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ROAD DESIGN
STANDARD

PART 3. DRAINAGE
CAM PW.03.103.99

2003
This document has been produced for the Kingdom of Cambodia as a joint Australia –
Cambodia project sponsored by the Australian Agency for International Development
(AusAID).

Valuable assistance and operational advice was provided by the staff of the Cambodian
Ministry of Public Works and Transport (MPWT) as follow:
I. Steering Committee (Appendix C)
1. Mr. Tan Hay Sien, Director of Infrastructure Department ............................................ Chairman
2. Dr. Yit Bunna, Director of Public Works Research Centre ........................Deputy Chairman
3. Mr. Tauch Chan Kosal, Director of Heavy Equipment Centre ............................................... Member
4. Mr. Lim Sidenine, Deputy Director of Bridge Construction Unit.................................... Member
5. Dr. Phung Katry, Director of Waterway Department ................................................... Member
6. Mr. Prum Sakun, Deputy Director of Cambodian Royal Railway................................. Member
7. Representative from Sihanouk Ville Port (Mr. Ma Sun Huot)................................................... Member
8. Representative from Public Works Laboratory (Mr. Keo Leap)................................................ Member
9. Representative from Phnom Penh Institute of Technology (Mr. Chhouk Chhay Horng).......... Member
10. Representative from Phnom Penh Public Works Department (Mr. Heng Nguon) ................... Member
11. Representative from Ministry of Water Resources and Meteorology ....................................... Member

II. Assistance Preparation Staff from Public Works Research Centre:

1. Mr. Nou Vaddhanak
2. Mr. Kong Sophal
3. Mr. Chan Somardy
4. Mr. Tep Virith

Technical research and specialist input was provided by the Australian consulting firms of
McMillan Britton & Kell Pty Limited and Willing & Partners Pty Ltd.

Reproduction of extracts from this publication may be made subject to due acknowledgment
of the source.

Although this publication is believed to be correct at the time of printing, neither the MPWT nor
AusAID accept responsibility for any consequences arising from the use of the information
contained in it. People using the information should apply, and rely upon, their own skill and
judgement to the particular issue which they are considering.

SECOND PRINTING
FINANCED BY THE ASIAN DEVELOPMENT BANK LOAN NO. 1659 CAM (SF)
 ROAD DESIGN STANDARD

FOREWORD

The Cambodia Road Design Standard is intended to be used for the design of all
new roads in the Kingdom of Cambodia. The Cambodian Road Design Standard
consists of the following complementary documents which shall be considered
together:

- CAM PW.03.101.99 Road Design Standard – Part 1 - Geometry;

- CAM PW.03.102.99 Road Design Standard – Part 2 - Pavement;

- CAM PW.03.103.99 Road Design Standard – Part 3 - Drainage;

For the purpose of regulating and interpreting the provisions of this Standard, the
AUTHORITY shall be the Cambodian Ministry of Public Works and Transport.

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

FOREWORD
 ROAD DESIGN STANDARD

TABLE OF CONTENTS

3.1 INTRODUCTION .................................................................................. 3

3.1.1 General ................................................................................................ 3
3.1.2 Approach and Philosophy ................................................................. 3
3.2 HYDROLOGY....................................................................................... 4
3.2.1 General ................................................................................................ 4
3.2.2 Rational Method.................................................................................. 4
3.2.3 Regional Flood Frequency Analysis................................................. 7
3.3 DESIGN FLOOD STANDARDS........................................................... 8
3.3.1 Introduction......................................................................................... 8
3.3.2 Design Floods ..................................................................................... 8
3.3.2.1 General ................................................................................................. 8
3.3.2.2 Level of Serviceability to Traffic Design Standard................................ 9
3.3.2.3 Environmental Impact ......................................................................... 10
3.4 HYDRAULIC DESIGN........................................................................ 11
3.4.1 Introduction....................................................................................... 11
3.4.2 Design Considerations .................................................................... 11
3.4.2.1 Headwater .......................................................................................... 11
3.4.2.2 Culvert in Plan .................................................................................... 12
3.4.2.3 Vertical Profile..................................................................................... 13
3.4.2.4 Multiple Cells ...................................................................................... 14
3.4.2.5 Increasing Capacity of Culverts.......................................................... 14
3.4.2.6 Culverts in Flat Terrain ....................................................................... 14
3.4.2.7 Siltation ............................................................................................... 15
3.4.2.8 Site Investigation ................................................................................ 16
3.4.2.9 Safety.................................................................................................. 16
(a) Traffic safety ....................................................................................... 16
(b) Child safety ......................................................................................... 17
3.4.3 Hydraulics ......................................................................................... 17
3.4.3.1 General ............................................................................................... 17
3.4.3.2 Control at Inlet .................................................................................... 18
3.4.3.3 Control at Outlet.................................................................................. 19
3.4.3.3.1 Determination of energy head (H) ...................................................... 20
3.4.3.3.2 Determination of Headwater Depth .................................................... 22
3.4.3.3.3 Determination of h0 ............................................................................. 22

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3.4.4 Design Procedure ............................................................................. 23

3.4.5 Culvert End Treatment ..................................................................... 28
3.4.5.1 Introduction ......................................................................................... 28
3.4.5.2 Typical End Treatments...................................................................... 28
3.4.5.3 Scour at Inlets..................................................................................... 29
3.4.5.4 Scour at Outlets .................................................................................. 29
3.4.5.5 Scour Protection ................................................................................. 29
3.5 CHARTS AND NOMOGRAPHS ........................................................ 32
APPENDIX A Rainfall Intensity - Duration - Frequency........................................50
APPENDIX B.....................................................................................................................52
APPENDIX C.....................................................................................................................54

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TABLE OF CONTENTS
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3.1 INTRODUCTION

3.1.1 GENERAL

This document provides guidance on the selection of the design floods required for the
various aspects of the design of waterway structures and the hydraulic design of culverts.

The hydraulic design of waterway structures is of considerable economic importance, as

these structures can comprise up to 30 % of total road construction costs. The selection of
the appropriate design flood and good practice in the design of these structures
determines the initial cost, ongoing maintenance costs, the provision of the desired level of
serviceability to traffic and the safety of road users.

3.1.2 APPROACH AND PHILOSOPHY

This document incorporates guideline and recommended design values for the estimation
of design floods and the design of culverts. Prescriptive limiting values are not provided so
designers may use different design criteria based on project specific investigation or
additional information not available when this document was drafted. Where circumstances
warrant designers have the liberty, and perhaps the duty to use other procedures and data
not provided in this standard. Where they are based on observed data the use of new or
improved procedures is encouraged, especially where these are more appropriate than the
methods described in this standard.

It is expected that designers of small projects where extensive investigation cannot be

justified will employ the guideline and recommended design values provided.

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3.2 HYDROLOGY

3.2.1 GENERAL

Users of this standard should be aware of the difference between design floods and floods
resulting from actual rains. A design flood is a probabilistic or statistical estimate, being
generally based on some form of probability analysis of flood or rainfall data. An average
recurrence interval or exceedance probability is attributed to the estimate. If a design
rainfall is used in the estimation of a flood it is not intended to imply that if a rainfall of that
amount occurred vat a given time, the estimated flood would result. Occurrence of the
rainfall when the catchment was wet might result in a very large flood of magnitude greater
than the design estimate, while the occurrence of the rainfall when the catchment was dry
might result in relatively littler, or even no runoff.

The approach to estimating an actual flood from a particular rainfall is quite different in
concept and is of a deterministic nature. All causes and effects require consideration.

Due to the lack of rainfall and other climatic data little rainfall intensity data is available and
thus only limited rainfall intensity duration and frequency design values are provided.
Considerable investigation and analysis is needed to provide rainfall design values for the
majority of the country for which design values has not been provided.

