.

4.3.6 Hydraulic Stress on a Bendway

The ratio of bend radius of curvature to flow width provides insight into the force on the
meander bend margin, but this parameter does not include discharge. A quantitative
technique which considers a single-event discharge and an estimate of the radial stress on a
meander bend margin was developed to evaluate the performance of alternative streambank
erosion protection techniques for the U.S. Army Corps of Engineers, Vicksburg District
(WET 1990). This technique could also be used by highway engineers to evaluate
alternative channel instability countermeasures for a bridge located in a meander bend.

Begin (1981) defines radial stress as the centripetal force divided by the outer bank area.
The centripetal force is responsible for deflecting the flow around the bend and is equal to
the apparent reactive force of the flow on the bend. Based on this concept of centripetal
force, the equation for the radial stress (N
r
) of flow on a meander bend is:

r
b c
F
A
QV
Y R W
= =
+ ( / ) 2
(4.6)
4.12
where:

F

= Centripetal force, lbs, (N)
A
b
= Area of outer bank, ft
2

(m
2
)
D

= Fluid density, lbs/ft
3
(kg/m
3
)
Q = Discharge, ft
3
/s (m
3
/s)
V = Flow velocity, ft/s (m/s)
Y = Mean flow depth, ft (m)
R
c
= Radius of curvature, ft (m)
W

= Topwidth, ft (m)

Thus, the radial stress is defined as a force per unit area (lbs/ft
2
or N/m
2
). Although it is not
suggested that the radial stress is directly responsible for meander bend migration or failure
of bank protection countermeasures, Begin did show that the radial stress is related to
meander migration (Begin 1981). It is assumed that shear stress is related to radial stress
because of water surface superelevation and increased near-bank velocity gradients.

Field investigations and computation of radial stress on banklines for channels in the Yazoo
River basin in Mississippi clearly showed that rudimentary countermeasures, such as used-
tire revetment were generally unsuccessful in bends with even low to moderate radial stress
(WET 1990). The study also showed that stone structures including longitudinal stone dikes
and stone spurs performed well in reaches of high radial stress. Isolated failures of stone
structures did occur at locations with the highest radial stress. The 2-year storm discharge
was used in the computations for radial stress at these sites.

As an alternative, the increased shear force on the outside of bends can be calculated by
multiplying the bed shear stress
0
by a dimensionless bend coefficient K
b
. The sharper the
bend, the greater the shear stress imposed on the outer bank. The bend coefficient K
b
is
related to the ratio of the bend radius of curvature R
c
divided by the top width of the channel
T, as shown in Figure 4.4.

1
1.5
2
2 3 4 5 6 7 8 9 10 11 12
R
c
/T
B
e
n
d

C
o
e
f
f
i
c
i
e
n
t

K
b

=


b
/

0

K
b
= 2.0 for R
c
/T < 2
K
b
= 2.38 - 0.206(R
c
/T) + 0.0073(R
c
/T)
2
for 2 < R
c
/T < 10
K
b
= 1.05 for R
c
/T >10

Figure 4.4. Shear stress multiplier K
b
for bends (HEC-15, 2005).
5.1
CHAPTER 5

RIPRAP DESIGN, FILTERS, FAILURE MODES, AND ALTERNATIVES


5.1 OVERVIEW

Riprap consists of a layer or facing of rock, dumped or hand-placed on channel and
structure boundaries to limit the effects of erosion. It is the most common type of
countermeasure due to its general availability, ease of installation and relatively low cost.
Any successful riprap design must account for several possible modes of failure. These
include riprap particle erosion, substrate material erosion and mass failure. Riprap particle
erosion is minimized by sizing the riprap to withstand hydraulic and turbulence forces, but is
also affected by riprap slope, impact and abrasion, ice, waves and vandalism. Substrate
particle erosion occurs when the base material erodes and migrates through the riprap voids
causing the riprap to settle. Substrate particle erosion is limited by placing a granular or
geotextile filter between the riprap and the base material. Mass failure occurs when large
sections of the riprap and/or base material slide or slump due to gravity forces. Mass failure
can be caused by excess pore water pressures, bank steepness and loss of basal support
through scour or channel migration. Also, a filter fabric that is too fine can clog and cause
the buildup of pore water pressures in the underlying soil.

Riprap that is large enough to resist all the hydraulic forces can fail if channel migration or
scour undermines the toe support. When the riprap toe is undermined it can shift and remain
functional to some degree. Often an extra volume of riprap is included at the toe for this
purpose, or the riprap toe is trenched to the depth of potential degradation and contraction
scour.

Graded riprap is more stable than uniform riprap because the range of sizes helps the riprap
layer to interlock. Care must be taken during construction to ensure that the graded rocks
are uniformly distributed. If large rocks roll to the base of the bank and the smaller rocks
accumulate at the top, the benefits of using graded riprap will be lost. Also, a relatively
uniform riprap surface will be more stable than an extremely uneven riprap surface.

Riprap design requires hydraulic, scour, and stream instability analyses as well as
geotechnical investigations of channel and bank stability. Pier riprap can fail if contraction
scour or channel bed degradation causes the stones to launch and roll away from the pier,
or on rivers with mobile bed forms, by bedform undermining. Abutment riprap can fail if
channel migration undermines the toe support of the rock. Channel bank riprap can fail if
excess pore pressures or toe scour produce a mechanically unstable bank. These failures
could occur even if the riprap size was appropriate for the particular application.

In summary, design of a riprap erosion control system requires knowledge of: river bed,
bank, and foundation material; flow conditions including velocity, depth and orientation;
riprap characteristics of size, density, durability, and availability; location, orientation and
dimensions of piers, abutments, guide banks, and spurs; and the type of interface material
between riprap and underlying foundation which may be geotextile fabric or a filter of sand
and/or gravel. Adequate "toe down" and termination details are essential to the performance
of the riprap system. Thus, riprap should be considered an integrated system where
successful performance of a riprap installation depends on the response of each component
of the system to hydraulic and environmental stresses throughout its service life.
5.2
5.2 RIPRAP DESIGN

5.2.1 Introduction

Most of the guidelines and recommendations of this chapter are derived from NCHRP
Report 568, "Riprap Design Criteria, Recommended Specifications, and Quality Control," the
final report for NCHRP Project 24-23 (Lagasse et al. 2006). The basic objectives of this
study were to develop design guidelines, material specifications and test methods,
construction specifications, and construction, inspection and quality control guidelines for
riprap for a range of applications, including: revetment on streams and riverbanks, bridge
piers and abutments, and bridge scour countermeasures such as guide banks and spurs.
NCHRP Project 24-23 was a synthesis study and did not involve any original laboratory
experimental work, but some analytical work (specifically 1- and 2-dimensional computer
modeling) was necessary to address issues related to input hydraulic variables for design.

Additional guidance for riprap at bridge piers is based on results of the NCHRP Project 24-
07(2) provided in NCHRP Report 593, "Countermeasures to Protect Bridge Piers from
Scour" (Lagasse et al. 2007). This study involved extensive laboratory testing at Colorado
State University of a range of bridge pier scour countermeasures, including: riprap, partially
grouted riprap, articulating concrete blocks, gabion mattresses, and grout-filled mattresses.
NCHRP Report 593 includes detailed, stand-alone guidelines for the design of these five pier
scour countermeasure alternatives.

Sizing the stone is only the first step in the comprehensive design, production, installation,
inspection, and maintenance process required for a successful riprap armoring system.
Filter requirements, material and testing specifications, construction and installation
guidelines, and inspection and quality control procedures are also necessary. This section
recommends riprap design approaches for a range of riprap applications. Riprap design
(sizing) is covered in more detail in application-specific design guidelines in Volume 2.

Subsequent sections provide an overview of filter design requirements and
recommendations for specification, testing, and quality control for revetment riprap
installations. In general, these recommendations are applicable to riprap for other
applications such as at bridge piers and abutments, and for countermeasures such as spurs
and guide banks.

Generalized construction/installation guidance is also summarized in this chapter. Failure
modes for revetment and bridge pier riprap are described to underscore the integrated
nature of riprap armoring systems and as a basis for developing inspection and maintenance
guidance. Finally, an overview of concrete armor units (artificial riprap) that could be used in
lieu of rock for selected applications is provided.

5.2.2 Riprap Revetment

Based on a screening of the many riprap revetment design equations found in the literature,
seven equations were evaluated with sensitivity analyses using both field and laboratory data
during NCHRP Project 24-23 (Lagasse et al. 2006). One, the U.S. Army Corps of Engineers
EM 1601, was recommended for streambank revetment design (USACE 1991). Factors
considered were the ability of the basic equation to discriminate between stable and failed
riprap using field and laboratory data, bank and bend correction factors, and the
reasonableness of safety/stability factors. Detailed design guidance using the EM 1601
equation is provided in Volume 2, Design Guideline 4. A standard riprap gradation
specification which considers design, production, and installation requirements is
recommended in Design Guideline 4, together with a standardized riprap size classification
system. Installation guidance for toe down and transitions is also provided for the revetment
5.3
application. General riprap specifications, testing, and quality control guidance can be found
in Design Guideline 4.

5.2.3 Riprap for Bridge Piers

According to Hoffmans and Verheij (1997) riprap can be sized using either the Isbash or
Shields stability criteria if turbulence intensity is incorporated into the velocity component.
The effect of turbulence is to increase instantaneous velocities well above the levels for
unobstructed flow. This concept is particularly applicable to the pier riprap equations.

The standard Isbash (1936) formula for sizing riprap on a channel bed is:

)) 1 S ( g 2 (
) KV ( 692 . 0
D
s
2
50

= (5.1)

where:

D
50
= Riprap size, ft (m)
V = Velocity, ft/s (m/s)
S
s
= Specific gravity of the riprap (usually 2.65)
K = 1.0

To incorporate the effects of turbulence intensity, Hoffmans and Verheij (1997) recommend
that the value of K be adjusted above a value of 1.0. In the specific case of circular piers,
they recommend using the local velocity upstream of the pier and values of K up to 2.0. This
amount of adjustment is equivalent to increasing shear stress by a factor of four.

This approach is similar to the equations presented in Design Guideline 8 and in the riprap
sizing formula presented by Parola (1993). The only difference is the recommended values
of K in the design guideline are 1.5 for circular piers and 1.7 for square piers. The
recommended values of K by Parola ranged from 1.44 to 1.90 depending on pier and footing
geometry and approach flow angle of attack.

After a preliminary screening during NCHRP Project 24-23, the HEC-23 (Second Edition)
equation, which was derived from work by Parola and Jones, was compared to several other
equations using three laboratory data sets. Based on this sensitivity analysis, it was
concluded that the HEC-23 and Parola equations provide the best balance between the
objective of rarely (if ever) undersizing bridge pier riprap and the desire to not be overly
conservative. As these equations are very similar, the HEC-23 (Second Edition) equation
was recommended for design practice.

The laboratory results and design recommendations from a concurrent study of
countermeasures to protect bridge piers from scour (NCHRP 24-07(2)) were evaluated
regarding filter requirements, riprap extent, and other construction/ installation guidelines for
pier riprap (Lagasse et al. 2007). Specifically, guidelines for the use of sand-filled geotextile
containers as a means of placing a filter for pier riprap derived from European practice were
investigated. Construction and installation guidelines and constructability issues were also
addressed. The findings and recommendations from NCHRP Projects 24-23 and 24-07(2)
are combined in Volume 2, Design Guideline 11 to provide comprehensive design guidance
for bridge pier riprap.

5.4
5.2.4 Riprap for Bridge Abutments

For NCHRP 24-23, only the abutment riprap sizing approach as developed by FHWA
(Pagn-Ortiz 1991, Atayee 1993) and presented in HEC-23 (Second Edition) was considered
to be a candidate for further investigation. The approach consists of two equations, one for
Froude numbers less than 0.8 and the other for higher Froude numbers. There are no field
data available to test these equations and the only available laboratory data set was used to
develop the equations. The FHWA equations rely on an estimated velocity, known as the
characteristic average velocity, at the abutment toe. Rather than evaluating these equations
using the same laboratory data set used to develop them, the method for estimating the
velocity at the abutment was investigated in detail. Two-dimensional modeling was
performed to evaluate the flow field around an abutment and to verify or improve the Set
Back Ratio (SBR) method for estimating velocity for the design equations. Results of the
modeling indicated that if the estimated velocity does not exceed the maximum velocity in
the channel, the SBR method is well suited for determining velocity at an abutment as a
basis for riprap design.

The findings and recommendations from NCHRP Project 24-23 (Lagasse et al. 2006) and
NCHRP Project 24-18 (Barkdoll et al. 2007) are presented in Volume 2, Design Guideline 14
for the sizing, filter, and layout of abutment riprap installations. Material and testing
specifications, construction and installation guidelines, and inspection and quality control for
revetment riprap are suitable for abutment riprap (see Section 5.5 and Design Guideline 4).

5.2.5 Riprap Protection for Countermeasures

In general, design guidelines and specifications for riprap to protect countermeasures are
similar to those for bankline revetment or abutments. Consequently, recommendations for
revetment riprap can be adapted to the countermeasure application. Guidance for sizing
and placing riprap at zones of high stress on countermeasures (e.g., the nose of a guide
bank or spur) was developed during NCHRP Project 24-23 (Lagasse et al. 2006). The
feasibility of using an abutment-related characteristic average velocity for countermeasure
riprap sizing was also evaluated, and a recommendation on an adjustment to the
characteristic average velocity approach for guide bank riprap design was developed.
Guidance from the U.S. Army Corps of Engineers (EM 1601) can be used for sizing riprap
for spurs (USACE 1991). The findings and recommendations from NCHRP Project 24-23
are the basis for design guidance for sizing riprap for spurs in Design Guideline 2 and for
guide banks in Design Guideline 15.

NCHRP 24-23 also investigated methods for sizing riprap under overtopping conditions on
roadway embankments and the embankment portion of countermeasures. The
recommended methodology, based on laboratory testing at Colorado State University, is
presented in Design Guideline 5.

5.2.6 Riprap for Special Applications

Environments subject to wave attack frequently require some type of protection to ensure
the stability of highway and/or bridge infrastructure. Design Guideline 17 provides
information on wave characteristics and procedures for designing rock riprap as protection
against wave attack.
5.5
Bottomless (or three-sided) culverts are structures that have natural channel materials as the
bottom. These structures may be rectangular in shape or may have a more rounded top.
They are typically founded on spread footings and can be highly susceptible to scour.
Recent laboratory studies by FHWA (Kerenyi 2003, 2007) show that scour is greatest at the
upstream corners of the culvert entrance. Based on these studies and other guidance
(MDSHA 2005), Design Guideline 18 presents riprap sizing, filter, and layout details to
protect against scour at bottomless culverts.

5.2.7 Termination Details

Undermining of the edges of armoring countermeasures like riprap is one of the primary
mechanisms of failure (see Section 5.4). The edges of the armoring material (head, toe, and
flanks) should be designed so that undermining will not occur. For channel bed armoring,
this is accomplished by keying the edges into the subgrade to a depth which extends below
the combined expected contraction scour and long-term degradation depth. For side slope
protection, this is achieved by trenching the toe of the revetment below the channel bed to a
depth which extends below the combined expected contraction scour and long-term
degradation depth. When excavation to the contraction scour and degradation depth is
impractical, a launching apron can be used to provide enough volume of rock to launch into
the channel while maintaining sufficient protection of the exposed portion of the bank.
Additional guidelines on edge treatment for riprap countermeasures can be found in Design
Guidelines 4, 11, and 14.

5.2.8 Riprap Size, Shape, and Gradation

Riprap design methods typically yield a required size of stone that will result in stable
performance under the design loadings. Because stone is produced and delivered in a
range of sizes and shapes, the required size of stone is often stated in terms of a minimum
allowable representative size. For example, the designer may specify a minimum d
50
or d
30

for the rock comprising the riprap, thus indicating the size for which 50% or 30% (by weight)
of the particles are smaller. Stone sizes can also be specified in terms of weight (e.g., W
50

or W
30
) using an accepted relationship between size and volume, and the known (or
assumed) density of the particle.

Shape: The shape of a stone can be generally described by designating three axes of
measurement: Major, intermediate, and minor, also known as the "A, B, and C" axes, as
shown in Figure 5.1.


Figure 5.1. Riprap shape described by three axes.
5.6
Riprap stones should not be thin and platy, nor should they be long and needle-like.
Therefore, specifying a maximum allowable value for the ratio A/C, also known as the shape
factor, provides a suitable measure of particle shape, since the B axis is intermediate
between the two extremes of length A and thickness C. A maximum allowable value of 3.0
is recommended:

0 . 3
C
A
(5.2)

For riprap applications, stones tending toward subangular to angular are preferred, due to
the higher degree of interlocking, hence greater stability, compared to rounded particles of
the same weight.

Density: A measure of density of natural rock is the specific gravity S
g
, which is the ratio of
the density of a single (solid) rock particle
s
to the density of water
w
:

w
s
g
S

=
(5.3)

Typically, a minimum allowable specific gravity of 2.5 is required for riprap applications.
Where quarry sources uniformly produce rock with a specific gravity significantly greater
than 2.5 (such as dolomite, S
g
= 2.7 to 2.8), the equivalent stone size can be substantially
reduced and still achieve the same particle weight gradation.

Size and weight: Based on field studies, the recommended relationship between size and
weight is given by:

) d ( 85 . 0 W
3
s
=
(5.4)

where: W = Weight of stone, lb (kg)

s
= Density of stone, lb/ft
3
(kg/m
3
)
d = Size of intermediate ("B") axis, ft (m)

Table 5.1 provides recommended gradations for ten standard classes of riprap based on the
median particle diameter d
50
as determined by the dimension of the intermediate ("B") axis.
These gradations were developed under NCHRP Project 24-23, "Riprap Design Criteria,
Recommended Specifications, and Quality Control" (Lagasse et al. 2006). The proposed
gradation criteria are based on a nominal or "target" d
50
and a uniformity ratio d
85
/d
15
that
results in riprap that is well graded. The target uniformity ratio d
85
/d
15
is 2.0 and the allowable
range is from 1.5 to 2.5.

To specify riprap using the standard classes shown in Table 5.1, the "next larger size"
approach should be adopted. For example, if a riprap sizing calculation results in a required
d
50
of 16.8 inches, Class V riprap should be specified because it has a nominal d
50
of 18
inches.

Based on Equation 5.4, which assumes the volume of the stone is 85% of a cube, Table 5.2
provides the equivalent particle weights for the same ten classes, using a specific gravity of
2.65 for the particle density.
5.7


Table 5.1. Minimum and Maximum Allowable Particle Size in Inches.
Nominal Riprap Class
by Median Particle
Diameter
d
15
d
50
d
85
d
100

Class Size Min Max Min Max Min Max Max

I 6 in 3.7 5.2 5.7 6.9 7.8 9.2 12.0
II 9 in 5.5 7.8 8.5 10.5 11.5 14.0 18.0
III 12 in 7.3 10.5 11.5 14.0 15.5 18.5 24.0
IV 15 in 9.2 13.0 14.5 17.5 19.5 23.0 30.0
V 18 in 11.0 15.5 17.0 20.5 23.5 27.5 36.0
VI 21 in 13.0 18.5 20.0 24.0 27.5 32.5 42.0
VII 24 in 14.5 21.0 23.0 27.5 31.0 37.0 48.0
VIII 30 in 18.5 26.0 28.5 34.5 39.0 46.0 60.0
IX 36 in 22.0 31.5 34.0 41.5 47.0 55.5 72.0
X 42 in 25.5 36.5 40.0 48.5 54.5 64.5 84.0
Note: Particle size d corresponds to the intermediate ("B") axis of the particle.

Table 5.2. Minimum and Maximum Allowable Particle Weight in Pounds.
Nominal Riprap
Class by Median
Particle Weight
W
15
W
50
W
85
W
100

Class Weight Min Max Min Max Min Max Max

I 20 lb 4 12 15 27 39 64 140
II 60 lb 13 39 51 90 130 220 470
III 150 lb 32 93 120 210 310 510 1100
IV 300 lb 62 180 240 420 600 1000 2200
V 1/4 ton 110 310 410 720 1050 1750 3800
VI 3/8 ton 170 500 650 1150 1650 2800 6000
VII 1/2 ton 260 740 950 1700 2500 4100 9000
VIII 1 ton 500 1450 1900 3300 4800 8000 17600
IX 2 ton 860 2500 3300 5800 8300 13900 30400
X 3 ton 1350 4000 5200 9200 13200 22000 48200
Note: Weight limits for each class are estimated from particle size by: W = 0.85(sd
3
) where d corresponds to
the intermediate ("B") axis of the particle, and particle specific gravity is taken as 2.65.

5.3 FILTER REQUIREMENTS

5.3.1 Overview

The importance of the filter component of a countermeasure for stream instability or bridge
scour installation should not be underestimated. Filters are essential to the successful long-
term performance of countermeasures, especially armoring countermeasures. There are two
basic types of filters: granular filters and geotextile filters. Some situations call for a
composite filter consisting of both a granular layer and a geotextile. The specific
characteristics of the base soil determine the design considerations for the filter layer. In
general, where dune-type bedforms may be present during flood events, it is strongly
recommended that only a geotextile filter be considered for use with
countermeasures.
5.8
The filter must retain the coarser particles of the subgrade while remaining permeable
enough to allow infiltration and exfiltration to occur freely. It is not necessary to retain all the
particle sizes in the subgrade; in fact, it is beneficial to allow the smaller particles to pass
through the filter, leaving a coarser substrate behind. The filter prevents excessive migration
of the base soil particles through the voids in the armor layer, permits relief of hydrostatic
pressure beneath the armor, and distributes the weight of the armor to provide more uniform
settlement.

Guidance for the design of both granular and geotextile filters is provided in National
Cooperative Highway Research Program (NCHRP) Report 568, "Riprap Design Criteria,
Recommended Specifications, and Quality Control" (Lagasse et al. 2006), and is found in
Volume 2 as Design Guide 16. When using a granular filter, the layer should have a
minimum thickness of 4 times the d
50
of the filter stone or 6 inches, whichever is greater.
When placement must occur under water, the layer thickness should be increased by 50%.
In flowing water, the placement of both granular and geotextile filters becomes challenging.
Under these conditions, special materials and placement techniques have been developed
to ensure that a quality filter installation is achieved, as discussed in the next section.

5.3.2 Placing Geotextiles Under Water

Placing geotextiles under water is problematic for a number of reasons. Most geotextiles
that are used as filters beneath riprap are made of polyethylene or polypropylene. These
materials have specific gravities ranging from 0.90 to 0.96, meaning that they will float
unless weighted down or otherwise anchored to the subgrade prior to placement of the
armor layer (Koerner 1998). In addition, unless the work area is isolated from river currents
by a cofferdam, flow velocities greater than about 1.0 ft/s (0.3 m/s) create large forces on the
geotextile. These forces cause the geotextile to act like a sail, often resulting in wavelike
undulations of the fabric (a condition that contractors refer to as "galloping") that are
extremely difficult to control. In mild currents, geotextiles (precut to length) have been placed
using a roller assembly, with sandbags to hold the fabric temporarily.

To overcome these problems, engineers in Germany have developed a product known as
SandMat
TM
. This blanket-like product consists of two nonwoven needle-punched geotextiles
(or a woven and a nonwoven) with sand in between. The layers are stitch-bonded or sewn
together to form a heavy, filtering geocomposite. The composite blanket exhibits an overall
specific gravity ranging from approximately 1.5 to 2.0, so it sinks readily.

According to Heibaum (2002), this composite geotextile has sufficient stability to be handled
even when loaded by currents up to approximately 3.3 ft/s (1 m/s). At the geotextile base
soil interface, a nonwoven fabric should be used because of the higher angle of friction
compared to woven geotextiles. Figure 5.2 shows a close-up photo of the SandMat
TM

material. Figure 5.3 shows the SandMat
TM
blanket being rolled out using conventional
geotextile placement equipment.

In deep water or in currents greater than 3.3 ft/s (1 m/s), German practice calls for the use of
sand-filled geotextile containers. For specific project conditions, geotextile containers can be
chosen that combine the resistance against hydraulic loads with the filtration capacity
demanded by the application. Geotextile containers have proven to give sufficient stability
against erosive forces in many applications, including wave-attack environments. The size
of the geotextile container must be chosen such that the expected hydraulic load will not
transport the container during placement (Heibaum 2002). Once placed, the geotextile
containers are overlaid with the final armoring material (e.g., riprap or partially grouted
riprap) as shown in Figure 5.4.

5.9

Figure 5.2. Close-up photo of SandMat
TM
geocomposite blanket.
(photo from NCHRP Project 24-07(2), courtesy Colcrete Von Essen Inc.)






Figure 5.3. SandMat
TM
geocomposite blanket being unrolled.
(photo from NCHRP Project 24-07(2), courtesy Colcrete Von Essen Inc.)
5.10
FLOW
Geotextile containers
filled with sand
Rock riprap placed
flush with channel bed
Pier
FLOW
Geotextile containers
filled with sand
Rock riprap placed
flush with channel bed
Pier

Figure 5.4. Sand-filled geotextile containers.

Figure 5.5 shows a geotextile container being filled with sand. Figure 5.6 shows the sand-
filled geotextile container being handled with an articulated-arm clam grapple. The filled
geotextile container in the photograph is a nominal 1-metric-tonne (1,000 kg or 2,200 lb) unit.
The preferred geotextile for these applications is always a non-woven needle punched fabric,
with a minimum mass per unit area of 500 grams per square meter. Smaller geotextile
containers can be fabricated and handled by one or two people for smaller-sized
applications.

As a practical minimum, a 200-lb (91 kg) geotextile container covering a surface area of
about 6 to 8 square ft (0.56 to 0.74 m
2
) can be fashioned from nonwoven needle punched
geotextile having a minimum mass per unit area of 200 grams per square meter, filled at the
job site and field-stitched with a hand-held machine. Figure 5.7 illustrates the smaller
geotextile containers being installed at a prototype-scale test installation for NCHRP Project
24-07(2) (Lagasse et al. 2007) in a pier scour countermeasure application (see also Design
Guidelines 11 and 12, Volume 2).

5.4 RIPRAP FAILURE MODES

As discussed in Section 5.1, riprap can be considered an integrated system for which
successful performance of a riprap installation depends on the response of each component
of the system to hydraulic and environmental stresses throughout its service life. This
section provides an overview of failure modes for revetment and bridge pier riprap to
underscore the integrated nature of riprap armoring systems and support development of
inspection guidance.

5.11

Figure 5.5. Filling 1.0 metric tonne geotextile container with sand.
(photo from NCHRP Project 24-07(2), courtesy Colcrete Von Essen Inc.)



Figure 5.6. Handling a 1.0 metric tonne sand-filled geotextile container.
(photo from NCHRP Project 24-07(2), courtesy Colcrete Von Essen Inc.)

5.12


Demonstrating puncture resistance of
geotextile containers

Placing geotextile containers with small
front-end loader into scour hole at pier
Figure 5.7. Two hundred lb (91 kg) sand-filled geotextile containers,
NCHRP Project 24-07(2).

5.4.1 Riprap Revetment Failure Modes

In a preliminary evaluation of various riprap design techniques, Blodgett and McConaughy
(1986) concluded that a major shortcoming of all present design techniques is the assumption
that failures of riprap revetment are due only to particle erosion. Procedures for the design of
riprap protection need to consider all the various causes of failures which include: (1) particle
erosion; (2) translational slide; (3) modified slump; and (4) slump.

Particle erosion is the most commonly considered erosion mechanism (Figure 5.8). Particle
erosion occurs when individual particles are dislodged by the hydraulic forces generated by the
flowing water. Particle erosion can be initiated by abrasion, impingement of flowing water,
eddy action/reverse flow, local flow acceleration, freeze/thaw action, ice, or toe erosion.
Probable causes of particle erosion include: (1) stone size not large enough; (2) individual
stones removed by impact or abrasion; (3) side slope of the bank so steep that the angle of
repose of the riprap material is easily exceeded; and (4) gradation of riprap too uniform. Figure
5.9 provides a photograph of a riprap failure due to particle displacement.


Figure 5.8. Riprap failure by particle erosion (Blodgett and McConaughy 1986).
5.13

Figure 5.9. Damaged riprap on left bank of Pinole Creek at Pinole, CA, following flood of
January 4, 1982. Note deposition of displaced riprap from upstream locations
in channel bed (photographed March 1982) (Blodgett & McConaughy 1986).


A translational slide is a failure of riprap caused by the downslope movement of a mass of
stones, with the fault line on a horizontal plane (Figure 5.10). The initial phases of a
translational slide are indicated by cracks in the upper part of the riprap bank that extend
parallel to the channel. This type of riprap failure is usually initiated when the channel bed
scours and undermines the toe of the riprap blanket. This could be caused by particle
erosion of the toe material, or some other mechanism which causes displacement of toe
material. Any other mechanism which would cause the shear resistance along the interface
between the riprap blanket and base material to be reduced to less than the gravitational
force could also cause a translational slide. Probable causes of translational slides are as
follows: (1) bank side slope too steep; (2) presence of excess hydrostatic (pore) pressure;
and (3) loss of foundation support at the toe of the riprap blanket caused by erosion of the
lower part of the riprap blanket. Figure 5.11 provides a photograph of a riprap failure due to
a translational sliding-type failure.

