United States

Department of
Agriculture
Bridge Scour
Forest Service

Technology &
Evaluation:
Development
Program

7700—Transportation Systems
Screening, Analysis,
September 1998
9877 1207—SDTDC & Countermeasures
Bridge Scour
Evaluation:
Screening, Analysis,
& Countermeasures
John Kattell, P.E.
Regional Bridge Engineer
USDA Forest Service Region 1

Merv Eriksson, P.E.

Project Leader
San Dimas Technology and Development Center/
Bridge Engineer
Wood in Transportation Program

USDA Forest Service

San Dimas Technology and Development Center
San Dimas, California

September 1998

Information contained in this document has been developed for the guidance of
employees of the Forest Service, USDA, its contractors, and cooperating Federal
and State agencies. The Department of Agriculture assumes no responsibility
for the interpretation or use of this information by other than its own employees.
The use of trade, firm, or corporation names is for the information and convenience
of the reader. Such use does not constitute an official evaluation, conclusion,
recommendation, endorsement, or approval of any product or service to the
exclusion of others that may be suitable.

The United States Department of Agriculture (USDA) prohibits discrimination in

its programs on the basis of race, color, national origin, sex, religion, age, disability,
political beliefs, and marital or familial status. (Not all prohibited bases apply to all
programs.) Persons with disabilities who require alternative means for
communication of program information (braille, large print, audiotape, etc.) should
contact USDA’s TARGET Center at 202-720-2600 (voice and TDD).

To file a complaint, write the Secretary of Agriculture, U.S. Department of

Agriculture, Washington, DC 20250, or call 1-800-245-630 (voice) or 202-720-
1127 (TDD). USDA is an equal opportunity employer.
 ACKNOWLEDGMENTS

A project of this scope requires the efforts of many people. The authors gratefully acknowledge the Washington
Office Engineering Staff and the Technology and Development Program Engineering Steering Committee for
providing guidance and funding.

The following persons are thanked for project guidance, reviewing documentation, and providing comments on
use and applications:

Dick Jones, Regional Hydraulic/Geotechnical Engineer, USDA Forest Service Region 8

Nelson Hernandez, Structural Engineer, USDA Forest Service Washington Office

Bill Grabner, Regional Bridge Engineer, USDA Forest Service Region 6

Scott Mitchell, Regional Bridge Engineer, USDA Forest Service Region 2

iii
 CONTENTS

ACKNOWLEDGMENTS ..................................................................................................................... iii

INTRODUCTION ................................................................................................................................ 1
SCOUR EVALUATION REFERENCE STANDARDS ......................................................................... 2
FOREST SERVICE SCOUR EVALUATION PROGRAM ................................................................... 2
Required FHWA Reporting ..................................................................................................... 4
Documentation and Program Monitoring ................................................................................ 4
STEP 1 - OFFICE SCREENING AND MANAGEMENT PRIORITY ANALYSIS ................................. 4
STEP 1A - OFFICE SCREENING ...................................................................................................... 4
Objective ................................................................................................................................ 4
Office Screening Flow Chart .................................................................................................. 5
Suggested Responsible Person(s) ......................................................................................... 5
Recommended Documentation .............................................................................................. 5
STEP 1B - MANAGEMENT PRIORITY ANALYSIS ........................................................................... 5
Objective ................................................................................................................................ 5
Management Priority Analysis Flow Chart .............................................................................. 8
Suggested Responsible Person(s) ......................................................................................... 9
Recommended Documentation .............................................................................................. 9
STEP 2 - FIELD REVIEW, SCOUR VULNERABILITY ANALYSIS, & PRIORITIZE ........................... 9
STEP 2A - FIELD REVIEW ................................................................................................................ 9
Objective ................................................................................................................................ 9
Suggested Responsible Person(s) ....................................................................................... 10
Recommended Documentation ............................................................................................ 10
STEP 2B - SCOUR VULNERABILITY ANALYSIS ........................................................................... 10
Objective .............................................................................................................................. 10
Scour Vulnerability Appraisal ................................................................................................ 10
Available Methodologies ...................................................................................................... 10
CAESAR ............................................................................................................................... 10
Colorado Highway Department Scour Vulnerability Ranking Flow Charts ........................... 10
Rapid-Estimation Method For Assessing Scour at Highway Bridges Based on Limited Site Data ...... 10
Suggested Responsible Person(s) ....................................................................................... 11
Recommended Documentation ............................................................................................ 11
STEP 2C - PRIORITIZE BRIDGES .................................................................................................. 11
Objective .............................................................................................................................. 11
Priority Guidelines and Criteria ............................................................................................. 11
Suggested Responsible Persons ......................................................................................... 11
STEP 3 - DETAILED SCOUR EVALUATION ................................................................................... 11
Objective .............................................................................................................................. 11
Evaluation Criteria, and Tools ............................................................................................... 12
Suggested Responsible Person(s) ....................................................................................... 12
STEP 4 - PLAN OF ACTION ............................................................................................................ 12
Objective .............................................................................................................................. 12
Suggested Responsible Person(s) ....................................................................................... 12

22 v
 Recommended Documentation ............................................................................................ 12
TYPICAL FOREST SERVICE BRIDGE ........................................................................................... 12
COMMON PROBLEMS AND CHARACTERISTICS ........................................................................ 14
Stream Channel Instability ................................................................................................... 14
Bridge Geometry and Scour ................................................................................................. 14
Aggradation .......................................................................................................................... 15
Long-Term Degradation ........................................................................................................ 15
Contraction Scour ................................................................................................................. 15
Abutment Scour .................................................................................................................... 15
Debris ................................................................................................................................... 17
Abutment Fill Failures ........................................................................................................... 17
COMMON COUNTERMEASURES ................................................................................................. 18
Riprap ................................................................................................................................... 18
Spur Dikes, Barbs, Groins, and Vanes ................................................................................. 18
Foundation Strengthening .................................................................................................... 18
LITERATURE CITED ....................................................................................................................... 20
APPENDIX A ................................................................................................................................... A-1
APPENDIX B................................................................................................................................... B-1
APPENDIX C ................................................................................................................................. C-1

vi 23
INTRODUCTION Prior to 1998, the Forest Service had not
Scour, defined as “the erosion or removal of implemented a scour evaluation program. In 1998,
streambed or bank material form bridge foundations an Engineering Technology Development Proposal
due to flowing water” is the most common cause of was funded to develop a scour evaluation program,
highway bridge failures in the United States. The specifically for the Forest Service, that all Regions
Forest Service, U.S. Department of Agriculture, of the Forest Service could implement. The project
administers 7,650 bridges on National Forest lands was to outline a single process and establish criteria,
and virtually all of them are over water. Scour is methods, and guidelines that would ensure
also the single most common cause for bridge consistency throughout the agency and eliminate
damage and failure on National Forest lands duplication of effort.
(Figure 1). Many bridges will experience floods
which can cause damage each year. To minimize The project was completed in cooperation with the
future bridge flood damage and ensure public safety Regional Bridge Engineers and was organized into
requires developing and implementing improved three phases.
procedures for designing bridges and inspecting
them for scour. “Every bridge over water, should be Phase 1. Review the FHWA guidelines and
assessed as to its vulnerability to scour in order to existing public road agency scour
determine the prudent measures to be taken for that programs.
bridge and the entire inventory” (Richardson and Phase 2. Develop a scour evaluation program
Davis␣ 1995). specifically for the Forest Service based
on the information from Phase I.
Realizing this need, the Federal Highway Phase 3. Provide support for the program during
Administration (FHWA) issued a Technical Advisory implementation by the Regions.
in 1988 revising the National Bridge Inspection
Standards (NBIS) to require evaluation of all bridges This document is the culmination of Phases 1 and 2.
for susceptibility to damage resulting from scour. In
accordance with the Memorandum of Understanding Implementing this process will provide valuable
between the Forest Service and the Federal information and initiate pro-active management of
Highway Administration, the Forest Service is our bridge inventories. Managers will be able to
required to implement the Technical Advisory, prioritize needs and avoid many future bridge
establish a scour evaluation program, and submit problems and failures. In addition, the process will
reports to FHWA discussing the progress of the provide valuable training, experience, and tools that
evaluation program. will enhance the skills of employees who implement
the program. This will benefit many future projects
in our role of “Caring For The Land, and Serving
People.”

R9800110

Figure 1—Little Salmon River Bridge, Nez Perce National Forest. A January 1997 flood event
scoured the abutment and one of the intermediate piers causing failure.