3.2.2 RATIONAL METHOD

The “Rational Equation Method” is used for calculation of the maximum water discharge
for a specific run off area, formula follows;

Q = 0.278 C I A ................................................................ (3-1)

Where:
Q = flow in cubic meters per second (m3 / sec )
C = run off coefficient, expressing the fraction of the rainfall that is assumed to
become direct runoff.
I = intensity of rainfall in mm/hour, for the duration corresponding to the time of
concentration (Tc) of each catchment area. See formula (3-2) below.
A = the drainage catchment area in km2
Tc = the time of concentration is the time period ( duration ) required for the rain water
to reach the outlet from the most remote point of the area.
The formula used is:

Tc = (0.87 L3 / H )0.385 .......................................................... (3-2)

Where:
L = the length of the catchment area in Km
H = the corresponding level difference in metres

Notes:
1) The Rational Formula is based on the theory that the runoff rate is linearly related to
rainfall intensity. This means that the runoff rate would become constant if a uniform
rain of a constant intensity falls on an impervious specific area. The actual runoff,
which varies over the area, is however far more complex than the formula indicates.

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3.2 HYDROLOGY
 ROAD DESIGN STANDARD

2) Since the error of runoff estimate increases with increasing size of the drainage area ,
the Rational Formula is normally limited to an area size of about 25 km2 (2500 ha ).
3) For larger areas the formula should be used with care and the catchment split into
small areas with uniform runoff coefficient rates. Other empirical, graphical or
statistical formulas should be considered where catchment areas exceed 25 km2.
4) Rainfall intensity, duration and frequency values for some centres are provided in
Appendix 1. Typical values of runoff coefficients for use in either rural or urban areas
are shown in Tables 3.2.1 and 3.2.2. The latter should be used carefully to provide
indicative values only.

Table 3.2.1 Typical C Coefficients Urban Areas

Description of Area Runoff Coefficients
Business
Downtown areas 0.70 - 0.95
Neighbourhood areas 0.50 - 0.70
Residential
Single family areas 0.60 - 0.50
Multiunit, detached 0.40 - 0.60
Multi-unit , attached 0.60 - 0.75
Residential ( suburban ) 0.25 - 0.40
Apartment dwelling areas 0.50 - 0.70
Industrial
Light areas 0.50 - 0.80
Heavy areas 0.60 - 0.90
Parks, cemeteries 0.10 – 0.25
Playgrounds 0.20 – 0.35
Railroad yard areas 0.20 – 0.40
Unimproved areas 0.10 – 0.30
Streets
Asphaltic 0.70 – 0.95
Concrete 0.80 – 0.95
Brick 0.70 – 0.85
Drives and walks 0.75 – 0.85
Roofs 0.75 – 0.95
Lawns; sandy soil
Flat, 2 % 0.05 – 0.10
Average 2-7 % 0.10 – 0.15
Steep 7 % 0.15 – 0.20
Lawns; heavy soil
Flat, 2 % 0.13 – 0.17
Average 2-7 % 0.18 – 0.22
Steep 7 % 0.25 – 0.35

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Table 3.2.2: Typical C Coefficients Rural Areas

Watershed Characteristics
A B C D
Relief Soil Infiltration Vegetation Cover Surface Storage
0.40 0.20 0.20 0.20

Steep rugged No effective soil No effective plant Negligible surface

terrain; average cover. Either rock or cover. Bare or very depression.
slopes greater than thin soil mantle. sparse soil cover Drainage paths with
30 % Negligible infiltration steep banks and
capacity small storage
capacity. No ponds
or marshes.
0.30 0.15 0.15 0.15

Hilly with average Slow to take up Poor to fair; clean Low well defined
slopes of 10 % to water, clay, or other cultivated crops or system of small
30 % soil of low infiltration poor natural cover ; drainage paths, no
capacity. less than 10 % of ponds or marshes
area under good
cover
0.20 0.10 0.10 0.10

Rolling with Normal , deep loam Fair to good. About Normal.

average slopes of 50 % cover in good Considerable
5 % to 10 % grassland, surface depression.
woodland or Storage typical of
equivalent cover prairie lands. Lakes,
ponds, and
marshes less than
20 % of area
0.10 0.05 0.05 0.05

Relatively flat land High, deep sand or Good to excellent; High surface
average slopes 0 % other soil that takes about 50 % of area depression storage
to 5 % up water readily in good grassland; capacity high.
and rapidly woodland or Drainage system
equivalent cover not sharply defined,
large flood plain
storage; large
number of ponds
and marshes

Note:
Runoff coefficient is equal to sum of coefficients from the appropriate block in Rows A,.B,.C
and D.

These runoff coefficients shall be proportioned to the percentage of area covered.

Example : A watershed consists of 20 ha of light industrial areas, 30 ha of apartment

dwelling areas , and 15 ha of residential single family areas. Total Area = 65 ha

Area Type Percent Of Total Coefficient

20 ha Light industrial 20/65 = 0.31 0.65
30 ha Apartment dwelling 30/65 = 0.46 0.60
15 ha Residential single family 15/65 = 0.23 0.40

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3.2.3 REGIONAL FLOOD FREQUENCY ANALYSIS

Regional flood analysis is a commonly used procedure to develop flood estimates for
catchments where little or no flood data exists. It is also a useful procedure providing an
independent assessment of design floods that are computed by other methods.

The regional flood frequency curves have their most useful applications in estimating the
flood potential of an ungauged catchment. Regional flood frequency curves show the ratio
of floods for a given return period relative to the mean annual flood (Qm). It is therefore
necessary to make an estimate of the mean annual flood (Qm) for the ungauged
catchment. The mean annual flood (Qm) is dependent upon many variables, the most
important and commonly available being the drainage area. The mean annual flood (Qm)
for a particular catchment is determined graphically by plotting mean annual floods against
respective drainage areas of all gauged stations in the region on logarithmic paper. The
flood of any given frequency for the ungauged area is then obtained by determining the
corresponding flood ratio from the regional frequency curve for the region of which the
ungauged basin is a part and multiplying it by the estimated mean annual flood of the
ungauged basin.

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3.3 DESIGN FLOOD STANDARDS

3.3.1 INTRODUCTION

The probability terms used in this Guide are the same as those adopted in Australian
Rainfall and Runoff (Institution of' Engineers, Australia, 1987). They are "average
recurrence interval" and "annual exceedance probability". The definitions of these two
terms are as follows:

Average recurrence interval (ARI) is the average or expected value of the period
between exceedances of a given discharge

Annual exceedance probability (AEP) is the probability of exceedance of a given

discharge within a period of one year.

The word "average" is the key part of the definition of recurrence interval. This is because
hydrological events are generally random occurrences and it cannot be inferred that a
flood of a particular ARI will be exceeded at regular intervals. It is important that design
engineers understand that the periods between exceedances are generally random, and
that they convey and explain this to those who make decisions on the basis of their
investigations and designs, and to members of the public who are affected by them.

Average recurrence interval is usually expressed as the reciprocal of the annual

exceedance probability. The following examples are provided to explain the relationship
between average recurrence interval (ARI) and annual exceedance probability(AEP). The
probability that the 100 year ARI peak flow will be equaled or exceeded in any one year is
0.01 or 1 % and the probability that the 50 year ARI peak flow will be equaled or exceeded
in any one year is 0.02 or 2 %.

3.3.2 DESIGN FLOODS

3.3.2.1 General

In designing stream crossings and associated waterway structures, there are several
aspects of the design that may require the use of design floods with different average
recurrence intervals. These various aspects of design are as follows:

• Overall design of the total waterway of a stream crossing, including protection works
to bridge abutments, culvert inlets and outlets, and floodways.
• Level of serviceability to be provided to traffic.
• Serviceability limit state for the bridge structure.
• Ultimate limit state for structural strength and stability of the bridge structure.
• Environmental impact of the waterway structure on the stream and its environs.

Due to the wide variation in conditions throughout Cambodia and the different standards
that may be adopted from time to time by the road authorities it is not possible to make
specific recommendations on the ARIs of the floods that should be used for the various
aspects of design. Hence, the following recommendations should only be taken as a
general guide to what is desirable.

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3.3 DESIGN FLOOD STANDARDS
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3.3.2.2 Level of Serviceability to Traffic

The level of serviceability to be provided to traffic at a stream crossing will depend on the
serviceability requirements of the road system. Although the probability of closure of a road
link is dependent on the failure of the road as a whole, and not the failure of a particular
stream crossing, it is normal practice to design each stream crossing on a road link for
some predetermined level of serviceability.