Modified slump failure of riprap (Figure 5.12) is the mass movement of material along an
internal slip surface within the riprap blanket. The base soil underlying the riprap does not
fail. This type of failure is similar in many respects to the translational slide, but the geometry
of the damaged riprap is similar in shape to initial stages of failure caused by particle
erosion. Probable causes of modified slump are: (1) bank side slope is so steep that the
riprap is resting very near the angle of repose, and any imbalance or movement of individual
stones creates a situation of instability for other stones in the blanket; and (2) material critical
to the support of upslope riprap is dislodged by settlement of the submerged riprap, impact,
abrasion, particle erosion, or some other cause. Figure 5.13 provides a photograph of a
riprap failure due to a modified slump-type failure.


5.14


Figure 5.10 . Riprap failure by translational slide (Blodgett and McConaughy 1986).









Figure 5.11. Riprap on Cosumnes River at Site 2 near Sloughhouse, CA, looking
downstream, showing translational slide failure (photographed May 31,
1983) (Blodgett & McConaughy 1986).


5.15


Figure 5.12. Riprap failure by modified slump (Blodgett and McConaughy 1986).



Figure 5.13. Riprap on Consumnes River at Site 3 near Sloughhouse, CA, looking
downstream, showing modified slump failure (photographed May 31,
1983) (Blodgett & McConaughy 1986).

Slump failure is a rotational-gravitational movement of material along a surface of rupture
that has a concave upward curve (Figure 5.14). The cause of slump failures is related to
shear failure of the underlying base soil that supports the riprap. The primary feature of a
slump failure is the localized displacement of base material along a slip surface, which is
usually caused by excess pore pressure that reduces friction along a fault line in the base
material. Probable causes of slump failures are: (1) non-homogeneous base material with
layers of impermeable material that act as a fault line when subject to excess pore pressure;
(2) side slopes too steep and gravitational forces exceeding the inertia forces of the riprap
and base material along a friction plane; and (3) too much overburden at the top of the slope
(may be caused in part by the riprap). Figure 5.15 provides a photograph of a riprap failure
due to a slump-type failure.
5.16

Figure 5.14. Riprap failure due to slump (Blodgett and McConaughy 1986).



Figure 5.15. Riprap on left bank Cosumnes River at Site 1 near Sloughhouse, CA,
showing slump failure (photographed May 31, 1983) (Blodgett &
McConaughy 1986).

Summary: Blodgett and McConaughy (1986) conclude that certain hydraulic factors are
associated with each of the four types of riprap failure (particle erosion, translational slide,
modified slump, and true slump). While the specific mechanism causing failure of the riprap
is difficult to determine, and a number of factors, acting either individually or combined, may
be involved, they identify the reasons for riprap failures as:

1. Particle size was too small because:
Shear stress was underestimated
Velocity was underestimated
Inadequate allowance was made for channel curvature
Design channel capacity was too low
Design discharge was too low
Inadequate assessment was made of abrasive forces
Inadequate allowance was made for effect of obstructions
5.17
2. Channel changes caused:
Impinging flow
Flow to be directed at ends of protected reach
Decreased channel capacity or increased depth
Scour
3. Riprap material had improper gradation
4. Material was placed improperly
5. Side slopes were too steep
6. No filter blanket was installed or blanket was inadequate or damaged
7. Excess pore pressure caused failure of base material or toe of riprap
8. Differential settlement occurred during submergence or periods of excessive precipitation

5.4.2 Pier Riprap Failure Modes

A study of the causes of riprap failure at model bridge piers (Chiew 1995) under clear-water
conditions with gradually increasing approach flow velocities defined three modes of pier
riprap failure:

1. Riprap shear failure whereby the riprap stones cannot withstand the downflow and
horseshoe vortex associated with the pier scour mechanism.
2. Winnowing failure whereby the underlying finer bed material is removed through voids
or interstices in the riprap layer.
3. Edge failure whereby instability at the edge of the coarse riprap layer and the bed
sediment initiates a scour hole beginning at the perimeter and working inward that
ultimately destabilizes the entire layer.

Since live-bed conditions are more likely to occur during flood flows, additional experiments
were conducted to evaluate the stability of pier riprap under live-bed conditions with
migrating bed forms (Lim and Chiew 1996). These experiments and subsequent research
Melville et al. (1997), Lauchlan (1999), and Lauchlan and Melville (2001) indicates that bed-
form undermining is the controlling failure mechanism at bridge piers on rivers with mobile
bed forms, especially sand bed rivers. The most important factors affecting the stability of
the riprap layer under live-bed conditions were the turbulent flow field around the pier and
the fluctuations of the bed level caused by bed forms (e.g., dunes) as they migrate past the
pier. The three failure modes defined for clear-water conditions also exist under live-bed
conditions and they may act independently or jointly with migrating bed forms to destabilize
the riprap layer.

Once sediment transport starts and bed forms associated with the lower flow regime (i.e.,
ripples and dunes) begin to form, the movement of sediments at the edge of the riprap layer
remove the support of the edge stones. When the trough of a bed feature migrated past the
riprap layer, stones would slide into the trough, causing the riprap layer to thin. Depending
on the thickness of the remaining riprap layer following stone sliding and layer thinning,
winnowing may occur as a result of exposure of the underlying fine sediments to the flow.
Winnowing can cause the entire remaining riprap layer to subside into the bed. With thicker
riprap layers winnowing is not a factor and there is no subsidence.
5.18
Under steady flow conditions, the inherent flexibility of a riprap layer can provide a self-
healing process (Chiew 1995). As scour occurs and sediment is removed from around the
riprap layer through the three modes of erosion described above, the riprap layer, if it has
sufficient thickness, can adjust itself to the mobile channel bed and remain relatively intact
while providing continued scour protection for the pier. However, when flow velocity is
steadily increased, riprap shear, winnowing, and edge erosion combine to cause either a
total disintegration or embedment failure of the riprap layer in the absence of an underlying
filter (either geotextile or granular).

Total disintegration, which is characterized by a complete breakup of the riprap layer
whereby the stones are washed away by the flow, occurs when the self-healing ability of the
riprap layer is exceeded by the erosive power created by higher flow velocity. Total
disintegration occurs when the riprap stone size to sediment size ratio is small. Embedment
failure occurs when: (1) the riprap stones are large compared to the bed sediment and local
erosion around the individual stones causes them to embed into the channel bed (i.e.,
differential mobility); and (2) the riprap stones lose their stability as bed forms pass and drop
into the troughs of the migrating bed forms (i.e., bed feature destabilization).

5.4.3 Pier Riprap Failure Modes Schoharie Creek Case Study

The failure of the I-90 bridge over Schoharie Creek near Albany, New York on April 5, 1987,
which cost 10 lives, was investigated by the National Transportation Safety Board (NTSB)
(Richardson and Davis 2001). The peak flow was 64,900 cfs (1,838 m
3
/s) with a 70- to 100-
year return period. The foundations of the four bridge piers were large spread footings 82 ft
(25 m) long, 18 ft (5.5 m) wide, and 5 ft (1.5 m) deep without piles. The footings were set 5
ft (1.5 m) into the stream bed in very dense ice contact stratified glacial drift, which was
considered nonerodible by the designers (Figure 5.16). However, flume studies of samples
of the stratified drift showed that some material would be eroded at a velocity of 4 ft/s (1.5
m/s), and at a velocity of 8 ft/s (2.4 m/s) the erosion rates were high.


Figure 5.16. South elevation - Schoharie Creek Bridge showing key structural features
and a schematic geological section (Richardson et al. 1987).
5.19
A 1 to 50 scale, 3-dimensional, model study indicated a prototype flow velocity of 10.8 ft/s
(3.3 m/s) at the pier that failed. Also, the 1 to 50 scale and a 1 to 15 scale, 2-dimensional
model study gave 15 ft (4.6 m) of maximum scour depth. The scour depth of the prototype
pier (pier 3) at failure was 14 ft (4.3 m) (Figure 5.17).


Figure 5.17. Pier scour holes at Schoharie Creek Bridge in 1987. Pier 2 in
the foreground with pier 3 in the background.

Design plans called for the footings to be protected with riprap. Over time (1953 to 1987),
much of the riprap was removed by high flows. NTSB gave as the probable cause ".... the
failure of the New York State Thruway authority to maintain adequate riprap around the
bridge piers, which led to severe erosion in the soil beneath the spread footings.
Contributing to the severity of the accident was the lack of structural redundancy in the
bridge" (NTSB 1988).

The NYSTA inspected the bridge annually or biennially with the last inspection on April 1,
1986. A 1979 inspection by a consultant hired by NYSDOT indicated that most of the riprap
around the piers was missing (Figures 5.18 and 5.19); however, the 1986 inspection failed to
detect any problems with the condition of the riprap at the piers. Based on the NTSB
findings, the conclusions from this failure are that inspectors and their supervisors must
recognize that riprap does not necessarily make a bridge safe from scour, and inspectors
must be trained to recognize when riprap is missing and the significance of this condition.

Summary: Examples of the most common modes of riprap failure at piers provide guidance for
post-flood and post-construction performance evaluation. Inspectors need to be aware of, and
understand, the causes of riprap inadequacies that they see in the field. While the specific
mechanism causing failure of the riprap is difficult to determine, and a number of factors,
acting either individually or combined, may be involved, the reasons for riprap failures at
bridge piers can be summarized as follows:
5.20


Figure 5.18. Photograph of riprap at pier 2, October 1956.




Figure 5.19. Photograph of riprap at pier 2, August 1977 (flow is from right to left).

5.21
1. Particle size was too small because:
Shear stress was underestimated
Velocity was underestimated
Inadequate allowance was made for channel curvature
Design channel capacity was too low
Design discharge was too low
Inadequate assessment was made of abrasive forces
Inadequate allowance was made for effect of obstructions (such as debris)
2. Channel changes caused:
Increased angle of attack (skew)
Decreased channel capacity or increased depth
Scour
3. Riprap material had improper gradation
4. Material was placed improperly
5. No filter blanket was installed or blanket was inadequate or damaged

5.5 RIPRAP INSPECTION GUIDANCE

5.5.1 General

Inspection of riprap placement typically consists of visual inspection of the installation
procedures and the finished surface. Inspection must ensure that a dense, rough surface of
well-keyed graded rock of the specified quality and sizes is obtained, that the layers are
placed such that voids are minimized, and that the layers are the specified thickness.

If the riprap installation is part of channel stability works in the vicinity of a bridge, it is
typically inspected during the biennial bridge inspection program. However, more frequent
inspection might be required by the Plan of Action for a particular bridge or group of bridges.
In some cases, inspection may be required after every flood that exceeds a specified
magnitude. Underwater inspection of a riprap system should only be performed by divers
specifically trained and certified for such work.

The following general guidance for inspecting riprap is presented in the National Highway
Institute (NHI) training course 135047, "Stream Stability and Scour at Highway Bridges for
Bridge Inspectors:"

1. Riprap should be angular and interlocking (Old bowling balls would not make good
riprap). Flat sections of broken concrete paving do not make good riprap.
2. Riprap should have a granular or synthetic geotextile filter between the riprap and the
subgrade material.
3. Riprap should be well graded (a wide range of rock sizes). The maximum rock size
should be no greater than about twice the median (d
50
) size.
4. For bridge piers, riprap should generally extend up to the bed elevation so that the top of
the riprap is visible to the inspector during and after floods.
5.22
5. When inspecting riprap, the following are strong indicators of problems:
Has riprap been displaced downstream?
Has angular riprap blanket slumped down slope?
Has angular riprap material been replaced over time by smoother river run material?
Has riprap material physically deteriorated, disintegrated, or been abraded over
time?
Are there holes in the riprap blanket where the filter has been exposed or breached?

5.5.2 Guidance for Recording Riprap Condition

To guide the inspection of a riprap installation, a recording system is presented in Appendix
D. This guidance establishes numerical ratings from 0 (worst) to 9 (best). Recommended
action items based on the numerical rating are also provided (Lagasse et al. 2006).

5.5.3 Performance Evaluation

The evaluation of any revetment system's performance should be based on its design
parameters as compared to actual field experience, longevity, and inspection / maintenance
history. To properly assess the performance of revetment riprap, the history of hydraulic
loading on the installation, in terms of flood magnitudes and frequencies, must also be
considered and compared to the design loading. Guidance for the performance evaluation of
riprap armoring systems is provided in NCHRP Report 593 (Lagasse et al. 2007).

Changes in channel morphology may have occurred over time subsequent to the installation
of the riprap. Present-day channel cross-section geometry and planform should be
compared to those at the time of installation. Both lateral and vertical instability of the
channel can significantly alter hydraulic conditions at the site. Approach flows may exhibit
an increasingly severe angle of attack (impinging flow) over time, increasing the hydraulic
loading on the riprap.

Deficiencies noted during the inspection should be corrected as soon as possible. As
with any armor system, progressive failure from successive flows must be avoided by
providing timely maintenance intervention.

5.6 GROUTED AND PARTIALLY GROUTED RIPRAP

Grouted riprap is rock slope paving with voids filled with concrete grout forming a monolithic
armor. Because fully grouted riprap is a rigid structure, it will not conform to bank settlement
or toe undermining as loose riprap does. Therefore, fully grouted riprap is susceptible to
mass failure, especially if pore water is not allowed to drain properly. Although the revetment
is rigid, it is not particularly strong and even a small loss of toe or bank support can result in
the failure of large portions of the structure.

The primary advantage of fully grouted riprap is that the grout anchors the rock and
eliminates particle erosion of the revetment. Therefore, smaller rock can be used for the
revetment, and the total thickness of the revetment can be reduced as compared with
traditional riprap revetment. Another advantage is that a relatively smooth surface can be
achieved and, therefore, the hydraulic efficiency of the waterway is improved. Filters are not
required for fully grouted riprap but drainage of pore water must be provided. A significant
disadvantage of fully grouted riprap is that a complete layer of grout converts a
5.23
flexible revetment to a rigid cover, subject to the potential problems of any rigid slope
paving, including undercutting at the toe, out flanking, and the possibility of sudden
catastrophic failure.

An alternative to fully grouted riprap is partially grouted riprap. In general, the objective is to
increase the stability of the riprap without sacrificing its flexibility. Partial grouting of riprap
may be well suited for areas where rock of sufficient size is not available to construct a loose
riprap revetment.

The River and Channel Revetments design manual published by H.R. Wallingford in the
United Kingdom (Escarameia 1998) provides design guidance for grouting "hand pitched
stone" with both bituminous and cement grout. For grouting riprap in the United Kingdom,
bitumen is the material most commonly used. Although various degrees of grouting are
possible, effective solutions are usually produced when the bituminous mortar envelopes the
loose stone and leaves relatively large voids between rock particles. The degrees of
bituminous grouting available are:

Surface grouting (which does not penetrate the whole thickness of the revetment and
corresponds to about one-third of the voids filled)

Various forms of pattern grouting (where only some of the surface area of the revetment
is filled, between 50 to 80% of voids)
Full grouting (an impermeable type of revetment)

Partial grouting of riprap with a cement slurry is presented as one of several standard design
approaches for permeable revetments in a discussion of considerations regarding the
experience and design of German inland waterways (Heibaum 2000). Partially grouted
riprap consists of appropriately sized rocks that are grouted together with grout filling only
1/3 to 1/2 of the total void space (Figure 5.20). In contrast to fully grouted riprap, partial
grouting increases the overall stability of the riprap installation unit without sacrificing
flexibility or permeability. It also allows for the use of smaller rock compared to standard
riprap, resulting in decreased layer thickness. Design, specification, and construction
guidance for partially grouted riprap is provided in Design Guideline 12, Volume 2.

The holes in the grout allow for drainage of pore water so a filter is required. The grout
forms conglomerates of riprap so the stability against particle erosion is greatly improved
and, as with fully grouted riprap, a smaller thickness of stone can be used (Figure 5.21).
Although not as flexible as riprap, partially grouted riprap will conform somewhat to bank
settlement and toe exposure.

An important consideration for partially grouted riprap is that construction methods must be
closely monitored to insure that the appropriate voids and surface openings are provided.
Contractors in Germany have developed techniques and equipment to achieve the desired
grout coverage and the right penetration. Various European countries have developed
special grout mixes and construction methods for underwater installation of partially grouted
riprap (see Design Guideline 12).


5.24

Figure 5.20. Close-up view of partially grouted riprap.





Figure 5.21. "Conglomerate" of partially grouted riprap, Federal Waterway Engineering and
Research Institute, Karlsruhe, Germany (Heibaum 2000).


5.25
5.7 CONCRETE ARMOR UNITS

Concrete armor units, also known as artificial riprap, consist of individual pre-cast concrete
units with complex shapes that are placed individually or in interconnected groups. These
units were originally developed for shore protection to resist wave action during extreme
storms. All are designed to give a maximum amount of interlocking using a minimum
amount of material. These devices are used where natural riprap is unavailable or is more
costly to obtain than fabrication of the artificial riprap units. Parker et al. (1998) provide a
review of studies conducted on the use of concrete armor units as pier scour
countermeasures.

Various designs for size and shape of concrete armor units are available and include such
commercial names as Tetrapods, Tetrahedrons, Toskanes, Dolos, Tribars, Accropodes,
Core-Loc
TM
, and A-Jacks

(Figure 5.22). Because concrete armor units are similar to riprap,

they can be susceptible to the same failure mechanisms as riprap. The use of a filter layer
or geotextile in conjunction with these types of devices is often required, especially in coastal
applications, and a geotextile or filter may be critical to the stability of these devices when
used as pier scour protection (see Design Guideline 19 for design procedures for Toskanes
and A-Jacks

).

Tetrapod Tetrahedron Toskane A-Jacks

Core-Loc
TM
Accropode Dolos Tribar

Figure 5.22. Concrete armor units.

The primary advantage of armor units is that they usually have greater stability compared to
riprap particles of equivalent weight. This is due to the interlocking characteristics of their
complex shapes. The increased stability allows their placement on steeper slopes or the use
of lighter weight units for equivalent flow conditions as compared to riprap. This is significant
when riprap of a required size is not available.

The design of armor units in open channels is based on the selection of appropriate sizes
and placement patterns to be stable in flowing water. The armor units should be able to
withstand the flow velocities without being displaced. Hydraulic testing is used to measure
the hydraulic conditions at which the armor units begin to move or "fail," and dimensional
5.26
analysis allows extrapolation of the results to other hydraulic conditions. Although a standard
approach to the stability analysis has not been established, design criteria have been
developed for various armor units using the following dimensionless parameters:

Isbash stability number (Parola 1993; Ruff and Fotherby 1995; Bertoldi et al. 1996)
Shields parameter (Bertoldi et al. 1996)
Froude number

( Brown and Clyde 1989)

The Isbash stability number and Shields parameter are indicative of the interlocking
characteristics of the armor units. Froude number scaling is based on similitude of stabilizing
and destabilizing forces. Quantification of these parameters requires hydraulic testing and,
typically, regression analysis of the data. Prior research and hydraulic testing have provided
guidance on the selection of the Isbash stability number and Shields parameter for riprap
and river sediment particles, but stability values are not available for all concrete armor units.
Therefore, manufacturers of concrete armor units have a responsibility to test their products
and to develop design criteria based on the results of these tests. Since armor units vary in
shape and performance from one proprietary system to the next, each system will have
unique performance properties.

Installation guidelines for concrete armor units in streambank revetment and channel armor
applications should consider subgrade preparation, edge treatment (toe down and flank)
details, armor layer thickness, and filter requirements. Subgrade preparation and edge
treatment for armor units is similar to that required for riprap. Considerations for armor layer
thickness and filter requirements are product specific and should be provided by the armor
unit manufacturer.

Concrete armor units have shown potential for mitigating the effects of local scour in the
laboratory; however, only limited data are available on their performance in the field.
Research efforts are currently being conducted to test the performance of concrete armor
units as pier scour countermeasures in the field.

Design methods which incorporate velocity (a variable which can be directly measured) are
commonly used to select local scour countermeasures. Normally an approach velocity is
used in the design equation (generally a modified Isbash equation) with a correction factor
for flow acceleration around the pier or abutment (see Section 5.2.3).

Although tetrahedrons are currently used for bank protection (Fotherby 1995), they have
garnered very little interest with regard to pier scour protection in the United States. This
may be primarily related to their lack of appendages and interlock (i.e., their simple compact
shape is similar to riprap and spheres). Dolos also have not been seriously considered for
use as pier scour protection because they have no inherent interlocking property to resist
movement under steady state turbulent flow (Brebner 1978). Extensive testing and research
has been conducted on the Core-Loc

system, which was developed by the U.S. Army

Engineer Waterways Experiment Station, but the testing was limited exclusively to coastal
applications. Accropode and tribar systems are used almost exclusively in coastal
applications as well.

In contrast, tetrapods have been extensively studied and evaluated for use as pier scour
protection (Fotherby 1992, 1993; Bertoldi et al. 1996; Jones et al. 1995; Bertoldi and Kilgore
1993). Fotherby (1992, 1993) and Stein et al. (1998) suggest that tetrapods offer little
advantage compared to riprap in terms of stability. Layering and density had no appreciable
5.27
effect on the stability of the tetrapods, although the stability increased with the size of the
tetrapod pad. Work by Bertoldi et al. (1996) and Stein et al. (1998) indicates that riprap and
tetrapods behaved comparably when both stability number and spherical stability number
were compared, and also suggest that fixing the perimeter and varying the number of
tetrapod layers may have an effect on stability.

A specific design procedure for Toskanes has been developed for application at bridge piers
and abutments where the Toskanes are installed as individual, interlocking units. The design
procedures for Toskanes are based on extensive research conducted at Colorado State
University (Ruff and Fotherby 1995; Fotherby 1995; Burns et al. 1996; Fotherby and Ruff
1998, 1999). Based on hydraulic model studies conducted at Colorado State University for
the Pennsylvania Department of Transportation, Burns et al. (1996) presented procedures
for the design of Toskane pads, provided criteria for sizing Toskanes, and suggested
techniques for installation of Toskanes (Figure 5.23). No other concrete armor unit has been
as extensively tested and evaluated for use as a pier scour countermeasure (see Design
Guideline 19).

Another approach to using concrete armor units for pier scour protection involves the
installation of banded modules of the A-Jacks

armor unit (Ayres Associates 1999; Thornton

et al. 1999). Laboratory testing results and installation guidelines developed at Colorado
State University by Ayres Associates (1999) for the A-Jacks

system illustrate the "modular"

design approach in contrast with the "discrete particle" approach for Toskanes (Figure 5.24).

The discrete particle design approach illustrated by the Toskane design guidelines
concentrates on the size, shape, and weight of individual armor units, whether randomly
placed or in stacked or interlocked configurations. In contrast, the basic construction element
of A-Jacks

for pier scour applications is a "module" comprised of a minimum of 14 individual

A-Jacks

banded together in a densely-interlocked cluster, described as a 5x4x5 module.

The banded module thus forms the individual design element as illustrated in Figure 5.24
(see Design Guideline 19).

As with any countermeasure, the armor units must withstand several potential failure modes.
Providing resistance against the hydraulic stresses may not be sufficient for structure
success. If the armor units are used to counter pier scour, they must also remain stable for
channel degradation, contraction scour, and the passage of bed forms (dunes). If armor
units are used as bank revetment, then the stability of the bank must be analyzed for
potential toe scour, pore water pressures and saturated soil strengths.

It should also be noted that concrete armor units, depending on their size, may be very
susceptible to vandalism. In addition, there may be maintenance and degradation issues
associated with any cables used to tie groups of concrete armor units together.
5.28

Figure 5.23. Laboratory study of Toskanes for pier scour protection.







Figure 5.24. Installation of A-Jacks modular units installed by Kentucky DOT
for pier scour protection.


6.1
CHAPTER 6

BIOTECHNICAL ENGINEERING


6.1 OVERVIEW

Vegetation is the most natural method for protecting streambanks because it is relatively
easy to establish and maintain and is visually attractive. However, vegetation alone should
not be seriously considered as a countermeasure against severe bank erosion where a
highway facility is at risk. At such locations, vegetation can best serve to supplement other
countermeasures.

Vegetation can effectively protect a bank below the design water line in two ways. First, the
root system helps to hold the soil together and increases overall bank stability by forming a
binding network. Second, the exposed stalks, stems, branches and foliage provide
resistance to flow, causing the flow to lose energy by deforming the plants rather than by
removing soil particles. Above the water line, vegetation prevents surface erosion by
absorbing the impact of falling raindrops and reducing the velocity of overbank flow and
rainfall runoff.

Vegetation is generally divided into two broad categories: (1) grasses, and (2) woody plants
(trees and shrubs). A major factor affecting species selection is the length of time required
for the plant to become established on the slope. Grasses are less costly to plant on an
eroding bank and require a shorter period of time to become established. Woody plants offer
greater protection against erosion because of more extensive root systems; however, under
some conditions the weight of the plant will offset the advantage of the root system. On high
banks, tree root systems may not penetrate to the toe of the bank. If the toe becomes
eroded, the weight of the tree and its root mass may cause a bank failure.

There are several synonymous terms that describe the field of vegetative streambank
stabilization and countermeasures. Terms for the use of 'soft' revetments (consisting solely
of living plant materials or plant products) include bioengineering, soil bioengineering,
ground bioengineering, and ecological bioengineering. Terms describing the techniques that
combine the use of vegetation with structural (hard) elements include biotechnical
engineering, biotechnical slope protection, bioengineered slope stabilization, and
biotechnical revetment. The terms soil bioengineering and biotechnical engineering are
most commonly used to describe stream bank erosion countermeasures and bank
stabilization methods that incorporate vegetation. Where riprap constitutes the "hard"
component of biotechnical slope protection, the term vegetated riprap is also used.

6.2 CURRENT PRACTICE

Due to a lack of technical training and experience, there is a reluctance on the part of many
engineers to resort to soil bioengineering and biotechnical engineering techniques and
stability methods. In addition, bank stabilization systems using vegetation have not been
standardized for general application under particular flow conditions. There is a lack of
knowledge about the properties of the materials being used in relation to force and stress
generated by flowing water and there may be difficulties in obtaining consistent performance
from countermeasures that rely on living materials. Nonetheless, stabilization of eroding
stream banks using vegetative countermeasures has proven effective in many documented
cases in Europe and the United States.
6.2
Most hydraulic engineers in Europe would not recommend the reliance on bioengineering
countermeasures as the only countermeasure technique when there is a risk of damage to
property or a structure, or where there is potential for loss of life if the countermeasure fails
(TRB 1999). Soil bioengineering is not suitable where flow velocities exceed the strength of
the bank material or where pore water pressure causes failures in the lower bank. In
contrast, biotechnical engineering is particularly suitable where some sort of engineered
structural solution is required because the risk associated with using just vegetation is
considered too high. However, the use of soil bioengineering and biotechnical engineering
with respect to scour and stream instability at highway bridges is a relatively new field.
Research has been conducted, but these techniques have generally not been tested
specifically as a countermeasure to protect bridges in the river environment.

Design of biotechnically engineered countermeasures to minimize rates of stream bank
erosion requires accounting for hydrologic, hydraulic, geomorphic, geotechnical, vegetative,
and construction factors. Although most of the literature dealing with biotechnical
engineering on rivers is associated with stream bank stabilization relative to channel
restoration and rehabilitation projects, it is also generally applicable to bank stabilization
associated with bridge crossings. Bentrup and Hoag (1998), Johnson and Stypula (1998),
U.S. Army Engineer Waterways Experiment Station (1998), and the Federal Interagency
Stream Restoration Working Group (1998) provide detailed guidelines, techniques, and
methods of biotechnical engineering for bank stabilization in the United States. Guidelines,
methodology, and design of biotechnically engineered streambank stabilization in Europe
and the United Kingdom are discussed in Schiechtl and Stern (1997), Morgan et al. (1997),
and Escarameia (1998).

6.3 GENERAL CONCEPTS

The following discussion is drawn primarily from concepts presented in NCHRP Report 544
entitled, "Environmentally Sensitive Channel- and Bank-Protection Measures," (McCullah
and Gray 2005) where biotechnical techniques are referred to as "vegetated riprap."

Continuous and resistive bank protection measures, such as riprap and longitudinal rock
toes are primarily used to armor outer bends or areas with impinging flows. These
continuous and concentrated high velocity areas will generally result in reduced aquatic
habitat. It has been widely documented that resistive techniques in general and riprap in
particular, provide minimal aquatic habitat benefits (Shields et al. 1995). Recently the
concerns over the poor aquatic-habitat value of riprap, both locally and cumulatively, have
made the use of riprap alone controversial (Washington Department of Fish and Wildlife
2003).

Since streambank protection designs that consist of riprap, concrete, or other inert structures
alone may be unacceptable for lack of environmental and aesthetic benefits, there is
increasing interest in designs that combine vegetation with inert materials into living systems
that can reduce erosion while providing environmental and aesthetic benefits (Sotir and
Nunnally 1995).