1
SCOUR EVALUATION REFERENCE FOREST SERVICE SCOUR EVALUATION
STANDARDS PROGRAM
The Technical Advisory issued by FHWA in 1988 The Forest Service Scour Evaluation Program has
provided recommendations for developing and been developed into a four-step process similar to
implementing a scour evaluation program. Since the five-step process recommended in HEC-18,
that time, FHWA developed two additional Chapter 5. The objective of the process is to provide
documents that have become the reference a consistent, efficient method to review and evaluate
standards for all scour evaluation programs. These all bridges over water, determine the scour potential
documents are: of each bridge, assist in establishing priorities and
identifying appropriate countermeasures, and
1. Hydraulic Engineering Circular No. 18 (HEC-18) documenting the results. Figure 3 presents a flow
- Evaluating Scour at Bridges. (Richardson and chart of the process. The four steps are:
Davis 1995)
2. Hydraulic Engineering Circular No. 20 (HEC-20) Step 1 - Office Screening and Management Priority
- Stream Stability at Highway Structures. Analysis
(Lagasse et al. 1995) Step 2 - Field Review, Scour Vulnerability Analysis,
and Prioritizing.
HEC-18 is the technical standard for knowledge and Step 3 - Detailed Scour Evaluation.
practice in the design, evaluation, and inspection of Step 4 - Plan of Action.
bridges for scour. HEC-20 provides guidelines for
identifying stream instability problems at stream Each step will be discussed in more detail in the
crossings that may cause scour damage to bridges following sections. Within each step of the process,
or culverts (Figure 2). bridges or major culverts are categorized with
respect to the determined scour potential.
The Scour Evaluation Program outlined within this (Categories are discussed under section Step␣ 1)
document also uses the two HEC documents as the These categories also correspond to the Scour
primary reference standards. Successful Critical Bridge field on the Structure Inventory and
implementation of the program will require Appraisal form, Item␣ 113. As a bridge proceeds
knowledge, understanding, and use of these through the evaluation process, a structure may be
references. placed in a different category from the previous step

R9800111

Figure 2—Left - HEC 18, Evaluating Scour at Bridges; Center - An Assessment

Methodology for Determining Historical Changes in Mountain Streams; Right - HEC 20,
Stream Stability at Highway Structures.

2
 Step 1a
Office Screening

Category 1 Bridges needing further evaluation Category 5

Low Catagories 2 - Scour Susceptible Tidal
Scour Risk 3 - Scour Critical
4 - Unknown Foundation

STOP STOP

Step 1b Low Monitor

Mgmt Priority Bridge
Priority
Analysis

Step 2a
Field Review

Step 2b
Scour Vulnerability
Analysis

Bridges needing further evaluation

Category 1 Catagories 2 - Scour Susceptible Category 5
Low Scour Risk 3 - Scour Critical Tidal
4 - Unknown Foundation

STOP
STOP

Step 2c
Prioritize

Step 4
Step 3 Plan of Action
Detailed Scour Evaluation for Bridge

R9800107

Figure 3—Forest Service Scour Evaluation Program Four Step Process.

3
and the Scour Critical Bridges field code may also Add new field “Scour Vulnerability” to the Appraisal
change. The process will be complete for a bridge fields. This field will assist in prioritizing the bridges
when the coding of the Scour Critical Bridges field during Step 2c of the scour evaluation process.
on the Structure Inventory and Appraisal form, Proper codes are:
Item 113, is any value other than code 6, which is
“Scour calculation/evaluation has not been made.” • H High Scour Vulnerability
• M/H Moderate to High Scour Vulnerability
Steps 1 and 2 provide guidelines and criteria to • M Moderate Scour Vulnerability
efficiently and economically screen as many bridges • M/L Moderate to Low Scour Vulnerability
and major culverts as possible into the appropriate • L Low Scour Vulnerability
categories, identify proper Scour Critical Bridge • ND Not Determined
codes, and prioritize bridges. Step 3, Detailed Scour
Evaluations, requires an experienced inter- With the revisions to INFRA-BMC proposed, various
disciplinary team. In most instances, this team will reports can be generated from the data to assist in
include technical expertise outside the Forest Service monitoring the program process. A supplement to
and is anticipated to be expensive. Steps 1 and 2 this report will be a standard report to query INFRA-
can be completed by Forest Service personnel or BMC for the data needed to generate FHWA
consultants. progress reports. Maintaining the proper coding of
INFRA-BMC for each bridge during the scour
All bridges should be first evaluated using Steps 1 evaluation process should result in adequate
and 2 since appropriate countermeasures can often monitoring capability.
be easily identified before proceeding to Step 3 on
many of the single-span, smaller bridges that are STEP 1 - OFFICE SCREENING AND
rated scour critical. MANAGEMENT PRIORITY ANALYSIS
STEP 1A - OFFICE SCREENING
Required FHWA Reporting
In addition to coding the Scour Critical Bridges field
Objective
on the Structure Inventory and Appraisal form, Item The objective of Step 1a is to quickly review the
113, for each bridge, FHWA requires the Forest current available documents within the bridge or
Service to submit progress reports outlining the major culvert file and screen them into five
progress of the evaluation program. In Appendix A categories. As mentioned above, each category has
is an outline of the format of the report and corresponding codes with respect to the Scour
explanations for the different reporting categories. Critical Bridge field on the Structure Inventory and
The progress reports are to reflect the number of Appraisal form, Item 113. It should be noted, that
bridges and the appropriate coding for each bridge. any bridge which has not proceeded to Step 1 in the
process, should have a code 6 for the Scour Critical
Documentation and Program Monitoring Bridge field (scour calculation/evaluation has not
been made) . The five categories and corresponding
Documentation is recommended for each bridge at
Scour Critical Bridge (Item 113) codes are as follows:
each step of the process. Recommended
documentation methods are described for each step
1. Low Scour Risk
in this report. In addition, INFRAstructure-Bridge
Item 113 corresponding codes 4, 5, 7, 8, 9
and Major Culvert (INFRA-BMC) will remain the
2. Scour-susceptible
database and inventory for all Forest Service bridges
Item 113 corresponding codes 6, LP
and major culverts. Monitoring of each bridge
3. Scour-critical
through the scour evaluation process should be
Item 113 corresponding codes 0, 1, 2, 3
possible using INFRA-BMC with a few proposed
4. Unknown Foundations
changes. The proposed changes are as follows:
Item 113 corresponding codes U
5. Tidal
Revise Scour Critical Bridge, Item 113:
Item 113 corresponding codes T
• Add new code “U” for Unknown Foundation.
These five categories are the same as recommended
• Add new code “T” for Tidal.
in HEC-18, Chapter 5.
• Add new code “LP” for Low Priority. (This code will
reflect bridges that are identified as low priority in
Step 1b - Management Priority Analysis, in the
proposed Forest Service scour evaluation process).