The selection of the level of serviceability to be provided at each waterway structure (as
distinct from the stream crossing) on a road link is generally based on the following criteria:

• The level of service expected by the community

• The availability of alternative routes and period of closure.
• The importance of the road as access in emergency situations, such as to hospitals,
airports etc.
• Relationship between traffic density and composition and the wet season.
• Economic considerations.

In addition, the requirements of Local Authorities, Environmental Protection Authorities,

and those responsible for navigation and flood control, will also influence the size and type
of waterway structures and, hence, impact on the level of serviceability provided.

Where it is likely that a higher level of serviceability will be required on a road in the future,
consideration should be given to staging the construction of the waterway structures at
stream crossings. This can be achieved by designing the initial stage so that it can be
upgraded without major structural changes.

There are two interrelated aspects to be considered when determining the level of
serviceability:
1. the frequency with which the road is closed to traffic, and
2. the time of closure.

It should be noted that for a particular class of road, frequent closures of short duration
may be acceptable, whereas, long duration closures of the same frequency may not be.
Conversely, there are situations where long duration, very infrequent closures may not
cause problems,

Typical levels of serviceability are provided in Table 3.3.1

Table 3.3.1 Recommended Design Recurrence Interval

Design Recurrence Interval

Design Standard U/R6 U/R5 U/R4 U/R3 U/R2 U/R1
Expressway 100
Highway 100
Rural

Provincial 50 50 20
District 5 5
Expressway 100
Arterial 100 50
Urban

Collector 50 20
Local 5 5

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3.3.2.3 Environmental Impact

There is no published information available on the ARI of the flood that should be used for
assessing and minimising possible environmental damage to a stream from the
construction of a road crossing. Each site should be investigated for possible problems that
might occur with a range of flood events, with emphasis on the more frequent events.
These factors, which are not only applicable to bridges but any waterway structure,
include:

• Selection of a suitable site.

• Provision of an adequate waterway opening to limit backwater effects and excessive
localised bed scour (Section 3.4.2.1).
• Protection of banks from erosion resulting from the redirection of flow and turbulence,
or from excessive increase in velocity,
• Protection of natural vegetation especially where it protects or stabilises natural
banks.
• Control of roadside drainage, where it enters the stream, to limit bank erosion.
• Provision of adequate waterway openings to maintain a natural supply of floodwater to
wetland areas.
• Provision of an adequate number of waterway openings, in wide flood areas in and
regions, to ensure that water is not prevented from reaching areas downstream from
the road, which could lead to the death of vegetation.

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3.3 DESIGN FLOOD STANDARDS
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3.4 HYDRAULIC DESIGN

3.4.1 INTRODUCTION

This Chapter provides guidance on the hydraulic design of culverts, culvert end treatments,
the design of scour protection, debris control and an introduction to improved culvert inlets.
The procedures for the hydraulic design of culverts are based on "Hydraulic Design of
Highway Culvert “, Hydraulic Engineering Circular No 5 (US Federal Highway
Administration 1985).

3.4.2 DESIGN CONSIDERATIONS

3.4.2.1 Headwater

Any culvert that constricts the natural stream flow will cause a rise in the upstream water
surface. The total flow depth in the stream measured from the invert of the culvert inlet is
termed headwater.

The available headwater will depend on the topography of the site and vertical road profile
in relation to topography. In flat or undulating country or where a high standard vertical
profile is used the available headwater may be limited by the height of the surrounding
ground or elevation at which the road formation cuts through higher ground some distance
from the culvert site. In such situations levee banks may be necessary to maintain the
headwater depth required as indicated in Section 3.4.2.6.

The most economical culvert is one which utilises all of the available headwater to pass the
design discharge, since the discharge increases with increasing head. However, it is not
always possible to utilise all of the available headwater, because of constraints which limit
the upstream water level. The two main factors to be considered when selecting the design
headwater are:

• Limits on backwater resulting from the presence of buildings upstream and /or the
inundation of agricultural land.
• The outlet velocity and the potential for scour.

Potential damage to adjacent property or inconvenience to owners should be of primary

concern in the design of all culverts. Expensive legal action and compensation may result,
if property owner's rights are neglected. In urban areas, the potential for damage to
adjacent property is greater, because of the number and value of properties that can be
affected.

Culvert installations under high embankments in rural areas may present the design
engineer with an opportunity to adopt a high headwater and allow ponding upstream to
attenuate flood peaks downstream. If deep ponding is considered, the consequences of
scour at the outlet and catastrophic failure of the embankment should be investigated.
When culverts are installed under high embankments, an appropriate investigation should
be made to evaluate the risk of a larger flood occurring or blockage of the culverts by
debris.

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3.4.2.2 Culvert in Plan

Ideally, a culvert should be placed in the natural channel (Figure 3.4.1). A culvert in this
location is usually aligned with the flow and little structural excavation and channel work
are required at the inlet and outlet, especially for shorter culverts.

In the case, where location in the natural channel would require an inordinately long
culvert, some stream realignment may be required (Figure 3.4.2). Culvert skew should not
generally exceed 45 degrees measured from a line perpendicular to the roadway
centreline. If the skew is greater than 45 degrees special consideration needs to be given
to the hydraulic efficiency of the wingwalls.

Culvert alignments square to the road centreline are not recommended where severe or
abrupt changes in channel alignment are required upstream or downstream of the culvert.
Small radius bends are subject to erosion on the concave bank and deposition on the
inside of the bend. Such changes, upstream of the culvert, result in poor alignment of the
approach flow to the culvert with resulting loss of hydraulic efficiency, subject the
embankment to erosion and increase the probability of deposition in the culvert cell. Abrupt
changes in channel alignment downstream of culverts may also cause erosion or
deposition of material in adjacent properties.

CHANNEL
CHANNEL

ROAD
C
L

Figure 3.4.1

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3.4 HYDRAULIC DESIGN
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NATURAL
CHANNEL

CHANNEL
CHANGE

ROAD
CL

ALTERNATE
ALTERNATE CULVERT
CULVERT LOCATION
LOCATION

CHANNEL
RELOCATED CHANGE
CHANNEL

RECOMMENDED NOT RECOMMENDED

Figure 3.4.2

3.4.2.3 Vertical Profile

chosen for either economic or hydraulic reasons. Modified culvert slopes, or slopes other
than that of the natural stream, can be used to prevent stream degradation, minimise
sedimentation, improve the hydraulic performance of the culvert, shorten the culvert, or
reduce structural requirements. Modified slopes can also cause stream erosion and
deposition. Slope alterations should, therefore, be given special attention to ensure that
detrimental effects do not result from the change.

Channel changes often result in culverts steeper than the natural channel. A modified
culvert slope can be used to achieve a flatter gradient to prevent channel degradation.
Figure 3.4.3 illustrates possible culvert profiles.

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PAVED
STREAMBED LOCATION DEPRESSED INLET

USE CHUTE
WHERE
NECESSARY
DEPOSITION

OPEN OR
CLOSED
CHUTE

SIDEHILL LOCATIONS

CHANNEL EXCAVATION
HEADCUT
STABLE CHANNEL
GRADIENT

DEGRADING CHANNEL

Figure 3.4.3

3.4.2.4 Multiple Cells

It is important to select a culvert shape that will best fit the waterway of the channel or
stream. In narrow deep channels, a small number of large diameter pipes or box culverts
are usually appropriate. In flat areas having no well defined waterway the flood may be
larger in volume, but of shallow depth. A number of separate culverts spread over the width
of the flooded area may be more appropriate for these conditions.

Special consideration should be given to multiple cell culverts where the approach flow is
of high velocity, particularly if supercritical. These sites are best suited to a single cell or
special inlet treatment to avoid adverse hydraulic jump effects.

3.4.2.5 Increasing Capacity of Culverts

Changed land use, such as urbanisation upstream from an existing crossing may increase
the magnitude of flooding and necessitate increasing the culvert capacity to accommodate
additional flow without exceeding a given headwater elevation. Before deciding that the
culvert has to be replaced by a larger structure, (assuming relief flow is not feasible), the
possibility of improving the inlet of the existing culvert should be investigated.