The negative environmental consequences of riprap can be reduced by minimizing the
height of the rock revetment up the bank and/or including biotechnical methods, such as
vegetated riprap with brush layering and pole planting, vegetated riprap with soil, grass and
ground cover, vegetated riprap with willow (Salix spp.) bundles, and vegetated riprap with
bent poles.

6.3
Combining riprap with deep vegetative planting (e.g., brush layering and pole planting) is
also appropriate for banks with geotechnical problems, because additional tensile strength is
often contributed by roots, stems, and branches. In contrast, trees and riparian vegetation
planted only on top of the bank can sometimes have a negative impact (Simon and Collison
2002).

Correctly designed and installed, vegetated riprap offers an opportunity for the designer to
attain the immediate and long-term protection afforded by riprap with the habitat benefits
inherent with the establishment of a healthy riparian buffer. The riprap will resist the hydraulic
forces, while roots and branches increase geotechnical stability, prevent soil loss (or piping)
from behind the structures, and increase pullout resistance.

Above ground components of the plants will create habitat for both aquatic and terrestrial
wildlife, provide shade (reducing thermal pollution), and improve aesthetic and recreational
opportunities. The roots, stems, and shoots will help anchor the rocks and resist 'plucking'
and gouging by ice and debris.

6.4 ADVANTAGES AND LIMITATIONS OF BIOTECHNICAL ENGINEERING

Specific ways vegetation can protect stream banks as part of a biotechnical engineering
approach include:

The root system binds soil particles together and increases the overall stability and shear
strength of the bank.
The exposed vegetation increases surface roughness and reduces local flow velocities
close to the bank, which reduces the transport capacity and shear stress near the bank,
thereby inducing sediment deposition.
Vegetation dissipates the kinetic energy of falling raindrops, and depletes soil water by
uptake and transpiration.
Vegetation reduces surface runoff through increased retention of water on the surface
and increases groundwater recharge.
Vegetation deflects high-velocity flow away from the bank and acts as a buffer against
the abrasive effect of transported material.
Vegetation improves the conditions for fisheries and wildlife and helps improve water
quality.

In addition, biotechnical engineering is often less expensive than most methods that are
entirely structural and it is often less expensive to construct and maintain when considered
over the long-term.

The critical threats to the successful performance of biotechnical engineering projects are
improper site assessment, design or installation, and lack of monitoring and maintenance
(especially following floods and during droughts).
6.4
Some of the specific limitations to the use of vegetation for streambank erosion control
include:

Lack of design criteria and knowledge about properties of vegetative materials
Lack of long-term quantitative monitoring and performance assessment
Difficulty in obtaining consistent performance from countermeasures relying on live
materials
Possible failure to grow and susceptibility to drought conditions
Depredation by wildlife or livestock
Significant maintenance may be required

More importantly, the type of plants that can survive at various submersions during the
normal cycle of low, medium, and high stream flows is critical to the design, implementation,
and success of biotechnical engineering techniques. In addition, the combination of riprap
and vegetation may be inappropriate if flow capacity is an issue, since bank vegetation can
reduce flow capacity, especially when in full leaf along a narrow channel.

6.5 DESIGN CONSIDERATIONS FOR BIOTECHNICAL COUNTERMEASURES

In an unstable watershed, careful study should be made of the causes of instability before
biotechnical countermeasures are contemplated (see HEC-20) (Lagasse et al. 2001a). Since
bank erosion is tied to channel stability, a stable channel bed must be achieved before the
banks are addressed. Scour and erosion of the bank toe produce the dominant failure
modes (see HEC-20), consequently, most biotechnical engineering projects documented in
the literature contain some form of structural (hard) toe stabilization, such as rock riprap
(Figure 6.1), rock gabions, cribs, cable anchored logs, or logs with root wads anchored by
boulders (Figure 6.2). Note the use of a fascine bundle in Figure 6.1 as part of the rock toe
protection. Toe protection should be keyed into the channel bed sufficiently deep to
withstand significant scour, and the biotechnically engineered revetment should be keyed
into the bank at both the upstream and downstream ends (called refusals) to prevent
flanking. Deflectors such as fences, dikes, and pilings may also be utilized to deflect flow
away from the bankline.

Other factors that need to be considered when selecting a design option include climate and
hydrology, soils, cross-sectional dimensions (is there sufficient room for the
countermeasure), flow depth, flow velocity (both magnitude and direction), and slope of the
bankline being protected. Most methods of biotechnical engineering will require some
amount of bank regrading. Because structure design is based on flood velocities and depths,
one or more design flows will need to be analyzed. Of particular interest is the bankfull or
overtopping event, since this event generates the greatest velocities and tractive forces.
Local (at or near the project site) flow velocities should be used for the design, especially
along the outside of bends. The erosion protection should extend far enough downstream,
particularly on the outer banks of bends. The highest velocities generally occur at the
downstream arc of a bend and on the outer bank of the exit reach immediately downstream.
As noted, the countermeasures should be tied into the bank at both ends to prevent flanking.

6.5


Figure 6.1. Details of brush mattress technique with stone toe protection (FISRWG 1998).




Figure 6.2. Details of root wad and boulder revetment technique (FISRWG 1998).

6.6
6.6 COMMONLY USED VEGETATIVE METHODS

Five methods for constructing vegetated riprap have proven effectiveness. Typical design
concept sketches of the five methods are provided as Figures 6.3 through 6.7. These
sketches are reproduced from NCHRP Report 544 (McCullah and Gray 2005). It should be
noted that the key hydraulic design variable "design high water" is not defined in these
sketches, and "average high water" (AHW) and "average low water" are only qualitatively
described.

1. Vegetated riprap with willow bundles (Figure 6.3): Vegetated riprap with willow
bundles is the simplest to install, but it has a few drawbacks. This technique typically
requires very long 10-23 ft (3-7 m) poles and branches, as the cuttings should reach
from 6 inches (15 cm) below the low water table to 1 ft (30 cm) above the top of the
rocks. In addition, only those cuttings that are in contact with the soil will take root,
and therefore, the geotechnical benefits of the roots from those cuttings on the top of
the bundle may not be realized.
2. Vegetated riprap with bent poles (Figure 6.4): Vegetated riprap with bent poles is
slightly more complex to install, and is the only method that can be installed with filter
fabric. Additionally, a variety of different lengths of willow cuttings can be used
because they will protrude from the rock at different elevations.
3. Vegetated riprap with brush layering and pole planting (Figure 6.5): Vegetated riprap
with brush layering and pole planting is the most complex type of riprap to install, but
also provides the most immediate habitat benefits. The installation of this technique
is separated into two methods; one method describes installation when building a
bank back up, while the other is for a well-established bank. If immediate aquatic
habitat benefits are desired, this technique should be used. However, vegetated
riprap with brush layering and pole planting may not provide the greatest amount of
root reinforcement, as the stem-contact with soil does not extend up the entire slope.
Combination of this technique with pole- or bundle-planted riprap will perform well, as
the latter techniques typically have higher rooting success.
4. Vegetated riprap with soil cover, grass and ground cover (Figure 6.6): This technique
is also known as "buried riprap," and consists of infilling and covering a standard rock
riprap installation with soil and subsequently establishing grass vegetation. Some
stripping of the soil and grass may be expected during severe events.
5. Joint or Live Stake Planted Riprap (Figure 6.7): Joint or live stake planted riprap is
revegetated riprap, as opposed to the other techniques, which are true vegetated
riprap methods. This technique should be used only when attempting to get
vegetative growth on previously installed riprap.

6.7. ENVIRONMENTAL CONSIDERATIONS AND BENEFITS

There are many environmental benefits offered by vegetated riprap, most of which are
derived from the planting of willows or other woody species in the installation. Willow
provides canopy cover to the stream, which gives fish and other aquatic fauna cool places to
hide. The vegetation also supplies the river with carbon-based debris, which is integral to
many aquatic food webs, and birds that catch fish or aquatic insects will be attracted by the
increased perching space next to the stream (Gray and Sotir 1996).
6.7

Figure 6.3. Vegetated riprap - willow bundle method (McCullah and Gray 2005).


Figure 6.4. Vegetated riprap - bent pole method (McCullah and Gray 2005).
6.8

Figure 6.5. Vegetated riprap - brush layering with pole planting (McCullah and Gray 2005).


Figure 6.6. Vegetated riprap - construction techniques (McCullah and Gray 2005).

6.9


Figure 6.7. Vegetated riprap with joint planting (McCullah and Gray 2005).


6.10
The exclusive placement of predator-perching type habitat may not be appropriate where
fish-rearing habitat is desired. In that situation, large rocks and logs located above the
average high water line (AHW) might be replaced with shrubby-type protective vegetation.
An additional environmental benefit is derived from the use of rock, as the surface area of
the rocks is substrate that is available for colonization by invertebrates (Freeman and
Fischenich 2000). The small spaces between the rocks also provide benthic habitat and
hiding places for small fish and fry.

6.8 APPLICATION GUIDANCE FOR BIOTECHNICAL COUNTERMEASURES

6.8.1 Streambank Zones

As indicated by U.S. Army Engineers Waterways Experiment Station (WES 1998), plants
should be positioned in various elevational zones of the bank based on their ability to
tolerate certain frequencies and durations of flooding, and their attributes of dissipating
current- and wave-energies. The stream bank is generally broken into three or four zones to
facilitate prescription of the biotechnical erosion control treatment. Because of daily and
seasonal variations in flow, the zones are not precise and distinct. The zones are based on
their bank position and are defined as the toe, splash, bank and overbank zones (Figure
6.8).

The toe zone is the area between the bed and the average normal stage. This zone is often
under water more than six months of the year. It is a zone of high stress and is susceptible
to undercutting and scour resulting in bank failure.

The splash zone is located between the normal high-water and normal low-water stages and
is inundated throughout much of the year (at least six months). Water depths fluctuate daily,
seasonally, and by location within the zone. This zone is also an area of high stress, being
exposed frequently to wave-wash, erosive currents, ice and debris movement, wet-dry
cycles, and freeze-thaw cycles.

Because the toe and splash zones are the zones of highest stress, these zones are treated
as one zone with a structural revetment, such as rock, stone, logs, cribs, gabions, or some
other 'hard' treatment. Within the splash zone, flood-resistant herbaceous emergent aquatic
plants like reeds, rushes, and sedges may be planted in the structural element of the bank
protection.

The bank zone is usually located above the normal high-water level, but is exposed
periodically to wave-wash, erosive flows, ice and debris movement, and traffic by animals or
man. This zone is inundated for at least a 60-day duration once every two to three years and
is influenced by a shallow water table. Herbaceous (i.e., grasses, clovers, some sedges,
and other herbs) and woody plants (i.e., willows, alder, and dogwood) that are flood tolerant
and able to withstand partial to complete submergence for up to several weeks are used in
this zone. Whitlow and Harris (1979) provide a listing of very flood-tolerant woody species
and a few herbaceous species by geographic area within the United States.

The overbank zone includes the top bank area and the area inland from the bank zone, and
is usually not subjected to erosive forces except during occasional flooding. Vegetation in
this zone is extremely important for intercepting overbank floodwater, binding the soil in the
upper bank together through its root system, helping reduce super-saturation of the bank,
and decreasing the weight of unstable banks through evapotranspiration processes. This
zone can contain grasses, herbs, shrubs, and trees that are less flood-tolerant than those in
the bank zone. The rooting depth of trees can be an extremely important part of bank
stability. Besides erosion control, wildlife habitat diversity, aesthetics, and access for project
construction and long-term maintenance are important considerations in this zone.
6.11
OVERBANK
ZONE
BANK
ZONE
SPLASH
ZONE
TOE
ZONE
WATER LEVELS
Above Normal Stage
Average Normal Stage
Below Normal Stage
OVERBANK
ZONE
BANK
ZONE
SPLASH
ZONE
TOE
ZONE
WATER LEVELS
Above Normal Stage
Average Normal Stage
Below Normal Stage

Figure 6.8. Bank zones defined for slope protection (WES 1998).

6.8.2 Biotechnical Engineering Treatments

Descriptions and guidelines for biotechnical engineering treatments or combinations of
treatments, and plant species that can be used in the treatments are described in detail by
WES (1998), Bentrup and Hoag (1998), and Schiechtl and Stern (1997). The following is a
brief summary of some of the major types of biotechnical engineering treatments that can be
used separately or in some combination.

Toe Zone. Structural revetments such as riprap, gabions, cribs, logs, or root wads in a
biotechnical engineering application are used at the toe in the zone below normal water
levels and up to where normal water levels occur. There are no definitive guidelines for how
far up the bank to extend the structural revetment. Instead, it is common practice to extend
the revetment from below the predicted contraction and local scour depth up to at least
where the water flows the majority of the year. Vegetative treatments are placed above or
behind this structural toe protection (see Figures 6.1, 6.2, and 6.7).

Splash Zone. Several treatments may be used individually or in combination with other
treatments in the splash zone above or behind the structural toe protection. These include
coir rolls and mats, brush mattresses, wattles or fascines, brush layering, vegetative geogrid,
dormant posts, dormant cuttings, and root pads.

Coir is a biodegradable geotextile fabric made of woven fibers of coconut husks and is
formed into either rolls (coir roll) or mats (coir fiber mats). Coir rolls are often placed above
the structural toe protection parallel to the bank with wetland vegetation planted or grown in
the roll. Coir fiber mats are made in various thicknesses and are often prevegetated at a
nursery with emergent aquatic plants or sometimes sprigged on-site with emergent aquatic
plants harvested from local sources.

Brush mattresses, sometimes called brush matting or brush barriers, are a combination of a
thick layer of long, interlaced live willow switches or branches and wattling. Wattling, also
known as fascine, is a cigar-shaped bundle of live, shrubby material made from species that
root rapidly from the stem. The branches in the mattress are placed perpendicular to the
bank with their basal ends inserted into a trench at the bottom of the slope in the splash
zone, just above the structural toe protection. The fascines are laid over the basal ends of
the brush mattress in the ditch and staked. The mattress and fascines are kept in place by
either woven wire or tie wire that is held in place by wedge-shaped construction stakes.
6.12
Both are covered with soil and tamped. Figures 6.1, 6.3, and 6.7 show examples of this type
of treatment.

Brush layering, also called branch layering or branch packing, is used in the splash zone as
well as in the bank zone. This treatment consists of live branches or brush that quickly
sprout, such as willow or dogwood species, placed in trenches dug into the slope, on
contour, with their basal ends pointed inward and the tips extending beyond the fill face.
Branches should be arranged in a cris-cross fashion and covered with firmly compacted soil.
This treatment can also be used in combination with live fascines and live pegs.

Vegetative geogrid is also used in the splash zone and can extend farther up into the bank
zone and possibly the overbank zone. This system is also referred to as "fabric encapsulated
soil" and consists of successive walls of several lifts of fabric reinforcement with intervening
long, live willow whips. The fabric consists of two layers of coir fabric which provide both
structural strength and resistance to piping of fine sediments.

Dormant post treatment consists of placing dormant, but living stems of woody species that
sprout stems and roots from the stem, such as willow or cottonwood, in the splash zone and
the lower part of the bank zone. Post holes are formed in the bank so that the end of the
post is below the maximum predicted scour depth. Posts can also be planted in riprap
revetments.

Dormant cuttings, also known as live stakes, consists of inserting and tamping live, single
stem, rootable cuttings into the ground or sometimes geotextile substrates. In the splash
zone of high velocity streams, this method is used in combination with other treatments,
such as brush mattresses and root wads. Dormant cuttings can be used as live stakes in
the brush mattress and fascines in the place of or in combination with the wedge-shaped
construction stakes (Figures 6.1, 6.5, 6.6, and 6.7).

Root pads are clumps of shrubbery composed of woody species that are often placed in the
splash zone between root wads (Figure 6.2). Root pads can also be used in the bank and
overbank zones, but should be secured with stakes on slopes greater than 1V:6H.

Bank Zone. This zone can be stabilized with the treatments previously described as well as
with sodding, mulching, or a combination of treatments. Sodding of flood-tolerant grasses
can be used to provide rapid bank stabilization where only mild currents and wave action are
expected. The sod usually must be held in place with some sort of wire mesh, geotextile
mesh such as a coir fabric, or stakes. Coir mats may extend into this zone. Shrub-like
woody transplants or rooted cuttings are also effective in this zone and are often placed in
combination with tied-down and staked mulch that is used to temporarily reduce surface
erosion. For areas where severe erosion or high currents are expected, methods such as
brush mattress should be carried into the bank zone.

Contour wattling consists of fascines, often used independent of the brush mattress, placed
along contours, and buried across the slope, parallel or nearly parallel to the stream course.
The bundles can be living or constructed from wood and are staked to the bank. Contour
wattles are often installed in combination with a coir fiber blanket over seed and a straw
mulch to prevent the development of rills or gullies (WES 1998) (Figure 6.5).

Brush layering with some modifications can be used in the bank zone. Geotextile fabrics
should be used between the brush layers and keyed into each branch layer trench to prevent
unraveling of the bank between the layers (WES 1998).

6.13
Overbank Zone. Bioengineered treatments are generally not used in this zone except to
control gullying or where slopes are greater than 1V:3H. In these cases, brush layering or
contour wattling may be employed across the gully or on the contour of the slope.

Deep-rooting plants, such as larger flood-tolerant trees, are required in this zone in order to
hold the bank together. Care should be taken in the placement of trees that may grow to be
fairly large since their shade can kill out vegetation in the splash and bank zones. Trees
planted in the overbank zone are planted either as container-grown or bare-root plants.

Depending on their shade tolerance, grasses, herbs, and shrubs can be planted between the
trees. Hydroseeding and hydromulching are useful and effective means of direct seeding in
the overbank zone.

6.9 SUMMARY

Biotechnical engineering can be a useful and cost-effective tool in controlling bank or
channel erosion, while increasing the aesthetics and habitat diversity of the site. However,
where failure of the countermeasure could lead to failure of a bridge or highway structure,
the only acceptable solution in the immediate vicinity of a structure is a traditional, "hard"
engineering approach. Biotechnical countermeasures need to be applied in a prudent
manner, in conjunction with channel planform and bed stability-analysis, and rigorous
engineering design. Designs must account for a multitude of factors associated with the
geotechnical characteristics of the site, the local and watershed geomorphology, local soils,
plant biology, hydrology, and site hydraulics. Finally, programs for monitoring and
maintenance, which are essential to the success and effectiveness of any biotechnical
engineering project, must be included in the project and strictly adhered to.
6.14



























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7.1
CHAPTER 7

COUNTERMEASURE DESIGN GUIDELINES


7.1 INTRODUCTION

In Volume 2, design guidelines are provided for a variety of stream instability and bridge
scour countermeasures. Most of these countermeasures have been applied successfully on
a state or regional basis, but, in several cases, only limited design references are available in
published handbooks, manuals, or reports. No attempt has been made to include in this
document design guidelines for all the countermeasures listed in the matrix (Table 2.1).
There are, however, references in Chapters 8 and 10 to publications that contain at least a
sketch or photograph of a particular countermeasure, and in many cases contain more
detailed design guidelines.

Countermeasure design guidelines formerly presented in HEC-20 (Lagasse et al. 2001a)
(spurs, guide banks, drop structures) and in HEC-18 (Richardson and Davis 2001) (riprap at
abutments and piers) are now consolidated in this document. Since many bridge scour and
stream instability countermeasures require riprap revetment as an integral component of the
countermeasure, riprap revetment design guidance is summarized in Design Guideline 4.
An appropriate granular or geotextile filter is essential for any countermeasure requiring a
protective armor layer (e.g., riprap, articulating concrete blocks, etc.). Filter design guidance
is provided in Design Guideline 16.

A number of DOTs provided specifications, procedures, or design guidelines for bridge scour
and stream instability countermeasures that have been used successfully locally, but for
which only limited design guidance is available outside the agency. Several of these are
presented as design guidelines for the consideration of and possible adaptation to the needs
of other DOTs (see for example, Design Guideline 6, Wire Enclosed Riprap Mattress, and
Design Guideline 13, Grout/Cement Filled Bags). These specifications, procedures, or
guidelines have not been evaluated, tested, or endorsed by the authors of this document or
by the FHWA. They are presented here in the interests of information transfer on
countermeasures that may have application in another state or region.

Since publication of the Second Edition of HEC-23 in 2001, both the Transportation
Research Board through the NCHRP Program and FHWA have sponsored a number of
research projects to improve the state of practice in bridge scour and stream instability
countermeasure technology and provide definitive guidance to bridge owners in
countermeasure design. Among the projects that represent advances in countermeasure
technology that have been incorporated into the Design Guidelines in Volume 2 are:

NCHRP Report 544 - Environmentally Sensitive Channel and Bank Protection Measures
(McCullah and Gray 2005)
NCHRP Report 568 - Riprap Design Criteria, Recommended Specifications, and Quality
Control (Lagasse et al. 2006)
NCHRP Report 587 - Countermeasures to Protect Bridge Abutments from Scour
(Barkdoll et al. 2007)
NCHRP Report 593 - Countermeasures to Protect Bridge Piers from Scour (Lagasse et
al. 2007)

7.2
7.2 DESIGN GUIDELINES

The following specifications, procedures, or design guidelines are included in Volume 2. The
application of the countermeasure and the contributing source(s) of information are also
indicated below.

7.2.1 Countermeasures for Stream Instability

Design Guideline 1
Bendway Weirs/Stream Barbs
- Source(s): Colorado Department of Transportation
Washington State Department of Transportation
Tennessee Department of Transportation
Soil Conservation Service (now Natural Resources Conservation
Service
U.S. Army Corps of Engineers
- Application: Bankline protection and flow alignment in
meandering channel bends

Design Guideline 2
Spurs
- Source(s): HEC-20 Stream Stability at Highway Structures (Second Edition)
NCHRP Report 568
- Application: Bankline stabilization and flow alignment

Design Guideline 3
Check Dams/Drop Structures
- Source(s): HEC-20 Stream Stability at Highway Structures (Second Edition)
HEC-14 Hydraulic Design of Energy Dissipators for Culverts and
Channels
- Application: Correcting or preventing channel degradation

7.2.2 Countermeasures for Streambank and Roadway Embankment Protection

Design Guideline 4
Riprap Revetment
- Source(s): NCHRP Report 568
- Application: Bankline/abutment protection and riprap component of many
other countermeasures

Design Guideline 5
Riprap Design for Embankment Overtopping
- Source(s): NCHRP Report 568
- Application: Protection for roadway approach embankments and flow
control countermeasures

Design Guideline 6
Wire Enclosed Riprap Mattress
- Source(s): New Mexico State Highway and Transportation Department
- Application: Revetment for banklines, guide banks, and sloping abutments
7.3
Design Guideline 7
Soil Cement
- Source(s): Portland Cement Association
Pima County Arizona
Maricopa County Arizona
- Application: Revetment for banklines and sloping abutments, drop structures,
and bed armor

Design Guideline 8
Articulating Concrete Block Systems for Bank Revetment or Bed Armor
- Source(s): Harris County Flood Control District (2001)
Federal Highway Administration
Maine Department of Transportation
Minnesota Department of Transportation
- Application 1: Bankline revetment and bed armor

Design Guideline 9
Grout-Filled Mattresses for Bank Revetment or Bed Armor
- Source(s): Federal Highway Administration
- Application 1: Bankline revetment and bed armor

Design Guideline 10
Gabion Mattresses for Bank Revetment or Bed Armor
- Source(s): Federal Highway Administration
- Application 1: Bankline revetment and bed armor

7.2.3 Countermeasures for Bridge Pier Protection

Design Guideline 8
Articulating Concrete Block Systems at Bridge Piers
- Source(s): NCHRP Report 593
- Application 2: Pier scour protection

Design Guideline 9
Grout-Filled Mattresses at Bridge Piers
- Source(s): NCHRP Report 593
- Application 2: Pier scour protection

Design Guideline 10
Gabion Mattresses at Bridge Piers
- Source(s): NCHRP Report 593
- Application 2: Pier Scour Protection

Design Guideline 11
Rock Riprap at Bridge Piers
- Source(s): HEC-18 Scour at Bridges (Third Edition)
NCHRP Report 593
NCHRP Report 568
- Application: Pier scour protection

Design Guideline 12
Partially Grouted Riprap at Bridge Piers
- Source(s): NCHRP Report 593
- Application: Pier Scour Protection

7.4
7.2.4 Countermeasures for Abutment Protection

Design Guideline 13
Grout/Cement Filled Bags
- Source(s): Maryland State Highway Administration
Maine Department of Transportation
- Application: Protection of undermined areas at piers and abutments,
and bed armor

Design Guideline 14
Rock Riprap at Bridge Abutments
- Source(s): NCHRP Report 568
NCHRP Report 587
- Application: Abutment scour protection

Design Guideline 15
Guide Banks
- Source(s): HEC-20 Stream Stability at Highway Structures (Second Edition)
NCHRP Report 568
- Application: Abutment scour protection

7.2.5 Filter Design

Design Guideline 16
Filter Design
- Source(s): NCHRP Report 568
NCHRP Report 593
- Application: Filter for revetment or countermeasure armor

7.2.6 Special Applications

Design Guideline 17
Riprap Design for Wave Attack
- Source(s): HEC-25 (1st and 2nd Editions)
- Application: Protection for coastal roadway embankments

Design Guideline 18
Riprap Protection for Bottomless Culverts
- Source(s): FHWA Reports FHWA-RD-02-078 and FHWA-HRT-07-026
- Application: Scour protection at bottomless culverts

Design Guideline 19
Concrete Armor Units/Toskanes and A-Jacks

- Source(s): Testing at Colorado State University
- Application: Pier scour protection

8.1
CHAPTER 8

OTHER COUNTERMEASURES AND CASE HISTORIES OF PERFORMANCE


8.1 INTRODUCTION

Design Guidelines 1 through 19 contain specific design procedures for a variety of stream
instability and bridge scour countermeasures that have been applied successfully on a state
or regional basis. Other countermeasures such as retarder structures, longitudinal dikes,
bulkheads, and even channel relocations may be used to mitigate scour at bridges or stream
bank erosion. Some of these measures are discussed and general guidance is summarized
in this chapter. Chapter 4 (Section 4.3.6) illustrates the use of the concept of radial stress
on a meander bend to evaluate the performance of fence, dike, and retarder type structures
in protecting an eroding bankline.

Case histories of hydraulic problems at bridge sites can provide information on the success
(or failure) of the various countermeasures used to stabilize streams. This chapter also
summarizes the evaluation of countermeasure performance compiled for FHWA from case
histories at 224 bridge sites (Brice and Blodgett 1978).

8.2 HARDPOINTS

Hardpoints consist of stone fills spaced along an eroding bank line, protruding only short
distances into the channel. A root section extends landward to preclude flanking. The crown
elevation of hardpoints used by the USACE at demonstration sites on the Missouri River was
generally at the normal water surface elevation at the toe, sloping up at a rate of about 1 ft in
10 ft (1 m in 10 m) toward the bank. Hardpoints are most effective along straight or relatively
flat convex banks where the streamlines are parallel to the bank lines and velocities are not
greater than 10 ft/s (3 m/s) within 50 ft (15 m) of the bank line. Hardpoints may be
appropriate for use in long, straight reaches where bank erosion occurs mainly from a
wandering thalweg at lower flow rates. They would not be effective in halting or reversing
bank erosion in a meander bend unless they were closely spaced, in which case spurs,
retarder structures, or bank revetment would probably cost less. Figure 8.1 is a perspective
of a hardpoint installation. Hardpoints have been used effectively as the first "spur" in a spur
field (see Design Guideline 2).

8.3 RETARDER STRUCTURES

Retarder structures are permeable or impermeable devices generally placed parallel to
streambanks to reduce velocities and cause deposition near the bank. They are best suited
for protecting low banks or the lower portions of streambanks. Retarder structures can be
used to protect an existing bank line or to establish a different flow path or alignment.
Retards do not require grading of the streambank, and they create an environment which is
favorable to the establishment of vegetation.
8.2


Figure 8.1. Perspective view of hardpoint installation with section detail (after Brown 1985).

8.3.1 Jacks and Tetrahedrons

Jacks most commonly consist of three linear members fixed together at their midpoints so
that each member is perpendicular to the other two. Wires are strung on the members to
resist distortion and to collect debris. Cables are used to tie individual jacks together and for
anchoring key units to deadmen. Tetrahedrons consist of six members of equal length fixed
together so as to form three faces, each of which is an equilateral triangle, i.e., a
tetrahedron. The tetrahedron unit may be braced as shown in Figure 8.2 and wire mesh
added to enhance flow retardance. Tetrahedrons are not as widely used as are jacks.

Jacks and tetrahedrons are effective in protecting banks from erosion only if light debris
collects on the structures thereby enhancing their performance in retarding flow. However,
heavy debris and ice can damage the structures severely. They are most effective on mild
bends and in wide, shallow streams which carry a large sediment load.

