Kuwait Highway Drainage Design Manual Summary

Ministry of Public Works
Roads Administration
PART 3
Kuwait Highway Drainage Design Manual
Edition 2
January 2012

Design Manual for Roads and Bridges Table of Contents
Part 3 Kuwait Highway Drainage Design Manual
Page i
TABLE OF CONTENTS
1. INTRODUCTION.................................................................................................................... 1-1
1.1 Purpose......................................................................................................................1-1
1.2 Principles....................................................................................................................1-1
1.3 Contents ....................................................................................................................1-1
1.4 Planning and Coordination.............................................................................................1-2
2. HYDROLOGY ........................................................................................................................ 2-1
2.1 Design Storm..............................................................................................................2-1
2.2 Catchment Hydrology...................................................................................................2-5
3. SYSTEM DESIGN................................................................................................................... 3-1
3.1 Permissible Site Discharge (PSD) ...................................................................................3-1
3.2 Streets and Open Channels ...........................................................................................3-4
3.3 Storm Drains ..............................................................................................................3-8
3.4 Culverts and Cross Drainage ....................................................................................... 3-14
4. OTHER FACTORS AFFECTING DESIGN ........................................................................................ 4-1
4.1 Utilities ......................................................................................................................4-1
4.2 Right-of-Way ..............................................................................................................4-1
4.3 Service Life.................................................................................................................4-1
4.4 Environmental Issues ...................................................................................................4-1
5. DOCUMENTATION, CONSTRUCTION AND MAINTENANCE ................................................................ 5-1
5.1 Construction ...............................................................................................................5-1
5.2 Maintenance ...............................................................................................................5-1
6. REFERENCES........................................................................................................................ 6-1

Design Manual for Roads and Bridges Chapter 1
Part 3 Kuwait Highway Drainage Design Manual Introduction
Page 1-1
1 INTRODUCTION
1.1 PURPOSE
The purpose of this manual is to establish minimum stormwater management requirements to guide
drainage design in Kuwait. Stormwater drainage should be designed to be compatible with existing drainage
patterns and facilities. It should also protect the highway and the road user from the hazards of flooding.
The objectives of the stormwater drainage system are to:
 Collect and convey stormwater and discharge to its receiving waters with minimal nuisance, danger or
damage, and at a development and environmental cost which is acceptable to the community as a whole;
 Limit flooding of public and private property to acceptable levels;
 Ensure a reasonable level of pedestrian and vehicular traffic safety and accessibility;
 Minimize pollutant inflows to the receiving waters, and to control scour and depositional effects.
Stormwater drainage for Kuwait shall be designed in accordance with Highway Drainage Guidelines,
AASHTO. The Kuwait Stormwater Drainage Design Manual is not intended to replace AASHTO. It is a
supplement that gives guidance on the interpretation of AASHTO's requirements as they relate to local
conditions in Kuwait.
1.2 PRINCIPLES
The overall drainage strategy is based on a framework of drainage principles that provide guidance on the
use of technical criteria. These principles are listed below:
 Stormwater drainage systems will be considered an integral part of the total urban framework and
drainage planning will be required for all development.
 Drainage considerations are regional in nature, meaning that storm runoff does not necessarily follow
jurisdictional boundaries. Therefore, jurisdictional cooperation and unified drainage standards are needed
to accomplish planning goals.
 Development should minimize and mitigate increases in flow, depth, and velocity unless downstream
facilities exist to accommodate these increases. Furthermore, downstream properties are not allowed to
block historic flow paths. Storm runoff will be maintained within its natural drainage path unless
reasonable use is demonstrated otherwise.
 New developments will design and construct drainage facilities that are in alignment with the approved
Stormwater Master Plan. All drainage plans, studies, construction drawings, and specifications should be
reviewed and approved for concurrence with the Master Plan.
 Water quality measures and water harvesting may be considered in future drainage designs.
 Ongoing maintenance of drainage facilities is essential to the operation of the overall stormwater drainage
system.
 Technical standards and criteria should be evaluated and amended to keep current with the latest
technology that is relevant to drainage design in Kuwait. For example, it may be decided that a full review
and update of the stormwater master plan should be completed every seven (7) years.
1.3 CONTENTS
This manual is primarily concerned with stormwater from rain falling within the right of way and from
adjacent overland flow. Guidance is given on how to design drainage systems that collect, convey, and
discharge stormwater runoff. The following topics are addressed in this manual:
 Streets and Open Channels
 Inlets
 Storm Drains
 Appurtenant Structures
 Roadside Channels
 Storage Facilities
 Pumping stations
 Outfalls