3.4.2.6 Culverts in Flat Terrain

In flat terrain, drainage channels are often ill defined or non-existent and culverts should be
located and designed for least disruption of the existing flow conditions. In these locations

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multiple culverts can be considered to have a common headwater elevation, although this
will not be precisely so. Figure 3.4.4 illustrates a design technique that can be used to
determine the combined capacity of multiple culverts with different invert levels and
capacities. The total discharge at any point of the headwater elevation for culverts 1 and 2,
on Figure 3.4.4, is the sum of the discharges Q1 and Q2.

Figure 3.4.4

In flat terrain it may be necessary to construct levee banks, as shown on Figure 3.4.5, to
achieve the design headwater at the culvert location. Where necessary, approval of the
local drainage authority should be obtained prior to construction of any levee banks.

LEVEE BANK TO MAINTAIN

DESIGN HEADWATER − SHOULD
BE EXTENDED FAR ENOUGH OUT
FROM EMBANKMENT TO MATCH
NATURAL SURFACE.
ROAD
DESIGN H.W.

Figure 3.4.5
3.4.2.7 Siltation

Culvert location in both plan and profile is of particular importance to the maintenance of
sediment-free culvert cells. Deposition occurs in culverts, because the sediment transport
capacity of flow within the culvert is often less than in the stream. The following factors
contribute to deposition in culverts:

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• Culverts often provide a wider flow width at low flows than natural streams. This
results in the flow depth and sediment transport capacity being reduced.
• Point bars (deposition) form on the inside of stream bends and culvert inlets placed at
bends in the stream will be subjected to deposition in the same manner. This effect is
most pronounced in multiple-cell culverts with the cell on the inside of the curve often
becoming almost totally plugged with sediment deposits.
• Abrupt changes to a flatter grade in the culvert or in the channel upstream of the
culvert will induce deposition. Gravel and sand deposits are common downstream
from the break in grade because of the reduced transport capacity in the flatter
section.

Deposition usually occurs at flow rates smaller than the design flow rate. The deposits may
be removed during larger floods, depending upon the relative transport capacity of flow in
the stream and in the culvert, compaction and composition of the deposits, flow duration,
ponding depth above the culvert and other factors.

3.4.2.8 Site Investigation

A site investigation must be carried out at each proposed culvert site. The extent and
complexity of the investigation will depend on the size, importance and cost of the
proposed culvert, site conditions, the height of the embankment, and the loading that will
be imposed on the foundation material and on the culvert itself.

Survey information should be sufficient to permit the culvert to be located in plan and
profile, and should include relevant physical features. In flat terrain the elevations of
important buildings upstream, such as houses, commercial property, roads or railways
should be recorded, if they are likely to be affected by backwater.

In scour prone areas, soil characteristics should be assessed to enable stream protection
strategies to be formulated. The design engineer should also know the nature of the
subsoil material underlying the streambed, unless it is obvious that it is sound bedrock or
other material that will not cause foundation problems. Detailed foundation investigations
should be carried out for all large culverts, unless it is certain that they will be founded on
sound bedrock.

3.4.2.9 Safety

(a) Traffic safety

An exposed culvert end (projecting from the plane of the batters) acts as an unyielding
obstruction, likely to bring an out of control vehicle to an abrupt stop, causing considerable
damage to the vehicle and high, deceleration forces on the occupants

Where a road safety barrier is not provided, culvert ends should be designed so that they
will not present an obstruction to vehicles running off the road. This can be achieved by
covering exposed sides with fill, providing headwalls or wingwalls which will not present an
obstruction, or mitering culvert ends flush with the embankment surface.

The location of culvert ends placed flush with the embankment slope should be indicated
by markers to reduce hazards to equipment operators and others. High culverts in
populated areas should be fenced whenever possible.

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The hazard presented by culverts under private and side-road entrances should be
minimised by placing them as far as practicable from the roadway, and avoiding the use of
headwalls.

(b) Child safety

Culverts can also be an attraction for adventurous and inquisitive children. At locations
where long culverts could be a hazard, especially in urban areas, fencing, swing gates or
grates at upstream ends should be considered to prevent entry. However , this may cause
blockages and reduce the efficiency of the culvert.

3.4.3 HYDRAULICS

3.4.3.1 General

The most important consideration in culvert hydraulics is whether the flow is subject to inlet
or outlet control. Figures 3.4.6 and 3.4.7 show the range of flow types commonly
encountered in culverts. For inlet control two distinct regimes exist, depending on whether
the inlet is submerged or not submerged. Outlet control occurs in long culverts, laid on flat
grades and with high tail-water depths. In designing culverts, the type of control is
determined by adopting the greater of the headwater depths calculated for both inlet
control and outlet control.

For the two types of control, different factors and formulae are used to calculate the
hydraulic capacity of a culvert. Under inlet control, the cross-sectional area of the culvert
cell, the inlet geometry and the amount of headwater or ponding at the entrance are of
primary importance. Outlet control involves the additional consideration of the elevation of
the tail-water in the outlet channel and the slope, roughness and length of the culvert cell.

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WATER SURFAC
E

A. PROJECTING END − UNSUBMERGED INLET

B. PROJECTING END − SUBMERGED INLET

C. MITRED END − SUBMERGED INLET

Figure 3.4.6 Flow Profiles for Culverts under Inlet Control

3.4.3.2 Control at Inlet

With culverts subject to inlet control, the important factors are the entrance conditions,
including the entrance type, existence and angle of headwalls and wing-walls, and the
projection of the culvert into the headwater pond.

For one dimensional flow, the theoretical relation between discharge and upstream energy
can be computed by an iterative process or by the use of nomographs. Sketches of inlet
control flow for both unsubmerged and submerged projecting entrances are shown on
Figures 3.4.6A and 3.4.6B. Figure 3.4.6C shows a mitred entrance flowing submerged
with inlet control.

Inlet control can occur with the inlet submerged and the outlet not submerged (Figure
3.4.6B). Under these conditions, the flow contracts to a supercritical jet immediately
downstream from the inlet. When the tail water depth exceeds critical depth, h and the
culvert is laid on a steep grade, flow remains supercritical in the cell and a hydraulic jump
will form near the outlet. If the culvert is laid on a slope less than critical, then a hydraulic
jump will form in the cell,

In inlet control the roughness and length of the culvert cell and the outlet conditions
(including depth of tail water) are not factors in determining culvert capacity. An increase in
the slope of the culvert reduces headwater only to a small degree, and can normally be
neglected for conventional culverts flowing under inlet control.

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WATER SURFACE

W.S.

A. CULVERT FLOWING FULL, SUBMERGED OUTLET

W.S.

B. CULVERT FLOWING FULL, UNSUBMERGED OUTLET

W.S.

C. CULVERT FLOWING FULL FOR PART OF LENGTH

W.S.

D. CULVERT NOT FLOWING FULL

Figure 3 4.7 Flow Profiles for Culverts under Outlet Control

3.4.3.3 Control at Outlet

Culverts flowing with outlet control can flow with the culvert cell full or with the cell part full
for all of the culvert length. With outlet control and both inlet and outlet submerged (Figure
3.4.7A) the culvert flows full under pressure. The culvert can also flow full over part of its
length then part-full at the outlet (Figure 3.4.7C). The point at which the water surface
breaks away from the culvert crown depends on the tailwater depth and culvert grade, and
can be determined by using backwater calculations.

If the Culvert is laid at a flat grade, outlet control can occur with both inlet and outlet not
submerged (Figure 3.4.7D), and part full flow throughout the cell is sub-critical. Minor

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variation of these main types can occur, depending on the relative value of critical slope,
normal depth, culvert height and tail-water depth.

The procedure given in Section 3.4.4 provides methods for the accurate determination of
headwater depths for the full flow condition and for the case of the cell part-full over part of
the culvert length. The method given for the condition of the cell part full over the total
length, gives a solution for headwater depth that decreases in accuracy as the headwater
decreases.

3.4.3.3.1 Determination of energy head (H)

The head, H (Figure 3.4.8) or energy required to pass a given flow through a culvert
operating under outlet control is made up of three major parts. These three parts are
usually expressed in metres of water and include a velocity head , Hv , an entrance loss,
He, and a friction loss Hf .