Where jacks are used to stabilize meandering streams, both lateral and longitudinal rows are
often installed to form an area retarder structure rather than a linear structure. Lateral rows
of jacks are usually oriented in a downstream direction from 45 to 70. Spacing of the lateral
rows of jacks may be 50 to 200 ft (15 to 75 m) depending on the debris and sediment load
carried by the stream. A typical jack unit is shown in Figure 8.3 and a typical area installation
is shown in Figure 8.4.

Outflanking of jack installations is a common problem. Adequate transitions should be
provided between the upstream bank and the structure, and the jack field should be
extended to the overbank area to retard flow velocities and provide additional anchorage.
Jacks are not recommended for use in corrosive environments or at locations where they
would constitute a hazard to recreational use of the stream.
8.3


Figure 8.2. Typical tetrahedron design (after Brown 1985).









Figure 8.3. Typical jack unit (after Brown 1985).

8.4

Figure 8.4. Retarder field schematic (after HDS 6, Richardson et al. 2001).

8.3.2 Fence Retarder Structures

Fence retarder structures provide protection to the lower portions of banks of relatively small
streams. Posts may be of wood, steel, or concrete and fencing may be composed of wood
planks or wire.

Scour and the development of flow channels behind linear structures are common causes of
failure of longitudinal fences. Scour at the supporting members of the structure can be
reduced by placing rock along the fence or the effects of scour can be overcome by driving
supporting members to depths below expected scour. Tiebacks can be used to retard
velocities between the linear structure and the streambank, thus reducing the ability of the
stream to develop flow channels behind the structure.

8.3.3 Timber Pile

Timber pile retarder structures may be of a single, double, or triple row of piles with the
outside of the upstream row faced with wire mesh or other fencing material. They have been
found to be effective at sharp bends in the channel and where flows are directly attacking a
bank. They are effective in streams which carry heavy debris and ice loads and where
barges or other shipping vessels could damage other countermeasures or a bridge. As with
other retarder structures, protection against scour failure is essential. Figure 8.5 illustrates a
design.

8.3.4 Wood Fence

Wood fence retarder structures have been found to provide a more positive action in
maintaining an existing flow alignment and to be more effective in preventing lateral erosion
at sharp bends than other retarder structures. Figure 8.6 is an end view of a typical wood
fence design with rock provided to protect against scour.

8.5


Figure 8.5. Timber pile bent retarder structure (after Brown 1985).

Wire fence retarder structures may be of linear or area configuration, and linear
configurations may be of single or multiple fence rows. Double-row fence retards are
sometimes filled with brush to increase the flow retardance. Figures 8.7 and 8.8 illustrate
two types of wire fence retarder structures.

8.4 LONGITUDINAL DIKES

Longitudinal dikes are essentially impermeable linear structures constructed parallel with the
streambank or along the desired flow path. They protect the streambank in a bend by
moving the flow current away from the bank. Longitudinal dikes may be classified as earth or
rock embankment dikes, crib dikes, or rock toe-dikes.

8.4.1 Earth or Rock Embankments

As the name implies, these dikes are constructed of earth with rock revetment or of rock.
They are usually as high or higher than the original bank. Because of their size and cost,
they are useful only for large-scale channel realignment projects.
8.6


Figure 8.6. Typical wood fence retarder structure (modified from USACE 1981,
after Brown 1985).





Figure 8.7. Light double row wire fence retarder structure (after Brown 1985).

8.7



Figure 8.8. Heavy timber-pile and wire fence retarder structures (Modified from USACE
1981, after Brown 1985).

8.4.2 Rock Toe-Dikes

Rock toe-dikes are low structures of rock riprap placed along the toe of a channel bank.
They are useful where erosion of the toe of the channel bank is the primary cause of the loss
of bank material. The USACE has found that longitudinal stone dikes provide the most
successful bank stabilization measure studied for channels which are actively degrading and
for those having very dynamic beds. Where protection of higher portions of the channel bank
is necessary, rock toe-dikes have been used in combination with other measures such as
vegetative cover and retarder structures.

8.8
Figure 8.9 shows the typical placement and sections of rock toe-dikes. The volume of
material required is 1.5 to 2 times the volume of material that would be required to armor the
sides of the anticipated scour to a thickness of 1.5 times the diameter of the largest stone
specified. Rock sizes should be similar to those specified for riprap revetments (see Design
Guideline 4). Tiebacks are often used with rock toe-dikes to prevent flanking, as illustrated
in Figure 8.10. Tiebacks should be used if the toe-dike is not constructed at the toe of the
channel bank.

Rock toe-dikes are useful on channels where it is necessary to maintain as wide a
conveyance channel as possible. Where this is not important, spurs could be more
economical since scour is a problem only at the end projected into the channel. However,
spurs may not be a viable alternative in actively degrading streams (Design Guideline 2).

8.4.3 Crib Dikes

Longitudinal crib dikes consist of a linear crib structure filled with rock, straw, brush,
automobile tires or other materials. They are usually used to protect low banks or the lower
portions of high banks. At sharp bends, high banks would need additional protection against
erosion and outflanking of the crib dike. Tiebacks can be used to counter outflanking.

Crib dikes are susceptible to undermining, causing loss of material inside the crib, thereby
reducing the effectiveness of the dike in retarding flow. Figure 8.11 illustrates a crib dike
with tiebacks and a rock toe on the stream side to prevent undermining.

8.4.4 Bulkheads

Bulkheads are used for purposes of supporting the channel bank and protecting it from
erosion. They are generally used as protection for the lower bank and toe, often in
combination with other countermeasures that provide protection for higher portions of the
bank. Bulkheads are most frequently used at bridge abutments as protection against
slumping and undermining at locations where there is insufficient space for the use of other
types of bank stabilization measures, and where saturated fill slopes or channel banks
cannot otherwise be stabilized.

Bulkheads are classified on the basis of construction methods and materials. They may be
constructed of concrete, masonry, cribs, sheet metal, piling, reinforced earth, used tires,
gabions, or other materials. They must be protected against scour or supported at elevations
below anticipated total scour, and where sections of the installation are intermittently above
water, provisions must be made for seepage through the wall. Some bulkhead types, such
as crib walls and gabions, should be provided with safeguards against leaching of materials
from behind the wall.

Bulkheads must be designed to resist the forces of overturning, bending and sliding, either
by their mass or by structural design. Figure 8.12 illustrates anchorage schemes for a
sheetpile bulkhead. Because of costs, they should be used as countermeasures against
meander migration only where space is not available to construct other types of measures.


8.9


Figure 8.9. Typical longitudinal rock toe-dike geometries (after Brown 1985).








Figure 8.10. Longitudinal rock toe-dike tiebacks (after Brown 1985).

8.10

Figure 8.11. Timber pile, wire mesh crib dike with tiebacks (modified from USACE
1981, after Brown 1985).




Figure 8.12. Anchorage schemes for a sheetpile bulkhead (after Brown 1985).
8.11
8.5 CHANNEL RELOCATION

At some locations, it may be advantageous to realign a stream channel, either in
combination with the use of other countermeasures against meander migration or in lieu of
other countermeasures.

Figure 8.13 illustrates hypothetical highway locations fixed by considerations other than
stream stability. To create better flow alignment with the bridge, consideration could be given
to channel realignment as shown in this figure (parts a and b). Similarly, consideration for
realignment of the channel would also be advisable for a hypothetical lateral encroachment
of a highway as depicted in part c of the figure. In either case, criteria are needed to
establish the cross-sectional dimensions.


Figure 8.13. Encroachments on meandering streams (after HDS 6, Richardson et al. 2001).

Before realigning a stream channel, the stability of the existing channel must be examined.
The stream classification, recent and older aerial photographs, and field surveys are
necessary. The realigned channel may be made straight without curves, or may include one
or more curves. If curves are included, decisions regarding the radius of curvature, the
number of bends, the limits of realignment (hence the length and slope of the channel) and
the cross-sectional area have to be made. Different streams have different historical
backgrounds and characteristics with regard to bend migration, discharge, stage, geometry,
and sediment transport, and an understanding and appreciation of river hydraulics and
morphology is important to decision making. It is difficult to state generalized criteria for
channel relocation applicable to all streams. HEC-20 (Lagasse et al. 2001a) provides
quantitative techniques for evaluating and predicting lateral channel migration and analyzing
vertical channel stability, as well as an introduction to channel restoration concepts that
should be considered for channel relocation projects.


8.12
Based on a study of the stability of relocated channels, Brice presented the following
recommendations and conclusions regarding specific aspects of planning and construction
of channel realignment (Brice 1981):

Channel Stability Prior to Realignment.

Assessment of the stability of a channel prior to realignment is needed to assess the risk
of instability. An unstable channel is likely to respond unfavorably. Bank stability is
assessed by field study and by stereoscopic examination of aerial photographs (see
HEC-20). The most useful indicators of bank instability are cut or slumped banks, fallen
trees along the bank line, and exposed wide point bars. Bank recession rates are
measured by comparison of time-sequential aerial photographs. Vertical instability is
equally important but more difficult to determine. It is indicated by changes in channel
elevation at bridges and gaging stations. Serious degradation is usually accompanied by
generally cut or slumped banks along a channel and by increased debris transport.

Erosional Resistance of Channel Boundary Materials.

The stability of a channel, whether natural or relocated, is partly determined by the
erosional resistance of materials that form the wetted perimeter of the channel (see
HEC-20). Resistant bedrock outcrops in the channel bottom or that lie at shallow depths
will provide protection against degradation, but not all bedrock is resistant. Erosion of
shale, or of other sedimentary rock types interbedded with shale, has been observed.
Degradation is not a problem at most sites where bed sediment is of cobble and boulder
size. However, degradation may result from the relocation of any alluvial channel,
whatever the size of bed material, but the incidence of serious degradation of channels
relocated by DOTs is small in number. The erosional resistance of channel beds tends to
increase with clay content. Banks of weakly cohesive sand or silt are clearly subject to
rapid erosion, unless protected with vegetation. No consistent relation has been found
between channel stability and the cohesion of bank materials, probably because of the
effects of vegetation.

Length of Realignment.

The length of realignment contributes significantly to channel instability at sites where its
value exceeds 250 channel widths. When the value is below 100 channel widths, the
effects of length of relocation are dominated by other factors. The probability of local
bank erosion at some point along a channel increases with the length of the channel.
The importance of vegetation, both in appearance and in erosion control, would seem to
justify a serious and possibly sustained effort to establish it as soon as possible on
graded banks.

Bank Revetment.

Revetment makes a critical contribution to stability of relocated stream channels at many
sites. Rock riprap is by far the most commonly used and effective revetment (see Design
Guideline 4). Concrete slope paving is prone to failure. Articulating concrete block is
effective where vegetation can establish within the open cells of the blocks (Design
Guideline 8).

8.13
Check Dams (drop structures).

In general, check dams are effective in preventing channel degradation in relocated
channels. The potential for erosion at a check dam depends on its design and
construction, its height and the use of revetment on adjoining banks. A series of low
check dams, less than about 1.5 ft (0.5 m) in height, is probably preferable to a single
higher structure, because of increased safety and reduced potential for erosion and
failure. By simulating rapids, low check dams may add visual interest to the flow in a
channel. One critical problem arising with check dams relates to improper design for
large flows. Higher flows have worked around the ends of many installations to produce
failure (see Design Guideline 3).

Maintenance.

Problems which could be resolved by routine maintenance were observed along
relocated channels. These were problems with the growth of annual vegetation,
reduction of channel conveyance by overhanging trees, local bank cutting, and bank
slumping. The expense of routine maintenance or inspection of relocated channels
beyond the highway right-of-way may be prohibitive; however, most of the serious
problems could be detected by periodic inspection, perhaps by aerial photography,
during the first five to ten years after construction. Hydraulic engineers responsible for
the design of relocated stream channels should monitor their performance to gain
experience and expertise.

8.6 CASE HISTORIES OF COUNTERMEASURE PERFORMANCE

Case histories of hydraulic problems at bridge sites can provide information on the relative
success of the various countermeasures used to stabilize streams. The following case
histories are taken from Brice and Blodgett (1978), Brice (1984), and Brown et al. (1980).
Site data are from Brice and Blodgett (1978). This compilation of case histories at 224 bridge
sites is recommended reference material for those responsible for selecting
countermeasures for stream instability. Additional case histories are given in HDS 6
(Richardson et al. 2001).

8.6.1 Flexible Revetment

Rock Riprap. Dumped rock riprap is the most widely used revetment in the United States.
Its effectiveness has been well established where it is of adequate size, of suitable size
gradation, and properly installed. Brice and Blodgett (1978) documented the use of rock
riprap at 110 sites (Volume 1, Table 2). They rated the performance at 58 sites and found
satisfactory performance at 34 sites, partially satisfactory performance at 12 sites, and
failure to perform satisfactorily at 12 sites. Keeley concluded that riprap used in Oklahoma
performed without significant failure and provides basic and efficient bank control on the
meandering streams in Oklahoma (Keeley 1971). Additional discussion of riprap revetment
failure modes and inspection guidance can be found in Chapter 5 and Appendix D.

A review of the causes of failure at the sites studied by Brice and Blodgett (1978) is
instructive. They found the absence of a filter blanket clearly the cause of the failure at a
site subject to tides and wave action. The riprap was placed on a fill of sand and fine gravel
which eroded through the interstices of the riprap.

8.14
Internal slope failure was the cause of failure of riprap at the abutment of bridges at two
sites. At one site, failure was attributed to saturation of a high fill by impounded water in a
reservoir. Wave action also probably contributed to the failure. The other site is difficult to
include as a riprap failure because the rock was not placed as riprap revetment. Thirty-three
freight car loads of rock were dumped as an emergency measure to stop erosion at a bridge
abutment during high-flow releases from a reservoir. The rock was displaced, and the high
streambanks and highway fill were still susceptible to slumps. At both sites, riprap failed to
prevent slumps in high fills.

Inadequate rock size and size gradation was given as the cause of failure at eight sites. All
of these sites are complex, and it is difficult to assign failure to one cause, but rock size was
definitely a factor.

Channel degradation accounted for failure at three sites in Mississippi. Channel degradation
at these sites is due to channel straightening and clearing by the SCS (NRCS) and USACE.
Riprap installations on the streambanks, at bridge abutments and in the streambed have
failed to stop lateral erosion. At one site, riprap placed on the banks and bed of the stream
resulted in severe bed scour and bank erosion downstream of the riprap.

Failure of riprap at one site was attributed to the steep slope on which the riprap was placed.
At this site, rock riprap failed to stop slumping of the steep banks downstream of a check
dam in a degrading stream.

Successful rock riprap installations at bends were found at five sites. Bank erosion was
controlled at these sites by rock riprap alone. Installations rated as failing were damaged at
the toe and upstream end, indicating inadequate design and/or construction, and damage to
an installation of rounded boulders, indicating inadequate attention to riprap specifications.
Other successful rock riprap study sites were sites where bank revetment was used in
conjunction with other countermeasures, such as spurs or retards. The success of these
installations was attributed more to the spurs or retards, but the contribution of the bank
revetment was not discounted.

Broken Concrete. Broken concrete is commonly used in emergencies and where rock is
unavailable or very expensive. No specifications were found for its use. Performance was
found to be more or less unsatisfactory at three sites.

Rock-and-Wire Mattress and Gabions. The distinction made between rock-and-wire mattress
and gabions is in the dimensions of the devices (see Design Guideline 10). Rock-and-wire
mattress is usually 1.0 ft (0.3 m) or less in thickness and a gabion is thicker and nearly
equidimensional. The economic use of rock-and-wire mattress is favored by an arid climate,
availability of stones of cobble size, and unavailability of rock for dumped rock riprap.
Corrosion of wire mesh is slow in arid climates, and ephemeral streams do not subject the
wire to continuous abrasion. Where large rock is not available, the use of rock-and-wire
mattress may be advantageous in spite of eventual corrosion or abrasion of the wire.

Rock-and-wire mattress performance was found to be generally satisfactory although local
failure of the wire mesh and spilling out of the rock was not uncommon. Mattresses are held
in place against the bank by railroad rails at sites in New Mexico and Arizona where good
performance was documented (see Design Guideline 6). This is known locally as "railbank
protection." The steel rail supported rock-and-wire mattress stays in place better than
dumped rock riprap on the unstable vertical banks found on the ephemeral streams of this
area. Mattress held in place by stakes has been found to be effective in Wyoming.

8.15
The use of rock-and-wire mattress has diminished in California because of the questionable
service of wire mesh, the high cost of labor for installation, and the efficiency of modern
methods of excavating for dumped riprap toe protection. The Los Angeles Flood Control
District, however, has had installations in-place for 15 years or more with no evidence of wire
corrosion. On the other hand, Montana and Maryland reported abrasion damage of wire.
These experiences illustrate that economical use of countermeasures is dependent on the
availability of materials, costs, and the stream environment in which the measure is placed.

Several sites were identified where gabions were installed, but the countermeasures had
been tested by floods at only one site where gabions placed on the downstream slope of a
roadway overflow section performed satisfactorily.

Other Flexible Revetment. Favorable performance of precast-concrete blocks at bridges was
reported in Louisiana. Vegetation is reported to grow between blocks and contribute to
appearance and stability. Vegetation apparently is seldom used alone at bridges. Iowa relies
on sod protection of spur dikes, but Arkansas reported failure of sod as bank protection.

8.6.2 Rigid Revetments

Failure of rigid revetment tends to be progressive; therefore, special precautions to prevent
undermining at the toe and termini and failure from unstable soils or hydrostatic pressure are
warranted.

Concrete Pavement. Well-designed concrete paving is satisfactory as fill slope revetment, as
revetment on streams having low gradients, and in other circumstances where it is well
protected against undermining at the toe and ends. The case histories include at least one
location where riprap launching aprons were successful in preventing undermining at the toe
from damaging the concrete pavement revetment. Weep holes for relief of hydrostatic
pressure are required for many situations (see Design Guideline 4).

Documented causes of failure in the case histories are undermining at the toe (six sites),
erosion at termini (five sites), eddy action at downstream end (two sites), channel
degradation (two sites), high water velocities (two sites), overtopping (two sites), and
hydrostatic pressure (one site). Good success is reported with concrete slope paving in
Florida, Illinois, and Texas.

Sacked Concrete. No DOT reported a general use of sacked concrete as revetment.
California was reported to regard this as an expensive revetment almost never used unless
satisfactory riprap was not available. Sacked concrete revetment failures were reported
from undermining of the toe (two sites), erosion at termini (one site), channel degradation
(two sites), and wave action (one site) (see Design Guideline 4).

Concrete-Grouted Riprap. Fully-grouted riprap permits the use of smaller rock, a lesser
thickness, and more latitude in gradation of rock than in dumped rock riprap. No failures of
grouted riprap were documented in the case histories, but it is subject to the same types of
failures as other rigid revetments (see Chapter 5, Section 5.6 and Design Guideline 12).

Concrete-Filled Fabric Mat. Concrete-filled fabric mat is a patented product (Fabriform)
consisting of porous, pre-assembled nylon fabric forms which are placed on the surface to
be protected and then filled with high-strength mortar by injection. Variations of Fabriform
and Fabricast consist of nylon bags similarly filled. Successful installations were reported by
the manufacturer of Fabriform in Iowa, and North Dakota reported successful installations
(see Design Guideline 9).

8.16
Soil Cement. In areas where any type of riprap is scarce, use of in-place soil combined with
cement provides a practical alternative. The resulting mixture, soil cement, has been
successfully used as bank protection in many areas of the Southwest (see Design Guideline
7). Unlike other types of bank revetment, where milder side slopes are desirable, soil cement
in a stairstep construction can be used on steeper slopes (i.e., typically one to one), which
reduces channel excavation costs. For many applications, soil cement is generally more
aesthetically pleasing than other types of revetment.

8.6.3 Bulkheads

A bulkhead is a steep or vertical wall used to support a slope and/or protect it from erosion
(See Section 8.4). Bulkheads usually project above ground, although the distinction between
bulkheads and cutoff walls is not always sharp. Most bulkhead applications were found at
abutments. They were found to be most useful at the following locations: (1) on braided
streams with erodible sandy banks, (2) where banks or abutment fill slopes have failed by
slumping, and (3) where stream alignment with the bridge opening was poor, to provide a
transition between streambanks and the bridge opening. It was not clear what caused
failures at five sites summarized in Brice and Blodgett (1978), but in each case, the probable
cause was undermining.

8.6.4 Spurs

Spurs are permeable or impermeable structures which project from the bank into the
channel. Spurs may be used to alter flow direction, induce deposition, or reduce flow
velocity. A combination of these purposes is generally served. Where spurs project from
embankments to decrease flow along the embankment, they are called embankment spurs.
These may project into the floodplain rather than the channel, and thus function as spurs
only during overbank flow. According to a summary prepared for the Transportation
Research Board, spurs may protect a streambank at less cost than riprap revetment, and by
deflecting current away from the bank and causing deposition, they may more effectively
protect banks from erosion than revetment (Richardson and Simons 1984). Uses other than
bank protection include the constriction of long reaches of wide, braided streams to establish
a stable channel, constriction of short reaches to establish a desired flow path and to
increase sediment transport capacity, and control of flow at a bend. Where used to constrict
a braided stream to a narrow flow channel, t he structure may be more correctly
referred to as a dike or a retard in some locations (see Design Guideline 2).

Several factors enter into the performance of spurs, such as permeability, orientation,
spacing, height, shape, length, construction materials, and the stream environment in which
the spur is placed.

Impermeable Spurs. The case histories show good success with well-designed impermeable
spurs at bends and at crossings of braided stream channels (eight sites). At one site,
hardpoints barely projecting into the stream and spaced at about 100 to 150 ft (30 to 45 m)
failed to stop bank erosion at a severe bend. At another site, spurs projecting 40 ft (12 m)
into the channel, spaced at 100 ft (30 m), and constructed of rock with a maximum diameter
of 1.5 ft (0.5 m) experienced erosion between spurs and erosion of the spurs. At a third site,
spurs constructed of timber piling filled with rock were destroyed. Failure was attributed to
the inability to get enough penetration in the sand-bed channel with timber piles and the
unstable wide channel in which the thalweg wanders unpredictably. Spurs (or other
countermeasures) are not likely to be effective over the long term in such an unstable
channel unless well-designed, well-built, and deployed over a substantial reach of stream.
8.17
Although no failures from ice damage were cited for impermeable spurs, North Dakota uses
steel sheet pile enclosed earth fill spurs because of the potential for ice damage. At one
site, such a spur sustained only minor damage from 2.5 ft (0.75 m) of ice.

Permeable Spurs. A wide variety of permeable spur designs were also shown to successfully
control bank erosion by the case histories. Failures were experienced at a site which is
highly unstable with rapid lateral migration, abundant debris, and extreme scour depths.
Bank revetments of riprap and car bodies and debris deflectors at bridge piers, as well as
bridges, have also failed at this site. At another site, steel H-pile spurs with wire mesh have
partially failed on a degrading stream.

8.6.5 Retardance Structures

A retardance structure (retard) can be a permeable or impermeable linear structure in a
channel, parallel with and usually at the toe of the bank. The purposes of retardance
structures are to reduce flow velocity, induce deposition, or to maintain an existing flow
alignment. They may be constructed of earth, rock, timber pile, sheet pile, or steel pile.
Steel jacks or tetrahedrons are also used (see Section 8.3).

Most retardance structures are permeable and most have good performance records. They
have proved to be useful in the following situations: (1) for alignment problems very near a
bridge or roadway embankment, particularly those involving rather sharp channel bends and
direct impingement of flow against a bank (ten sites), and (2) for other bank erosion
problems that occur very near a bridge, particularly on streams that have a wandering
thalweg or very unstable banks (seven sites).

The case histories include a site where a rock retardance structure similar to a rock toe dike
was successful in protecting a bank on a highly unstable channel where spurs had failed.
There were, however, deficiencies in the design and construction of the spur installation. At
another site, a rock retardance structure similar to a rock toe-dike has reversed bank erosion
at a bend in a degrading stream. The USACE (1981) reported that longitudinal rock toe
dikes were the most effective bank stabilization measure studied for channels having very
dynamic and/or actively degrading beds.

8.6.6 Dikes

Dikes are impermeable linear structures for the control or containment of overbank flow (see
Section 8.4). Most are in floodplains, but they may be within channels, as in braided streams
or on alluvial fans. Dikes at study sites were used to prevent flood water from bypassing a
bridge at four sites, or to confine channel width and maintain channel alignment at two sites.
Performance of dikes at study sites was judged generally satisfactory.

8.6.7 Guide Banks

The major use of guide banks (formerly referred to as spur dikes) in the United States is to
prevent erosion by eddy action at bridge abutments or piers where concentrated flood flow
traveling along the upstream side of an approach embankment enters the main flow at the
bridge (see Design Guideline 15). By establishing smooth parallel streamlines in the
approaching flow, guide banks improve flow conditions in the bridge waterway. Scour, if it
occurs, is near the upstream end of the guide bank away from the bridge. A guide bank
differs from dikes described above in that a dike is intended to contain overbank flow while a
guide bank only seeks to align overbank flow with flow through the bridge opening. An
8.18
extension of the usual concept of the purpose for guide banks, but not in conflict with that
concept, is the use of guide banks and highway fill to constrict braided channels to one
channel. At three sites studied, guide banks only or guide banks plus revetment on the
highway fill were used to constrict wide braided channels rather severely, and the
installations have performed well.

Guide bank performance was found to be generally satisfactory at all study sites.
Performance is theoretically affected by construction materials, shape, orientation, and
length.

Most guide banks are constructed of earth with revetment to inhibit erosion of the dike. At
two sites, guide banks of concrete rubble masonry performed well. Riprap revetment is most
common, but concrete revetment with rock riprap toe protection, rock-and-wire mattress,
gabions, and grass sod have also performed satisfactorily. Since partial failure of a guide
bank during a flood usually will not endanger the bridge, wider consideration should be given
to the use of vegetative cover for protection. Partial failure of any countermeasure is usually
of little significance so long as the purpose of protecting the highway stream crossing is
accomplished.

Guide banks of elliptical shape, straight, and straight with curved ends performed
satisfactorily at study sites, although there is evidence at one site that flow does not follow
the nose of the straight guide bank. Clear evidence of the effect of guide bank orientation
was not found at study sites although the conclusion from a study of guide banks in
Mississippi that guide banks should be oriented with valley flow for skewed crossings of
wooded floodplains was cited (Colson and Wilson 1973). There was evidence at one site
that a guide bank may be severely tested where a large flow is diverted along the roadway
embankment, as at a skewed crossing or on a wide floodplain which is severely constricted
by the bridge. At these locations, embankment spurs may be advisable to protect the
embankment from erosion and to reduce the potential for failure of the guide bank.

Guide banks at study sites tended to be longer than recommended by Bradley (1978) at
most sites, except at five sites where they ranged from 16 to 75 ft (5 to 23 m). All guide
banks appeared to perform satisfactorily. Not enough short guide banks were included in
the study to reach conclusions regarding length.

8.6.8 Check Dams

Check dams are usually used to stop degradation in the channel in order to protect the
substructure foundation of bridges (see Design Guideline 3). At one site, however, a check
dam was apparently used to inhibit contraction scour in a bridge waterway. The problem
with vertical scour was resolved, but lateral scour became a problem and riprap revetment
on the streambanks failed (Brice and Blodgett 1978).

Scour downstream of check dams was found to be a problem at two sites, especially lateral
erosion of the channel banks. Riprap placed on the streambanks at the scour holes also
failed, at least in part because of the steep slopes on which the riprap was placed. At the
time of the study, lateral erosion threatened damage to bridge abutments and highway fills.
At another site, a check dam placed at the mouth of a tributary stream failed to stop
degradation in the tributary and the delivery of damaging volumes of sediment to the main
stream just upstream of a bridge.

No structural failure of check dams was documented. Failures are known to have occurred,
however, and the absence of documented failures in this study should not be given undue
weight. Failure can occur by bank erosion around the ends of the structure resulting in
8.19
outflanking; by seepage or piping under or around the structure resulting in undermining and
structural or functional failure; by overturning, especially after degradation of the channel
downstream of the structure; by bending of sheet pile; by erosion and abrasion of wire fabric
in gabions or rock-and-wire mattress; or by any number of structural causes for failure.

8.6.9 Jack or Tetrahedron Fields

Jacks and tetrahedrons function as flow control measures by reducing the water velocity
along a bank, which in turn results in an accumulation of sediment and the establishment of
vegetation. Steel jacks, or Kellner jacks which consist of six mutually perpendicular arms
rigidly fixed at the midpoints and strung with wire are the most commonly used (see Section
8.3). Tetrahedrons apparently are not currently used by DOTs. Jacks are usually deployed
in fields consisting of rows of jacks tied together with cables.

Four sites where steel jack fields were used are included in the case histories. At two sites,
the jack fields performed satisfactorily. Jacks were buried in the streambed and rendered
ineffective at one site, and jacks were damaged by ice at one site, but apparently continued
to perform satisfactorily. From Keeley's (1971) observations of the performance of jack fields
used in Oklahoma and findings of the study of countermeasures by Brice and Blodgett
(1978), the following conclusions were reached regarding performance:


C The probability of satisfactory performance of jack fields is greatly enhanced if the
stream transports small floating debris and sediment load in sufficient quantity to form
accumulations during the first few years after construction.