Page 1-2
 Subsurface drainage
 Culverts and Cross Drainage
 Other Factors
Other factors affecting drainage design are also discussed in Chapter 4 that includes: utilities, right of way,
service life, environmental issues, computer modelling, hydrology, inlets, conduits, other losses, and cost
estimating. Advice is also given on culvert design for cross drainage provision at wadi crossings or open
channels, computer modelling, and system construction and maintenance.
1.4 PLANNING AND COORDINATION
Drainage is a regional feature that crosses over governmental jurisdictions and property boundaries. This
characteristic of drainage requires coordination between different entities and cooperation from both the
public and private sectors. Planning and coordination should begin early on in the project with a
comprehensive study of the existing drainage patterns and facilities, followed by an examination of the
potential impacts of the proposed highway.
Close communication and coordination with all agencies that have interests in drainage matters will help the
designer provide a drainage system that will benefit both the highway user and the local residents or
businesses. Agencies include MPW, EPA and Kuwait Municipality. The following planning and coordination
guidelines have been referenced directly from the U.S. Federal Highway Administration’s (FHWA) Urban
Drainage Design Manual (September 2009).
1.4.1 System Planning
Stormwater drainage design is an integral component in the design of highway and transportation
networks. Drainage design for highway facilities must strive to: maintain compatibility with and minimize
interference with existing drainage patterns; control flooding of the roadway surface for design flood
events; and minimize potential environmental impacts from highway-related stormwater runoff.
To meet these goals, the planning and coordination of storm drainage systems must begin in the early
planning phases of projects.
1.4.2 Design Objectives
The objective of stormwater drainage design is to provide for safe passage of vehicles and protect human
life and property during a storm event. The drainage system is designed to collect stormwater runoff and
discharge it to an adequate receiving body without causing adverse impacts.
1.4.3 Design Approach
The design of stormwater drainage systems is a process which evolves as an overall design develops. The
primary elements of the process include: data collection; agency coordination; preliminary concept
development; concept refinement and design and final design documentation.
Each of these elements is briefly described in the following steps:
Step 1: Data Collection
This step involves assembling and reviewing technical data and background information as necessary to
perform the design. This information may include but is not limited to:
 Watershed mapping
 Land use mapping
 Soils maps
 Flood history records
 Existing drainage reports and design drawings
 Services plans
 Existing right of way and property boundaries
 Survey data

Page 1-3
Step 2: Agency Coordination
This step includes coordination with regulatory and other reviewing agencies and stakeholders. Prior to the
design of a storm drainage system, it is essential to coordinate with regulatory agencies or others that have
interests in drainage matters. Regulatory agency involvement may come from any level of government.
The concerns of these agencies are generally related to potential impacts resulting from highway drainage,
and centre on stormwater quantity and quality issues.
Others with interests in storm drainage systems include local municipalities, and developers. Local
municipalities may desire to use portions of the highway storm drainage system to provide for new or better
drainage, or to augment old municipal drainage systems. Local municipalities may be interested in
developing cooperative projects where a mutual economic benefit may exist. Local municipalities may also
be aware of proposed private development in the vicinity of the road project which may impact drainage
design.
These groups may wish to improve or change drainage patterns, redirect stormwater to the right of way, or
propose joint projects which could require the highway storm drainage system to carry water for which it
would not usually be designed. Early planning and coordination is required to identify and coordinate
cooperative projects.
Step 3: Preliminary Concept Design
Layout and design of a storm drainage system begins with the development of sketches or schematics
identifying the basic components of the intended design. This section provides an overview of the concepts
involved in the development of a preliminary concept plan, by preparing a base map. The base map should
identify watershed areas and subareas, land use and cover types, soil types, existing drainage patterns, and
other topographic features. This base information is then supplemented with underground utility locations, a
preliminary roadway plan and profile, and locations of existing and proposed structures.
Major Versus Minor Systems
A complete storm drainage system design includes consideration of both major and minor drainage
systems. The minor system, sometimes referred to as the “convenience system”, consists of the
components that have been historically considered as part of the storm drainage system. These
components include kerbs, gutters, ditches, inlets, access holes, pipes and other conduits, open channels,
pumps, detention basins, water quality control facilities, etc. The minor system is normally designed to
carry runoff from 10 year frequency storm events.
The major system provides overland relief for stormwater flows exceeding the capacity of the minor system.
This usually occurs during more infrequent storm events, such as the 25, 50, and 100-year storms.
The major systems are composed of pathways that are provided for the runoff to flow to natural or
manmade receiving channels such as streams, creeks, or rivers. The designer should determine these
emergency overflow paths for major storm events.
Concept Plan
With the preliminary base map completed and the difference between major and minor system components
determined, a conceptual storm drainage plan can be prepared. The development of this plan includes
consideration of both major and minor drainage systems and should consist of the following preliminary
activities:
 Locate and space inlets
 Locate main outfall
 Locate storm mains and other conveyance elements
 Define detention strategy and storage locations
 Define water quality control strategy and facility locations
 Define elements of major drainage system