4
Office Screening Flow Chart Unknown foundations should be placed in the same
Figure 4 provides a flow chart of Step 1 indicating category as shallow foundations, requiring further
the recommended steps in an office screening and review.
the criteria for placing bridges and major culverts
into the five categories. Documents needed are Inspection Reports: Prior to placing any structure
bridge plans and past inspection reports. Below is in Category 1, Low Scour Risk, the inspection reports
some discussion on several of the various decision should be reviewed for indications of past or current
nodes within the flow chart. scour problems. Scour damage should include not
only damage to the structure itself, but also to
Pier/Abut/Ftg in Channel or Floodplain: Plans approach fills. An abutment on a deep foundation
that indicate all foundations are outside the channel may have a low vulnerability to scour damage
and flood plain and well above flood water elevations structurally, but the approach fills may scour away
can be categorized as Low Scour Risk (Figure 5). leaving a serious safety hazard even though the
bridge itself is not damaged. Specific items in the
Pier/Abut/Ftg on Scourable Material: Plans that inspection reports to be reviewed are the
indicate all foundations are on non-scourable Substructure, Channel & Channel Protection, and
material can be categorized as Low Scour Risk. Waterway Adequacy fields. A numeric code of 5 or
Non-scourable material is considered to be durable less in any of these fields (substructure must be due
rock that is not susceptible to significant deterioration to observed scour) indicates potential scour
due to weathering and that scours at such a slow problems and those bridges should proceed to the
rate that changes occur over a long period of time next step of the process. If a bridge has deep
(measured in centuries). foundations, and the inspection reports do not
indicate any potential scour problems, the bridges
Stream Velocity: “Slow” is generally associated with are placed in Category 1, Low Scour Risk and no
lakes, tidal zones, or ditches and canals which further review is necessary.
experience very slow moving, predominately static-
flow conditions. All other streams, creeks, and rivers Suggested Responsible Person(s)
should be considered rapid (Figure 6). The office screening can be completed by Forest
Service Bridge Inspection Team Leaders or Program
Foundations: The foundation type is a primary Managers or consultants with assistance from the
influencing factor in determining the vulnerability to Regional Bridge Engineer.
scour damage. Deep foundations such as long piles
or drilled shafts are considered to have low Recommended Documentation
vulnerability to scour damage and may be placed in A simple method of documenting the office screening
the low risk category upon review of the inspection step is to highlight the decisions, path, and ending
reports, thus eliminating the bridges further from the Category that the bridge was placed in on the Office
scour evaluation process. Shallow foundations such Screening Flow Chart, Figure 4. A flow chart should
as spread footings, short piles, mud sills, or cribs be prepared for each individual bridge. Notes could
are considered to have high vulnerability to scour also be written on the flow chart. If electronic
and are not recommended to be considered low risk documentation is preferred, highlighting, shading,
without proceeding to the Field Review, Step 2a. and typed notes can be added to the flow chart
Lengths defining a long pile vs. a short pile have indicating the decisions, path, and ending Category
not been provided. Guidance should be provided that the bridge was placed in.
on a Regional basis, however, in the predominately
glacier till soils of the Rocky Mountain States,
15 to 20 feet is being used to distinguish between
STEP 1B - MANAGEMENT PRIORITY
deep and shallow pile foundations.
ANALYSIS
Bridges and major culverts are separated prior to Objective
reviewing the foundations. Major culverts usually Realizing that funding and resources for detailed field
have no foundations or are on shallow spread reviews, scour evaluation studies, and implementing
footings and are not recommended to be considered on-site scour countermeasures will be limited, the
low risk without proceeding to the Field Review, objective of Step 1b is to recognize that some Forest
Step 2a. Service bridges and major culverts will have a much
lower priority, regardless of the bridge’s scour
vulnerability rating. Many Forest Service bridges are

5
 Region_________________________
Forest__________________________ Bridge over NO
STOP
Route ID & MP___________________ water
Name__________________________ YES
Feature Crossed__________________
Pier/Abut/Ftg.
NO in channel
or Floodplain

YES

NO Pier/Abut/Ftg.
on Scourable
material
YES

Stream Slow (lake, canal, tidal)

Velocity

Rapid

Bridge Major Culvert

Deep Foundations

Unknown or shallow

footings, short piles,

foundations (spread
(long piles, drilled

mud sills, cribs)

shafts)

Inspection Reports Inspection Reports

No No

Item 60 Item 60
Substructure YES YES Substructure
≤ 5 due to ≤ 5 due to
observed scour observed scour

No No

Item 61 Item 61
Channel & YES YES Channel &
Channel Channel
Protection Protection
≤5 ≤5
No No

Item 71 YES YES Item 71

Waterway Waterway
Adequacy Adequacy
≤5 ≤5

No Bridges needing further evaluation No

Catagories 2 - Scour Susceptible
3 - Scour Critical
Category 1 4 - Unknown Foundation
Category 5
Low Scour Risk Tidal

STOP To Step 1b STOP

R9800108

Figure 4—Forest Service Scour Evaluation Program -Office Screening Flow Chart. This figure breaks
down step 1a from the overall diagram in Figure 3.

6
 R9800112

Figure 5—Libby Creek Bridge, Kootenai National Forest. Bridge footings are set into non-erodible bedrock
as well as being outside the channel and well above flood waters. Bridge is categorized as Low Scour Risk.

R9800113

Figure 6—Lake Koocanusa Bridge, Kootenai National Forest. Illustration of bridge crossing a body of
water with “slow” stream velocities.

7
 behind locked gates, have low traffic volumes, are NBIS vs. Non-NBIS Structure: To evaluate bridges
not vital access routes, are older, or are small bridges and major culverts with respect to traffic volumes
with low present-worth values, where scour damage and public use, distinguishing between a NBIS
or complete washout would not create significant (National Bridge Inspection Standards) or Non-NBIS
resource damage. These structures are of low bridge is recommended. (A NBIS bridge or major
priority and do not economically justify further culvert is one that is considered “open to public
evaluation or installation of any scour travel” and subject to the National Bridge Inspection
countermeasures. An acceptable mitigation plan for Standards). A NBIS structure should remain in the
these structures is monitoring after flood events and scour evaluation process and proceed to the Field
closure if necessary. Figure 7 outlines a process for Review of Step 2.
Step 1b in which bridges and major culverts that have
been screened into Categories 2, 3 or 4 in Step 1a, Potential For Resource Damage: The bridge or
are quickly evaluated with respect to priority. major culvert should be reviewed for potential to
Structures meeting certain criteria can be considered resource damage if significant scour or complete
low priority without further review or evaluation. The washout occurs. Several possibilities to consider
action plan for these structures is monitoring. are:

Management Priority Analysis Flow Chart • The amount of sediment that would be added to
Figure 7 presents a flow chart of the recommended the creek or river with scour damage. In general,
process and criteria for the Management Priority sediment comes from the approach fills, which
Analysis. Below is some discussion on several of many times are small; however, major culverts
the various decision nodes within the flow chart. may be buried in high fills that would contribute
much more sediment (Figure 8).

• The debris from a bridge or major culvert that may

damage other structures below.

Non-NBIS Acceptable No Low Acceptable

Bridges needing Monitor
Potential for Vital Bridge Other
further evaluation NBIS or Bridge
Resource Access Present Worth Considerations
Categories Non-NBIS (Low Priority)
Damage or Value
2, 3, or 4
(From Step 1a)

NBIS Unacceptable Yes High Unacceptable

Proceed to Step 2a
Field Review R9800109

Region__________________________
Forest___________________________
Route ID & MP____________________
Name___________________________ Completed by_____________________
Feature Crossed__________________ Date____________________________

Figure 7—Forest Service Scour Evaluation Program - Management Priority Analysis. This
figure breaks down Step 1b from the overall diagram shown in Figure 3.

8
 R9800114

Figure 8—Moss Creek, Idaho Panhandle National Forest. Piping and subsequent scour of this culvert resulted in
failure of the entire fill introducing a large amount of sediment into the stream. With Step 1b, Management Priority
Analysis, the potential for resource damage at similar sites could be considered as unacceptable.

The potential resource damage should be evaluated Recommended Documentation

as Acceptable or Unacceptable. Structures with
A simple method of documenting the Management
unacceptable potential for resource damage should
Priority Analysis is to highlight the decisions and
remain in the scour evaluation process and proceed
path for the bridge on the Management Priority
to the Field Review of Step 2.
Analysis Flow Chart, Figure 7. Notes could also be
written on the flow chart. If electronic documentation
Vital Access: A bridge or major culvert that may be
is preferred, highlighting, shading, and typed notes
closed to public travel (Non-NBIS) but is on a vital
can be added, indicating the decisions.
administrative route and would severely impact
access to critical management areas should remain
in the scour evaluation process and proceed to the
STEP 2 - FIELD REVIEW, SCOUR
Field Review of Step 2. VULNERABILITY ANALYSIS, & PRIORITIZE
The bridges and major culverts placed in
Bridge Present Worth / Value: Non-NBIS bridges Categories 2, 3, and 4 in Step 1a and identified to
that are large, in good condition, or can be proceed to Step 2 in Step 1b, may be screened
economically rehabilitated and would have a high further by completing Field Reviews and Scour
replacement cost should remain in the scour Vulnerability Analyses. In addition, bridges are
evaluation process and proceed to the Field Review prioritized for further evaluation and/or
of Step 2. However, Non-NBIS bridges that are implementation of scour countermeasures.
small, in poor condition, or can not be economically
rehabilitated and would have a low replacement cost STEP 2A - FIELD REVIEW
would be good bridges to rate as low priority and
Objective
monitor.
The objective of the Field Review is to verify the
Suggested Responsible Person(s) inspection report information used during Step 1 and
to gather additional field data necessary to complete
The Management Priority Analysis should be the a Scour Vulnerability Analysis. The Field Review is
responsibility of the Engineering Forest Staff Officer, a comprehensive study of current scour problems
with assistance from the Forest Bridge Inspection as well as an analysis of the fluvial geomorphology
Team Leader or Program Manager, Forest of the stream.
Transportation Planner, and Resource Specialists.