C Jack fields may serve to protect an existing bank line, or to alter the course of a stream if
the stream course is realigned and the former channel backfilled before the jack field is
installed.

C On wide shallow channels, which are commonly braided, jack fields may serve to shift
the bank line channelward if jacks of large dimensions are used.

8.6.10 Special Devices for Protection of Piers

Countermeasures at piers have been used to combat abrasion of piers, to deflect debris, to
reduce local scour, and to restore structural integrity threatened by scour. Retrofit
countermeasures installed after problems develop are common. The usual countermeasure
against abrasion consists of steel armor on the upstream face of a pier in the area affected
by bed load. At one site, a pointed, sloping nose on a massive pier, called a special
"cutwater" design, and a concrete fender debris deflector has functioned to prevent debris
accumulation at the pier. At another site, a steel rail debris deflector worked until channel
degradation caused all countermeasures to fail.

Countermeasures used to restore structural integrity of bridge foundations included in the
case histories include underpinning, sheet pile driven around the pier, and a grout curtain
around the pier foundation.


8.20
8.6.11 Channel Alterations

Although channel alterations or modifications have been curtailed due to environmental
concerns, their judicious use can be a viable countermeasure not to be dismissed. It is
recognized that extensive channelization projects, usually made to reduce floodplain
damage, have resulted in serious channel degradation and lateral erosion. However, there
is little documentation of upstream or downstream environmental damage of an alteration of
a short reach in the vicinity of a bridge (Brice and Blodgett 1978).

In a United States Geological Survey study for FHWA of 103 stream channels that were
altered for purpose of bridge construction mostly during the period of 1960-1970, the stability
of the relocated channel was rated as good at 36 sites, as fair to good at 42 sites, as fair at
15 sites, and as poor at 7 sites. In comparison with bank stability of the channels where such
data was available before and after relocation, bank stability was about the same at 45 sites,
better at 28 sites, and worse at 14 sites. At sites where the value of channel relocation
length to channel width was below 100, the effects of length of channel relocation were
dominated by other factors (Brice 1981) (see Section 8.5).

8.6.12 Modification of Bridge Length and Relief Structures

A countermeasure for contraction scour and lateral movement of stream banks that may not
always be considered for an existing bridge but may be needed is to increase its length.
Increased bridge length was used at 11 sites and increased freeboard was provided at 2
sites.

Other techniques that have been used by State DOTs include the design of abutments
as piers so that the bridge may be extended to accommodate future movement of the
stream. Other means of providing additional relief to flow would be the use of a relief bridge.

8.6.13 Investment in Countermeasures

While it may be possible to predict that bank erosion will occur at or near a given location in
an alluvial stream, one can frequently be in error about the location or magnitude of potential
erosion. At some locations, unexpected lateral erosion occurs because of a large flood, a
shifting thalweg, or from other actions of the stream or activities of man. Therefore, where
the investment in a highway crossing is not in imminent danger of being lost, it is often
prudent to delay the installation of countermeasures until the magnitude and location of the
problem becomes obvious. In many, if not most, of the case histories collected by Brice et
al., DOTs invested in countermeasures after a problem developed rather than in anticipation
of a problem (Brice and Blodgett 1978, Brice 1984).


9.1
CHAPTER 9

SCOUR MONITORING AND INSTRUMENTATION

9.1 INTRODUCTION

There are many scour critical bridges on spread footings or shallow piles in the United
States and a large number of bridges with unknown foundation conditions (Lagasse et al.
1995). With limited funds available, these bridges cannot all be replaced or repaired.
Therefore, they must be monitored and inspected following high flows. During a flood, scour
is generally not visible and during the falling stage of a flood, scour holes generally fill in.
Visual monitoring during a flood and inspection after a flood cannot fully determine that a
bridge is safe. Instruments to measure or monitor maximum scour would resolve this
uncertainty. As introduced in Chapter 2 (Section 2.3), monitoring as a countermeasure for a
scour critical bridge involves two basic categories of instruments: portable instruments and
fixed instruments.

Whether to use fixed or portable instruments in a scour monitoring program depends on
many different factors. Unfortunately, there is not one type of instrument that works in every
situation encountered in the field. Each instrument has advantages and limitations that
influence when and where they should be used. The idea of a toolbox, with various
instruments that can be used under specific conditions, best illustrates the strategy to use
when trying to select instrumentation for a scour monitoring program. Specific factors to
consider include the frequency of data collection desired, the physical conditions at the
bridge and stream channel, and traffic safety issues.

Fixed instrumentation is used when frequent measurements or regular, ongoing monitoring
(e.g., weekly, daily, or continuous) are required. Portable instruments would be preferred
when only occasional measurements are required, such as after a major flood, or when
many different bridges must be monitored on a relatively infrequent basis. The physical
conditions at the bridge, such as height off the water and type of superstructure, can
influence the decision to use fixed or portable equipment. For example, bridges that are
very high off the water, or that have large deck overhang or projecting geometries, would
complicate portable measurements from the bridge deck. Making portable measurements
from a boat assumes that a boat ramp is located near the bridge, and/or there are no issues
with limited clearance under the bridge that would prohibit safe passage of a boat. Bridges
with large spread footings or pile caps, or those in very deep water can complicate the
installation of some types of fixed instruments. Stream channel characteristics include
sediment and debris loading, air entrainment, ice accumulation, or high velocity flow, all of
which can adversely influence various measurement sensors used in fixed or portable
instruments. Traffic safety issues include the need for traffic control or lane closures when
either installing or servicing fixed instruments, or attempting to make a portable
measurement from the bridge deck.

It is apparent that the selection of the instrument category (fixed or portable) and the specific
instrument types to be used in a monitoring plan is not always straightforward. In some
situations there is no clearly definable plan that will be successful, and the monitoring plan is
developed knowing that the equipment may not always work as well as might be desired.
Ultimately, the selection of any type of instrumentation must be based on a clear
understanding of its advantages and limitations, and in consideration of the conditions that
exist at the bridge and in the channel.
9.2
To improve the state-of-practice when adopting fixed instrumentation as a countermeasure,
the Transportation Research Board (TRB) under the National Cooperative Highway
Research Program (NCHRP) completed NCHRP Project 21-3 "Instrumentation for
Measuring Scour at Bridge Piers and Abutments" in 1997 (Lagasse et al. 1997, Schall et al.
1997a, 1997b). NCHRP Project 21-7, "Portable Scour Monitoring Equipment," was
completed in 2004 and developed an articulated arm truck as a platform for deploying a
variety of portable scour monitoring instruments (Schall and Price 2004). In addition, to
facilitate the technology transfer of instrumentation-related research to the highway industry,
particularly those in inspection and maintenance operations, the Federal Highway
Administration (FHWA) developed a Demonstration Project (DP97) on scour monitoring and
instrumentation. The purpose of Demonstration Project 97 was to promote the use of new
and innovative equipment, both fixed and portable, to measure scour, monitor changes in
scour over time, detect the extent of past scour, and serve as countermeasures (FHWA
1998, Ginsberg and Schall 1998). This chapter provides information on the use of portable
and fixed instrumentation for scour monitoring. The portable instrument discussion includes
lessons learned from NCHRP 21-7 and concepts developed during DP97. The fixed
instrument discussion includes results from NCHRP Project 21-3 and highlights fixed
instrument installations completed in the last ten years. Information on implementation and
experience of several State DOTs with scour monitoring instrumentation is also summarized.

9.2 PORTABLE INSTRUMENTATION

9.2.1 Components of a Portable Instrument System

Portable instrumentation is typically used when a fixed instrument has not been installed at a
bridge; however, portable instruments are also useful when it is necessary to supplement
fixed instrument data at other locations along the bridge. Physical probing has been used
for many years as the primary method for portable scour monitoring by many DOTs. More
recently, sonar has seen increased use, in part due to the technology transfer provided
through FHWA's Demonstration Project 97 (FHWA 1998). The use of these methods during
low-flow conditions has been very successful, for example during the 2-year inspection
cycle; however, their success during flood conditions, when the worst scour often occurs,
has been more limited. When appropriate, portable instrumentation is an important part of a
scour monitoring program.

A portable scour measuring system typically includes four components (Mueller and Landers
1999):

1. Instrument for making the measurement
2. System for deploying the instrument(s)
3. Method to identify and record the horizontal position of the measurement
4. Data-storage device

9.2.2 Instrument for Making the Measurement

A wide variety of instruments have been used for making portable scour measurements. In
general, the methods for making a portable scour measurement can be classified as:

1. Physical probing
2. Sonar
3. Geophysical
9.3
Physical Probes. Physical probes refer to any type of device that extends the reach of the
inspector, the most common being sounding poles and sounding weights. Sounding poles
are long poles used to probe the bottom (Figure 9.1). Sounding weights, sometimes
referred to as lead lines, are typically a torpedo shaped weight suspended by a
measurement cable (Figure 9.2). This category of device can be used from the bridge or
from a boat. An engineer diver with a probe bar is another example of physical probing.
Physical probes only collect discrete data (not a continuous profile), and can be limited by
large depth and velocity (e.g., during flood flow condition) or debris and/or ice accumulation.
Advantages of physical probing include not being affected by air entrainment or high
sediment loads, and it can be effective in fast, shallow water.

Sonar. Sonar instruments (also called echo sounders, fathometers or acoustic depth
sounders) measure the elapsed time that an acoustic pulse takes to travel from a generating
transducer to the channel bottom and back (FHWA 1998). Sonar is an acronym for SOund
NAvigation and Ranging that was developed largely during World War II. However, early
sonar systems were used during World War I to find both submarines and icebergs and
called ASDICs (named for the Antisubmarine Detection Investigation Committee). As
technology has improved in recent years better methods of transmitting and receiving sonar
and processing the signal have developed, including the use of digital signal processing
(DSP). The issues of transducer frequency (typically around 200 kHz) and beam width are
important considerations in the use of sonar for scour monitoring work. Additionally, sonar
may be adversely impacted by high sediment loads or air entrainment.

Applications of single beam sonar range from fish finders to precision survey-grade
hydrographic survey fathometers. Low-cost fish-finder type sonar instruments have been
widely used for bridge scour investigations (Figure 9.3) with a tethered float to deploy the
transducer. Float platforms have included kneeboards (Figure 9.4) and pontoon-style floats
(Figure 9.5).

Other types of sonar, such as side scan, multi-beam and scanning sonar, are specialized
applications of basic sonar theory. These devices are commonly used for oceanographic
and hydrographic survey work, but have not been widely utilized for portable scour
monitoring. Side scan sonar transmits a specially shaped acoustic beam to either side of the
support craft. These applications often deploy the transducer in a towfish, normally
positioned behind and below the surface vessel.

While side scan sonar is one of the most accurate systems for imaging large areas of
channel bottom, most side scan systems do not provide depth information. Multi-beam
systems provide a fan-shaped coverage similar to side scan, but output depths rather than
images. Additionally, multi-beam systems are typically attached to the surface vessel, rather
than being towed. Scanning sonar works by rotation of the transducer assembly or sonar
"head," emitting a beam while the head moves in an arc. Since the scanning is accomplished
by moving the transducer, rather than towing, it can be used from a fixed, stationary position.
Scanning sonar is often used as a forward looking sonar for navigation, collision avoidance
and target delineation.

The Sonar Scour Vision system was developed by American Inland Divers, Inc (AIDI) using
a rotating and sweeping 675 Khz high resolution sonar (Barksdale 1994). The transducer is
mounted in a relatively large hydrodynamic submersible, or fish, that creates a downward
force adequate to submerge the transducer in velocities exceeding 20 ft/s (6 m/s) (Figure
9.6). Given the forces created, the fish must be suspended from a crane or boom truck on
the bridge. From a single point of survey, the system can survey up to 328 ft (100 m)
radially. Data collected along the face of the bridge can be merged into a real-time 3-
dimensional image with a range of 295 ft (90 m) both upstream and downstream of the
bridge.
9.4


Figure 9.1 Sounding pole measurement.






Figure 9.2. Lead-line sounding weight.

9.5


Figure 9.3. Portable sonar in use.








Figure 9.4. Kneeboard float.
9.6


Figure 9.5. Pontoon float.








Figure 9.6. AIDI system (Barksdale 1994).

9.7
Geophysical. Surface geophysical instruments are based on wave propagation and reflection
measurements. A signal transmitted into the water is reflected back by interfaces between
materials with different physical properties. A primary difference between sonar and
geophysical techniques is that geophysical methods provide sub-bottom, while sonar can
only "see" the water-soil interface and is not able to penetrate the sediment layer. The main
difference between different geophysical techniques are the types of signals transmitted and
the physical property changes that cause reflections. A seismic instrument uses acoustic
signals, similar to sonar, but at a lower frequency (typically 2-16 kHz). Like sonar, seismic
signals can be scattered by air bubbles and high sediment concentrations. A ground
penetrating radar (GPR) instrument uses electromagnetic signals (typically 60-300 mHz),
and reflections are caused by interfaces between materials with different electrical
properties. In general, GPR will penetrate resistive materials and not conductive materials.
Therefore, it does not work well in dense, moist clays, or saltwater conditions.

The best application of geophysical technology in scour monitoring may be as a forensic
evaluation tool, used after the flood during lower flow conditions to locate scour holes and
areas of infilling. In general, the cost and complexity of the equipment and interpretation of
the data are limiting factors for widespread use and application as a portable scour
monitoring device. These issues have moderated as newer, lower cost GPR devices with
computerized data processing capabilities have been developed. However, GPR may still
be limited by cost and complexity, and often the need for bore hole data and accurate bridge
plan information to properly calibrate and interpret the results.

9.2.3 System for Deploying the Instrument

The system for deploying the scour instrument is a critical component in a successful
portable scour measurement system. In practical application, particularly under flood flow
conditions, the inability to properly position the instrument is often the limiting factor in
making a good measurement. The use of different measurement technologies from different
deployment platforms can produce a wide variety of alternative measurement approaches.

Deployment methods for portable instruments can be divided into two primary categories:

1. From the bridge deck
2. From the water surface

Bridge Deck Deployment. Bridge deck deployment can be defined by two categories, non-
floating and floating. Non-floating systems generally involved standard stream gaging
equipment and procedures, including the use of various equipment cranes and sounding
weights for positioning a sensor in the water. This category could also include devices that
use a probe or arm with the scour measurement device attached to the end. Probes or arms
include things as simple as an extendable pole or rod (such as a painter's pole), to a
remotely controlled articulated arm. Hand held probes or arms are not generally useable at
flood flow conditions.

A prototype articulated arm to position a sonar transducer was developed under an FHWA
research project (Bath 1999). An onboard computer calculated the position of the transducer
based on the angle of the boom and the distance between the boom pivot and transducer.
Additionally, the system could calculate the position of the boom pivot relative to a known
position on the bridge deck. The system was mounted on a trailer for transport and could be
used on bridge decks from 16 - 50 ft (5 - 15 m) above the water surface (Figure 9.7). Field
testing during the 1994 floods in Georgia indicated that a truck mounted system would
provide better maneuverability, and that a submersible head or the ability to raise the boom
pivot was necessary to allow data collection at bridges with low clearance (less than 16 ft (5
m)).
9.8

Figure 9.7. FHWA articulated arm in use.

NCHRP Project 21-7 (Schall and Price 2004) resulted in a truck mounted articulated arm to
facilitate portable scour measurements during flood conditions (Figure 9.8). The truck was
designed to operate in high velocity flow while providing accurate positioning information and
efficient data collection procedures. These measurements can be completed from a variety
of bridge geometries including limited clearance, overhanging geometry, and high bridges.
Scour measurement can be by a streamlined sonar probe, sounding weights, kneeboards, or
physical probing. A dual winch system was developed to facilitate cable suspended
operations. Crane location is tracked by a variety of sensors installed on the articulated arm,
and by a survey wheel on the back of the truck. Data loggers manage data, and a laptop is
used for data reduction. Sonar data and positioning data collected at the end of the crane
are transmitted by a wireless link that eliminates any wires from the water surface to the
truck (Figure 9.9).


Figure 9.8. NCHRP 21-3 articulated arm truck (Schall and Price 2004).
9.9

Figure 9.9. Sonar instrument deployed from articulated arm truck.

Float based systems permit measurement beneath the bridge and along side the bridge
piers. Tethered floats are a low-cost approach that have been used with some success
during flood flow conditions. A variety of float designs have been proposed and used to
varying degrees for scour measurements, typically to deploy a sonar transducer. Common
designs include foam boards, PVC pontoon configurations, spherical floats, water skis and
kneeboards (FHWA 1998). The size of the float is important to stability in fast moving,
turbulent water.

Floating or non-floating systems can be also be deployed from a bridge inspection truck, an
approach that is particularly useful when the bridge is high off the water. For example,
bridges that are greater than 50 ft (15 m) off the water are typically not accessible from the
bridge deck without using this approach.

Water Surface Deployment. Water surface deployment typically involves a manned boat,
however, safety issues under flood conditions have suggested the use of unmanned vessels.
The use of manned boats generally requires adequate clearance under the bridge and
nearby launch facilities. This can be a problem at flood conditions when the river stage may
approach or submerge the bridge low chord, and/or boat ramps may be underwater.
Smaller boats may be easier to launch, but safety at high flow conditions may dictate use of
a larger boat, further complicating these problems.

When clearance is not an issue, the current and turbulence in the bridge opening may be
avoided using one of the tethered floating or nonfloating methods described above from a
boat positioned upstream of the bridge. For example, a pontoon or kneeboard float with a
sonar transducer could be maneuvered into position from a boat holding position upstream
of the bridge, thereby avoiding the current and turbulence problems at the bridge itself.

The safety, launching and clearance issues have suggested that an unmanned or remote
control boat might be a viable alternative. A prototype unmanned boat using a small flat
bottom jon boat and an 8 hp outboard motor with remote controls (Figure 9.10) was
successfully tested during six flood events (Mueller and Landers 1999).


9.10

Figure 9.10. Unmanned, remote control boat.

9.2.4 Positioning Information

In order to evaluate the potential risk associated with a measured scour depth it is necessary
to know the location of the measurement, particularly relative to the bridge foundation.
Location measurements can range from approximate methods, such as "3 ft (1 m) upstream
of pier 3," to precise locations based on standard land and hydrographic surveying
technology.

The most significant advancement for portable scour measurement positioning may be in the
use of Global Positioning Systems (GPS). GPS is a positioning system based on a
constellation of satellites orbiting the earth. An advantage of GPS over traditional land-
based surveying techniques is that line-of-sight between control points is not necessary. A
GPS survey can be completed between control points without having to traverse or even see
the other point. GPS also works at night and during inclement weather, which could be a
real advantage for scour monitoring during flood conditions. The most significant
disadvantage of GPS is the inability to get a measurement in locations where overhead
obstructions exist, such as tree canopy or bridge decks. However, GPS measurements up
to the bridge face, without venturing under the bridge, have been successful.
9.11
9.2.5 Data Storage Devices

Portable scour monitoring data are typically manually recorded in a field book, however,
there has been a growing interest in more automated data collection using various data
storage devices. Available data storage devices include hydrometeorological data loggers,
laptop computers and more recently palm computers and organizers. Data loggers provide
a compact storage device, however, they are generally not very user friendly with each
company typically having a unique programming language and approach. In field
applications, laptop computers are bulky and need to be ruggedized to survive the rain, dirt
and dust of a field environment. Palm computers and organizers may have an application as
their capability and user-friendliness continue to improve. The advantage of laptop
computers and palm computers is the ability to integrate data reduction software, such as
plotting or topographic mapping programs to display the results, often in real time mode
while the data collection occurs.

9.3 FIXED INSTRUMENTATION

9.3.1 NCHRP Project 21-3

The basic objective of NCHRP Project 21-3 was to develop, test, and evaluate fixed
instrumentation that would be both technically and economically feasible for use in
measuring or monitoring maximum scour depth at bridge piers and abutments (Lagasse et
al. 1997). The scour measuring or monitoring device(s) were required to meet the following
mandatory criteria.

Mandatory Criteria

C Capability for installation on or near a bridge pier or abutment
C Ability to measure maximum scour depth within an accuracy of 1 ft ( 0.3 m)
C Ability to obtain scour depth readings from above the water or from a remote site
C Operable during storm and flood conditions

Where possible, the devices should meet the following desirable criteria:

Desirable Criteria

C Capability to be installed on most existing bridges or during construction of new bridges
C Capability to operate in a range of flow conditions
C Capability to withstand ice and debris
C Relatively low cost
C Vandal resistant
C Operable and maintainable by highway maintenance personnel

Since the mandatory criteria required that the instruments be capable of installation on or
near a bridge pier or abutment, the research was limited to fixed instruments only. While the
research was conducted in phases, a final project report was prepared to integrate and
summarize the findings, interpretation, conclusions and recommendations for the total
research effort (Lagasse et al. 1997). A separate Installation, Operation, and Fabrication
Manual was developed for both the magnetic sliding collar device and low-cost sonic
instrument system that resulted from this research (Schall et al. 1997a, 1997b).

9.12
9.3.2 Scour Measurement

Although a vast literature exists relating to bridge scour, relatively few reports deal
specifically with instrumentation. The final report for NCHRP Project 21-3 includes an
extensive bibliography on equipment for scour measurement and monitoring (Lagasse et al.
1997). A detailed survey of the evolution of scour measuring instrumentation was presented
at the Transportation Research Board Third Bridge Engineering Conference in 1991
(Lagasse et al. 1991). This section summarizes the development of scour measuring
equipment and techniques that had particular relevance to the instrumentation developed
under the NCHRP project.

Major advances in instrumentation such as sonar, sonic sounders, electronic positioning
equipment, and radar occurred during World War II. By the mid 1950s, many devices
became commercially available and were introduced into scientific studies of rivers. A dual
channel sonic stream monitor was used in the 1960s to study alluvial channel bed
configurations and the scour and fill associated with migrating sand waves. Commercial
sonic sounders became available about the same time and soon were used extensively in
hydrographic surveys.

In the 1970s, many scour studies were undertaken in New Zealand. One of the instruments
used in the field to measure maximum scour depth at bridge piers was called the
"Scubamouse." The device consists of a vertical pipe buried or driven into the streambed in
front of the bridge pier around which is placed a horseshoe-shaped collar that initially rests
on the streambed. The collar slides down the pipe and sinks to the bottom of the scour hole
as scour progresses during a flood. The position of the collar is determined by sending a
detector down the inside of the pipe after the flood. Earlier models involved a metal detector
inside a PVC pipe, but the pipe was sometimes damaged by debris, so the current models
use a steel pipe, a radioactive collar, and a radiation detector inside the pipe. This device
has been installed on many bridges in New Zealand (Melville et al. 1989).

In the United Kingdom, Hydraulic Research Limited of Wallingford has developed and
deployed a buried rod instrument system to monitor bed scour during flood events (Waters
1994). This 'Tell Tail' scour monitoring system is based on omni-directional motion sensors,
buried in the river or sea bed adjacent to the structure. The sensors are mounted on flexible
'tails' and are connected to the water surface via protected cables. Under normal flow
conditions, the detectors remain buried and do not move. When a scour hole develops, the
sensors are exposed and transmit alarm signals to the surface.

In the early 1990s, there were no accepted methods or off-the-shelf equipment for collecting
scour data in the United States. In part, this was because there had been no coordinated
long-term effort to study scour processes. Also, most scour studies were site-specific and
the equipment and techniques that were used were tailored to the geometry of the site and
its hydrology and hydraulic conditions. The Brisco Monitor, a sounding rod device, was
available, but had not been tested extensively in the field.

Scour studies in the United States were carried out with a great variety of portable
equipment and techniques, and, through the U.S. Geological Survey (USGS) National Scour
Study, conducted in cooperation with the Federal Highway Administration (FHWA), efforts
were made to standardize the collection of scour data (Landers and Trent 1991).
Techniques for determining the extent of local scour include the use of divers and visual
inspection, direct measures of scour with mechanical and electronic devices, and indirect
observations using ground-penetrating radar and other geophysical techniques (Gorin and
Haeni 1989).
9.13
In the early 1990s, the USGS investigated the use of fixed instruments for scour
measurement at a new bridge on U.S. Highway 101 across Alsea Bay near Walport,
Oregon. Depth soundings were made using commercially available sonic sounders. The
transducers for sounding were mounted on brackets attached to the piers and pointed out
slightly to avoid interference from the side of the pier. The system worked well, but the
installation was not subject to debris, ice, or air entrainment from highly turbulent flows
(Crumrine et al. 1996).

The initial literature search on scour instrumentation in 1990 revealed, and a resurvey of
technology in 1994 confirmed, that fixed scour-measuring and -monitoring instruments can
be grouped into four broad categories:

1. Sounding rods - manual or mechanical device (rod) to probe streambed
2. Buried or driven rods - device with sensors on a vertical support, placed or driven into
streambed
3. Fathometers - commercially available sonic depth finder
4. Other Buried Devices - active or inert buried sensor (e.g., buried transmitter)

As a result of the literature review a laboratory testing program was designed to test at least
one device from each category and to select devices for field testing that would have the
greatest potential for meeting mandatory and desirable criteria.

9.3.3 Summary of NCHRP Project 21-3 Results

No single methodology or instrument can be utilized to solve the scour monitoring problems
for all situations encountered in the field. Considering the wide range of operating conditions
necessary, environmental hazards such as debris and ice, and the variety of stream types
and bridge geometries encountered in the field, it is obvious that several instrument systems
using different approaches to detecting scour will be required.

Under NCHRP Project 21-3, a variety of scour measuring and monitoring methods were
tested in the laboratory and in the field, including sounding rods, driven rod devices,
fathometers, and buried devices. Two instrument systems, a low-cost bridge deck (above
water) serviceable fathometer and a magnetic sliding collar device using a driven rod
approach were installed under a wide range of bridge substructure geometry, flow, and
geomorphic conditions. Both instrument systems met all of the mandatory criteria and most
of the desirable criteria established for the project and were considered fully field deployable
in 1997.

The Installation, Operation, Fabrication Manuals for the low-cost sonic system and magnetic
sliding collar devices (Schall et al. 1997a, 1997b) provide complete instrument
documentation, including specifications and assembly drawings. That information, together
with the findings, appraisal, and applications information of the final report (Lagasse et al.
1997), provide a potential user of a scour monitoring device complete guidance on selection,
installation, operation, maintenance, and if desired, fabrication of two effective systems, one
of which could meet the need for a fixed scour instrument at most sites in the field.

Of the devices tested extensively in the field, the low-cost sonic system and the manual-
readout sliding collar device are both vulnerable to ice and debris; however, both proved to
be surprisingly resistant to damage from debris or ice impact at field test sites. The sonic
system can be rendered inoperative by the accumulation of debris, and presumably ice,
between the transducer face and streambed. The manual-readout sliding collar requires an
extension conduit, generally up the front face of a pier, which can be susceptible to debris or
ice impact damage unless the extension can be firmly anchored to a substructure element.
9.14
From this perspective, the automated sliding collar device has the distinct advantage of
having a configuration which places most of the device below the streambed, and therefore,
less vulnerable to ice or debris. The connecting cable from the device to a data logger on
the bridge deck can be routed through a buried conduit and up the downstream face of a
bridge pier or abutment where it is much less vulnerable to damage. An overview of these
and other operational instrument systems is provided in the next section.

9.3.4 Operational Fixed Instrument Systems

A scour monitoring system at a bridge may be comprised of one or more types of fixed
instruments. The various devices are either mounted on the bridge or installed in the
streambed in the vicinity of the bridge. Remote units with data loggers may be installed so
that the scour measuring device transmits data to the unit. The data from any of these fixed
instruments may be downloaded manually at the site, or it can be telemetered to another
location. The early scour monitoring devices measured streambed elevations using simple
on-site manually read units. The more recent installations use remote technology for data
retrieval. Each bridge may have one or more remote sensor units that transmit data to a
master unit on or near the bridge (Figure 9.11). The scour monitoring data is then
transmitted from the master unit to a central office for data analysis. Remote technology
transmits data by modem using cellular or landline telephones, or by satellite. Recent
installations include systems that post the data on the Internet so that authorized persons
may access the data from any location using a computer with Internet access.


Figure 9.11. Master station with data logger for use with any of the fixed instruments.

Sonars. The sonar scour monitors are mounted onto the pier or abutment face (Figures
9.12 and 13) to take streambed measurements. Currently new sonar monitors range from
fish finders to smart sonar transducers, both of which are commercially available. The sonar
transducer is connected to the sonar instrument or directly to a data logger. The sonar
instrument measures distance based on the travel time of a sound wave through water to the
stream bed and back to the transducer. The data logger controls the sonar system
operation and data collection functions and can be programmed to take measurements at
prescribed intervals. Sonar sensors normally take a rapid series of measurements and use
an averaging scheme to determine the distance from the sonar transducer to the streambed.
These instruments can track both the scour and refill (deposition) processes. The early
sonar monitors used off-the-shelf "fish finders."
9.15

Figure 9.12. Above-water serviceable low-cost fathometer system (Schall et al. 1997b).
9.16

Figure 9.13. Sonar scour monitor installation including remote station and solar panel.