Page 1-4
Step 4: Concept Refinement
This step comprises the primary design phase which generally proceeds in the following sequence:
 Computation of runoff parameters and quantities based on the preliminary concept layout.
 Refine inlet location and spacing.
 Refine the storm drain system layout including access holes, connecting mains, outfall structures, and any
other system components.
 Size pipes, channels, pump stations, discharge control structures, and other storm drain system
components
 Compute and review the hydraulic grade line
 Revise plan and recompute design parameters as necessary.
Step 5: Final Design Documentation
This step includes preparation of final documentation for the design files and construction plans. Final
design documentation requirements are typically defined by a sponsoring agency, and can vary depending
on project scope. However, a detailed discussion of final design documentation is beyond the scope of this
document.

Part 3 Kuwait Highway Drainage Design Manual Hydrology
Page 2-1
2 HYDROLOGY
Stormwater drainage design is dependent on an estimate of magnitude, volume and distribution of
stormwater runoff. An overestimate of run-off may result in excessive expenditure of construction funds. An
underestimate may result in storm damage and traffic disruption due to poor performance of the drainage
system.
Kuwait is located in an arid region and experiences very little rainfall. When rain occurs, it is generally
characterised by severe thunderstorms of limited geographic extent. An instantaneous peak flow rate, such
as that determined by the Rational Method, is therefore considered sufficient for use in the design of
highway drainage systems in Kuwait.
Complex drainage systems employing pumping stations and storage facilities may require the use of
hydrographs. Information supporting the Soil Conservation Service (SCS) Synthetic Unit Hydrograph
Method is provided in Section 2.2.2_b. Techniques applied should be commensurate with cost, risk and
importance of the system. For additional information, the designer is referred to the hydrology chapter in
AASHTO Highway Drainage Guidelines fourth edition 2006.
Given the lack of rain in Kuwait and the length of most projects, it may be impractical or even impossible to
calibrate or validate either hydrologic or hydraulic data. The designer should consult with MPW to determine
an appropriate methodology.
2.1 DESIGN STORM
In order to calculate peak flow rates, it is necessary to identify the relationship between rainfall intensity,
frequency and duration. Rainfall intensity for a given duration and frequency shall be selected from the
curves developed by MPW. Rainfall intensity / frequency / duration curves for Kuwait have been prepared
by MPW for frequencies of 2, 5, 10, 25, 50 and 100 years, as shown in Figure 2.1. The rainfall data is given
in Table 2.1.

Page 2-2

Page 2-3
Table 2.1: Rainfall Intensity / Duration / Frequency
Duration
(mins)
Intensity (I / s / ha)
2 -Year Return
Period
5 - Year
Return
Period
10 - Year
Return
Period
25 - Year
Return
Period
50 - Year
Return
Period
100 -Year
Return
Period
5 129.4 209.5 257.9 314.0 352.4 387.9
6 123.5 200.1 246.8 301.7 339.6 375.0
7 117.6 190.7 235.8 289.3 326.8 362.1
8 111.8 181.5 225.1 277.5 314.5 349.8
9 106.3 172.9 215.1 266.4 303.1 338.4
10 101.1 164.8 205.8 256.2 292.7 328.2
11 96.3 157.6 197.5 247.2 283.6 319.4
12 92.0 151.0 190.0 239.2 275.7 311.9
13 88.0 145.1 183.2 232.0 268.6 305.2
14 84.3 139.5 177.0 225.3 262.0 299.1
15 80.9 134.4 171.1 219.0 255.8 293.4
16 77.8 129.6 165.5 212.9 249.7 287.6
17 75.0 125.0 160.2 207.1 243.8 281.9
18 72.4 120.8 155.2 201.5 238.1 276.4
19 70.0 116.8 150.5 196.3 232.8 271.3
20 67.9 113.3 146.2 191.5 227.9 266.6
21 65.9 110.1 142.4 187.2 223.5 262.4
22 64.1 107.2 139.0 183.3 219.6 258.7
23 62.4 104.5 135.8 179.7 215.9 255.2
24 60.8 101.9 132.7 176.4 212.5 251.3
25 59.3 99.5 129.8 173.1 209.2 247.7
26 57.7 97.0 126.9 169.8 205.9 244.8
27 56.2 94.6 124.0 166.6 202.6 241.2
28 54.7 92.2 121.2 163.4 199.4 238.3
29 53.3 90.0 118.5 160.3 196.2 234.7
30 52.0 87.9 116.0 157.4 193.1 231.1
31 50.9 86.0 113.7 154.7 190.2 228.2
32 49.9 84.3 111.6 152.0 187.3 225.0
33 49.0 82.7 109.5 149.5 184.4 221.8
34 48.1 81.2 107.6 147.1 181.7 218.9