9
Suggested Responsible Person(s) without completing a full scour evaluation with an
The Field Review can be completed by Forest interdisciplinary team. Of the methodologies
Service Bridge Inspection Team Leaders, or reviewed, a computer program developed by the
consultants. With some training, Forest Service University of Washington, called CAESAR, is
Bridge Inspection Team Leaders should be able to recommended to the Forest Service at this step in
gather the information in conjunction with the the program. A discussion of the CAESAR program
regularly scheduled bridge inspections. and two alternative methodologies follows.

Recommended Documentation CAESAR

Documentation of the Field Review will be necessary The University of Washington has developed a
to provide pertinent information for the Scour computer program called Cataloging and Expert
Vulnerability Analysis. More information is provided Evaluation of Scour Risk and River Stability at Bridge
in the Step␣ 2b discussion. Sites (CAESAR). The program operates in a
Windows environment and is structured in a question
and answer format. Basic bridge data and Field
STEP 2B - SCOUR VULNERABILITY
Review data are required as input. The program
ANALYSIS outputs weighted recommendations pertaining to
Objective scour vulnerability, stream stability, and waterway
With the information provided by the Field Review, adequacy. The program has two parts:
bridges and major culverts may be further screened
into appropriate categories and proper Scour Critical 1. The user interface, through which site information
Bridge field codes. In addition, for many single-span, is collected, stored, and retrieved. Textual and
smaller bridges, the Scour Vulnerability Analysis may visual (graphs and photographs) help is provided.
be adequate to determine the nature of a scour The Field Review data can be documented and
problem and the appropriate countermeasure or stored within the program.
mitigation.
2. An evaluation module assesses the site conditions
Scour Vulnerability Appraisal and provides recommendations (with confidence
values) and suggested actions.
Scour vulnerability is defined as “the degree to which
a bridge is open to attack or damage from forces The program has been “beta” tested on several
and conditions causing scour.” Scour vulnerability Forest Service bridges and was found to be an
is related to the scour critical codes but are not the efficient and effective tool for completing and
same. If appropriate countermeasures are installed, documenting the Field Review and Scour
the degree of scour vulnerability will probably Vulnerability Analysis.
decrease. Also, over time, site conditions may
change, generating new factors that effect the degree
of scour vulnerability. In addition, a bridge’s scour Colorado Highway Department Scour
vulnerability will influence a manager’s priority Vulnerability Ranking Flow Charts
decision. Therefore, a new field in the INFRA-BMC The Colorado Highway Department developed a
inventory has been proposed that will indicate the series of Scour Vulnerability Ranking Flow Charts
degree of scour vulnerability. The codes are: as part of their scour evaluation program. The flow
charts outline a method to determine a vulnerability
• H High Scour Vulnerability score for general site conditions, abutment scour
• M/H Moderate to High Scour Vulnerability vulnerability, and pier scour vulnerability. The flow
• M Moderate Scour Vulnerability charts are not as comprehensive as the CAESAR
• M/L Moderate to Low Scour Vulnerability analysis and do not document the site conditions of
• L Low Scour Vulnerability the Field Review. Appendix␣ B provides a copy of
• ND Not Determined the flow charts and accompanying documentation.

Available Methodologies Rapid-Estimation Method For Assessing

Various scour vulnerability analyses have been used Scour at Highway Bridges Based on
by many other agencies at similar points in their scour Limited Site Data
evaluation programs. A number of methodologies The Montana U.S. Geologic Survey (USGS), in
are available to complete the analysis and to cooperation with the Montana Department of
estimate scour potential with limited information and Transportation, developed a methodology for
10
estimating scour depths that would (1)␣ require only Review data can be stored electronically along with
limited on-site data, (2)␣ provide estimates of scour the evaluation of the bridge. The proposed new field
depth that would be reasonably comparable to in INFRA-BMC (Scour Vulnerability) will provide a
estimates from more detailed methods and would means for documentation within the bridge inventory.
tend to overestimate rather than underestimate scour
depths, and (3)␣ provide estimates for each site in a
STEP 2C - PRIORITIZE BRIDGES
few hours or less (Holnbeck and Parrett␣ 1997). The
method was developed using calculated scour Objective
depths from 122␣ detailed scour evaluations of bridge Throughout the scour evaluation process, bridge
sites in 10␣ states and formulating relationships program managers will need to evaluate available
between scour depth and hydraulic variables that resources, personnel, and the funding required to
can be rapidly measured in the field. “Although the complete more detailed bridge evaluations and
method was developed specifically for bridges in implement countermeasures. Step␣ 1b provides an
Montana, it is believed to be valid for a wide range initial priority screening. At this point in the process,
of hydrologic and hydraulic conditions throughout after the Scour Vulnerability Analysis, bridges
the United States” (Holnbeck and Parrett␣ 1997). The remaining in Categories␣ 2,␣ 3,␣ and␣ 4, which require
method uses a Standardized Scour Analysis and more detailed evaluations, need to be prioritized with
Reporting Form which includes a worksheet for respect to safety. Safety will be evaluated with
calculating the scour depths and a general summary respect to the determined scour vulnerability, the
sheet for general field investigation information. functional classification, and the road the bridge
resides on, along with National Forest road
This method will provide a good assessment of scour management policies. Due to the varied road
vulnerability for a bridge with good documentation. management policies within the National Forests, a
The method will require more time than either the strict method for developing a prioritized list is not
CAESAR program or the Colorado flow charts, good provided or recommended. Some general
judgement, and a high level of expertise. guidelines and criteria are suggested.

Suggested Responsible Person(s) Priority Guidelines and Criteria

• The CAESAR program can be used by field Each National Forest will need to determine how to
personnel with little formal training in river prioritize and evaluate the safety risks of a bridge. A
mechanics and scour processes. Forest Service simple listing of bridges separated between those
Bridge Inspection Team Leaders, Program on arterial, collector, or local roads and sorted by
Managers, or consultants can complete the Scour scour vulnerability from high to low will provide
Vulnerability Analysis. Completion of a training managers with a basis to plan for needed resources
session in stream stability and bridge scour is and prioritize individual bridges for further evaluation.
recommended. Additional information accompanying the listing, such
• Colorado DOT scour vulnerability ranking flow as the scour critical code, substructure condition
charts should be completed by Forest Service code, and average daily traffic (ADT) values may be
Bridge Inspection Team Leaders, Program helpful. A supplement to this document will be a
Mangers, or consultants. Completion of a training standard report to query INFRA-BMC as described
session in stream stability and bridge scour is above.
recommended.
• Rapid-Estimation Method For Assessing Scour at
Highway Bridges Based on Limited Site Data, Suggested Responsible Persons
developed by USGS, requires a higher degree of Prioritizing bridges should be the responsibility of
expertise than the CAESAR program or the the Forest Staff Officer for engineering activities with
Colorado flow charts. To complete the assistance from the Forest Bridge Inspection Team
vulnerability analysis using this method, qualified Leader or Program Manager, Forest Transportation
Forest Service hydraulic or bridge engineers or Planner, and Resource Specialists.
qualified consultants are recommended.
STEP 3 - DETAILED SCOUR EVALUATION
Recommended Documentation Objective
Documentation will be dependant on the method Bridges that have gone through Steps␣ 1␣ and␣ 2 and
used. An advantage of the CAESAR program is that remain in Categories␣ 2,␣ 3␣ and␣ 4 will require a
the Field Review report and any subsequent Field Detailed Scour Evaluation. HEC-18 indicates this