Magnetic Sliding Collars. Magnetic sliding collars (Figures 9.14, 9.15, and 9.16) are rods or
masts that are attached to the face of a pier or abutment and driven or augered into the
streambed. A collar with magnetic elements is placed on the streambed around the rod. If
the streambed erodes, the collar moves or slides down the rod into the scour hole. The
depth of the collar provides information on the scour that has occurred at that particular
location.

The early version of the sliding magnetic collar used a battery operated manual probe that
was inserted down from the top and a buzzer sounded when the probe tip sensed the level
of the magnetics on the collar (Figure 9.14). More recent instruments have a series of
magnetically activated switches inserted in the rod at known distances. Magnets on the steel
collar come into proximity with the switches as the collar slides into the scour hole. The
switches close sequentially as the collar slides by, and their position is sensed by the
electronics (Figures 9.15 and 9.16). A data logger reads the level of the collar via the auto
probe and tracks scour activity. Sonar scour monitors may be used to provide the infill scour
process at a bridge, whereas magnetic sliding collars can only be used to monitor the
maximum scour depth.

Float-out Devices. Buried devices may be active or inert buried sensors or transmitters.
Float-out devices (Figure 9.17) are buried transmitters. This device consists of a radio
transmitter buried in the channel bed at pre-determined depth(s). If the scour reaches that
particular depth, the float-out device floats to the stream surface and an onboard transmitter
is activated. It transmits the float-out device's digital identification number with a radio
signal. The signal is detected by a receiver in an instrument shelter on or near the bridge.
The receiver listens continuously for signals emitted by an activated float-out device. A
decoded interface decodes the activated float-out device's unique digital identification
number that will determine where the scour has occurred. A data logger controls and logs
all activity of the scour monitor. These devices are particularly easy to install in dry
riverbeds, during the installation of an armoring countermeasure such as riprap, and during
the construction of a new bridge. The float-out sensor is a small low-powered digital
electronics position sensor and transmitter. The electronics draws zero current from a
lithium battery which provides a 9-year life expectancy when in the inactive state buried in
the streambed.
9.17

Figure 9.14. Manual read out magnetic sliding collar device (Schall et al. 1997a).

9.18


Figure 9.15. Automated read out magnetic sliding collar system (Schall et al. 1997a).
9.19

Figure 9.16. Detail of magnetic sliding collar on the streambed.


Figure 9.17. Float-out devices prior to installation. Color coded
and numbered for identification purposes.


Tilt Sensors. Tilt sensors (Figure 9.18) measure movements and rotations of the bridge
itself. An X, Y tilt sensor or clinometer monitors the bridge position. Should the bridge be
subject to scour causing one of the support piers or abutments to settle, one of the tilt
sensors would detect the change. A pair of clinometers is installed on the bridge piers or
abutments (Figure 9.19). One tilt meter senses rotation parallel to the direction of traffic (the
longitudinal direction of the bridge), while the other senses rotation perpendicular to traffic
(usually parallel with the stream flow).

9.20

Figure 9.18. Tilt meter on California bridge.



Figure 9.19. Detail of tilt meter.

Time Domain Reflectometers. In Time Domain Reflectometry (TDR) an electromagnetic
pulse is sent down two parallel pipes that are buried vertically in the streambed (Figure 9.20)
(Zabilansky 2002). When the pulse encounters a change in the boundary conditions (i.e.,
the soil-water interface), a portion of the pulse's energy is reflected back to the source from
the boundary. The remainder of the pulse's energy propagates through the boundary until
another boundary condition (or the end of the probe) causes part or all of the energy to be
reflected back to the source. By monitoring the round-trip travel time of a pulse in real time,
the distance to the respective boundaries can be calculated and this provides information on
any changes in streambed elevation. Monitoring travel time in real time allows the
processes affecting sediment transport to be correlated with the change in bed elevation.
Using this procedure, the effects of hydraulic and ice conditions on the erosion of the
riverbed can be documented.

9.21

Figure 9.20. Time domain reflectometry probe.
(Courtesy of USACE Cold Regions Research and Engineering Laboratory)

Sounding Rods. Sounding-rod or falling-rod instruments are manual or mechanical
(automated) gravity based physical probes (Figure 9.21). As the streambed scours, the rod,
with its foot resting on the streambed, drops following the streambed and causing the system
counter to record the change. The foot must be of sufficient size to prevent penetration into
the streambed caused by the weight of the rod and the vibration of the rod from flowing
water. These devices are susceptible to streambed surface penetration in sand bed
channels and this influences their accuracy. Consequently, they are best suited to monitoring
coarse bed streams or riprap stability (as shown in Figure 9.21)


Figure 9.21. Brisco Monitor sounding rods installed at a bridge pier in
New York to monitor movement of riprap (Butch 1996).

9.22
Summary. If recording a series of streambed elevations over time is of interest, sonars,
magnetic sliding collars and sounding rod monitors may be used (only the sonar will record
scour and fill). If a bridge owner is interested only in when a certain streambed elevation is
reached, float-outs may be employed. For specific information on a pier or abutment, tilt
sensors record relative rotation and movement of the structure. Additional fixed instruments
may be added to the scour monitoring system to gather information on water elevations,
water velocities and/or temperature readings.

Data from any of these fixed instruments may be downloaded manually at the site, or may
be telemetered to another location. A scour monitoring system at a bridge may use one of
these devices, or include a combination of two or more of these fixed instruments all
transmitting data to a central control center. These types of scour monitors are being used
in a wide variety of climates and temperatures, and in a wide range of bridge and channel
types throughout the United States.

9.3.5 NCHRP Project 20-5

Most of the information in Section 9.3.4 is derived from NCHRP Project 20-5 (Topic 36-02)
which was a synthesis study entitled Practices for Monitoring Scour Critical Bridges (Hunt
2008). The study assessed the state of knowledge and practice for fixed scour monitoring of
scour critical bridges. It included a review of the literature and research, and a survey of
transportation agencies and other bridge owners to obtain their experience with fixed scour
monitoring systems.

The study found that 30 of the 50 states use, or have employed fixed scour monitoring
instrumentation for their highway bridges. A total of 120 bridge sites were identified that
have been instrumented with fixed monitors. The five types of fixed instruments being used
in 2007 included sonars, float-outs, tilt meters, magnetic sliding collars and time domain
reflectometers.

The site conditions and the types of bridges that were monitored with fixed scour
instrumentation varied in many aspects. There were small to long span bridges with lengths
ranging from 41 ft (12.5 m) to 12,865 ft (3,921 m). The Average Daily Traffic (ADT) ranged
from 400 to 175,000 vehicles per day, and the bridges were constructed between 1920 and
1986. The site conditions included both riverine and tidal waterways, intermittent to perennial
flows, and water depths ranging from less than 10 to 75 ft (3 m to 30 m). The soil conditions
ranged from clay to gravel, and some had riprap protection.

The scour monitors were installed between 1992 and 2007. The earlier installations included
sounding rods, magnetic sliding collars and sonars. More recent installations also include
float-outs, tilt sensors and TDRs. The sonar scour monitoring system is the most commonly
used device, installed at 70 of the 120 bridge sites. This was followed by the magnetic sliding
collar at 21 sites. The bridge owners reported that 90% of the structures monitored with
fixed instruments were piers. The remaining devices were on abutments, or in the vicinity of
the bridge on bulkheads or downstream countermeasure protection.

The survey respondents indicated that high velocity flows, debris, ice forces, sediment
loading and/or low water temperatures were extreme conditions that were present at the
monitored bridge sites. The debris and ice forces caused the majority of damage and
interference to the scour monitoring systems. They noted that the extent and frequency of
the damage was often not anticipated by the bridge owner and this resulted in much higher
maintenance and repair costs than anticipated.

9.23
The bridge owners provided information on their future needs for improved scour monitoring
technology which include:

More robust devices increased reliability and longevity
Decreased costs
Less maintenance
Devices more suitable for larger bridges
Devices that measure additional hydraulic variables and/or structural health

9.3.6 Application Guidelines

Bridge Pier and Abutment Geometry. It is clear that no single device is applicable to all
bridge pier and abutment geometries. However, most bridge geometries can be
accommodated with one of the scour measuring devices described in Section 9.3.4.

Most instruments are adaptable in some degree to vertical piers and abutments. Sloping
piers and spill-through abutments present difficulties for most instrument configurations;
however, driven rod instruments, such as the automated sliding collar, that are not fastened
to the substructure can be used on sloping piers and abutments. Adapting scour
instrumentation to a large spread footing or pile cap configuration also presents challenges.

Flow and Geomorphic Conditions. Each class of scour measuring instrument will not be
applicable to all flow and geomorphic conditions. While some limitations stem from the
capabilities of the device itself, some pertain to whether the device is installable given the
geomorphic and flow conditions. For example, sounding rods have not performed well in
sand-bed streams, although the addition of a large base plate to the sounding rod could help
correct the problem, and sonar devices may be best suited for tidal waterways where
problems with debris are not as common as they are for riverine bridges.

All devices using a driven rod configuration (including TDR) will have limitations imposed by
bed and substrate characteristics. Pre-drilling, jetting, or augering may permit installation
under a wide range of conditions, but these techniques may be expensive and could be
difficult over water. The connecting conduit required by the manual-readout sliding collar
device is vulnerable to ice and debris impact, but the instrument proved surprisingly durable
at field test sites with significant debris.

Low-cost fathometers are applicable to a wide range of streambed characteristics and flow
and geomorphic conditions, but ice and debris in the stream can quickly render a fathometer
inoperable. Strategies such as placing the transducer close to the streambed may reduce,
but won't eliminate, the vulnerability of this instrument to ice and debris.

Float out devices are simple to fabricate and relatively inexpensive. Installation in the dry on
an ephemeral stream or where a coffer dam can be installed can be accomplished with
drilling equipment available to most DOTs. Installation under water, however, would be
difficult.

Tilt Meters. Tilt meters are placed on the bridge superstructure and above-water
substructure components making installation easier and less expensive than other fixed
instruments. Tilt meters measure the movement of the bridge itself, therefore the bridge
must be redundant enough to withstand some movement without failure (Avila et al. 1999).
This will allow maintenance forces sufficient time to remotely observe the movement and
9.24
send crews to inspect the bridge and close it, if necessary. However, it is difficult to set the
magnitude of the angle at which the bridge is in danger. Bridges are not rigid structures, and
movement can be induced by traffic, temperature, wind, hydraulic and earthquake loads. It
is necessary to observe the "normal" movement of the bridge and then determine the
"alarm" angle that would provide sufficient time for crews to travel to and close the bridge to
traffic. The California Department of Transportation (Caltrans) has accomplished this by
installing the tilt meters, monitoring normal pier movement for several months (ideally, they
recommend one year), and setting the "alarm" angles based on the unique "signature" of
each monitored pier on any given bridge.

9.4 SELECTING INSTRUMENTATION

Developing the monitoring program in a Plan of Action requires identifying the specific
instruments, portable and/or fixed, and how they will be used to monitor scour. Selection of
the appropriate instrumentation will depend on site conditions (streambed composition,
bridge height off water surface, flow depth and velocity, etc.) and operational limitations of
specific instrumentation (e.g., as related to high sediment transport, debris, ice, specialized
training necessary to operate a piece of equipment, etc.).

Engineering judgment will always be required in designing instrument specifications to
maximize the scour information collected within the given resources. Specific issues related
to the use of either fixed or portable instruments include:

1. For fixed instrumentation, the number and location of instruments will have to be defined,
as it may not be practical or cost effective to instrument every pier and abutment.

2. For portable instrumentation, the frequency of data collection and the detail and accuracy
required will have to be defined, as it may not be possible to complete detailed
bathymetric surveys at every pier or abutment during every inspection.

Some monitoring programs will involve a mix of fixed, portable and geophysical instruments
to collect data in the most efficient manner possible. Furthermore, portable instrumentation
should be used to ground-truth fixed instrumentation to insure accurate results and to
evaluate potential shifting of the location of maximum scour.

Table 9.1 summarizes the advantages and limitations of the various instrumentation
categories. In general, fixed instrumentation is best used when ongoing monitoring is
required, recognizing that the location of maximum scour may not always be where the
instrument was originally installed. This could be the result of geomorphic conditions and
changes in the river over time, or an initial miscalculation when the instrument was installed.
Portable instruments are best used where more areal coverage is required, either at a given
bridge or at multiple bridges. Portable instruments provide flexibility and the capability to
respond quickly to flood conditions; however, if a portable monitoring program becomes
large, collecting data may become very labor intensive and costly. Additionally, deployment
of portable instruments may require specialized platforms, such as trucks with cranes or
booms, or the use of an under bridge inspection truck. Geophysical instrumentation is best
used as a forensic tool, to evaluate scour conditions that existed during a previous flood.
The primary limitation of geophysical equipment is the specialized training and cost involved
in deploying this type of measurement.


9.25

Table 9.1. Instrumentation Summary by Category.
Instrument
Category

Advantages

Limitations
Fixed Continuous monitoring, low
operational cost, easy to use
Maximum scour not at instrument location,
maintenance/loss of equipment
Portable Point measurement or complete
mapping, use at many bridges
Labor intensive, special platforms often
required
Geophysical Forensic investigations Specialized training required, labor
intensive
Positioning Necessary for portable and
geophysical instruments


9.4.1 Portable Instruments

Within the portable instrument category, the use of physical probes is generally limited to
smaller bridges and channels (Table 9.2). It is a simple technology that can be effectively
used by personnel with limited training, but may be of limited use as the flow depth or
velocity increase, such as during flood conditions. Portable sonar instruments may be better
suited for large bridges and channels, but they too can be limited by flow conditions based
on the deployment options available. Sonar may also be limited in high sediment or air
entrainment conditions, or when debris or ice accumulation are present.

Table 9.2. Portable Instrumentation Summary.
Best Application Advantages Limitations
Physical Probes Small bridges and
channels
Simple technology Accuracy, high flow
application
Sonar Larger bridges and
channels
Point data or complete
mapping, accurate
High flow application

Positioning equipment is required to provide location information with any portable or
geophysical measurement (Table 9.3). The approximate methods are useful for any type of
reconnaissance or inspection level monitoring, but are obviously limited by accuracy. The
use of standard land survey techniques, using a total station type instrument or in the case
of hydrographic surveying, an automated range-azimuth type device, can provide very
accurate positional data. However, these instruments require a setup location on the
shoreline that may be difficult to find during flooding, when overbank water and/or riparian
vegetation limit access and line-of-sight. These approaches can also be somewhat slow and
labor intensive. In contrast, the use of GPS provides a fast, accurate measurement, but will
not work under the bridge.

Table 9.3. Positioning System Summary.
Best Application Advantages Limitations
Approximate
methods
Recon or inspection No special training or
equipment
Accuracy
Traditional land
survey methods
Small channels or areal
surveys
Common technique
using established
equipment
Shore station
locations, labor
intensive
GPS Measurement up to
bridge face
Fast, accurate Cannot work under
bridge
9.26
Another important factor in designing a monitoring program is the cost of the instrumentation
and data collection program. Portable instrument costs can be readily identified, but the cost
of installation and operation are more difficult to quantify, since this will depend on site
specific conditions and the amount of data needed. Based on field experience, Table 9.4
provides general guidelines on cost information. These costs should be used cautiously in
an absolute sense, given unique site-specific conditions and/or the changes in cost that can
occur with time and new research and development. This information may be most useful
as a relative comparison between different approaches.

Table 9.4. Estimated Cost Information for Portable Instruments.
Instrument
Cost
Cost for
Installation or
Use
Operation
Cost
Physical Probes < $500 varies by use varies, minimum
2-person crew for safety
Portable Sonar fish-finder - $500;
survey grade - $15,000 +/-
varies by use varies, minimum
2-person crew for safety
Traditional land survey $10,000 +/- varies 2-3 person crew
GPS $5,000 for submeter accuracy,
$20,000 + for centimeter
varies 1-2 person crew

9.4.2 Fixed Instruments

Fixed instrument devices include sonar, sounding rods (automated physical probe),
magnetic sliding collar, float out devices, tilt sensors, and time domain reflectometers (Table
9.5). Based on field experience, the sonar type devices work best in coastal regions and can
be built using readily available components. They provide a time history of scour, yet have
difficulty in conditions with high debris, ice, and air entrainment (Zabilansky 1996).
Therefore, if a sonar device is selected for a riverine environment, these conditions may limit
when data is collected and the quality of the data record.

Table 9.5. Fixed Instrumentation Summary.
Type of Fixed
Instrumentation
Best
Application

Advantages

Limitations
Sonar Coastal regions Records infilling; time history;
can be built with off the shelf
components
Debris, high sediment loading,
ice, and air entrainment can
interfere with readings
Magnetic Sliding Collar Fine bed channels Simple, mechanical device Vulnerable to ice and debris
impact; only measures
maximum scour; unsupported
length, binding
Tilt Sensors All May be installed on the bridge
structure and not in the stream-
bed and/or underwater
Provides bridge movement
data which may or may not be
related to scour
Float-Out Device Ephemeral
channels
Lower cost; ease of installation;
buried portions are low main-
tenance and not affected by
debris, ice or vandalism
Does not provide continuous
monitoring of scour; battery life
Sounding Rods Coarse bed
channels
Simple, mechanical device Unsupported length, binding,
augering
Time Domain
Reflectometers
Riverine ice
channels
Robust; resistance to ice,
debris, and high flows
Limit on maximum lengths for
signal reliability of both cable
and scour probe
9.27
Sounding rods, typically a dropping rod with a method to measure the displacement
occurring, have been found to work best in coarse bed channels, and are a simple
mechanical type of device. They have had difficultly in channels with fine sediments where
sediment accumulation around the sliding components has led to binding. Additionally, they
are limited by the maximum amount of travel that the sounding rod can realistically achieve,
given problems with unsupported length vibration and augering. In contrast, the magnetic
sliding collar device works best in fine bed channels, where it is possible to drive the
supporting rod into the streambed. It, too, is a simple mechanical type device, but it is also
limited by concerns with unsupported length, binding and debris.

The float-out type sensors have worked well in ephemeral channels, and are a low-cost
addition to any other type of fixed instrument installation. They have been successfully used
when buried either in the channel bed, or in riprap, and can be placed at locations away from
structural members of the bridge, which may not be possible with the other types of fixed
instruments.

Tilt sensors are installed above water on the bridge superstructure or substructure and may
be used on a wide variety of bridges and sites. They measure overall structure movement
and, therefore, do not have to be located at the specific location of the scour, as is the case
with the other fixed instruments discussed in this chapter. Tilt sensors do not provide
information on scour depths. The bridge must be redundant enough to ensure that when
movement is detected there is enough time to close or repair the bridge. In addition, bridge
movements and rotations may be caused by a variety of load factors, and it may take some
time to establish the "signature" movements particular to a bridge which are not due to
scour.

The fixed instrumentation selection matrix, Table 9.6 was developed during the synthesis
study (Hunt 2008) to complement the countermeasure selection matrix (Table 2.1). See
Chapter 2, Section 2.3 for a discussion of the various symbols used in both tables. If fixed
instrumentation is to be used to monitor a bridge, this table provides additional items to be
considered in deciding between the various fixed instrument options. It was developed
based on the results of the synthesis study state survey and literature search (Section 9.3.5).
Table 9.6 includes additional categories for suitable river environment for the various fixed
instruments:

Type of waterway riverine / tidal Bed material
Flow habit Extreme conditions
Water depth

The bridge geometry includes information on the characteristics of the bridges for the
different types of instruments:

Foundation type

The table includes additional items regarding the monitoring system capabilities which may
be mandatory or desirable criteria for a particular bridge site:

Continuous data monitoring
Remote technology

Additional river environment factors listed in Table 2.1 (river type, stream size, bend radius,
bank slope, and floodplain) are not listed in Table 9.6 as they do not directly influence the
selection of fixed instrumentation.
9.28

9.29
The installation experience by state for each type of fixed monitor for those that responded
to the synthesis survey (Section 9.3.5) and also from the literature search are included in the
last two columns of Table 9.6.

The cost of fixed instrumentation and the data collection program is an important factor in
the selection process. Table 9.7 provides general guidelines on cost information. This table
may be used with Table 9.4 for portable instruments to compare relative cost information
between the different monitoring approaches. As with portable instruments, the most
quantifiable cost for fixed instrumentation is the equipment cost. The installation, operation,
maintenance and repair costs are more difficult to ascertain.

Table 9.7. Estimated Cost Information for Fixed Instruments.

Typed of Fixed
Instrumentation
Instrument Cost
with Remote
Technology ($)
(1)

Instrument Cost for
Each Additional
Location ($)
Installation Cost Maintenance/
Operation Costs
Sonar 12,000 - 18,000 10,000 - 15,500 Medium to high; 5 to 10-
person days to install
Medium to High
Magnetic Sliding
Collar
13,000 15,500 10,500 12,500 Medium, minimum 5-person
days to install
Medium
Tilt Sensors 10,000 11,000 8,000 9,000 Low Low
Float-Out
Device
10,100 10,600 1,100 1,600 Medium; varies with number
installed
Low
Sounding Rods 7,500 10,000 7,500 10,000 Medium; minimum 5-person
days to install
High
Time Domain
Reflectometers
5,500 21,700 500 Low Medium
(1)
Cost per device will decrease when multiple devices share remote stations and/or the master station.

Instrument costs generally include the basic scour monitoring instrument and mounting
hardware, as well as power supply, data logger and instrument shelter/enclosure, where
applicable. This cost may not include miscellaneous items to install the equipment such as
electrical conduit, brackets and anchor bolts which may be included as part of the contractor
installation cost.

The cost of the scour monitoring installations can vary dramatically due to different factors
such as site conditions, the group and/or the experience of the personnel installing the
equipment, the type of contract, and the installation requirements. Larger bridges and
deeper waterways are more expensive to instrument than smaller bridges in ephemeral or
low water crossings. Scour monitors may be installed at certain sites by the state
maintenance group, or another agency with equipment they own, or by students. More
complicated installations and sites may require specialized contractors and construction
equipment to install the scour monitoring devices.

Most recent installations of fixed instrumentation have used remote technology to download
data to avoid repeated visits to the bridge site. Although this increases the initial equipment
cost, it can substantially reduce the long-term operational costs of data retrieval. Site data
retrieval involves sending crews to the bridge and access may include security clearance,
lane or bridge closures, and equipment such as snooper trucks or boats. Remote
technology can also increase safety to the traveling public because it permits real-time
monitoring during the storm events which may result in earlier detection of scour.

9.30
Factors that contribute to increased scour monitoring installation, inspection, maintenance
and repair costs include: larger bridges; complex pier geometries; bridges with large deck
heights off the water; deeper waterways; long distance electrical conduit runs; more durable
materials required for underwater tidal installations; the type of data retrieval required (i.e.,
Internet, satellite); lane or bridge closures and maintenance-of-traffic; and installation and
access equipment such as boats, barges, snooper trucks, drills and diving teams.

9.5 FIXED INSTRUMENT CASE HISTORIES

9.5.1 Introduction

The following case histories were selected for this section because they cover a range of
geographical locations and types of fixed scour monitoring instrumentation. This section
provides descriptions of the systems as well as details on the installations and
implementation of the scour monitoring programs.

9.5.2 Typical Field Installations

Alaska Installations. To better understand the scour process and to monitor bed elevation at
bridge piers, the U.S. Geological Survey (USGS) and the Alaska Department of
Transportation and Public Facilities operate a network of streambed scour-monitoring
stations in Alaska (Conaway 2006a). To date they have instrumented 20 bridges with sonar
and river stage instrumentation (Figure 9.22). In 2007, 16 bridges remained in the scour
monitoring program. These stations provide state engineers with near real-time bed
elevation data to remotely assess scour at bridge piers during high flows. The data also
provide a nearly continuous record of bed elevation in response to changes in discharge and
sediment supply. Seasonal changes as well as shorter duration scour and fill have been
recorded. In addition to the near real-time data, channel bathymetry and velocity profiles are
collected at each site several times per year.

Each bridge is instrumented with a retractable, pier-mounted sonar device. At locations with
multiple scour critical piers, sonar transducers were mounted at each pier. The sonar
transducers were mounted either at an angle on the side of the piers near the nose or on the
pier nose in order to collect data just upstream of the pier footing. Many of Alaska's bridges
are situated in locations too remote for landline or cellular telephone coverage. The scour
monitoring instrumentation on the remote bridges has incorporated Orbcomm, a
constellation of low-earth-orbiting satellites. Data is sent from the bridge to a passing
satellite, which then relays it on to an earth station which then forwards the data to specified
email addresses. The network of scour monitoring sites is dynamic, with locations being
added and removed annually based on monitoring priority and the installation of scour
countermeasures. Instrumentation is subject to damage by high flows, debris and ice, and
repairs at some sites can only be made during low-flow conditions.

In 2002 one sonar scour monitor was installed at the Old Glenn Highway Bridge over the
Knik River near Palmer (Figure 9.23) (Conaway 2006b). There are two bridges that cross
the Knik River at this location. The active bridge was built in 1975, is 505 ft (154 m) in
length, and is supported by two piers. The roadway approaches to the active bridge
significantly contract the channel. Approximately 98 ft (30 m) upstream is the original bridge
which is no longer open to vehicular traffic. Two guide banks extend upstream of both
bridges and route flow through a riprap lined bridge reach. All piers are approximately
aligned with the flow. The Knik River is a braided sand and gravel channel that transports
large quantities of sediment from the Knik Glacier. The braided channel narrows from
approximately 3 mi (4.8 km) wide at the glacier mouth to 0.07 mi (0.12 km) at the Old Glenn
Highway Bridge where the channel is subject to a 4:1 contraction during summer high flows.
9.31

Figure 9.22. Active streambed scour monitoring locations in Alaska (Conaway 2006a).


9.32

Figure 9.23. Oblique aerial photograph of the Knik River Old Glenn Highway bridges during
a summer high flow (Courtesy of U.S. Geological Survey).

The right-bank pier of the new bridge was instrumented with a retractable, pier-mounted
sonar monitor. This retractable arm was designed to prevent ice and debris flows from
damaging the sonar bracket, as had occurred in other scour monitoring installations in
Alaska. Stage data were measured by a nearby USGS stream gage. The sonar was
mounted at an angle on the side of pier near the nose in order to collect data just upstream
of the pier footing. Data are collected every 30 minutes and transmitted every 6 hours via
satellite. When bed elevation or stage thresholds are exceeded, data transmissions
increase in frequency. The Knik River was the only site within the monitoring network that
had large changes in bed elevation each year. Annual scour ranged from 17.2 to 20 ft (5.2
m to 6.0 m). These near real-time data for the Knik River and other sites in Alaska are
available on the USGS website.

New York Installations. NYSDOT has installed twenty-seven sonar scour monitors at three
bridges on the South Shore of Long Island in Nassau and Suffolk Counties in New York
(Hunt 2003). Wantagh Parkway over Goose Creek is a 93 ft (28.3 m) bascule bridge (Figure
9.24), Wantagh Parkway over Sloop Channel was a 576 ft (175.6 m) long bridge, and Robert
Moses Causeway over Fire Island Inlet, a 1,068 ft (326 m) bridge (Figure 9.25). These have
served as both short and long-term solutions to the scour problems at these bridges. In
1998, following a partial pier collapse at Wantagh Parkway over Goose Creek, it was found
that the streambed at one pier had experienced approximately 29 ft (8.8m) of localized scour
since it was built in 1929. In order to ensure that these bascule piers were safe, several
options were investigated and a scour monitoring system and program was designed for the
bridge.

9.33
A nearby bridge, Wantagh Parkway over Sloop Channel was also examined. It was found to
have similar problems with respect to scour of the piers. As a result, four scour monitors
were installed at the bascule piers of Goose Creek, and ten monitors were installed at Sloop
Channel. In addition, a water stage sensor was installed at each bridge. The scour monitors
were approved by NYSDOT within one week of the 1998 failure, and they were designed,
custom-built and arrived at the site ten weeks later. The sonar mounting brackets were
made of stainless steel due to the harsh tidal environment. For data retrieval the system
employed remote telemetry via a modem and telephone landline. The power was supplied
using solar panels for the fixed bridge at Sloop Channel and using the electrical system on
the bascule bridge at Goose Creek.


Figure 9.24. Conduit to sonar scour monitor at Wantagh Parkway
over Goose Creek.

A scour monitoring program and manual were developed for the Wantagh Parkway Bridges.
This was the first procedural manual to be developed for scour monitors. The manual
provided the opportunity to work through various scenarios should these bridges continue to
experience scour. The program included round-the-clock monitoring even during storms. It
included critical streambed elevations for each pier, procedures for normal and emergency
situations, a Plan of Action should certain scour elevations be reached and troubleshooting,
maintenance and servicing instructions. An effective communication system for all
responsible parties was established.