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Table 2.1: Rainfall Intensity / Duration / Frequency
Duration
(mins)
Intensity (I / s / ha)
2 -Year Return
Period
5 - Year
Return
Period
10 - Year
Return
Period
25 - Year
Return
Period
50 - Year
Return
Period
100 -Year
Return
Period
35 47.3 79.7 105.7 144.7 178.9 216.0
36 46.4 78.3 103.8 142.3 176.2 213.1
37 45.6 76.8 102.0 140.0 173.4 210.2
38 44.7 75.4 100.1 137.6 170.7 207.4
39 43.9 73.9 98.2 135.2 168.0 204.8
40 43.0 72.4 96.3 132.8 165.2 202.3
41 42.2 71.0 94.4 130.4 162.5 199.4
42 41.4 69.6 92.6 128.1 159.9 196.9
43 40.6 68.2 90.9 126.0 157.4 194.7
44 40.0 67.2 89.6 124.2 155.4 192.2
45 39.3 66.1 88.2 122.5 153.5 190.0
50 37.0 62.4 83.5 116.5 146.3 181.5
60 33.1 56.2 75.6 106.1 133.9 166.9
70 29.7 50.8 68.6 96.8 122.6 153.5
80 26.8 46.0 62.3 88.2 112.1 140.7
90 24.5 42.2 57.3 81.3 105.3 130.2
100 22.6 38.8 52.6 74.8 95.3 120.1
110 20.8 35.7 48.4 68.9 87.8 110.7
120 19.3 33.0 44.7 63.6 81.1 102.3
130 18.0 30.8 41.7 59.2 75.5 95.2
140 16.8 28.6 38.6 54.8 69.9 88.1
150 16.0 27.0 36.4 51.6 65.8 82.9
160 15.1 25.5 34.4 48.7 62.0 78.1
170 14.7 24.7 33.3 47.0 59.9 75.3
180 14.0 23.5 31.7 44.7 56.9 71.5
Storm duration should not be less than 5 minutes and is subject to justification by design engineer.
The design storm frequency, or return period, should be selected according to the importance of the road,
expected traffic volumes, land use type, anticipated development in the area and the potential for damage
to adjacent facilities. For highway drainage systems, the frequencies are as follows:

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Road Classification
Design Frequency
(yr)
Expressway & Freeway
Roads Network
10
Arterial Road Network 10
Collector Road 10
Local Road Network 5
Atypical situations may require further consideration. The drainage of a depressed roadway, for example,
may warrant a higher return period than that of an at grade roadway.
Acceptable frequency limits for crossing culverts where roads cross watercourses (wadis) or open channels
are as follows:
Road Classification
Design Frequency
(yr)
Expressway & Freeway
Road Network
50
Arterial Road Network 50
Collector Road Network 50
Local Road Network 25
Certain land uses may require additional protection from overland flow. Frequencies for these areas are as
follows:
Land Use
Design Frequency
(yr)
Hospitals, Airport 100
Kuwait Oil Company 50
These design frequencies are intended to be a minimum return period. Higher frequencies may also be
required where adjacent facilities need additional protection. These facilities include fresh water storage
reservoirs, electricity substations, and foul or stormwater pumping stations or treatment plants.
Frequencies at these locations should be approved with MPW.
2.2 CATCHMENT HYDROLOGY
2.2.1 Catchment Areas
A catchment area is usually surrounded by an easily discernable topographic divide. This divide is the line
that separates the rainfall onto two adjacent catchment areas, ensuring the runoff is directed into one or
the others collection system. Determining the size of the catchment area that contributes flow to the
drainage system is an important step in hydrologic analysis.
Field inspections should be undertaken to confirm the boundaries of catchment areas, as topographic maps
are not always current. Plans of existing stormwater drain systems may also be a valuable source of
drainage boundary information. Once the boundaries of contributing areas have been established, they
should be delineated on a base map.
Rural catchment areas should include the areas within the right-of-way subject to direct precipitation and
the broader natural catchment areas within which the road runs. Urban catchment areas shall incorporate
the areas within the right-of-way subject to direct precipitation and any adjacent contributing areas