11
evaluation is to be completed by an interdisciplinary Suggested Responsible Person(s)
team of hydraulic, geotechnical and structural In most instances, the Detailed Scour Evaluation is
engineers. The evaluation typically includes a recommended to be completed by consultants with
detailed site review, estimated scour calculations, the interdisciplinary expertise required. Depending
structural evaluation of the foundations under the on the Forest or Region the responsible person may
estimated scour conditions, and the design of any be the National Forest Bridge Program Manager,
necessary scour countermeasures. In addition, the Forest Engineer, or Regional Bridge Engineer.
results of a Detailed Scour Evaluation will
subsequently establish proper Scour Critical Bridge
STEP 4 - PLAN OF ACTION
field codes for the INFRA-BMC database.
Objective
Evaluation Criteria, and Tools The final step in the Forest Service Scour Evaluation
Criteria and guidelines for a Detailed Scour Process is to develop a Plan of Action for a bridge to
Evaluation are outlined in the reference, HEC-18. correct scour problems. The Plan of Action is the
FHWA recommends that a bridge be evaluated for tool that “closes the loop” from evaluating and
the design flood and superflood conditions and have studying a bridge, to acknowledging and recognizing
suggested the 500-year flood event. a problem, and, finally, to implement field corrective
measures. The Plan of Action may include interim
HEC-18 presents the state-of-the-art in scour scour countermeasures until permanent measures
calculation methods and equations. There are a are installed, monitoring plans and/or inspections
number of hydraulic computer programs available after flood events, and procedures for closing bridges
for assisting in the calculation of scour depths, such if necessary.
as WSPRO, HEC-RAS, and BRI-STARS. Most of
these programs use one-dimensional models and Suggested Responsible Person(s)
do not have the capability to evaluate lateral flows Depending on the Forest or Region, the responsible
and channel instabilities or meanders of the stream. person to prepare the Plan of Action for a bridge
Engineers must account for these effects separately, may be the Forest Bridge Program Manager, Forest
as well as evaluate the structure foundations for Engineer, or Regional Bridge Engineer.
instability at the calculated scour depths.
Recommended Documentation
For many streams and rivers, typically associated
with mountainous bridge sites of the Forest Service, A simple Plan of Action form is included in
the evaluation of scour at a site is considered more Appendix␣ C. The form is to be completed for each
of an art than a science. At these sites, stream bridge and retained in the bridge file. The form
morphology is a significant factor. Also, hydrologic includes:
estimates of these mountainous stream flood-event
flows can have significant error. Therefore, in • Basic bridge identification information.
addition to the criteria and guidelines outlined in • The BMC/INFRA Scour Critical field coding.
HEC-18 for a Detailed Scour Evaluation, the • The proposed Scour Vulnerability coding with a
following is recommended: brief description of the critical elements vulnerable
to scour.
• The interdisciplinary team should include a person • Recommended scour countermeasures and
with expertise in stream morphology such as a implementation plan. Any design or drawings
Wildland Hydrologist or a Fluvial Geomorphologist should be referenced. If scour countermeasures
to assist in evaluating the potential of scour from have been completed, some basic information
lateral stream instabilities, long-term aggregation, should be included for future reference.
or degradation, etc. • Bridge Closure Plan. A bridge closure plan should
identify the acceptable method of closing the
• The scour evaluation should envelop estimated bridge, such as gates or barricades, and any
scour depths by calculating depths for the 50-year, needed detour or safety signing.
100-year, and 500-year flood events and apply
engineering judgement to achieve a reasonable TYPICAL FOREST SERVICE BRIDGE
and prudent evaluation of the bridge. The Forest Service road system is similar in some
ways to other public road systems because it
includes roads “open to the public” with standards
and traffic volumes similar to local county systems.

12
But the Forest Service road system is also unique The majority of Forest Service stream crossings are
because it includes many roads not open to the on mountain streams with channel gradients
public, which have reduced standards and very low between 0.01 and 0.10. “Mountain streams are
traffic volumes. Forest Service bridges also reflect subject to highly variable discharges and are
this diversity with some major, important, arterial susceptible to large sediment loads from slope
bridges and many small, local road bridges. One failures and debris flows” (Smelser and Schmidt
objective in developing the Forest Service Scour 1998). The streams typically flow between steep
Evaluation Program was to maintain flexibility in the forest slopes with incised channels and nearly
process so that the wide variety of bridges could be vertical, non-cohesive banks composed of gravel and
evaluated efficiently with practical results. cobbles before entering valleys where the streams
The most significant difference in this program, when flow into larger streams and rivers. As the channel
compared to the programs developed by each state, gradient drops, stream transport drops, causing
is Step 1b - Management Priority Analysis. This aggradation and channel braiding. Meandering, slow
step recognizes the fact that many Forest Service moving streams also occur in the large flatter valley
bridges are of a low priority, regardless of the scour bottoms as well as occasionally in higher elevation
vulnerability of the bridge. The step provides open parks and swamps. Each of these stream
flexibility early in the process to assess priorities channel types (steep incised, braiding, and
and resources needed to complete the program. meandering) have their own scour problems. Steep
incised streams experience long-term degradation,
Even though there are a wide variety of bridges in braided streams (occurring at gradient changes)
the Forest Service, a review of the Forest Service experience aggradation and stream instability, and
inventory indicates a few basic characteristics of a meandering streams experience lateral channel
typical Forest Service bridge (Figure␣ 9). migration and have very wide flood plains.

• Typically a single-span bridge. Scour associated with typical Forest Service bridges
• Virtually all cross a stream or river. in similar channel types will many times be of similar
• Average span is 50␣ feet with 80% of the inventory nature and cause. Therefore, it is also reasonable
under 80␣ feet. to assume that there may be common solutions or
• Most common abutment substructure type is vertical countermeasures. The following is a discussion of
walls supported on spread footings or mud sills. some of the most common problems and
• Bridges are on typically low volume roads with characteristics that may be evident on a typical
ADT’s less than␣ 100. Forest Service bridge.

R9800115

Figure 9—LaMarche Creek Bridge, Beaverhead National Forest. Typical single-span, treated timber bridge supported
on vertical wall abutments, with wingwalls, on mudsills crossing a mountainous stream.

13
COMMON PROBLEMS AND The coefficients indicate that spill-through abutments
CHARACTERISTICS (trapezoidal-shaped channel through a bridge)
decrease local scour depths significantly, compared
Stream Channel Instability to vertical wall abutments. Spill through abutments
As described above, stream channel instability is a provide a smoother transition through a bridge
problem most commonly associated with braiding opening, eliminating abrupt corners that cause
streams. Many Forest Service roads are located turbulent areas. Recent stream mechanics theory
adjacent to larger rivers, and thus, many Forest suggests that bridge abutments should span outside
Service bridges cross the tributaries. As described the “bankfull” stage of the stream, which
above, many of these tributaries have grade changes “corresponds to the discharge at which channel
as they approach the flood plain of the larger rivers, maintenance is the most effective, that is, the
have braiding characteristics, and experience shifting discharge at which moving sediment, forming or
and lateral migration. Bank erosion and changing removing bars, forming or changing bends and
angles of attach of the stream to the bridge cause meanders, and generally doing work that results in
local scour problems. the average morphologic characteristics of the
channels” (Rosgen 1996). Flows above the
Bridge Geometry and Scour “bankfull” stage are accommodated with adequate
Scour depth equations in HEC-18 use a coefficient freeboard through the bridge or overflow channels.
for abutment shape. The coefficients are Again, spill-through abutments are more efficient
(Richardson and Davis 1995): hydraulically at higher flood stages, allowing more
area and capacity than a vertical wall abutment
• Vertical wall abutment 1.00 (Figure 10).
• Vertical wall abutment with wingwalls 0.82
• Spill-through abutment 0.55

R9800116

Figure 10—Independence Creek Bridge, Idaho Panhandle National Forest. Example of a spill through
(trapezoidal) abutment configuration. See Figure 9 for an example of a vertical wall abutment with wingwalls.

14
Aggradation Contraction Scour
Mountainous streams generally will have variable Contraction scour occurs when a channel narrows
grades. Many will have steep grades, yet flatten and stream velocities increase. Many Forest Service
out substantially within a short distance of their bridge spans are undersized by today’s standards
confluence with a larger river. Aggradation within and contraction scour is present (Figure 13).
this flatter stream section can be a problem as the
stream transports bedload off the steeper grades and
Abutment Scour
deposits it as velocities slow along the flatter grades.
High flows in a larger river where a smaller tributary Abutment Scour is commonly termed local scour.
joins can cause back-water in the smaller tributary, Local scour involves removal of material from
which can also cause aggradation. Over time, the isolated areas caused by an acceleration of flow past
aggradation may be balanced by isolated storm an obstruction and the subsequent turbulent water
events which will flush (downgrade) out the tributary (vortices). Local scour is accentuated by debris
stream section. However, in the short term, this buildup or stream instabilities that shift the stream
aggradation can be a problem to bridges. Continued towards one abutment or change the angle of attack.
aggradation can minimize clearance for debris The most common locations for local scour on a
passage, cause overtopping or scour damage to the typical Forest Service single-span bridge with vertical
bridge, and approach roadways (Figure 11). wall abutments is adjacent to the upstream and
downstream corners intersecting the wingwalls.
Long-Term Degradation
HEC-18 provides equations for predicting abutment
Another characteristic of mountainous streams is scour, however, HEC-18 also states that the
long-term degradation. Steep, incised channels will laboratory research to date has failed to replicate
experience long-term degradation. When evaluating field conditions and these equations generally give
bridges for scour vulnerability, long-term degradation excessively conservative estimates of scour depths.
should be a factor. Many typical Forest Service Therefore, engineering judgement is required in the
bridges were built on spread footings with an use of these equations when evaluating or designing
embedment depth of only a few feet. Today, many abutment foundations for scour.
of these bridges are of the age in which these footings
will be exposed or undermined, mainly due to long-
term channel degradation (Figure 12).