The energy head is expressed as follows:

H = Hv + He + H f
......................................................... (3-3)

The velocity head, H v is given by,

2
H =V
2g ......................................................................... (3-4)

Where V is the mean velocity in the culvert cell and g is the acceleration due to gravity.
The mean velocity is the discharge, Q, divided by the cross-sectional area, A, of the cell.

The entrance loss is expressed as,

V2
He = K e
2g ........................................................................ (3-5)

The loss coefficient, Ke depends on the inlet geometry, primarily through the effect it has on
contraction of the flow. Values of Ke, determined from experiment, range from 0.2 for a
well rounded entrance, through 0.5 for a square edged inlet in a vertical headwall to 0.9 for
a sharp pipe (e.g. corrugated steel) projecting from an embankment. K coefficients are
given in Table 3.5.1.

Since most engineers are familiar with Manning's n, the following expression is used to
calculate the friction loss, Hf along the conduit:

2 g n2 L V 2
Hf = ×
R1.33 2 g ............................................................ (3-6)
A
R =
Wp .......................................................................... (3-7)

Where,

n = Manning's friction factor. Refer Table 3.5.2

L = length (m) of culvert cell

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V = mean velocity (m/sec) of flow in culvert cell

g = acceleration due to gravity = 9.80 m/s2,
R = hydraulic radius (m)
A = area (m2 ) of flow for full cross section
Wp = wetted perimeter (m)

Substituting in Equation (3-3) and simplifying, we get for full flow:

 2 g n2 L  V 2
H = 1 + K e + ×
 R1.33  2 g
........................................... (3-8)

Figure 3.4.8 shows the terms of Equation (3-8), the energy line, the hydraulic grade
line and the headwater depth, HW. The energy line represents the total energy at any
point along the culvert cell. The hydraulic grade line is defined as the pressure line to
which water would rise in small vertical pipes attached to the culvert wall along its length.
The difference in elevation between these two lines is the velocity head, V2 / 2g.

By referring to Figure 3.4.8 and using the culvert invert the outlet as datum, we get:

2
h1 + V1 + LS = h2 + H v + H e + H f
2g .............................. (3-9)

then,
2
h1 + V1 + LS − h2 = H v + H e + H f
2g ............................ (3-10)

and
2
H = h1 + V1 + LS − h2 = H v + H e + H f
2g .................... (3-11)

From the development of this energy equation and Figure 3.4.8, H is the difference
between the elevation of the hydraulic grade line at the outlet and the energy line at the
inlet. Since the velocity head in the entrance pool is usually small under ponded
conditions, the water surface of headwater pool elevation can be assumed to equal the
elevation of the energy line.

Equation (3-8) can be readily solved for H by the use of the full flow nomographs
shown on Figures 3.5.6 to 3.5.7.

2
V1 2
2g V
2g He

W.S.
V1
ENERGY LIN
HYDRAULIC GRA
E Hf
DE LINE
HW h1
Hv W.S.
V
S h2
LS DATUM

Figure 3.4.8 Hydraulics of a Culvert Flowing Full under Outlet Control

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3.4.3.3.2 Determination of Headwater Depth

HW can be determined from an equation for outlet control:

HWo = H + h0 − LS ............................................................ (3-12)

Where,
H = head (m) determined from Figures 3.5.6 to 3.5.7 or from Equation (3.11).
h0 = greater of TW and (hc + D)/2 in which hc ≤ D (see following discussion and
Figure 3.4. 0)
hc = critical depth (m) from Figures 3.5.4 and 3.5.5.
D = culvert height (m),
L = length (m) of culvert
S = slope (m/m) of cell

3.4.3.3.3 Determination of h0

The determination of h0 is an important factor in calculating both the headwater depth and
the hydraulic capacity of a culvert flowing under outlet control. Tailwater depth, TW, is the
depth from the culvert invert at the outlet to the water surface in the outlet channel.
Engineering judgement is required in evaluating possible tailwater depths. Tailwater is
often controlled by a downstream obstruction or by water levels in another stream. A field
inspection should be made to check on downstream conditions and flood levels. The
Slope Area Method can be used to calculate flow depths, if downstream conditions do not
provide an obvious control.

Fortunately, most natural streams are wide compared to the culvert and the depth of water
in the natural channel is considerably less than critical depth in the culvert section. In such
cases the natural tailwater does not govern.

Two tailwater conditions can occur with culverts operating under outlet control, (1) tailwater
above the top of the opening and (2) tailwater at or below top of opening:

(1) Tailwater above the top of opening - when the tailwater, TW in the outlet channel is
above the top of the culvert outlet, Figure 3.4.7A,

h0 = TW
............................................................................ (3-13)

The relationship of h0 to the other terms in Equation (3-12), for this situation, is illustrated
on Figure 3.4.9.

HW

D S TW = h0
LS

Figure 3.4.9 Determination of h0 for Higher Tailwater

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(2) Tailwater at or below top of opening - when the tailwater in the outlet channel is at
or below the top of the culvert outlet, as on Figure 3.4.7B, 3.4.7C and 3.4.7D, ho, is more
difficult to determine.

Full flow depth at the outlet, Figure 3.4.7B, will occur only when the flow rate is sufficient to
give critical depths equal or higher than the height of the culvert opening. For all such flows
the hydraulic gradeline will pass through the top of the culvert at the outlet and the head, H
can be added to the level of the top of the culvert opening in calculating HWo.

When critical depth is less than the height of the culvert opening, the water surface drops
as shown on Figures 3.4.7C and 3.4.7D, depending on the flow. For the condition shown
on Figure 3.4.7C, the culvert must flow full for part of its length. Flow profile computations
show that the hydraulic gradeline, if extended as a straight line from the point where the
water breaks away from the top of the culvert, will be at a height approximately halfway
between critical depth and the top of the culvert, at the culvert outlet, ie

h0 = 0.5 (hc + D ) .......................................................... (3-14)

This level should be used if it is greater than TW.

The head, H can be added to this level in calculating HWo. The relationship of ho to the
other terms in Equation (3-12) for this situation is illustrated on Figure 4.10.

As the discharge decreases the situation approaches that of Figure 3.4.7D. For design
purposes, this method is satisfactory for calculated headwater depths above 0.75D. For
smaller values of headwater, more accurate results can be obtained by flow profile
calculations or by the use of the capacity charts from Hydraulic Engineering Circular No 10
(US Federal Highway Administration, 1972).

HW
D S
LS hc TW

ho = GREATER OF hc + D AND TW
2

Figure3.4.10 Determination of h0 for Tailwater Below Top of Opening

3.4.4 DESIGN PROCEDURE

The design engineer should be familiar with all the equations in the previous Section
before using these procedures. Following the design method without an understanding of
culvert hydraulics can result in inadequate, unsafe, or costly structures. The procedure
does not address the effect of storage. The design procedure is summarised on the
Culvert Design Flow Chart, Figure 3.4.11.

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1. Assemble Site Data

• Site survey and locality map.

• Embankment cross-section.
• Roadway profile.
• Photographs, aerial photographs.
• Details from field visit (sediment, debris, and scour at existing structure).
• Studies by other authorities near the site, including small dams, canals, weirs,
floodplains, storm drains.
• Recorded and observed flood data

2. Determine Design Flood Discharge

• Determine ARI of design flood – see Section 3.3.2.

• Determine design flood, Q – see Section 3.3.2.

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Figure 3.4.11 Culvert design Flow Chart

3. Commence Summarising Data on Design Form

4. Select Trial Culvert

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• Choose culvert material, shape, size and entrance type.

• Determine the initial trial size of culvert, either by arbitrary selection or by assuming a
velocity (say 3 m/sec) and calculating a culvert area from A = Q/V.

5. Determine Inlet Control Headwater Depth, HWi - Use inlet Control Nomographs
Figure 3.5.2 to 3.5.3.

These nomographs cover various culvert types and inlet configurations. Each nomograph
has an example on it which is self-explanatory. Using the trial culvert size, the relevant
nomograph can be used to calculate HWi given a known Q. They can also be used in
reverse to calculate Q given a known HWI It should be noted that where the approach
velocity is considerable, the approach velocity head can be calculated and deducted from
the calculated HWi to give the actual physical head required.