Page 2-6
assessed from development plans or topographic maps.
The shape of the area affects the rate at which water is supplied to the storm drain system. Long narrow
watersheds may have lower runoff rates than fan or pear shaped watersheds. The slope of the catchment
area is also related to surface runoff. Steeper basins yield a quicker response time than flat basins. For
further information on drainage areas, refer to the Hydrology chapter in AASHTO Highway Drainage
Guidelines, fourth edition 2006.
2.2.2 Hydrologic Methods
The choice of hydrologic method must be appropriate to the type of catchment and the required degree of
accuracy. Simplified hydrologic methods such as the Rational Method should not be used whenever a full
design hydrograph is required for flood mapping or to assess flood storage issues. Instead the more reliable
runoff – routing techniques should be adopted.
The Rational Method provides a simplistic methodology for assessing the design peak flow rate.
Unfortunately the Rational Method has significant limitations, and it is the task of the designer to be familiar
with these limitations and to know when an alternative methodology is required. A brief description of some
commonly used hydrologic methods is given below:
a. Rational Method
There are a number of methods for estimating flood peaks, storm durations and runoff volumes. One of the
simplest, the Rational Method, is an empirical formula that expresses a relationship between rainfall
intensity, catchment area and runoff, as follows:
Q = C I A
Where:
Q = Peak discharge (l/s)
C = Average of the runoff coefficients assigned to different contributing areas
I = Rainfall intensity for the selected frequency and time of concentration (l/s/ha)
A = Catchment area (ha)
Discharge, as computed by the Rational Method, assumes that the discharge has the same frequency as the
selected rainfall intensity. Because of the assumption that rainfall is of equal intensity over the entire
watershed, and because its frequency is not truly related to flood frequency, this method is to be used only
for estimating runoff from areas of 80 ha or less.
Rain falling on the earth's surface is either retained where it falls, passes through the soil surface as
infiltration, or finds its way into the storm drain system. The amount of infiltration varies for differing
surfaces. Runoff coefficients are used to determine the percentage of the runoff that will reach the storm
drain system. The following runoff coefficients should be used in Kuwait:

Page 2-7
Type of Drainage Area Runoff Coefficient, C*
Business:
Downtown areas 0.70 – 0.95
Neighbourhood areas 0.50 – 0.70
Residential:
Single-family areas 0.30 – 0.50
Multi-units, detached 0.40 – 0.60
Multi – units, attached 0.60 – 0.75
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, cemetries 0.10 – 0.25
Playgrounds 0.20 – 0.40
Railroad yard areas 0.20 – 0.40
Unimproved areas 0.10 – 0.30
Lawns:
Sandy soil, flat, 2% 0.50 – 0.10
Sandy soil, average, 2-7% 0.10 – 0.15
Sandy soil, steep, 7% 0.15 – 0.20
Heavy soil, flat, 2% 0.13 – 0.17
Heavy soil, average, 2-7% 0.18 – 0.22
Heavy soil, steep, 7% 0.25 – 0.35
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
*Higher values are usually appropriate for steeply sloped areas and
longer return periods because infiltration and other losses have a
proportionally smaller effect on runoff in these cases.
Referenced from Table 3.1 of the Urban Drainage Design Manual, FHWA.
The use of average coefficients for different kinds of surfaces assumes that the coefficient does not vary.
The designer should be aware that, in practice, the runoff coefficient for any particular surface varies with
respect to the length of time of prior wetting.
Time of Concentration, (TC)
The total peak flow at any point is not the sum of the calculated sub-area flows contributing at that point,
but is dependent on the time of concentration at that point. Time of concentration is defined as the time
required for storm runoff to travel from the hydraulically most remote point of the drainage basin to the
point of interest. Time of concentration is typically the cumulative sum of three travel times, including:

Page 2-8
 Sheet flow travel time
 Shallow concentrated flow travel time
 Channel flow travel time
For impervious areas it is not necessary to calculate a separate shallow concentrated flow travel time
segment. Such flows will typically transition directly from sheet flow to channel flow or be intercepted at
inlets with negligible lengths of concentrated flow.
A minimum time of concentration can be assumed as extremely short travel times will lead to calculated
rainfall intensities that are overly conservative. For impervious areas a minimum time of concentration of 5
minutes is recommended. For undeveloped areas, a minimum of 10 minutes is recommended. However, for
slopes steeper than 1V:10H; or where there is limited opportunity for surface storage, a time of
concentration of 5 minutes should be assumed.
(1) Sheet flow travel time. Sheet flow is flow of uniform depth over plane surfaces and usually occurs for
some distance after rain falls on the ground. The maximum flow depth is usually less than 20 to 30mm. For
unpaved areas, sheet flow normally exists for a distance less than 25 to 30m. A common method to
estimate the travel time of sheet flow is based on kinematic wave theory and uses the Kinematic Wave
Equation:
Where:
Tt = travel time (min)
L= Length of flow path (m)
S = Slope of flow (m/m)
n= Manning’s roughness coefficient for sheet flow
i = Design storm rainfall intensity (mm/h)
If Tt is used (as part of TC) to determine the intensity of the design storm from the IDF curves, application
of the Kinematic Wave Equation becomes an iterative process: an assumed value of Tt is used to determine
“i” from the IDF curve; then the equation is used to calculate a new value of Tt which in turn yields an
updated “i”. The process is repeated until the calculated Tt is the same in two successive iterations. To
eliminate the iterations, use the following simplified form of the Manning’s kinematic solution:
Where:
P2 is the rainfall depth (mm)
(2) Shallow concentrated flow travel time. After short distances, sheet flow tends to concentrate, or the
depth exceeds the range where use of the Kinematic wave equation applies. At that point the flow becomes
defined as shallow concentrated flow. The Upland Method is commonly used when calculating flow velocity
for shallow concentrated flow. Average velocities for the Upland Method can be calculated from the following
equation:
3/10
2/5
3/5
3/5
S
i
n
L
6.92
Tt 
2/5
1/2
2
4/5
4/5
t
S
P
n
L
5.476
T 