The installation of sonar scour monitors at Robert Moses Causeway over Fire Island Inlet is
a long-term solution to the scour issues at that bridge. The flow rate was estimated to be
over 492,000 cfs (13,932 m
3
/sec) for the 100-year storm. Riprap scour protection had been
placed at some piers over the years, and according to the FHWA guidance, riprap should be
monitored when used as a countermeasure at piers. In 2001, sonar scour monitors were
placed at 13 piers, a water stage sensor was installed, and the Long Island scour monitoring
manual was revised to include the new system. This was a complex design and installation
due to the proximity of the bridge to the Atlantic Ocean, the deep-water conditions, the pier
configurations and the high flow rates. In order to ensure that the underwater sonar
brackets could clear the pier footings to measure the streambed elevations, this design
incorporated a new type of adjustable tripod stainless steel bracket (Figure 9.26).
9.34









Figure 9.25. Solar panel, remote station and (inset) conduit to sonar monitor at Robert
Moses Causeway over Fire Island Inlet.





Figure 9.26. Adjustable stainless steel sonar mounting
bracket prior to underwater installation.

9.35
Summary New York Installations. The scour monitoring systems at Goose Creek and Fire
Island have been in operation for since 1998 and 2001, respectively (Hunt and Price 2004).
When the Sloop Channel Bridge was replaced in 1999, the monitoring system was salvaged
and has been used for spare parts for the other bridges. The scour monitoring program
includes the routine monitoring of these bridges, including data acquisition and analysis;
round-the-clock monitoring during scour critical events; the preparation of bi-weekly graphs
of the streambed elevations and tide gage data; periodic data reduction analyses and
graphs; and routine maintenance, inspection, and repairs. In 2004, a total refurbishment of
the Goose Creek system was completed. This included the installation of the latest operating
system software and a new bracket for the sonar transducer at one monitor location. An
underwater contractor installed the new bracket and also strengthened the scour monitor
mountings at the other three pier locations. The condition of the scour monitors and the
accuracy of their streambed elevation readings are checked during the regularly scheduled
diving inspections at each bridge. Also, substantial marine growth and/or debris on the
underwater components is cleared away during these inspections.

California, Arizona and Nevada Installations. In preparation for El Nio driven storm events,
a variety of instruments were installed at bridges in the southwest in late 1997 and early
1998. Five bridges were instrumented in California, five in Arizona and four in Nevada.
The equipment included automated sliding collar devices, low-cost sonar, multi-channel
sonar, float-out transmitters and sliding rod devices (Figures 9.27 and 9.28). These
installations provided an opportunity to test a number of new concepts, including 2- and 4-
channel sonar devices, application of early warning concepts (by defining threshold scour
levels and automated calls to pagers when that threshold was exceeded), and development
and refinement of the float-out instrument concept.


Figure 9.27. Installation of a sonar scour monitor on Salinas River Bridge near Soledad,
California (Highway 101) by CALTRANS.

9.36

Figure 9.28. Close up of sonar scour monitor on Salinas River Bridge near Soledad, CA.

To support the California, Arizona, and Nevada installations, a buried transmitter float-out
device was developed for application on bridge piers over ephemeral stream systems. As
summarized in Section 9.3.4, this device consists of a radio transmitter buried in the channel
bed at a pre-determined depth. When the scour reaches that depth, the float out device
rises to the surface and begins transmitting a radio signal that is detected by a receiver in an
instrument shelter on the bridge. Installation requires using a conventional drill rig with a
hollow stem auger (Figure 9.29). After the auger reaches the desired depth, the float out
transmitter is dropped down the center of the auger (Figure 9.30). Substrate material refills
the hole as the auger is withdrawn.

The float out device can be monitored by the same type of instrument shelter/data logger
currently being used to telemeter low-cost fathometer or automated sliding collar data. The
instrument shelter contains the data logger, cell-phone telemetry, and a solar panel/gell-cell
battery for power (Figure 9.31). The data logger monitors the sliding collar and sonar scour
instruments, taking readings every hour and transmitting the data once per day to a
computer at a central location (e.g., DOT District). A threshold elevation is defined that,
when reached, initiates a phone call to a pager network. The bridge number is transmitted
as a numeric page, allowing identification of the bridge where scour has occurred. The float
out devices are monitored continuously, and if one of these devices floats to the surface, a
similar call is automatically made to the pager network.

Although the float out devices had not been tested extensively in the field, in late 1997 and
early 1998 more than 40 float-out devices were installed at bridges in Arizona (4 bridges),
California (1 bridge), and Nevada (4 bridges). Most devices were installed at various levels
below the streambed as described above; however, several devices at bridges in Nevada
were buried in riprap at the base of bridge piers to monitor riprap stability (Figure 9.32).
9.37

Figure 9.29. CALTRANS drilling with hollow stem auger for installation of float out devices
at Salinas River Bridge (Highway 101) near Soledad, CA.





Figure 9.30. Installation of float out device on Salinas River Bridge near Soledad, CA.
9.38



Figure 9.31. Typical instrument shelter with data logger, cell-phone telemetry, and a
solar panel/gel-cell for power.




Figure 9.32. Installation of a float out device by Nevada DOT to monitor riprap stability.

9.39
One of the bridges instrumented experienced several scour events that triggered threshold
warnings during February 1998. In one case the automated sliding collar dropped 5 ft (1.5
m) causing a pager call-out. Portable sonar measurements confirmed the scour recorded by
the sliding collar. Several days later, another pager call-out occurred from a float-out device
buried about 13 ft (4 m) below the streambed.

In both cases, the critical scour depth was about 20 ft (6 m) below the streambed. However;
pager call-out was ineffective in alerting maintenance personnel during non-office hours and
no emergency action was called for to insure public and/or bridge safety. Consequently, a
programmed voice synthesizer call-out to human-operated 24-hour communications centers
was implemented at other bridges. This illustrates the importance of effective and well-
defined communication procedures, and the on-going need for comprehensive scour training
at all levels of responsibility.

9.40


























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10.1
CHAPTER 10

REFERENCES


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presented at the 72nd Annual TRB meeting in Washington, D.C., January.

Avila, C.M.C., Racin, J.A. and Davies, P., 1999, "Talk to Your Bridges and They Will Talk
Back Caltrans Bridge Scour Monitoring Program," Proceedings from the ASCE Hydraulic
Engineering Conference, Seattle, WA.

Ayres Associates Inc, 1999, "A-Jacks Concrete Armor Units Channel Lining and Pier Scour
Design Manual," Prepared for Armortec, Inc., Bowling Green, KY.

Barkdoll, B.D., Ettema, R., and Melville, B.W., 2007, "Countermeasures to Protect Bridge
Abutments from Scour," NCHRP Report 587, Transportation Research Board, National
Academies of Science, Washington, D.C.

Barksdale, G. 1994, "Bridge Scour During the 1993 Floods in the Upper Mississippi River
Basin," proceeding of the 1994 ASCE Hydraulic Engineering Conference.

Bath, W.R., 1999, "Remote Methods of Underwater Inspection of Bridge Structures," FHWA
Report RD-99-100, Turner-Fairbank Highway Research Center, McLean, VA.

Begin, Z.B., 1981, "Stream Curvature and Bank Erosion: A Model Based on the Momentum
Equation," Journal of Geology, Vol. 89, pp. 497-504.

Bentrup, G. and Hoag, J.C., 1998, "The Practical Streambank Bioengineering Guide: User's
Guide for Natural Streambank Stabilization Techniques in the Arid and Semi-Arid Great
Basin and Intermountain West," Interagency Riparian/Wetland Plant Development Project,
USDA Natural Resources Conservation Service.

Bertoldi, D. and Kilgore, R., 1993, "Tetrapods as a Scour Countermeasure, Proceedings
Hydraulic Engineering '93," Vol. 2, 25-30 July 1993, San Francisco, CA, pp. 1385-1390.

Bertoldi, D.A., Jones, S.J., Stein, S.M., Kilgore, R.T., and Atayee, A.T., 1996, "An
Experimental Study of Scour Protection Alternatives at Bridge Piers," FHWA-RD-95-187,
Federal Highway Administration, Washington, D.C.

Blodgett, J.C. and McConaughy, C.E., 1986, "Rock Riprap Design for Protection of Stream
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Blodgett, J.C. and McConaughy, C.E., 1986, "Rock Riprap Design for Protection of Stream
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10.2
Bradley, J.N., 1978, "Hydraulics of Bridge Waterways," Hydraulic Design Series No. 1, U.S.
Department of Transportation, FHWA, Washington, D.C.

Brebner, A., 1978, "Performance of Dolos Blocks in an Open Channel Situation,"
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Brice, J.C., 1981, "Stability of Relocated Stream Channels," FHWA/RD-80/158, Federal
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Brice, J.C., 1984, "Assessment of Channel Stability at Bridge Sites," Transportation
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Brice, J.C. and Blodgett, J.C., 1978, "Countermeasure for Hydraulic Problems at Bridges,
Volumes 1 and 2," FHWA-RD-78-164 and 163, USGS, Menlo Park, CA.

Brown, S.A., 1985, "Streambank Stabilization Measures for Highway Engineers," FHWA/RD-
84/100, Federal Highway Administration, Washington, D.C.

Brown, S.A. and Clyde, E.S., 1989, "Design of Riprap Revetment," Hydraulic Engineering
Circular No.11, FHWA-IP-89-016, prepared for the Federal Highway Administration,
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Brown, S.A., R.S. McQuivey, and T.N. Keefer, 1980, "Stream Channel Degradation and
Aggradation: Analysis of Impacts to Highway Crossings," FHWA/RD-80-159, Federal
Highway Administration, Washington, D.C.

Butch, G., 1996, "Evaluation of Scour Monitoring Instruments in New York," 1996 North
American Water and Environment Congress '96--Bridge Scour Symposium, Hydraulics
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in Wadable Gravel- and Cobble-Bed Streams for Analyses in Sediment Transport,
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of Agriculture, Forest Service, Rocky Mountain Research Station, Fort Collins, CO, 428 p.

Burns, R.S., Fotherby, L.M., Ruff, J.F., and Carey, J.M., 1996, "Design Example for Bridge
Pier Scour Measures Using Toskanes," Transportation Research Board, Transportation
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Chiew, Y.M., 1995, "Mechanics of Riprap Failure at Bridge Piers," ASCE Journal of Hydraulic
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Colson, B.E. and Wilson, K.V., 1973, "Hydraulic Performance of Bridges - Efficiency of
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Comit Europen Normalisation (CEN), 2002, "European Standard for Armourstone," Report
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10.3
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Conaway, J.S., 2006(b), "Comparison of Long-Term Streambed Scour Monitoring Data with
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rd
International
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Crumrine, M.D., Lee, K.K., and Kittelson, R.L., 1996, "Bridge-Scour Instrumentation and
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Eisenhauer, N.O. and Rossbach, B. 1999, "Testing the Effectiveness of Scour Counter-
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(BAW), Karlsruhe, Germany, proceedings of the Transportation Research Board, 5th
International Bridge Engineering Conference, April 3-5, 2000, Tampa, FL.

Escarameia, M., 1998, "River and Channel Revetments - A Design Manual," Environment
Agency R&D Publication 16, Thomas Telford, London.

Federal Highway Administration, 1981, "Standard Specifications for Construction of Roads
and Bridges on Federal Highway Projects," U.S. Department of Transportation, FP-79
(Revised June 1981, Washington D.C.

Federal Highway Administration, 1995, "Recording and Coding Guide for the Structure
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Federal Highway Administration, 1998, "Scour Monitoring and Instrumentation,"
Demonstration Project 97 Participants Workbook, FHWA-SA-96-036, Office of Technology
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Federal Highway Administration, 2001, "Revision of Coding Guide, Item 113 Scour Critical
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Federal Interagency Stream Restoration Working Group (FISRWG), 1998, "Stream Corridor
Restoration: Principles, Processes, and Practices," U.S. Dept. of Commerce, National
Technical Information Service, Washington, D.C.

Fotherby, L.M., 1992, "Footings, Mats, Grout Bags, and Tetrapods; Protection Methods
Against Local Scour at Bridge Piers," MS Thesis, Colorado State University, Fort Collins,
CO.

Fotherby, L.M., 1993, "Alternatives to Riprap for Protection Against Local Scour at Bridge
Piers," Transportation Research Record 1420, pp. 32-38.

Fotherby, L.M., 1995, "Scour Protection at Bridge Piers: Riprap and Concrete Armor Units,"
Ph.D. Dissertation, Colorado State University, Fort Collins, CO.

Fotherby, L.M. and Ruff, J.F., 1998, "Sizing Riprap and Concrete Armor Units at Bridge
Piers," Transportation Research Board, 77th Annual Meeting, 11-15 January 1998,
Washington, D.C., Paper No. 980633.
10.4
Fotherby, L.M. and Ruff, J.F., 1999, "Riprap and Concrete Armor to Prevent Pier Scour," In:
Richardson, E.V. and Lagasse, P.F. (Eds.), Stream Stability and Scour at Highway Bridges,
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EMRRP Technical Notes Collection, ERDC TN-EMRRP-SR-22, U.S. Army Engineer
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Galay, V.J., Yaremko, E.K, and Quazi, M.E., 1987, "River Bed Scour and Construction of
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Ginsberg, A. and Schall, J.D., 1998, "Impact of Scour Monitoring and Instrumentation in the
United States," ASCE, Proceedings of Water Resources Engineering '98, Memphis, TN,
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Gorin, S.R. and Haeni, F.P., 1989, "Use of Surface-Geophysical Methods to Assess
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Gray, D.H. and Sotir, R., 1996, "Biotechnical and Soil Bioengineering Slope Stabilization,"
John Wiley and Sons, New York, NY.

Harris County Flood Control District, 2001, "Design Manual for Articulating Concrete Block
Systems," prepared by Ayres Associates, Project No. 32-0366.00, Fort Collins, CO.

Heibaum, M.H., 2000, "Scour Countermeasures Using Geosynthetics and Partially Grouted
Riprap," Federal Waterways Engineering and Research Institute (BAW), Karlsruhe,
Germany, proceedings of the Transportation Research Board, 5
th
International Bridge
Engineering Conference, April 3-5, 2000, Tampa, FL.

Heibaum, M.H., 2002, "Geotechnical Parameters of Scouring and Scour Countermeasures,"
Mitteilungsblatt der Bundesanstalt fr Wasserbau Nr. 85. (J. Federal Waterways Engineering
and Research Institute, No. 85), Karlsruhe, Germany.

Henry, K.S., 1999, "Geotextile Reinforcement of Low-Bearing-Capacity Soils," Special
Report 99-7, U.S. Army Corps of Engineers Cold Regions Research and Engineering
Laboratory, Hanover, NH 03755.

Hoffmans, G.J.C.M. and Verheij, H.J., 1997, "Scour Manual," A.A. Balkema: Rotterdam,
Brookfield.

Hunt, B.E., 2003, "Pier Pressure," Bridge Design & Engineering, Issue No. 30, First Quarter.

Hunt, B.E., 2008, "Practices for Monitoring Scour Critical Bridges," NCHRP Project 20-5,
Topic 36-02, Transportation Research Board, National Academies of Science, Washington,
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10.5
Hunt, B.E. and Price, G.R., 2004, "Scour Monitoring Lessons Learned," Proceedings from
the 2
nd
International Conference on Scour and Erosion, Nanyang Technological University,
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Jones, J.S., 1989, "Laboratory Studies of the Effect of Footings and Pile Groups on Bridge
Pier Scour," Proceeding of 1989 Bridge Scour Symposium, FHWA, Washington, D.C.

Jones, J.S., Bertoldi, D., and Stein, S., 1995, "Alternative Scour Countermeasures,"
Proceedings ASCE 1st International Conference Water Resources Engineering, Vol. 2, 14-
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Keeley, J.W., 1971, "Bank Protection and River Control in Oklahoma," Federal Highway
Administration, Washington, D.C.

Kellerhals, R. and Bray, D., 1971, "Sampling Procedures for Coarse Fluvial Sediments,"
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Laboratory Report," Federal Highway Administration, Report No. FHWA-HRT-07-026.

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Englewood Cliffs, NJ, 761 p.

Lagasse, P.F., Thompson, P.L., and Sabol, S.A., 1995, "Guarding Against Scour," Civil
Engineering, American Society of Civil Engineers, June, pp. 56-59.

Lagasse, P.F., Richardson, E.V., and Schall, J.D., 1998, "Instrumentation for Monitoring
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Lagasse, P.F., Nordin, C.F., Schall, J.D., and Sabol, G.V., 1991, "Scour Monitoring Devices
for Bridges," Third Bridge Engineering Conference, Transportation Research Record No.
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10.6
Lagasse, P.F., Richardson, E.V., Schall, J.D., Price, G.R., 1997, "Instrumentation for
Measuring Scour at Bridge Piers and Abutments," NCHRP Report 396, Transportation
Research Board, National Academies of Science, Washington, D.C.

Lagasse, P.F., Zevenbergen, L.W., Schall, J.D., and Clopper, P.E., 2001b. "Bridge Scour
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Lagasse, P.F., Clopper, P.E., Zevenbergen, L.W., and Ruff, J.F., 2006, "Riprap Design
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Lauchlan, C.S., 1999, "Pier Scour Countermeasures," Ph.D. Thesis, University of Auckland,
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Maynord, S.T., 1996, "Toe-Scour Estimation in Stabilized Bendways," ASCE Journal of
Hydraulic Engineering, Vol. 122, No. 8, pp. 460-464.

McCullah, J.A. and Gray, D., 2005, "Environmentally Sensitive Channel- and Bank-
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MDSHA (Maryland State Highway Agency) 2005, "Office of Bridge Development Manual for
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Melville B.W., Ettema, R., and Jain, S.C., 1989, "Measurement of Bridge Scour," Bridge
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10.7
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Molinas, A., 1990, "Bridge Stream Tube Model for Alluvial River Simulation" (BRI-STARS),
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Morgan, R.P.C., Collins, A.J., Hann, M.J., Morris, J., Dunderdale, J.A.L., and Gowing,
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Mueller, D.S. and Landers, M.N., 1999, "Portable Instrumentation for Real-Time
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National Transportation Safety Board (NTSB), 1988, "Collapse of the New York Thruway (I-
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Pagn-Ortiz, Jorge E., 1991, "Stability of Rock Riprap for Protection at the Toe of Abutments
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Parker, G., Toro-Escobar, C., and Voigt, R.L. Jr., 1998, "Countermeasures to Protect Bridge
Piers from Scour," Users Guide (revised 1999) and Final Report, NCHRP Project 24-07,
prepared for Transportation Research Board by St. Anthony Falls Laboratory, University of
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Parola, A.C., 1993, "Stability of Riprap at Bridge Piers," ASCE, Journal of Hydraulic
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Pearson, D.R., Jones, J.S., and Stein, S.M., 2000, "Risk-Based Design of Bridge Scour
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Racin, J.A., Hoover, T.P., and Crossett-Avila, C.M., 2000, "California Bank and Shore Rock
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Resource Consultants & Engineers (RCE), 1994, "Sediment and Erosion Design Guide,"
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Richardson, E.V. and Simons, D.B. 1984, "Use of Spurs and Guidebanks for Highway
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10.8
Richardson, E.V. and Davis, S.R., 2001, "Evaluating Scour at Bridges," Hydraulic
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Richardson, E.V., Simons, D.B., and Lagasse, P.F., 2001, "River Engineering for Highway
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Schall, J.D. and Price, G.R., 2004, "Portable Scour Monitoring Equipment," NCHRP Report
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Schall, J.D., Price, G.R., Fisher, G.A., Lagasse, P.F., and Richardson, E.V., 1997a,
"Magnetic Sliding Collar Scour Monitor-Installation, Operation, and Fabrication Manual,"
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Schall, J.D., Price, G.R., Fisher, G.A., Lagasse, P.F., and Richardson, E.V., 1997b, "Sonar
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10.9
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10.10
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APPENDICES








































A.1









APPENDIX A

METRIC SYSTEM, CONVERSION FACTORS, AND WATER PROPERTIES

A.2
























(page intentionally left blank)

A.3
APPENDIX A

Metric System, Conversion Factors, and Water Properties


The following information is summarized from the Federal Highway Administration, National
Highway Institute (NHI) Course No. 12301, "Metric (SI) Training for Highway Agencies." For
additional information, refer to the Participant Notebook for NHI Course No. 12301.

In SI there are seven base units, many derived units and two supplemental units (Table A.1).
Base units uniquely describe a property requiring measurement. One of the most common
units in civil engineering is length, with a base unit of meters in SI. Decimal multiples of
meter include the kilometer (1000m), the centimeter (1m/100) and the millimeter (1 m/1000).
The second base unit relevant to highway applications is the kilogram, a measure of mass
which is the inertial of an object. There is a subtle difference between mass and weight. In
SI, mass is a base unit, while weight is a derived quantity related to mass and the
acceleration of gravity, sometimes referred to as the force of gravity. In SI the unit of mass is
the kilogram and the unit of weight/force is the newton. Table A.2 illustrates the relationship
of mass and weight. The unit of time is the same in SI as in the English system (seconds).
The measurement of temperature is Centigrade. The following equation converts Fahrenheit
temperatures to Centigrade, EC = 5/9 (EF - 32).

Derived units are formed by combining base units to express other characteristics. Common
derived units in highway drainage engineering include area, volume, velocity, and density.
Some derived units have special names (Table A.3).

Table A.4 provides useful conversion factors from English to SI units. The symbols used in
this table for metric units, including the use of upper and lower case (e.g., kilometer is "km"
and a newton is "N") are the standards that should be followed. Table A.5 provides the
standard SI prefixes and their definitions.

Table A.6 provides physical properties of water at atmospheric pressure in SI system of
units. Table A.7 gives the sediment grade scale and Table A.8 gives some common
equivalent hydraulic units.

A.4


Table A.1. Overview of SI Units.



Units

Symbol

Base units
length
mass
time
temperature*
electrical current
luminous intensity
amount of material


meter
kilogram
second
kelvin
ampere
candela
mole


m
kg
s
K
A
cd
mol

Derived units





Supplementary units
angles in the plane
solid angles


radian
steradian


rad
sr

*Use degrees Celsius (EC), which has a more common usage than kelvin.






Table A.2. Relationship of Mass and Weight.

Mass
Weight or
Force of
Gravity

Force
English slug
pound-mass
pound
pound-force
pound
pound-force
metric kilogram newton newton



A.5

Table A.3. Derived Units With Special Names.
Quantity Name Symbol Expression
Frequency hertz Hz s
-1

Force newton N kg A m/s
2

Pressure, stress pascal Pa N/m
2

Energy, work, quantity of heat joule J N A m
Power, radiant flux watt W J/s
Electric charge, quantity coulomb C A A s
Electric potential volt V W/A
Capacitance farad F C/V
Electric resistance ohm V/A
Electric conductance siemens S A/V
Magnetic flux weber Wb V A s
Magnetic flux density tesla T Wb/m
2

Inductance henry H Wb/A
Luminous flux lumen lm cd A sr
Illuminance lux lx lm/m
2





























A.6


Table A.4. Useful Conversion Factors.

Quantity
From English
Units
To Metric
Units
Multiplied
By*
Length mile
yard
foot
inch
km
m
m
mm
1.609
0.9144
0.3048
25.40
Area square mile
acre
acre
square yard
square foot
square inch
km
2

m
2

hectare
m
2

m
2

mm
2

2.590
4047
0.4047
0.8361
0.09290
645.2
Volume acre foot
cubic yard
cubic foot
cubic foot
100 board feet
gallon
cubic inch
m
3

m
3

m
3

L (1000 cm
3
)
m
3

L (1000 cm
3
)
cm
3

1233
0.7646
0.02832
28.32
0.2360
3.785
16.39
Mass lb
kip (1000 lb)
kg
metric ton (1000
kg)
0.4536
0.4536
Mass/unit length plf kg/m 1.488
Mass/unit area
psf

kg/m
2


4.882
Mass density pcf kg/m
3
16.02
Force lb
kip
N
kN
4.448
4.448
Force/unit length plf
klf
N/m
kN/m
14.59
14.59
Pressure, stress,
modulus of elasticity
psf
ksf
psi
ksi
Pa
kPa
kPa
MPa
47.88
47.88
6.895
6.895
Bending moment,
torque, moment of
force
ft-lb
ft-kip
N A m
kN A m
1.356
1.356
Moment of mass lb A ft m 0.1383
Moment of inertia lb A ft
2
kg A m
2
0.04214
Second moment of
area
in
4
mm
4
416200
Section modulus in
3
mm
3
16390
Power ton (refrig)
Btu/s
hp (electric)
Btu/h
kW
kW
W
W
3.517
1.054
745.7
0.2931
*4 significant figures; underline denotes exact conversion
A.7


Table A.4. Useful Conversion Factors (continued).

Quantity

From English
Units

To Metric Units

Multiplied by*

Volume rate of flow

ft
3
/s
cfm
cfm
mgd

m
3
/s
m
3
/s
L/s
m
3
/s

0.02832
0.0004719
0.4719
0.0438

Velocity, speed

ft/s

m/s

0.3048

Acceleration

f/s
2


m/s
2


0.3048

Momentum

lb A ft/sec

kg A m/s

0.1383

Angular momentum

lb A ft
2
/s

kg A m
2
/s

0.04214

Plane angle

degree

rad
mrad

0.01745
17.45

*4 significant figures; underline denotes exact conversion








Table A.5. Prefixes.
Submultiples Multiples
deci 10
-1
d deka 10
1
da
centi 10
-2
c hecto 10
2
h
milli 10
-3
m kilo 10
3
k
micro 10
-6
mega 10
6
M
nano 10
-9
n giga 10
9
G
pica 10
-12
p tera 10
12
T
femto 10
-15
f peta 10
15
P
atto 10
-18
a exa 10
18
E
zepto 10
-21
z zetta 10
21
Z
yocto 10
-24
y yotto 10
24
Y

A.8





A.9

A.10

A.11

A.12
























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B.1









APPENDIX B

STANDARD TEMPLATE FOR A PLAN OF ACTION

B.2
B.3
STANDARD TEMPLATE FOR A PLAN OF ACTION

B.1 Overview

In order to facilitate the development of a POA, the FHWA has created a "standard"
template for bridges that are scour critical. This template includes the minimum information
recommended by FHWA for a POA.

The template is intended to be a guide and tool for bridge owners to use in developing their
POAs. The template provides the program manager with a summary of the type of
information required to develop a plan of action for bridges that are scour critical or have
unknown foundations.

All the fields in the template may be modified so that local terminology is employed, unique
information may be added regarding local and site-specific scour and stream stability
concerns, and local sources of information may be included. The electronic Microsoft Word
document template may be downloaded from the FHWA website:
http://www.fhwa.dot.gov/engineering/hydraulics/bridgehyd/poa.cfm

Blocks in this template will expand automatically to allow additional space. Where check
boxes are provided, they can be checked by double-clicking on the box and selecting the
"checked" option.

To provide guidance and training on preparation of a POA for scour critical bridges, the
FHWA National Highway Institute has prepared an on-line module (NHI Course No. 135085)
which includes the suggested template for a POA and illustrates its application to field case
studies. This training module can be accessed (see site reference, p. 2.6).

A state's Bridge Management System is a useful source of data for developing a POA.
Many DOTs are now using information technology (IT) systems that provide immediate
access via the bridge engineers desktop computer to an integrated system of bridge
management information and data bases. Much of the information outlined in the template
may be obtained from these systems.

B.2 Executive Summary

The standard template contains ten sections. Sections 1 through 4 are intended as an
executive summary for the busy reviewer/manager who may not need the details of Sections
5 through 10, and show:

Section 1: General information
Section 2: Who prepared the POA
Section 3: The source of the problem
Section 4: What actions are recommended and their status

To assist in completing a POA using the template, the remaining sections of this appendix
contain general guidance for each section of the template. Note that an abbreviated set of
instructions is appended to the template.

B.4
B.3 Standard Template Sections

Section 1 General Information

Section 1 of the POA Standard Template covers general information about the bridge. This
information is usually available from the bridge owners Bridge Management System or
bridge-specific files.

The bridge replacement information provides a framework for decisions regarding the need
for a structural or hydraulic countermeasure.

The bridge type information provides insight on how the bridge could fail, if significant scour
were to occur. For example, a simply-supported span bridge can fail suddenly, so the bridge
should be closed as soon as scour becomes close to critical. Bridges with structural
redundancy may allow more time to respond to an emergency situation.

Also, identified in this section is whether the bridge provides service to emergency facilities
or is part of an evacuation route. This information is important in bridge closure plans, since
early communication of a bridge closure to emergency responders could help them
effectively detour their normal routes.

Section 2 Responsibility for POA

Section 2 of the POA Standard Template provides information on who are the people
responsible for preparing and maintaining the POA.

Author or Authors refers to the individual or company that developed the POA, such as
the State Bridge Maintenance Engineer or a consultant.

Concurrences on POA refers to the individuals or organizations which must concur with the
contents of the POA. The individuals or organizations may also refer to upper management
who approve funding or county officials and law enforcement agencies who must agree with
the bridge closure plans and the disruption the closure will have on the public. Gaining
concurrence before an emergency occurs helps to minimize inter-agency conflicts during an
emergency.