6. Determine Depth, h0 for Outlet control

• Calculate both 0.5(hc + D) and the tailwater, TW, from known flood levels,
downstream controlling levels or from the Slope Area Method. If it is clear that the
downstream tailwater conditions do not control, take ho = 0.5(hc + D). hc can be
calculated from Figures 3.5.4 to 3.5.5. If hc exceeds D then take hc as D.
• h0 is the larger of TW or 0.5(hc + D)

7. Determine Outlet Control Headwater Depth at Inlet, HW0

• Determine entrance loss coefficient, Ke from Table 3.5.1.

• Calculate the losses through the culvert, H, using the outlet control nomographs,
Figures 3.5.6 and 3.5.7 (or Equation (3-8) if outside the range). As with the inlet
control nomographs, these nomographs cover various culvert types and each
nomograph has an self-explanatory example on it.
• If the Manning n value of the culvert under consideration differs from the Manning n
value shown on the nomograph, this can be allowed for by adjusting the culvert length
as follows:

n1
Lf = L
n ............................................................................ (3-15)

Where:
L1 = adjusted culvert length
L = actual culvert length
n1 = desired Manning n value
n = Manning n value given on the nomograph

• Calculate H W0,=H+ hc –LS

As with inlet control, where the approach velocity is considerable, the approach velocity
head can be calculated and deducted from the calculated HW0 to give the actual physical
head required.

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• If HW0 is less than 0.75D and the culvert is under outlet control, then the culvert may
be flowing only part full and using 0.5(hc +D) to calculate h, may not be applicable. If
required, more accurate results can be obtained by flow profile calculations or the use
of Hydraulic Engineering Circular No 10 (as discussed in Section 3.4.3.3.3 under (2)
Tailwater at or below top of opening).

8. Determine Controlling Headwater, H Wc

• Compare HWi and HW0 and use the higher:

• If HWi > HW0 the culvert is under inlet control and HWc = HWi
• If HW0 > HWi the culvert is under outlet control and HWc = HW0

9. Calculate Outlet Velocity, V0

The average outlet velocity will be the discharge divided by the cross-sectional area of flow
at the culvert outlet. The cross-sectional area of flow depends, in turn, on the flow depth at
the outlet.

If inlet control is the controlling headwater, the flow depth can be approximated by
calculating the normal depth, yn for the culvert cross-section using Manning's Equation,
The flow area, A is calculated using yn and the outlet velocity:

V0 = Q / A

The outlet velocity computed utilising the normal depth, yn will usually be high, because the
normal depth is seldom reached in the relatively short length of the average culvert.

If outlet control is the controlling headwater, the flow depth can be either critical depth, hc,
the tailwater depth, TW (if below the top of the culvert) or the full depth, D of the culvert
depending on the following relationships:

Use hc , if hc > TW

Use TW if hc < TW < D

Use D if D < TW

Calculate flow area using appropriate flow depth and then outlet velocity:

V0= Q / A

10. Review Results

Compare alternative design with varying constraints and assumptions. If any of the
following conditions are not met, repeat steps 4 to 9:

• The culvert must have adequate cover.

• The final length of the culvert should be close to the approximate length assumed in
design.
• The headwalls and wingwalls must fit the site.
• The allowable headwater should not be exceeded.
• The allowable overtopping flood frequency should not be exceeded.

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The performance of the culvert should also be considered, (i) with floods larger than the
design flood to ensure such rarer floods do not pose unacceptable risks to life or potential
for major damage, and (ii) with smaller floods than the design flood to ensure that there will
be no unacceptable problems of maintenance.

If outlet velocity is high, scour protection or an energy dissipator (see Section 3.4.5.5) may
be required.

11. Improved Designs

Under certain conditions more economic designs may be achieved by consideration of the
following:
• The use of improved inlets for culverts operating under inlet control.
• Level pool routing, if a large upstream headwater pool exists.

12. Documentation

Prepare report and file background information,

3.4.5 CULVERT END TREATMENT

3.4.5.1 Introduction

The term "end treatment" encompasses the shape of the culvert ends, end structures such
as wingwalls, cut-offs and anchorages, and erosion control measures for the adjoining fill
and channel. It does not include the design of hydraulically improved inlets.

Culvert end treatments may be required to perform one or more of the following functions:

• To prevent fill from encroaching on the culvert opening.

• To prevent erosion of the fill and adjacent channel.
• To prevent undermining of culvert ends.
• To inhibit the seepage and piping through the bedding and backfill.
• To meet traffic safety requirements (see Section 3.4.2.9).
• To improve the appearance of large culverts.
• To resist hydraulic uplift forces on corrugated metal pipe culverts.
• To strengthen the ends of large flexible culverts, especially those with mitred or
skewed ends.

Cutoffs in the form of a vertical wall, constructed below the end or apron of a culvert,
should always be provided at culverts inlets to prevent undermining and piping. For
corrugated metal pipe culverts, the cut-off walls also act to counteract uplift at the culvert
inlet.

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3.4.5.2 Typical End Treatments

Headwalls and wingwalls - are the most common end treatment. An apron is generally
incorporated between the wingwalls to limit scour of the stream bed. They are usually
constructed from reinforced concrete, but can be formed from masonry, or rock filled
gabions and mattresses, or concrete filled mattresses.

Mitred ends - these are generally limited to corrugated metal pipe culverts, where the end
of the pipe is cut parallel to the slope of the embankment. The area of embankment around
the ends of the culvert is usually paved with concrete or rock.

Projecting ends - where the ends of the culvert project from the face of the embankment.
Although they are the least costly end treatment, they are not commonly used because
they do not meet safety requirements and are visually objectionable.

3.4.5.3 Scour at Inlets

A culvert normally constricts the natural channel, forcing the flow through a reducing
opening. As the flow contracts, vortices and areas of high velocity flow impinge against the
upstream slopes of the embankment adjacent to the culvert. Scour can also occur
upstream of the culvert, as a result of the acceleration of the flow, as it leaves the natural
channel and enters the culvert.

Upstream wing walls, aprons, cut-off walls and embankment paving assist in protecting the
embankment and stream bed at the upstream end of a culvert.

3.4.5.4 Scour at Outlets

If the flow emerging from a culvert has a sufficiently high velocity and the channel is
erodible, the jet will scour a hole in the bed immediately downstream, and back eddies will
erode the stream banks to form a circular elongated scour hole. Coarse material scoured
from the hole will be deposited immediately downstream, often forming a low bar across
the stream, while finer material will be carried further downstream. Depending on the
supply of sediment, the scour hole may gradually refill until after the next major flood
occurs.

3.4.5.5 Scour Protection

The provision of wing walls, headwall, cut-off wall and apron is generally all the protection
that is required at culvert outlets. The judgement of design engineers, working in a
particular area, is required to determine the need for any further protection. As an aid in
evaluating the need for further protection, culvert outlet velocities should be computed and
compared with the natural velocities occurring in the stream. When comparing velocities, it
should be noted that in many streams the maximum velocity in the main channel is
considerably higher than the mean velocity for the whole channel cross section.
Investigation of scour and outlet protection at similar culverts in the vicinity of the culvert
being designed will provide guidance on whether further protection is required. Periodic
site visits and inspection after major flood events will also confirm whether the protection is
adequate or further protection is required.

If an unacceptable scour hole does develop, a decision must be made as to which type of
scour protection is suitable for the site. A choice must be made from the following:

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• Protection of the natural stream bed material.

• Flow expansion structure.
• An energy dissipating structure.

Stream bed protection can be achieved with a concrete apron, rock rip-rap, or rock
mattresses, or concrete filled mattresses. The Class of rock required to resist the velocity
of flow should be in accordance with the details given on Table 3.4.1, Design of Rock
Slope Protection. Details of the Class of Rock Protection are provided in Table 3.4.2,
Standard Classes of Rock Slope Protection.