Page 2-9
Where:
K = intercept coefficient (see table below)
S = slope (%)
Intercept Coefficient
Trash fallow or minimum tillage cultivation;
contour or strip cropped;
0.152
Short green pasture 0.213
Cultivated straight row 0.274
Nearly bare and untilled alluvial fans 0.305
Grassed waterway 0.457
The travel time for shallow concentrated flow can then be calculated from:
Where:
Tt = travel time (min)
L = length (m)
V = flow velocity (m/s)
(3) Channel flow travel time. When the channel characteristics and geometry are known the preferred
method of estimating channel flow time is to divide the channel length by the channel velocity obtained by
using the Manning equation, assuming full flow conditions.
For culvert or storm drain flow, flow velocities in a short culvert are generally higher than they would be in
the same length of natural channel and comparable to those in a lined channel. In most cases, including
short runs of culvert in the channel, the flow time calculation will not materially affect the overall time of
concentration. When it is appropriate to separate flow time calculations, such as for urban storm drains,
Manning's equation may be used to obtain flow velocities within pipes.
b. SCS Synthetic Unit Hydrograph
The Soil Conservation Service (SCS now known as the Natural Resources Conservation Service) has
developed a synthetic unit hydrograph procedure which can be applied to drainage areas greater than 80ha.
A unit hydrograph is used to represent the amount of stormwater runoff generated from a unit of rainfall on
a particular drainage area. The unit hydrograph used by this method is based upon an analysis of a large
number of natural unit hydrographs from a broad cross section of geographic locations and hydrologic
regions. The only parameters that need to be determined are the peak discharge and the time to peak. A
standard unit hydrograph is constructed using these two parameters.
For the development of the SCS Unit Hydrograph, the curvilinear unit hydrograph is approximated by a
triangular unit hydrograph (UH) that has similar characteristics. Figure 2.2 referenced from the FHWA Urban
Drainage Manual, shows a comparison of the two dimensionless unit hydrographs. Even though the time
base of the triangular UH is 8/3 of the time to peak and the time base of the curvilinear UH is five times the
time to peak, the area under the two UH type is the same.
KS
V 1/2

60V
L
T 
t

Page 2-10
Figure 2.2: Dimensionless Unit Hydrograph
*Referenced from Figure 3.7, FHWA Urban Drainage Manual
The area under a hydrograph equals the volume of direct runoff QD which is one millimetre for a unit
hydrograph. The peak flow is calculated as follows:
Where:
qP = Peak flow (m³/s)
Ak = Drainage area (Km)²
QD = volume of direct runoff (=1 for unit hydrograph), (mm)
tP = time to peak (hr)
Ku = 2.083
The constant 2.083 reflects a unit hydrograph that has 3/8 of its area under the rising limb. For
mountainous watersheds, the fraction could be expected to be greater than 3/8, and therefore the constant
may be near 2.6. For flat, swampy areas, the constant may be on the order of 1.3.
Time to peak, tp, can be expressed in terms of time of concentration, tc, as follows:
c
p t
3
2
t 
Expressing qp in terms of tc rather than tp yields:
c
D
k
u
p
t
Q
A
K
q 
p
D
k
u
p
t
Q
A
K
q 

Page 2-11
Where: Ku = 3.125
Curve Numbers
The runoff curve number (also called a curve number or simply CN) is an empirical parameter used
in hydrology for predicting direct runoff or infiltration from rainfall excess. The number is still popularly
known as a "SCS runoff curve number". The runoff curve number was developed from an empirical
analysis of runoff from small catchments and hillslope plots monitored by the USDA. It is widely used in
computer programs and is an efficient method for determining the approximate amount of direct runoff
from a rainfall event in a particular area.
The runoff curve number is based on the area's hydrologic soil group, land use, treatment and hydrologic
condition. References indicate the runoff curve numbers for characteristic land cover descriptions
and a hydrologic soil group. The runoff equation is:
S
I
-
P
2
)
I
-
(P
Q


a
a
Where: Q is runoff (in) P is rainfall (in) S is the potential maximum soil moisture retention after runoff
begins (in) Ia is the initial abstraction, or the amount of water before runoff, such as infiltration, or rainfall
interception by vegetation. It is generally assumed that Ia = 0.2S
The runoff curve number, CN, is then related to S: 10
CN
1000
S 