R9800115

Figure 11—Eagle Creek Bridge, Idaho Panhandle National Forest. Backwater effects from the main river caused
aggradation at the mouth of this tributary and inadequate clearance for the bridge. Note, the following year the tributary
flushed and the stream re-established the normal channel depth.

15
 R9800118

Figure 12—Shepherd Creek Bridge, Flathead National Forest. Long-term degradation has exposed the mudsill of this
treated timber bridge with vertical wall abutments.

R9800119

Figure 13—Vigilante Bridge, Beaverhead-Deerlodge National Forest. This 40 foot bridge constricts the channel and
has caused upstream aggradation and contraction scour through and below the bridge.

16
 R9800120

Figure 14—Irene Bridge over Cascade River, Mt. Baker - Snoqualmie National
Forest, Region 6. Drift build-up on upper side of pier.

Debris • Monitoring debris buildup for prompt removal.

Debris can have a significant impact on bridge scour • Clearing upstream debris.
in a number of ways (Figure 14): • Installing debris catchers/deflectors.

• A buildup of debris can reduce a bridge’s waterway A debris catcher/deflector on mountainous streams
opening causing contraction scour of the channel. requires maintenance and its use must be carefully
• A buildup of debris can increase the obstruction considered with respect to stream mechanics, since
area of a pier or abutment and increase local a catcher/deflector could cause the stream channel
scour. to shift, resulting in other scour problems.
• Debris can deflect the flow of the water, changing
the angle of attack, and increase local scour or Abutment Fill Failures
shift the entire channel around the bridge During many flood events, the structure and
altogether. foundations of the bridge will not be damaged, but
• Action of water against debris can place a the fill behind an abutment will scour (Figure␣ 15).
substantial lateral force on the bridge. This commonly occurs on a typical Forest Service
single-span bridge with vertical wall abutments.
In general, debris is associated with many scour Local scour occurs around the wingwalls or
problems on a typical Forest Service bridge and must undermines the abutment footing and subsequently
be considered carefully. However, debris problems scours the approach fill away. To a user on the road,
and the associated scour are difficult to anticipate an abutment fill failure can be just as hazardous as
and remedy on existing bridges. New bridge designs a bridge failure. For this reason, abutment fill failures
can account for potential debris problems by due to scour should be included in determining the
oversizing spans, providing additional freeboard, and scour vulnerability of a bridge.
minimizing or eliminating piers in the channel. Typical
countermeasures for an existing bridge with a debris
problem include:

17
 R9800121

Figure 15—Monture Bridge, Lolo National Forest. An example of abutment fill failure with
little damage to the bridge.

COMMON COUNTERMEASURES improperly placed riprap will reduce the hydraulic

opening significantly and create contraction scour
The Federal Highway Administration saw the need
problems.
to identify common scour countermeasures and
provide guidelines for their use. They recently
The use of riprap to protect intermediate piers is now
published HEC-23, Bridge Scour and Stream
considered only a temporary solution. Again, if
Instability Countermeasures (Lagasse et al. 1997).
placed improperly, riprap can increase local scour
HEC-23 provides guidance for scour counter-
forces.
measure applicability, design, installation, and
maintenance, highlighted by a countermeasure
matrix. Many of the countermeasures that will apply
Spur Dikes, Barbs, Groins, Vanes
to typical Forest Service bridges are included. Spur dikes, barbs, groins and vanes are considered
river training structures that alter stream hydraulics
Riprap to mitigate undesirable erosional and/or depositional
conditions. They are commonly used on unstable
Within HEC-18, riprap is included under Local Scour
stream channels to redirect stream flows to a more
Armoring. Riprap is, and probably will remain, one
desirable location through the bridge (Figure 17).
of the primary scour countermeasures to resist local
scour forces at abutments of typical Forest Service
bridges. Riprap is generally abundant, inexpensive,
Foundation Strengthening
and requires no special equipment. However, proper On a typical Forest Service bridge, foundation
design and placement is essential. HEC-18 and strengthening requires extending the footing deeper
HEC-23 provide guidelines for proper sizing and to offset long-term degradation, providing additional
placement (Figure 16). tiebacks on a vertical wall abutment if scour has
caused loss of support and the abutments have
When designing riprap countermeasures, begun to “kick in,” or installing a new abutment with
maintaining an adequate hydraulic opening through deeper footings or piles.
the bridge must be considered. Many times,

18
 R9800122

Figure 16—Monture Bridge, Lolo National Forest. Repair of abutment fill failure and use of riprap
as a scour countermeasure.

R9800123

Figure 17—Fisher River Bank Stabilization, Kootenai National Forest. Use of rock vanes to
protect the stream bank from scour.

19
 LITERATURE CITED

CAESAR, an Expert system for the Cataloging and Expert Evaluation of Scour Risk and River Stability at Bridge
Sites. University of Washington, Department of Civil Engineering, National Cooperative Highway Research
Program, Project 24-6. Transportation Research Board, National Research Council.
http://maximus.ce.washington.edu/~scour/.

Colorado Highway Department. 1990. “Colorado Bridge Safety Assurance Procedure.” Ref: 1514. April 1990.

Holnbeck, S.R., and C. Parrett. 1997. “Method For Rapid Estimation of Scour At Highway Bridges Based On
Limited Site Data.” U.S. Geological Survey, Water-Resources Investigations Report 96-4310, pp. 1-3.

Lagasse, P.F., J.D. Schall, F. Johnson, E.V. Richardson, and F. Chang. 1995. “Stream Stability at Highway
Bridges, Second Edition.” US Department of Transportation, Publication No. FHWA-IP-90-014. Hydraulic
Engineering Circular No. 20.

Lagasse, P.F., M.S. Byars, L.W. Zevenbergen, and P.E. Clopper. 1997. “Bridge Scour and Stream Instability
Countermeasures.” US Department of Transportation, Publication No. FHWA-HI-97-030. Hydraulic Engineering
Circular No. 23.

Richardson, E.V., and S.R. Davis. 1995. “Evaluating Scour at Bridges, Third Edition.” US Department of
Transportation, Publication No. FHWA-IP-90-017. Hydraulic Engineering Circular No. 18, pp. 2, 48.

Rosgen, D. 1996. “Applied River Morphology.” Lakewood, CO: Western Hydrology.

Smelser, M.G., and J.C. Schmidt. 1998. “An Assessment Methodology for Determining Historical Change in
Mountain Streams.” USDA, Forest Service, Rocky Mountain Research Station, General Technical Report
RMRS-GTR-6, p. 1.

20
APPENDICES
APPENDIX A

FHWA Scour Report

 REQUIRED FHWA REPORTING ENCLOSURE A

BRIDGE NEEDING UNDERWATER INSPECTION

REGION

DATE

NUMBER OF BRIDGES

NBI TOTAL
REPORTING CATEGORIES CODE NBIS NON-NBIS* NUMBER

In Master List 92B

Initial Inspection
Completed 93B
Observed to Have
Structural Problems ---
Observed to Have
Scour Problems ---
Corrective Actions
Completed ---

* Includes bridges over waterways which are less than 20’ in length.

Note 1: The total number of bridges needing underwater inspection is for those bridges requiring special
manpower, techniques or equipment for determining the condition of underwater elements with certainty. This
total would not include bridges than can be examined from above by wading, probing, or adequate visual
inspection.

A-1
 REQUIRED FHWA REPORTING ENCLOSURE B

BRIDGES SCOUR EVALUATIONS

REGION

DATE

NUMBER OF BRIDGES
NBI
ITEM 113 TOTAL
REPORTING CATEGORIES CODE NBIS NON-NBIS* NUMBER

1. Over waterways ---

2. Evaluation Total ---
A. Low Risk Total 4,5,7,8,9
B. Scour Susceptible 6
C. Unknown Foundations U
D. Scour Critical 0 -3
E. Tidal T
3. Analyzed for Scour ---
4. Countermeasures Installed 7
5. Monitoring Planned ---

NBI NUMBER OF BRIDGES

NOTE: LOW RISK TOTAL ITEM 113 TOTAL
IS INCLUDED ABOVE CODE NBIS NON-NBIS NUMBER

REPORTING CATEGORIES

2A. Low Risk Total ---

(1) Calculated or 4, 5, &
Assessed 7-9
(2) Screened 6
(3) Culverts 8

* Includes bridges over waterways which are less than 20’ in length.