Table 3.4.1, Design of Rock Slope Protection

Velocity (m/sec.) Class of Rock Protection, Section Thickness, T (m)
Wc (tonne)
<2 None ---
2.0 – 2.6 Facing 0.5
2.6 – 2.9 Light 0.75
2.9 – 3.9 0.25 1.00
3.9 – 4.5 0.5 1.25
4.5 – 5.1 1.0 1.60
5.1 – 5.7 2.0 2.00
5.7 – 6.4 4.0 2.50
> 6.4 Special ---

Table 3.4.2 Standard Classes of Rock Slope Protection

Rock Class Rock Size (m) Rock Mass (kg) Minimum
Percentage of
Rock Larger
Than
Facing 0.4 100 0
0.3 35 50
0.15 2.5 90
Light 0.55 250 0
0.40 100 50
0.20 10 90
0.25 tonne 0.75 500 0
0.55 250 50
0.30 35 90
0.50 tonne 0.90 1000 0
0.70 450 50
0.40 100 90
1 tonne 1.15 2000 0
0.90 1000 50
0.55 250 90
2 tonne 1.45 4000 0
1.15 2000 50
0.75 500 90
4 tonne 1.80 8000 0
1.45 4000 50
0.90 1000 90

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Mattresses should be selected on the basis of the details given on Table 3.4.3. It is
important that mattresses are anchored to the cut-off wall or apron at the culvert outlet, to
stop them moving downstream. A geotextile filter is usually provided under the mattresses
and may also be required under the rock rip-rap.

Table 3.4.3 Indicative Thickness of Rock Mattresses

Thickness Rock Fill Critical Velocity Limit Velocity
(m) Size (mm) D50 (mm) (m/sec) (m/sec)
0.15 – 0.17 70 – 100 85 3.5 4.2
70 – 150 110 4.2 4.5
0.23 – 0.25 70-100 85 3.6 5.5
70-150 120 4.5 6.1
0.3 70 - 120 100 4.2 5.5
100 - 150 120 5.0 6.4

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CAM PW 03-101-99
CHARTS AND NOMOGRAPHS ROAD DESIGN PART 3 - DRAINAGE

3.5 CHARTS AND NOMOGRAPHS

Item Description
Figure 3.5.1 Design Form for Culvert Calculations
Table 3.5.1 Entrance Loss Coefficients
Table 3.5.2 Roughness coefficients
Figure 3.5.2 Inlet Control Nomograph – Box Culvert
Figure 3.5.3 Inlet Control Nomograph – Concrete Pipe Culvert
Figure 3.5.4 Critical Depth in a Rectangular Section
Figure 3.5.5 Critical Depth in a Circular Pipe
Figure 3.5.6 Outlet Control Nomograph – Concrete Box Culvert Flowing Full
with n = 0.012
Figure 3.5.7 Outlet Control Nomograph – Concrete Pipe Culvert Flowing
Full with n = 0.012
Figure 3.5.8 Dimensions of Triangular Channel
Figure 3.5.9 Dimensions of Trapezoidal Channel with Side Slope 1:1
Figure 3.5.10 Dimensions of Trapezoidal Channel with Side Slope 1:2
Figure 3.5.11 Dimensions of Trapezoidal Channel with Side Slope 1:3
Figure 3.5.12 Dimensions of Trapezoidal Channel with Side Slope 1:4
Figure 3.5.13 Dimensions of Trapezoidal Channel with Side Slope 1:5
Figure 3.5.14 Dimensions of Trapezoidal Channel with Side Slope 1:6
Figure 3.5.15 Rip Rap Sizing

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 32-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

E N G IN E E R :
PRO JEC T : DATE:

H Y D R O L O G IC A L A N D C H A N N E L IN F O R M A T IO N SKETCH
S T A T IO N :
EL.

EL.
EL.

D E S IG N D IS C H A R G E
M E A N S T R E A M V E L O C IT Y =
C H E C K D IS C H A R G E M A X . S T R E A M V E L O C IT Y =

CONTROLLING
H E A D W A T E R C O M P U T A T IO N

VELOCITY
OUTLET
CULVERT
D E S C R IP T IO N S IZ E IN L E T C O N T . OUTLET CONTROL COST COM M ENTS
(E n tra n c e ty p e )

S U M M A R Y A N D R E C O M M E N D A T IO N S :

Figure 3.5.1 Design Form for Culvert Calculations

33-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Coefficient Ke to apply to velocity head V2 / 2g for determination of head loss, He, at

entrance to a culvert operating under outlet control.

V2
H e = Ke
2g

Table 3.5.1 Energy Loss Coefficients

Type of Barrel and Inlet Ke
Pipe, concrete:
0.2
Projecting from fill, socket end
0.5
Projecting from fill, socket cut end
Headwall or headwall and wingwalls
0.2
Socked end of pipe
0.5
Square-edge
0.2
Rounded (radius = 1/12D)
0.7
Mitred to conform to fill slope
0.5
End-section conforming to fill slope (standard precast )
0.2
Beveled edges , 33.7° or 45° bevels 0.2
Side-tapered or slope-tapered inlets
Pipe, or pipe-arch, corrugated steel:
0.9
Projecting from fill
0.5
Headwall or headwalls, square edge
0.7
Mitred to conform to fill slope
0.5
End-section conforming to fill slope (standard prefab)
0.25
Beveled edges, 33.7° or 45° bevels 0.2
Side-tapered or slope-tapered inlets
Box, reinforced concrete:
Headwall
0.5
Square-edged on 3 edges
Rounded on 3 edges to radius of 1/12 barrel dimension or
0.2
beveled edges on 3 sides
Wingwalls at 33° to 75° to barrel 0.4
Square-edged at crown
Crown edge rounded to radius of 1/12 barrel dimension 0.2
or beveled top edge 0.5
Wingwalls at 10° to 25° to barrel, square –edged at crown
Wingwalls parallel (extension of sides), square –edged at 0.7
crown 0.2
Side-tapered or slope-tapered inlet
Projecting 0.7
Square –edge 0.2
Beveled edges 33.7° or 45° to bevels

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 34-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Table3.5.2 – Drainage
Channel Description Mannings ‘n’ Vmax. m/sec
Natural Channels
A Without vegetation
Rock
Smooth and uniform 0.350-0.040 6.1
Jagged and irregular 0.040-0.045 4.5-5.5
Soils
Gravel 0.020-0.025 1.5-2.1
Sand 0.020-0.025 0.3-0.6
Silt 0.023-0.024 0.9-1.5
Clay 0.022-0.024 0.6-0.9
Organic clays & silts 0.022-0.024 0.6-0.9
Peat 0.022-0.025 0.6-0.9
B With vegetation
Average turf:
Erosion resistant soil 0.050-0.070 1.2-1.5
Easily eroded soil 0.030-0.050 0.9-1.2
Dense turf:
Erosion resistant soil 0.070-0.090 1.8-2.4
Easily eroded soil 0.040-0.050 1.5-1.8
Clean bottom with bushes on sides 0.050-0.080 1.2-1.5
Channel with tree stumps:
No sprouts 0.040-0.050 1.5-2.1
With sprouts 0.060-0.080 1.8-2.4
Dense weeds 0.080-0.120 1.5-1.8
Dense brush (flood plains) 0.100-0.140 1.2-1.5
Dense willows (flood plains) 0.150-0.200 2.4-2.7

Paved Channels
A Concrete, all surfaces: 0.012-0.017 6.1
Trowel finish
Float finish 0.013-1.015 6.1
Formed, no finish 0.014-0.016 6.1
B Concrete bottom, float finished, with sides of:
Dressed stone in mortar 0.015-0.017 5.5-6.1
Random stone in mortar 0.017-0.020 5.2-5.8
Dressed stone or smooth concrete 0.020-0.025 4.6
rubble (rip-rap)
Rubble or random stone (rip-rap) 0.025-0.030 4.6
C Gravel bottom, sides of:
Concrete 0.017-0.020 3.0
Random stone or rubble 0.020-0.023 2.4-3.0
Random stone or rubble (rip-rap) 0.023-0.033 2.4-3.0
D Brick 0.014-0.017 3
E Asphalt 0.013-0.016 5.5-6.1