Curve Numbers can range from 30 to 100; lower numbers indicate low runoff potential while larger
numbers are for increasing runoff potential. A list of Curve Numbers for various land uses, vegetation, and
soil conditions are compiled in Table 2.7. Additional Curve Number data can be referenced from various
sources including U.S. Natural Resources Conservation Service publications.
Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups
according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly
wet, and receive precipitation from long-duration storms. Soils can be assigned to four groups (A, B, C, and
D) and are defined as follows:
Group A: Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist
mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of
water transmission.
Group B: Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately
deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately
coarse texture. These soils have a moderate rate of water transmission.
Group C: Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a
layer that impedes the downward movement of water or soils of moderately fine texture or fine texture.
These soils have a slow rate of water transmission.
Group D: Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These
consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that
have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.

Page 2-12
Table 2.7: Curve Numbers Values (CN)*
Cover Type and Hydrologic Condition
Curve Number for Hydrologic
Soil Group**
A B C D
URBAN AREAS
Open Space (lawns, parks, golf courses, cemeteries, etc.)
Poor condition (vegetation cover <50%) 68 79 86 89
Fair condition (vegetation cover 50% to 75%) 49 69 79 84
Good condition (vegetation cover >75%) 39 61 74 80
Impervious Areas
Paved parking lots , roofs,driveways,etc. 98 98 98 98
Streets – paved, with curb and gutter 98 98 98 98
Streets – paved, with open side ditches 83 89 92 93
Roads-gravel surface 76 85 89 91
Roads- dirt surface 72 82 87 89
Desert Areas
Natural desert landscaping 63 77 85 88
Artificial desert landscaping 96 96 96 96
Urban Districts
Commercial (85% impervious area) 89 92 94 95
Industrial (72 % impervious area) 81 88 91 93
Residential Districts
Newly graded areas with no vegetation 77 86 91 94
Apartments / Condominiums(72 % impervious area) 81 88 91 93
Townhouses /Lot Size less than 550sqm (69%impervious area) 80 87 90 92
Lot Size of 650 sq m (63% impervious area) 76 84 89 91
Lot Size of 1,300 sq m (30% impervious area) 57 72 81 86
Lot Size of 1,850 sq m(25% impervious area) 54 70 80 85
Lot Size of 3,700 sq m ( 20% impervious area ) 51 68 79 84
Lot Size of 7,450 sq m (12% impervious area) 46 65 77 82
RURAL AREAS-Agricultural Land and Semi Arid Rangeland
Pasture ,grassland, or range – continuous forage for grazing
Meadow- continous grass, protected from grazing 30 58 71 78
Brush/brush/weed/grass mixture
Wood- grass combination (orchard or tree farm).
Farmstead- building ,lanes, driveways, and surrounding lots 59 74 82 86
Desert shrub
* CN values referenced from data provided by the U.S. Natural Resources Conservation Service (NRCS)
** See above for description for Hydrologic Soil Group.
c. Computer Models
Computer based runoff routing models are used for calculating flood hydrographs from rainfall, catchment
and channel inputs. These models use the concept of “critical storm duration” as opposed to the concept of
“time of concentration” used in the Rational Method. The critical storm duration for a given catchment may
be similar in duration to the time of concentration, but the two terms are different and should not be
confused. The critical storm duration is determined by testing the model for a range of storm durations.

Page 2-13
Computer-based models which incorporate the routing of the time–area relationship developed for the sub-
catchments under consideration can also be used. Calibration of these models with actual flow data is
recommended. Where this is not possible, model results may be compared with the output from other
runoff-routing models. For small catchments it is common to compare the results to a Rational Method peak
discharge.
As with all computer software, designers are expected to be familiar with the underlying concepts used, the
limitations of those concepts and the capabilities / limitations of the programs themselves. Designers should
be aware of the need for model calibration and the limitations which should be placed upon results where
such calibration is not available. Sensitivity analysis is recommended so that the sensitivity of the program’s
performance in any given situation can be measured against variation in uncertain pararmeters. Full details
of the design assumptions, including copies of input data and output data should be made available to the
MPW. Output data should include Hydraulic Grade line profiles indicating maximum water level along the
profile to be minimum 0.5m below the ground level.