Also, in Section 2 is information on the POA update: who will do the update, when, and what
was updated. The POA should be reviewed and updated on a routine basis to ensure that
the contents of the plan are still valid.

Section 3 Scour Vulnerability

Section 3 of the POA Standard Template provides a summary of the scour status of the
bridge.

Section 3a shows the current scour coding. If Other is selected, the appropriate code would
be provided. For example, if the foundation is unknown U would be entered in the Other
field.

In Section 3b indicate how the Item 113 code was determined. If the bridge has a U code,
unknown must be written in the Other field.

B.5
Section 3c, provides space for a narrative description to summarize the information from the
scour evaluation. The template has been developed to expand automatically as information
is entered. Some items to include are:

Scour critical flood flow and scour depth
100- or 500-year flood flows
Overtopping flow

Section 3d provides space to summarize the scour history, including when, where and how
much scour or stream instability has been observed at the bridge. Also, information on scour
countermeasures previously installed at the bridge and their performance should be
included.

Section 4 Recommended Action(s)

Section 4 of the POA Standard Template contains highlights from the recommended actions
from Sections 6 and 7 of the template. Items 4a, 4b, and 4c refer to the main parts of the
POA monitoring program. Item 4d refers to the hydraulic or structural countermeasure
selected in Section 7. This section cannot be completed until Sections 6 and 7 are
completed.

Section 5 NBIS Coding Information

Section 5 of the POA Standard Template contains previous and current codes for the
hydraulic related items of the NBI. This information provides a quick indication of observed
or potential long-term problems or adverse trends that may affect the stability of the bridge
foundations.

The Inspection Date corresponds to the date of the inspection when the NBI items were
coded. If the Items were coded on a date different than the inspection date, this different
date is indicated in the Comments block.

For additional details on Items 113 and 60, see the FHWA Policy Memorandum Revision of
Coding Guide, Item 113 Scour Critical Bridges dated April 27, 2001.

Section 6 Monitoring Program

There are three types of countermeasures which should be considered as part of the
countermeasure program:

1. Monitoring
2. Hydraulic
3. Structural

A monitoring countermeasure can be considered a key component of a POA, either alone or
in combination with other countermeasures. Monitoring is highlighted in its own section,
Section 6, in the POA Standard Template. Section 6 is subdivided into three additional
subsections including:

Inspection Frequency
Fixed Monitoring
Flood Monitoring

B.6
The first subsection of Section 6 covers information on the frequency of inspection. Bridges
are usually inspected biennially. Bridge owners may choose to keep this schedule but may
also specify inspectors look at certain items at the bridge to ensure stability with regard to
scour. In this case, the Regular Inspection Program box would be selected along with a list
of the items to be watched. These items may include countermeasures, channel bed
elevations, signs of movement or settlement.

Some bridge owners may choose to increase the frequency of inspection to less than the 2-
year cycle. In this case, the Increased Inspection Frequency box would be checked and the
number of months between inspections would be indicated. Usually only items pertinent to
scour and stream stability would be observed and inspected.

Underwater inspections may also be required at the bridge. If the Underwater Inspection
cycle remains on the regular schedule, then the Underwater Inspection Required box would
be checked. If an increased cycle is needed, then the Increased Underwater Inspection
Frequency box would be selected and the months between inspections indicated. In both
cases the items for inspection and observation are indicated on the POA.

The second subsection of Section 6 covers information on Fixed Monitoring Devices. Fixed
monitoring devices can provide continuous information about scour at the bridge site (see
Chapter 9). This information can lead to early identification of potential scour problems.

The Fixed Monitoring box would be selected if a bridge owner opts to use fixed monitoring
devices at the bridge. The type of devices and location of devices would be described in the
plan. Details about the devices may be included in Attachment F to the POA.

In most cases, the fixed monitoring device can send information continuously from the bridge
site. However, this amount of information can become cumbersome, so most bridge owners
obtain or sample the information periodically. The sampling interval should be indicated on
the POA and can be modified during flood events. If modified, the rationale for the change
would be noted on the POA.

The information received from the fixed monitoring device should be reviewed for developing
scour problems. During normal flow, the information may be reviewed daily, weekly, or
monthly. During flood events, the review frequency may increase. The POA should detail
the frequency of review and identify who is conducting the review.

Scour Critical Criteria should be determined from a scour evaluation study. This criteria
should be indicated on the plan. Selecting an elevation higher than the scour critical
elevation ensures sufficient time needed to take action in protecting the traveling public and
possibly the bridge. This elevation, called the Scour Alert Criteria, should also be presented
on the POA.

The third subsection of Section 6 describes monitoring actions that should be implemented
during an actual flood event. If the bridge owner inspects the bridge visually during a flood,
then the Visual Inspection box is selected. Those individuals visually observing the bridge
may look for movement or settlement in the bridge or for a certain elevation of water, which
could trigger the actions prescribed in the POA. If some kind of instrumentation is used to
measure scour or water elevation, the Instrument box is selected and the applicable
instrumentation type indicated. Both the Visual Inspection and the Instrument boxes may be
selected.
B.7
The POA should document thresholds for the start and end of flood monitoring and note the
frequency of the monitoring (see Chapter 2, Section 2.1.4). These thresholds may include:

Flood discharge
Stage
Water surface elevation
Rainfall data

The POA should clearly describe the thresholds and how the threshold is determined. For
example, the threshold discharge or stage may be tied to a nearby USGS gage. Some
bridge owners have opted to mark their bridges with the threshold water surface elevations
to ensure inspectors know when action should be taken. The POA should also describe the
actions required when threshold values are reached.

The agency, department, position, or person responsible for inspecting or reviewing
instrumentation data should be listed at the end of Section 6 in the POA Standard Template.
Some bridge owners may have maintenance staff rather than bridge inspectors monitor the
bridge during a flood and then have bridge inspection staff conduct the post-flood inspection.
All staff with responsibilities in implementing the POA should be listed. More than one person
may be listed, especially to provide back up points-of-contact.

In some cases, the person at the bridge site during a flood event must confer with someone
of greater authority in order to decide to close the bridge. This decision maker should also
be listed in Section 6 of the POA Standard Template.

Finally, if action must be taken, the agency, department, position, or person responsible for
taking the action would be listed. For example, if the local law enforcement is to close the
bridge and set up the detour route, this agency is listed in Section 6 of the POA Standard
Template.

Section 7 Countermeasure Recommendations

Section 7 of the POA Standard Template summarizes the alternative countermeasures
considered for the bridge, as well as the final countermeasures selected and rationale (see
Chapter 2, Sections 2.3 2.5).

If a monitoring countermeasure were selected for the countermeasure program, the
Monitoring box would be selected. If a structural and/or hydraulic countermeasure were
selected, then the Structural/Hydraulic Scour Countermeasures Considered box would be
selected. Both boxes may be selected, if needed.

Under the Priority Ranking and Estimated Cost columns, the various countermeasures for
consideration along with their corresponding estimated costs. The selected
countermeasures are indicated and the reasons for selecting the countermeasures should
be explained on the Basis for selection line. Supporting information for the considered
countermeasures can be included in Attachment F. Sufficient information to help
independent reviewers understand the countermeasure selection process and the resulting
decisions should be included.

This section also includes boxes for indicating the countermeasure implementation project
type.
B.8
The last part of Section 7 of the POA Standard Template requires information on
agency/department/position/person responsible for designing and implementing the
countermeasure program. More than one person may be listed, if appropriate.

Target design and construction dates should are also required.

Section 8 Bridge Closure Plan

Section 8 of the POA Standard Template provides instructions for closing a bridge.
Specifically, this section should include:

Specific criteria indicating when to close the bridge
Who should close the bridge
Contact information, such as management or local law enforcement

Several examples of bridge closing conditions are provided. If the bridge owner has closure
instructions specified in another document, these instructions should be referenced on the
Emergency repair plans line.

The POA should also detail the process for reopening the bridge. In some cases, the bridge
may be reopened when the floodwater has receded sufficiently. In other cases, the bridge
will require inspection to ensure it is structurally sound. The reopening criteria should be
listed. The agency or person who will inspect or make the decision to reopen the bridge
should be identified in the plan.

Section 9 Detour Route

Section 9 of the POA Standard Template describes potential detour routes, if the bridge is
closed. The description should include route numbers, from/to locations, distances from
closed bridge, as well as any other pertinent information. A map of the detour routes should
be provided in Attachment E to the POA.

The bridges on the detour route should be listed in the Bridges on Detour Route table, along
with restrictive factors for each bridge on the detour route, such as:

Load restrictions
Clearance restrictions
Scour vulnerability condition

Additional items to present in Section 9 include:

Required traffic control equipment
Critical issues, such as flood overtopping vulnerability for bridges and roadways along
the detour route, waterway adequacy of the detour bridges, and lane restrictions
Authority to communicate with the media and public

Concurrence from local law enforcement agencies on the proposed detour and the closing
procedures should be obtained and copies of the POA provided to these agencies.
B.9
Detour routes are not set in stone, but information should be provided on potential detours.
Some factors to be considered in documenting in advance a detour route for a particular
bridge include:

Detours must be set up, taking into consideration the conditions existing at the time a
detour is needed. For some bridges, it may not be possible to foresee what these
conditions might be, which roads might be flooded, which bridges may already be closed,
what will be the load and clearance restrictions for the proposed detour route, etc.
DOT or bridge owner may have a designated department/office which is intimately
familiar with their bridges and road systems, and are in the best position to quickly
decide upon and coordinate a detour route for the given set of circumstances. This would
be noted in the POA.
Decisions are made on the basis of the conditions that exist at the time of the closure.

Consideration needs to be given to floods that may overtop bridges and approach roadways
on the detour route. Actual detour routes should be based on roadway and bridge network
status at the time the detour is proposed.

Section 10 Attachments

Section 10 is the final section of the template and contains the following Attachments.

Attachment A should reference or include all available boring logs, probes, construction
inspection and monitoring records, monitoring well readings, test pits, soil laboratory data,
and anecdotal information.

Attachment B should reference or include all available channel cross sections for the
bridge. Historic cross section comparisons, if available, should also be included.

Attachments C, D, and E are for documenting bridge elevation and plan views, and maps
necessary to show detour routes.

Attachment F should include documentation on scour countermeasure alternatives. A
comprehensive plan of action should provide enough information that an independent
reviewer could arrive at the same conclusion regarding the preferred countermeasure
alternative. For proposed scour countermeasures, a conceptual design should be prepared
and details including reference to any hydraulic, structural or geotechnical studies that have
been completed for the purpose of scour mitigation should be provided. Estimated costs of
all proposed scour countermeasures should also be provided.

Details and dates of any recent scour countermeasure that has been implemented to
address the current scour critical/unknown foundation status of the bridge should be
included. All applicable studies, lead agencies, subcontractors and as-builts should be noted
or included in Appendix F.

Attachment G may also reference or include historic and current aerial photographs of the
site.

Attachment H may be used for any additional information such as: (1) standard closing and
reopening procedures, (2) information on public transit, or (3) special circumstances such as
access to emergency facilities, evacuation routes, etc. In some situations, public
transportation (e.g., bus routes) may be of importance to the public with respect to detours.
If additional information is included, indicate this in Section 10 of the template.
B.10


























(page intentionally left blank)



SCOUR CRITICAL BRIDGE - PLAN OF ACTION
1. GENERAL INFORMATION

Structure number:


City, County, State:


Waterway:

Structure name:

State highway or facility carried:

Owner:

Year built: Year rebuilt:
Bridge replacement plans (if scheduled):
Anticipated opening date:
Structure type: Bridge Culvert
Structure size and description:
Foundations: Known, type: Depth: Unknown
Subsurface soil information (check all that apply): Non-cohesive Cohesive Rock
Bridge ADT: Year/ADT: % Trucks:
Does the bridge provide service to emergency facilities and/or an evacuation route (Y/N)?

If so, describe:
2. RESPONSIBILITY FOR POA
Author(s) of POA (name, title, agency/organization, telephone, pager, email):
Date:

Concurrences on POA (name, title, agency/organization, telephone, pager, email):


POA updated by (name, title, agency/organization): Date of update:
Items updated:
POA to be updated every months by (name, title, agency/organization):
Date of next update:
3. SCOUR VULNERABILITY
a. Current Item 113 Code: 3 2 1 Other:
b. Source of Scour Critical Code: Observed Assessment Calculated Other:
c. Scour Evaluation Summary:
d. Scour History:


4. RECOMMENDED ACTION(S) (see Sections 6 and 7)
Recommended Implemented

a. Increased Inspection Frequency Yes No Yes No

b. Fixed Monitoring Device(s) Yes No Yes No

c. Flood Monitoring Program Yes No Yes No

d. Hydraulic/Structural Countermeasures Yes No Yes No

5. NBIS CODING INFORMATION

Current Previous

Inspection date


Item 113 Scour Critical


Item 60 Substructure


Item 61 Channel & Channel Protection


Item 71 Waterway Adequacy


Comments: (drift, scour holes, etc. - depict in
sketches in Section 10)




6. MONITORING PROGRAM
Regular Inspection Program w/surveyed cross sections
Items to Watch:
Increased Inspection Frequency of mo. w/surveyed cross sections
Items to Watch:

Underwater Inspection Required
Items to Watch:
Increased Underwater Inspection Frequency of mo.
Items to Watch:

Fixed Monitoring Device(s)
Type of Instrument:
Installation location(s):
Sample Interval: 30 min. 1 hr. 6 hrs. 12 hrs. Other:
Frequency of data download and review: Daily Weekly Monthly Other

Scour alert criteria for each pier/abutment:
Scour critical criteria for each pier/abutment
Survey ties:
Criteria for termination of fixed monitoring program:

Flood Monitoring Program
Type: Visual inspection
Instrument (check all that apply):
Portable Geophysical Sonar Other:

Flood monitoring required: Yes No
Flood monitoring event defined by (check all that apply):
Discharge Stage
Elev. measured from Rainfall (in/mm) per (hour)
Flood forecasting information:
Flood warning system:

Frequency of flood monitoring: 1 hr. 3 hrs. 6 hrs. Other:

Post-flood monitoring required: No Yes, within days
Frequency of post-flood monitoring: Daily Weekly Monthly Other:
Criteria for termination of flood monitoring:
Criteria for termination of post-flood monitoring:
Scour alert criteria for each pier/abutment:
Scour critical criteria for each pier/abutment:

Note: Additional details for action(s) required may be included in Section 8.

Action(s) required if scour alert criteria detected (include notification and closure
procedures):
Action(s) required if scour critical criteria detected (include notification and closure
procedures):

Agency and department responsible for monitoring:
Contact person (include name, title, telephone, pager, e-mail):

7. COUNTERMEASURE RECOMMENDATIONS
Prioritize alternatives below. Include information on any hydraulic, structural or monitoring
countermeasures.

Monitoring countermeasure (see Section 6 and Section 10 - Attachment F)
Estimated cost $

Structural/hydraulic countermeasures (see Section 10 - Attachment F):
Priority Ranking Estimated cost
(1) $

(2)



$

(3) $

(4)
$
(5)
$


Basis for the selection of the preferred scour countermeasure:
Countermeasure implementation project type:
Proposed Construction Project Maintenance Project
Programmed Construction - Project Lead Agency:
Bridge Bureau Road Design Other

Agency and department responsible for countermeasure program (if different from Section 6
contact for monitoring):

Contact person (include name, title, telephone, pager, e-mail):

Target design completion date:

Target construction completion date:
Countermeasures already completed:
8. BRIDGE CLOSURE PLAN
Scour monitoring criteria for consideration of bridge closure:
Water surface elevation reaches at
Overtopping road or structure
Scour measurement results / Monitoring device (See Section 6)
Observed structure movement / Settlement
Discharge: cfs/cms
Flood forecast:
Other: Debris accumulation Movement of riprap/other armor protection
Loss of road embankment
Emergency repair plans (include source(s), contact(s), cost, installation directions):
Agency and department responsible for closure:
Contact persons (name, title, agency/organization, telephone, pager, email):
Criteria for re-opening the bridge:
Agency and person responsible for re-opening the bridge after inspection:
9. DETOUR ROUTE
Detour route description (route number, from/to, distance from bridge, etc.) - Include map in
Section 10, Attachment E.
Bridges on Detour Route:
Bridge Number Waterway
Sufficiency Rating/
Load Limitations
Item 113 Code





Traffic control equipment (detour signing and barriers) and location(s):


Additional considerations or critical issues (susceptibility to overtopping, limited waterway
adequacy, lane restrictions, etc.):

News release, other public notice (include authorized person(s), information to be provided
and limitations):


10. ATTACHMENTS

Please indicate which materials are being submitted with this POA:

Attachment A: Boring logs and/or other subsurface information

Attachment B: Cross sections from current and previous inspection reports

Attachment C: Bridge elevation showing existing streambed, foundation depth(s) and
observed and/or calculated scour depths

Attachment D: Plan view showing location of scour holes, debris, etc.

Attachment E: Map showing detour route(s)

Attachment F: Supporting documentation, calculations, estimates and conceptual designs
for scour countermeasures.

Attachment G: Photos

Attachment H: Other information:






INSTRUCTIONS FOR COMPLETING THE PLAN OF ACTION


The existing bridge management system in your state will provide much of the information
required to fill out this template. Note that all blocks in this template will expand automatically
to allow as much space as you require. All fields can be modified to accommodate local
terminology, as desired. Where check boxes are provided, they can be checked by double-
clicking on the box and selecting the checked option. If you include additional attachments,
please indicate this in Section 10.

Section 1

Foundations It is recommended that substructure depths be shown in the bridge elevation,
Attachment C (see Section 10). The minimum depth should be reported in Section 1 as a
worst-case condition.

Subsurface soil information If conditions vary with depth and/or between substructure units,
this should be noted and included in Attachments A and/or C (see Section 10).

Sections 1, 2, 3, and 4

These sections are intended as an executive summary for the reviewer/manager who may not
need the details of Sections 5 through 10, and show:

Section 1: General information
Section 2: Who prepared the POA
Section 3: The source of the problem
Section 4: What actions are recommended and their status

Section 3

Reasons why the bridge has been coded scour critical for Item 113:

Scour Critical
Aggressive stream or tidal waterway (high velocity, steep slope, deep flow)
Actively degrading channel
Bed material is easily eroded
Large angle of attack (> 10)
Significant overbank or floodplain flow (floodplain >50 m or 150 ft wide)
Possibility of bridge overtopping (potential for pressure flow through bridge)
Evidence of scour and/or degradation
Evidence of structural damage due to scour
Foundations are spread footings on erodible soil, shallow piles, or embedment unknown
Exposed footing in erodible material
Exposed piles with unknown or insufficient embedment
Loss of abutment and/or pier protection
No countermeasures or countermeasures in poor condition
Needs countermeasures immediately

Unknown Foundations
No record of foundation type (spread footing vs. piles)
Depth of foundation or pile embedment unknown
Condition of foundation or pile embedment unknown
Subsurface soil strata not documented

Section 5

This section highlights recent changes in the scour/hydraulics coding items as an indication of
potential problems or adverse trends. See FHWA Policy Memorandum on Revision of Coding
Guide, Item 113 Scour Critical Bridges dated April 27, 2001 for details on Items 113 and 60.
A link to this memorandum is provided below:

(www.fhwa.dot.gov/engineering/hydraulics/policymemo/revguide.cfm).

Section 6

Multiple individuals responsible for various monitoring activities may be listed, as appropriate.

Section 7

Guidance on the selection and design of scour countermeasures may be found in FHWA
Hydraulic Engineering Circular No. 23, Bridge Scour and Stream Instability Countermeasures,
Third Edition, 2009. To facilitate the selection of alternative scour countermeasures, a matrix
describing the various countermeasures and their attributes is presented in this circular. A link
to this document is provided below:

http://isddc.dot.gov/OLPFiles/FHWA/010592.pdf

Section 8

Standard closure and reopening procedures, if available, may be appended to the POA (see
Section 10, Attachment H).

Section 9

In some situations, public transportation (e.g., bus routes) may be of importance to the public,
and therefore could be included in the POA (see Section 10, Attachment H).



























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C.1









APPENDIX C

PIER SCOUR COUNTERMEASURE SELECTION METHODOLOGY

C.2
























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C.3
PIER SCOUR COUNTERMEASURE SELECTION METHODOLOGY

C.1 Overview

This selection methodology provides a quantitative assessment of the suitability of six
armoring-type countermeasures for pier scour based on selection factors that consider river
environment, construction considerations, maintenance, performance, and estimated life-
cycle cost (Lagasse et al. 2007). With the exception of life-cycle costs, the methodology
analyzes the design factors by stepping the user through a series of decision branches,
ultimately resulting in a site-specific numerical rating for each selection factor (see Section
3.2.5). The following countermeasures are evaluated by this methodology:

Standard (loose) riprap
Partially grouted riprap
Articulating concrete blocks
Gabion mattresses
Grout-filled mattresses
Grout-filled bags

To facilitate the decision-making process, the procedure was automated using a Microsoft

Excel spreadsheet format. In the spreadsheet, the decision-making process can easily be
modified to consider new situations or include additional information. Detailed directions are
included in the program file, and automated features are incorporated in the program to step
the user through the process.

C.2 Selection Index

Five factors are used to compute a Selection Index (SI0 for each countermeasure:

S1: Bed Material size and transport
S2: Severity of debris or ice loading
S3: Constructability constraints
S4: Inspection and maintenance requirements
LCC: Life-cycle costs

The selection Index is calculated as:

SI = (S1 x S2 x S3 x S4)/LCC (C.1)

The countermeasure that has the highest value of SI is considered to be most appropriate for
a given site, based not only on its suitability to the specific riverine and project site
conditions, but also in consideration of its economy. The approach is sensitive to
assumptions regarding initial construction cost, remaining service life, assumed frequency of
maintenance events, and extent of maintenance required. Each of these factors requires
experience and engineering judgment, as well as site- or region-specific information on the
cost of materials and delivery, construction practices, and prevailing labor rates. It should be
noted that the methodology can be used simply to rank the countermeasures in terms of
suitability alone by assuming that the life-cycle costs are the same for all countermeasures.

C.4
In Section 3.2.5 the five factors that compose the methodology are described in detail. Flow
charts illustrating selection factors S1 through S4 and reference to Excel spread sheet
support follows.

C.3 Additional Considerations and Support

Federal or state regulations that preclude and use of a particular countermeasure because of
environmental considerations and permitting issues are beyond the scope of NCHRP Project
24-07(2). The practitioner in any particular state must be aware of circumstances that may
warrant the exclusion of a countermeasure for consideration at a specific site.

A feature allowing the user to easily include an additional design consideration, such as
state-specific environmental concerns, to the computation of the Selection Index was added
to the Excel-based selection methodology program. Inclusion of an additional selection
criterion will require the user to assign values in the context of the selection factors for all
countermeasures considered.

In addition, a feature was added to the selection methodology Excel spreadsheet capability
to permit a user to introduce another countermeasure and generate selection factor values
for that countermeasure. Inclusion of an additional countermeasure will require the user to
assign values in the context of the design considerations and selection factors. The
supplementary countermeasure feature and design consideration feature can be used
independently or together, as described in the countermeasure selection Excel file available
on the TRB website (http://trb.org/news/blurb_detail.asp?id=7998).

C.5


C.6


C.7



C.8



D.1









APPENDIX D

RIPRAP INSPECTION RECORDING GUIDANCE

D.2

D.3
APPENDIX D

Riprap Inspection Recording Guidance


To guide the inspection of a riprap installation, a recording system is presented in this
appendix. This guidance establishes numerical ratings from 0 (worst) to 9 (best).
Recommended action items based on the numerical rating are also provided (Lagasse et al.
2006).

A single-digit code is used as indicated below to identify the current status of the rock riprap
regarding its condition compared to the design intent, and the immediacy of need for
maintenance activities to return it to the design condition.

This guidance covers riprap installations that may be: (1) located on stream banks for lateral
stream stability purposes; (2) placed against bridge piers or abutments for protection against
scour at the structure; (3) placed across the stream to provide vertical grade stabilization; or
(4) other applications in riverine environments (e.g., guide banks or spurs).

Code Description

U UNINSPECTABLE:
The riprap is uninspectable, due to burial by sediment, debris, or other
circumstance. Until the condition of the riprap can be reliably determined, a plan
of action should be developed that considers the degree of risk posed by
potential failure of the installation.

9 THE RIPRAP INSTALLATION IS STABLE:
Riprap stones are angular to subangular with no evidence of deterioration or
segregation of sizes; and the distribution of stone sizes and overall thickness of
riprap layer conform to design specifications; and there is no evidence of
displacement of individual stones.

8 THE RIPRAP INSTALLATION IS STABLE:
Riprap stones are angular to subangular with no evidence of deterioration or
segregation of sizes; and the distribution of stone sizes and overall thickness of
riprap layer conform to design specifications; and some displacement of
individual stones is evident, but only smaller sized particles significantly smaller
than the design d
50
size have moved.

7 THE RIPRAP INSTALLATION IS STABLE:
Evidence of some deterioration of stones due to surficial weathering (abrasion,
freeze-thaw or wet-dry spalling); and stone shape is primarily subangular.
OR
A minor decrease in overall layer thickness is evident, and/or particle
displacement noted with displaced particles approaching the design d
50
size;
and the geotextile or granular filter has NOT been exposed.

6 THE RIPRAP INSTALLATION HAS EXPERIENCED EROSION:
Individual stones are primarily subrounded in shape due to surficial weathering;
and the distribution of stone sizes still exhibits a d
50
particle greater than the
minimum allowable d
50
size.
OR

D.4
Minor decrease in overall layer thickness is evident; and some particles greater
than the design d
50
size have been displaced; and the geotextile or granular
filter has NOT been exposed.

5 THE RIPRAP INSTALLATION HAS EXPERIENCED EROSION:
Similar condition as Code 6, except that the geotextile or granular filter has
been exposed in local areas, or around the periphery of the installation. The
inspector should attempt to identify whether stone displacement has occurred
due to gravity slump or slide, or by hydraulic forces.

4 THE RIPRAP INSTALLATION HAS EXPERIENCED SIGNIFICANT EROSION:
Individual stones are subrounded to rounded in shape due to significant
deterioration, and the distribution of stone sizes exhibits a d
50
particle smaller
than the minimum allowable d
50
size.
OR
Significant decrease in overall layer thickness is evident in local areas; and
some particles greater than the design d
50
size have been displaced; and the
geotextile or granular filter has been exposed in local areas.

3 THE RIPRAP INSTALLATION IS UNSTABLE:
The riprap matrix consists primarily of stones smaller than the minimum
allowable d
50
particle size; and the overall layer thickness is less than 50% of
specification.
OR
A significant portion of the particles greater than the design d
50
size has been
displaced, and the geotextile or granular filter has been exposed over more
than 20% of the installation area.

2 THE RIPRAP INSTALLATION IS UNSTABLE:
The riprap matrix consists almost entirely of stones smaller than the minimum
allowable d
50
particle size; and the overall layer thickness is less than 2 particles
thick.
OR
Most of the particles greater than the design d
50
size has been displaced, and
the geotextile or granular filter has been exposed over more than 50% of the
installation area.

1 THE RIPRAP INSTALLATION IS ERODED AND CAN NO LONGER SERVE
ITS FUNCTION. IMMEDIATE MAINTENANCE IS REQUIRED:
Most of the riprap matrix has been displaced or is missing; and native subgrade
soil is exposed.
OR
Large patches or voids in the riprap matrix have been opened; and stones are
no longer in contact with structural elements.

0 THE RIPRAP INSTALLATION IS ESSENTIALLY GONE AND SCOUR IS
IMMINENT. IMMEDIATE MAINTENANCE IS REQUIRED:
The riprap has deteriorated to the point that it cannot perform its protective
function even in minor events.

Particle Size Distribution. During inspection, the existing particle size distribution should be
determined and compared with the design particle size distribution to assess whether the
riprap particles have deteriorated over time. NCHRP Report 568 (Lagasse et al. 2006) and
Design Guideline 4 provide guidance for determining particle size distribution in the field.

D.5
Recommended action.

Code U: The riprap cannot be inspected. A plan of action should be developed to determine
the condition of the installation. Possible remedies may include: removal of debris,
excavation during low flow, probing, or nondestructive testing using ground penetrating radar
or seismic methods.

Codes 9, 8, or 7: Continue periodic inspection program at the specified interval.

Codes 6, 5, or 4: Increase inspection frequency. The rating history of the installation should
be tracked to determine if a downward trend in the rating is evident. Depending on the
nature of the riprap application, the installation of monitoring instruments might be
considered.

Codes 3 or 2: The Maintenance Engineer's office should be notified and maintenance should
be scheduled. The cause of the low rating should be determined, and consideration given to
redesign and replacement. Materials other than standard riprap might be considered.

Code 1 or 0: The Maintenance Engineer's office should be notified immediately. Depending
upon the nature of the riprap application, other local officials and/or law enforcement
agencies may also need to be notified.
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