35-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

3
(m /s per metre span)
4.00

70
EXAMPLE
3.50 60
2.00 x 0.80 m Box = 8.0 m3 /s (1) (2) (3)
50
= 4.0 m /s per m 3 8 9 10
3.00 40 8 9
7
7 8
30 inlet 6
7
6
(1) 4.5 3.60 5 6
2.50 5
20 (2) 4.8 3.84
4 5
(3) 5.6 4.48
4
4
m 3
r
2.00 pe 3
3 /s
10 m 3
0
9 4.
=
8
7 2
2
6 2
1.50 5
1.5
4 1.5
Angle of 1.5
3 Wingwall

HEADWATER DEPTH IN TERMS OF HEIGHT

E Flare
PL
AM
EX
HEIGHT OF BOX

m 2
8
0.
RATIO OF DISCHARGE TO WIDTH

= 1.0
1.00

0.9 1.0 1.0

0.90
1.0
0.8 0.9 0.9
0.9
0.80
0.8
0.7
0.7 0.8 0.8
0.70 0.6

0.5 0.7 0.7

0.6
0.60 0.4

0.3 0.6 0.6

0.5
0.50

0.2
0.5 0.5

0.40 0.1 0.4

0.09
0.08
0.07 0.4 0.4

0.06
0.05 0.35 0.35
0.30 B = Span per cell 0.3
0.04

Wingwall Flare Scale

30° − 75° 1
90° (headwall) 2
0° − (parallel) 3

Figure 3.5.2 Inlet Control Nomograph – Box Culvert

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 36-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

3
(m /s)

300
4.50
EXAMPLE (1) (2) (3)
200 6
4.00 6
= 0.80 m = 1.7 m 3 /s
5
6 5
3.50
100
inlet 4
(m) 5 4
80
3.00 (1) 2.60 2.08
60
(2) 2.18 1.74 4
50 3
(3) 2.20 1.76 3
40
2.50 3
30

20 2 2

2.00
2
E
PL
10 AM
EX 1.5 1.5
8 3 /s
m
7
6 1. 1.5
=
1.50
5
4

2 1.0 1.0
m
80 1.0
1.00 0.
=
0.9 0.9
1
0.90
0.8 INLET TYPE 0.9

0.6 0.8
0.80
0.5
(1) Headwall with 0.8
square edge. 0.8
0.4
0.70 (2) Headwall with
0.3 socket end. 0.7
0.7
0.7
0.60 (3) Projecting with
0.2 socket end.
0.15
0.6
0.50 0.6
0.1 0.6
0.09
0.08
0.07
0.06
0.05
0.40
0.04 0.5
0.5
0.5
0.03

0.02
0.30

Figure 3.5.3 Inlet Control Nomograph – Concrete Pipe Culverts

37-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

20
3
(m /s)
(m)
1000
7
15 800

600 6
500
400 5
10 300
9
4
200
8
150
7
3
100
6
80

5 60
50
40 2
4
30

20 = 1.50m
3
1.5
/s
3 15 = 11.5m

LE
EXAMP 10
8
= 2.00m 1.0
2 6
0.9
5
4 0.8

3 0.7
1.5

2 0.6

1.5
0.5
1.0 1.0
0.8
0.9 0.4
0.6
0.8
0.5
0.7 0.4
0.3
0.6 0.3

0.2
0.5
0.15
0.2
0.4 0.1
0.08
0.15
0.06
0.3 0.05

0.667
= 0.467
D
hc

CRITICAL DEPTH
RECTANGULAR SECTION

Figure 3.5.4 Critical depth in a Rectangular Section

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 38-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Figure 3.5.5 Critical Depth in a Circular Pipe

39-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Figure 3.5.6 Outlet Control Nomograph –

Concrete Box Culvert Flowing Full with n = 0.12

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 40-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Figure 3.5.7 Outlet Control Nomograph

– Concrete Pipe Culvert Flowing Full with n = 0.012

41-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Hydraulic radius (R), m

Figure 3.5.8 Dimension of Triangular Channels

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 42-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Hydraulic radius (R), m

Figure 3.5.9 Dimension of Trapezoidal Channels with 1 to 1 Side Slopes

43-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Hydraulic radius (R), m

Figure 3.5.10 Dimension of Trapezoidal Channels with 1 in 2 Side Slopes

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 44-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Hydraulic radius (R), m

Figure 3.5.11 Dimension of Trapezoidal Channels with 1 in 3 Side Slopes

45-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Hydraulic radius (R), m

Figure 3.5.12 Dimension of Trapezoidal Channels with 1 in 4 Side Slopes

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 46-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Hydraulic radius (R), m

Figure 3.5.13 Dimension of Trapezoidal Channels with 1 in 5 Side Slopes

47-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Hydraulic radius (R), m

Figure 3.5.14 Dimension of Trapezoidal Channels with 1 in 6 Side Slopes

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 48-55

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

Figure 3.5.15 Rip-rap Sizing

49-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

3.5 CHARTS AND NOMOGRAPHS
 ROAD DESIGN STANDARD

APPENDIX A
Rainfall Intensity – Duration – Frequency

Table A.1 Rainfall Intensity - Duration - Frequency at Battambang

Battambang Rainfall Intensity (mm /hr)

Time Frequency (Return Period) (years)
(hours) 5 yr. 10 yr. 20 yr. 50 yr. 100 yr.
0.5 53.9 69.7 88.2 115.1 138.4
1 34.2 43.3 53.7 68.5 81.1
3 16.6 20.4 24.4 30.1 34.8
6 10.5 12.6 14.9 17.9 20.4
12 6.7 7.8 9.1 10.7 11.9
18 5.1 5.9 6.8 7.9 8.7
24 4.3 4.9 5.5 6.3 7.0
48 2.7 3.0 3.4 3.8 4.1
72 2.1 2.3 2.5 2.8 3.0

Table A.2 Rainfall Intensity – Duration – Frequency at Kompong Thom

Kompong Thom Rainfall Intensity (mm/hr)

Time Frequency (Return Period) (years)
(hours) 5 – yr 10 – yr 20 – yr 50 – yr 100 - yr
0.5 56.6 59.6 61.6 65.9 68.4
1 35.5 37.8 39.5 42.6 44.6
3 17.0 18.4 19.6 21.4 22.6
6 10.7 11.7 12.6 13.8 14.7
12 6.7 7.4 8.1 9.0 9.6
18 5.1 5.7 6.2 6.9 7.5
24 4.2 4.7 5.2 5.8 6.3
48 2.6 3.0 3.3 3.8 4.1
72 2.0 2.3 2.6 2.9 3.2

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 50-55

APPENDIX A
 ROAD DESIGN STANDARD

Table A.3 Rainfall Intensity – Duration – Frequency at Pursat

Pursat Rainfall Intensity (mm/hr)

Time Frequency (Return Period) (years)
(hours)
5 – yr 10 – yr 20 – yr 50 – yr 100 - yr
0.5 59.5 68.1 79.8 94.0 105.7
1 37.8 42.9 49.8 58.2 65.0
3 18.2 20.6 23.5 27.2 30.1
6 11.4 12.9 14.7 16.8 18.5
12 7.2 8.1 9.1 10.4 11.4
18 5.5 6.2 6.9 7.9 8.6
24 4.5 5.2 5.8 6.5 7.1
48 2.9 3.3 3.6 4.1 4.4
72 2.2 2.4 2.6 2.9 3.2

51-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

APPENDIX A
 ROAD DESIGN STANDARD

APPENDIX B
Prakas No. 377, Dated 11th October, 2001

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 52-55

APPENDIX B
 ROAD DESIGN STANDARD

Prakas No. 377, Dated 11th October, 2001 (Cont.)

53-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

APPENDIX B
 ROAD DESIGN STANDARD

APPENDIX C
Decision No. 328, Dated 13th November, 1998

ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99 54-55

APPENDIX C
 ROAD DESIGN STANDARD

Decision No. 328, Dated 13th November, 1998 (Cont.)

END OF DOCUMENT

55-55 ROAD DESIGN PART 3 - DRAINAGE, CAM PW 03-103-99

APPENDIX C