A-2
 Notes for Enclosures A and B

The following notes are keyed to the National Bridge Inventory (NBI) as documented in the Recording and
Coding Guide for the Structure Inventory and Appraisal of the Nations Bridges, December 1995 (metric version).
Item numbers and codes are described in the coding guide. Category number refer to the reporting form categories.

CATEGORY EXPLANATION
1 Equals sum of bridges with NBI Item 42B, coded 5-9.
2 Equals the sum of Categories 2A, 2B, 2C, 2D, and 2E. This sum should also equal Category 1, Over
Waterways, when screening is complete. A bridge should be included in only one of the categories
under 2.
2A Equals the sum of Categories 2A (1), 2A(2) and 2A(3).
2A(1) Equals the sum of (a), (b), and (c) described below:
(a) bridges assessed during scour screening with code 8 (spread footing on competent rock) code 9
(foundation well above flood elevations)
(b) bridges analyzed as stable : codes 4, 5, and 8 and
(c) bridges protected by countermeasures, code 7
2A(2) Bridges assessed as low risk during scour screening activity, code 6. (These bridges are candidates
for scour analysis, but lower priority than category 2B)
2A(3) Culverts assessed during scour screening as code 8.
2B Bridges assessed during scour screening as requiring scour analysis, code 6
2C Bridges assessed with unknown foundations, code 6
2D Bridges analyzed as scour critical, codes 0-3
3 Total number of bridges that have been analyzed for scour
4 Scour critical bridges that have been protected with a structural countermeasure (riprap, paving, etc.)
5 Scour critical bridges to be monitored. (structural countermeasures have not been constructed.)
4&5 The sum of categories 4 and 5 should equal category 2D, scour critical bridges.

Definitions

Assessed The structure has been screened for obvious conditions and evaluated using engineering
judgement.
Analyzed The structure has received a full engineering evaluation which includes calculation of hydrology,
hydraulics, scour and foundation stability.

A-3
 APPENDIX B

Scour Vulnerability Ranking Flow Charts taken from:

Colorado Bridge Safety Assurance

Procedure for Colorado Highway Department

April 1990

Ref. 1514

Pages 15-24
2. Step Two

Ranking the bridges in each category as to scour vulnerability.

a. Scour Vulnerability Ranking Flow Charts.

The ranking of the scour vulnerability of those bridges determined to be scour susceptible, is obtained by flow
charts that evaluate the vulnerability on the basis of the bridges geologic, hydraulic and river conditions as well
as the conditions of the bridges foundation (abutments and piers).

The purpose of the Vulnerability Ranking Flow Charts is to provide a procedure to prioritize the list of scour
susceptible bridges by determining the relative scour vulnerability of all bridges in each scour susceptibility
category. The numerical values included in the flow chart were selected to give the relative effect of each
parameter on the potential to produce scour. For example, the river slope/velocity parameter for steep, medium
and mild conditions is valued at “2,” “1” and “0” respectively because a steep slope will produce deeper scour
than a mild slope. The values in each parameter are such that the most scour vulnerable bridge will have the
largest value. More than one bridge can have the same value of vulnerability.

The value of the vulnerability ranking is that it orders a bridge relative to other scour vulnerable bridges, and
other things being equal (traffic counts for example) determines what bridge should be repaired or replaced first.

The Scour Vulnerability Ranking has three flow charts. They are: 1) General Considerations, 2) Abutments and
3) Piers, which proceed sequentially. It is expected that field evaluation of the bridge will be required to complete
the ranking.

b. General Conditions Flow Chart.

The General Conditions Flow Chart addresses parameters that have a general impact on the potential scour
depth. The need for intermediate scour countermeasures is included in the flow chart to remind the evaluator
to identify this need. No vulnerability ranking value is assigned to this parameter because it is expected that the
countermeasures will be implemented before the detailed scour evaluation and installation of remedial measure
is complete. The intermediate scour countermeasures are intended to protect the bridge from catastrophic
failure until the design and construction of remedial measures is completed.

The remaining parameters are included for the following reasons:

(1) River Slope/Velocity - A steeper/faster flowing stream is expected to experience more severe scour than
one with a medium or mild slope. The stream slope is defined as follows:
i) Steep S>0.0015 ft/ft
ii) Medium 0.0015 < S > 0.0004 ft/ft
iii) Mild S < 0.0004 ft/ft

(2) Channel Bottom - An aggrading condition is given a value of 0 because the slight deposition represented
reflects a decrease in scour potential. Severe deposition that restricts capacity is addressed later in the
flow chart. A stable channel condition is, therefore, given a value of 1 because it represents a more
scour prone condition than aggradation. Similarly, a degrading channel is given a value of 2.

(3) The channel bed material are ranked because rock would take more time to erode to maximum scour
than sand. The other material also would take more time. Thus, in ranking bridges to scour vulnerability
the bridge that takes longer for scour to reach its maximum value would be less vulnerable.

(4) Channel Configuration - A meandering or braided channel is given a value of 2 because they have the
most potential to have scour problems. A straight channel, defined as exhibiting a sinuosity of less than
1.5, is given a value of 0 because it is the least likely to affect scour. However, if a straight channel has
bar formations that shift the thalweg, it should be given a values of 1.0.

(5) Debris/Ice Problem - Watershed, river conditions or pier and abutment configurations that promote debris
B-1
 and ice accumulation, primarily as indicated by historic records or field observations, warrant a value of 1
because the accumulation increases potential scour depth by either reducing the conveyance area or by
increasing the effective pier width.

(6) Near River Confluence - The potential for increased flow and river velocity near a river confluence and the
resultant scour potential, warrants use of the value of 1 for this condition.

(7) Affected by Backwater - Locations affected by backwater for all flow conditions, primarily resulting from
proximity to a dam, warrants use of a value of 0. For this condition backwater from a downstream
waterway should not be considered because it may not occur concurrently with peak flow and velocity on
the tributary and at the location being studied.

(8) Historic Scour Depth - Historic scour indicates a clear potential for continued and increased scour
activity. Historic scour depths in excess of 3’ are a concern because spread footings are seldom deeper
than this.

(9) Historic Maximum Flood Depth - Flow depth is a parameter in the scour prediction equations. Deeper
flow is expected to produce greater scour.

(10) Adequate Opening - An inadequate opening is expected to produce greater scour than a restricted one,
therefore, a value of 2 is assigned to this condition. This parameter also addresses the deposition of
material in the channel at the structure to the point that the capacity of the bridge opening is restricted.
Bridges that experience overtopping and thus have pressure flow should also be given a 2.

(11) Overflow/Relief Available - The ability of the design flow to proceed downstream by a means other than
through the structure, usually by way of a relief structure or by overtopping the roadway embankment,
reduces the scour potential at the structure being evaluated because the resultant discharge and velocity
are less than would otherwise be the case.

(12) Simple Spans - This parameter recognizes that the ramifications of scour at simple span structures is
more severe than would occur for structures with alternate load paths that probably would not experience
catastrophic failure due to the loss of some foundation material.

The sum of the vulnerability ranking scores is tabulated at the bottom of the form before proceeding to the
abutment vulnerability ranking flow chart.

B-2
 General Conditions
Scour Vulnerability Ranking Flow Chart

Bridge # Feature Carried Stream

Community County
Bridge Type Spans

River Slope/Velocity

Steep Medium Mild

2 1 0

Channel Bottom

Stable Aggrading Degrading

1 0 2

Intermediate Scour Counter Measures Required

Yes No
Implement Scour
Counter Measures

Channel Bed Material

Rock Boulders Cobbles Glacial Till Sand

0 1 2 3 4

Channel Configuration

Straight Meandering Braided

0 2 3

Debris/Ice Problem

Yes No
1 0

Near River Confluence

Yes No
1 0

Effected by Backwater

Yes No
1 0

Historic Scour Depth

None Small Medium Large

=<1' 1'-3' >3'
0 1 2 3

Historic Maximum Flood Depth

=< 5' 5'-9' 10'-19' 20'-40' > 40'

0 1 2 3 4

Adequate Opening

Yes No
1 0

Overflow/Relief Available
General Condition
Yes No Vulnerablility Score____________
1 0
(Proceed to Abutment Scour Vulnerability
Simple Spans Ranking Chart)

Yes No
1 0

R9800145

Figure B-1—General Conditions Scour Vulnerability Ranking Flow Chart

B-3
 c. Abutment Vulnerability Flow Chart.

The abutment vulnerability assessment flow chart is intended to evaluate the relative vulnerability of a bridge to
scour considering factors that affect abutment scour. A separate evaluation is provided for each abutment
because the scour producing parameters may vary at each one, although it is expected that the abutment
foundation configuration will remain the same. The left and right directions are established looking downstream.
The parameters evaluated in the abutment vulnerability ranking flow chart reflect their relative effect on scour
vulnerability as discussed for the office review flow chart. The rationale for their use follow:

(1) Scour Countermeasures - Installation of a wall or spur dike (guide bank) represent a relatively permanent
countermeasure and are, therefore, provided the lowest value. Riprap and other countermeasures are con-
sidered temporary and are, therefore, given a higher value. The absence of scour countermeasures warrant
assignment of the highest value. Location that do not require scour countermeasures, as indicated in the
general conditions flow chart, should be given a value of 0 for this parameter.

(2) Abutment Foundation - The value assigned to each classification of abutment configuration and foundation
type reflects their relative susceptibility to scour as discussed for the office review flow chart.

(3) Abutment Location on River Bend - An abutment located on the outside of a bend is more susceptible to scour
than one on the inside of the bend or one on a straight channel and is, therefore, given a higher value than the
other conditions.

(4) Angle of Inclination - The angle of inclination is determined in accordance with Figure 4.11 of the Technical
Advisory. Relative values are assigned to each range of angles.

(5) Embankment Encroachment - The magnitude of the scour encroachment is reflected in most of the abutment
scour equations, therefore, this parameter is included in the chart. A large encroachment would be consid-
ered one that substantially reduces the overbank flow area available for the conveyance of peak discharges.
A small encroachment would be considered one that impacts less than 10 percent of the total discharge for
the design discharge.

The abutment vulnerability score for each abutment is tabulated and summarized at the bottom of the form. The
intermediate vulnerability score from the general conditions flow chart is also tabulated and added to the total
abutment score to yield the subtotal, which is the final score, if the bridge does not have any piers. The
presence of piers necessitates continuation of the evaluation by proceeding to the pier vulnerability ranking
flow chart.

B-4
 Abutment Scour Vulnerability Ranking Flow Chart

Bridge # Feature Carried Stream

Community County
Bridge Type Spans

Left Abutment Right Abutment

Scour Countermeasures Scour Countermeasures

Riprap Wall Spur Other None Riprap Wall Spur Other None
1 0 0 1 2 1 0 0 1 2

Abutment Foundation (Left) Abutment Foundation (Right)

Vertical Vertical Vertical Vertical Spill thru Spill thru Vertical Vertical Vertical Vertical Spill thru Spill thru
Wall Wall Wall Long Wall Long Spread Other Wall Wall Wall Long Wall Long Spread Other
Spread Short Piles Piles Piles >20' Unknown Spread Short Piles Piles Piles >20' Unknown
Unknown <19' >20' Wood Not Wood Wood or Unknown <19' >20' Wood Not Wood Wood or
Short Piles Short Piles
5 4 3 2 1 0 5 4 3 2 1 0

Abutment Location on River Bend Abutment Location on River Bend

Inside Outside Inside Outside

0 1 0 1

Angle of Inclination (Degrees) Angle of Inclination (Degrees)

0 0-19 20-44 45-90 >90 0 0-19 20-44 45-90 >90

0 1 2 3 4 0 1 2 3 4

Embankment Encroachment Embankment Encroachment

Small Medium Large Small Medium Large

0
0 1 2 1 2

Left Abutment Right Abutment

Vulnerablility Score____________ Vulnerablility Score____________
Left and Right are established looking downstream

Abutment Scour Vulnerability

Left Abutment____________ Right Abutment____________ Total____________
General Conditions Vulnerability Score Total____________

Subtotal____________
(Final score if there are points)
Proceed to Pier Scour Vulnerability Ranking Flow Chart if Necessary
R9800146

Figure B-2—Abutment Scour Vulnerability Ranking Flow Chart

B-5
 Pier Vulnerability Ranking Flow Chart
Bridge # Feature Carried Stream
Community County
Bridge Type Spans
Pier #1 Pier #2
Scour Countermeasures Scour Countermeasures

Riprap Wall Cofferdam Other None Riprap Wall Cofferdam Other None
1 0 0 1 2 1 0 0 1 2

Pier Foundation Pier Foundation

Spread/Unkown Piles Spread/Unkown Piles

1 0 1 0

Skew Angle (Degrees) Skew Angle (Degrees)

0 0-9 10-20 >20 0 0-9 10-20 >20

0 1 2 3 0 1 2 3

Pier/Pile Bottom Pier/Pile Bottom

Below Streambed Below Streambed

<3 3-5 6-9 10-14 15-20 >20 <3 3-5 6-9 10-14 15-20 >20
5 4 3 2 1 0 5 4 3 2 1 0

Pier Width Pier Width

<3 3-4 5-7 8-9 >10 <3 3-4 5-7 8-9 >10
0 1 2 3 4 0 1 2 3 4
Pier #1 Vulnerability Pier #2 Vulnerability
Score: __________ Score: __________

Pier #3 Pier #4
Scour Countermeasures Scour Countermeasures

Riprap Wall Cofferdam Other None Riprap Wall Cofferdam Other None
1 0 0 1 2 1 0 0 1 2

Pier Foundation Pier Foundation

Spread/Unkown Piles Spread/Unkown Piles

1 0 1 0

Skew Angle (Degrees) Skew Angle (Degrees)

0 0-9 10-20 >20 0 0-9 10-20 >20

0 1 2 3 0 1 2 3

Pier/Pile Bottom Pier/Pile Bottom

Below Streambed Below Streambed

<3 3-5 6-9 10-14 15-20 >20 <3 3-5 6-9 10-14 15-20 >20
5 4 3 2 1 0 5 4 3 2 1 0

Pier Width Pier Width

<3 3-4 5-7 8-9 >10 <3 3-4 5-7 8-9 >10
0 1 2 3 4 0 1 2 3 4
Pier #3 Vulnerability Pier #4 Vulnerability
Score: __________ Score: __________

Pier Vulnerability Ranking Score Summary

Pier #1_____ Pier #2_____ Pier #3_____ Pier #4_____
Pier with maximum score: Pier #______
Subtotal from abutment scour vulnerability:

Total Vulnerability Score:

R9800148

Figure B-3—Pier Vulnerability Ranking Flow Chart

B-7
 d. Pier Vulnerability Flow Chart.

The pier vulnerability assessment flow chart is intended to evaluate the relative vulnerability of a bridge to scour
considering factors that affect pier scour. A separate evaluation is provided for each pier because the scour
producing parameters may vary at each one. The piers are numbered sequentially from the left abutment, with
the left side established looking downstream.

The parameters evaluated in the pier vulnerability ranking flow chart reflect their relative effect on scour. The
rationale for their use follows:

(1) Scour Countermeasures - The rationale is the same as presented for the abutment flow chart.

(2) Pier Foundation - A spread footing or unknown foundation condition warrants a higher value than a pile
foundation.

(3) Skew Angle - The skew angle ranges reflect the relative effect on scour potential as indicated in Table 4.3
of the Technical Advisory (FHWA 1987).

(4) Pier/Pile Bottom Below Streambed - This parameter reflects the relative susceptibility to scour based on
the depth of the footing or pile bottom to the streambed elevation. The highest value is assigned to a
depth of three feet or less because this is the normal depth of spread footings. Deeper footing or pile
bottom elevations warrant lower ranking values. Depths greater than twenty feet are arbitrarily assigned
the lowest value.

(5) Pier Width - The pier width reflects the maximum expected scour in accordance with pier scour questions
as indicated in the Technical Advisory. The range of three to five feet in the pier width represents the
normal dimensions expected. No adjustment for debris or ice accumulation is used here because it is
reflected in the general conditions flow chart.

The pier vulnerability score is tabulated for each pier evaluated. The values are summarized and the value of
the most vulnerable pier added to the subtotal from the abutment vulnerability flow chart to determine the total
vulnerability score.

B-6
APPENDIX C
 FOREST SERVICE SCOUR EVALUATION PROCESS
PLAN OF ACTION
Region ___________________________________
Forest ___________________________________
Route ID & MP __________________________
Name ___________________________________
Feature Crossed _________________________
NBIS / Non-NBIS ________________________

BMC / INFRA Codes

Scour Critical ____________
Scour Vulnerability ________

Description of Bridge Vulnerability

____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________

Recommended Scour Countermeasures

____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________

Implementation Plan
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
_____________________________________________________________________________________

Bridge Closure Plan

____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
______________________________________________________________________________________________________________________________________________________________________
C-1