Part 3 Kuwait Highway Drainage Design Manual System Design
Page 3-1
Chapter 3: System Design
3 SYSTEM DESIGN
The components of a stormwater drainage system can be separated into three main functions:
1 collection / inlets,
2 conveyance,
3 discharge.
The collection elements include the roadway pavement, channels and inlet structures. The conveyance and
release of stormwater can be achieved by using closed conduits or open channels. All of these elements are
discussed in detail in this section.
3.1 PERMISSIBLE SITE DISCHARGE (PSD)
Typically, the majority of the drainage network in Kuwait was designed years ago, when the extent of
development within the catchment was significantly less than is now allowable under current zonings. The
increase in development has meant that catchments now have a greater percentage of impervious surfaces
and a corresponding reduction in natural surface absorption. Changes in surface levels and land usage also
have the effect of disturbing natural overland flow paths.
In the past, stormwater drainage design criteria was not nearly as well researched as it is now, and rainfall
data gathered over the intervening years has caused a re-evaluation of design intensities and expected
recurrences. The net result is that the flow rates the systems are now expected to accommodate, in many
cases, exceed the capacity of the old pipe, culvert and channel systems.
Therefore, all new developments must comply with the approved current Stormwater Drainage System
Master plan in terms of design and construction of drainage facilities. That means that the Consultant must
confirm with MPW the Permissible Site Discharge (PSD) for all new developments prior to commencing of
any design or calculation processes.
Generally, PSD must satisfy the condition that discharge from the post-development area must not exceed
discharge from pre-developed area to ensure that new developments do not increase peak stormwater
flows in any downstream area during major storms. This condition must be satisfied unless otherwise
stated, confirmed and approved by MPW.
3.1.1 Storages
3.1.1.1 General
The main purpose for storing stormwater runoff is to reduce peak flow rates in order to satisfy the following
constraints:
 limited hydraulic capacity of downstream facilities
 post-development runoff is not allowed to exceed pre-development runoff
Storage of stormwater runoff is accomplished by either detention or retention. Detention facilities are
designed to temporarily store runoff and discharge at a controlled, lower flow rate. Retention facilities are
designed to hold the runoff “permanently”. Retention facilities are different from detention facilities because
there is no outlet, thereby relying on infiltration and evaporation to empty slowly after storm events. In
Kuwait, due to safety, sanitation, climatic and aesthetic reasons a detention design – underground storages
is adopted and applied to temporarily store runoff.
Detention facilities can take many forms, varying from large regional facilities, to smaller local facilities. An
example of a regional facility is a large detention basin or large underground storage tank. Examples of
local facilities are parking lots, or smaller underground tanks.
Sizing of smaller, local detention facilities is generally governed by the need to reduce the post-
development peak flow back down to pre-development conditions. For this situation, approximate methods
can be used.

Page 3-2
Sizing of larger, regional detention facilities should be based on a more sophisticated modelling approach
that takes into account the timing of the hydrograph peaks. Although a detention basin may reduce the
peak flow rate at one location, it can also change the timing of that peak outflow hydrograph, which can
change the point at which they combine with other hydrographs downstream. If the timing of the new peak
outflow is coincident with another downstream peak flow, there is a possibility that the detention basin can
actually increase the peak flow at some downstream location. Therefore, the location and size of major
detention basins should be analyzed and planned with a regional approach, as part of an overall master
planned storm water drainage system.
All effects of storage should be considered during the design of a storage facility. For example, the
reduction in peak discharges within the upper reaches by a storage facility may serve to increase the peak
discharge at some point downstream. Because tributaries contribute runoff at different time intervals, it is
possible to cause a greater downstream peak discharge when the discharges are delayed or altered from
natural conditions. The hydraulics engineer may find it necessary to employ one of the sophisticated
computer models to investigate this possibility in complex drainage systems. Close coordination with local
regulatory agencies responsible for controlling drainage development is strongly recommended.
3.1.1.2 An Approximate Method for Sizing Storages when using the Rational Method
For sizing drainage systems (especially for greater catchments) computer based runoff routing models are
to be used.
For smaller catchments (up to 80ha) the Rational Method can be applied. For the design of detention
storages in such catchments, a variety of simple and approximate methods may be adopted when using the
Rational Method.
3.1.1.3 Storages for small catchments – up to 80ha
Developments in remote isolated areas in Kuwait are common. Common characteristic of such
developments is that contributing catchments are relatively small and that there is no existing stormwater
drainage network within the area to which the development can be connected to.
For such cases where contributing catchments are up to 80ha, MPW requires discharge of the drainage
system to an underground storage. MPW Maintenance, or Project Owner, will then be required to implement
a future maintenance regime of pumping collected water from these underground storages at some time
after the storm event as discharge from these storages by gravity is not possible in most cases due to
topographic conditions.
Overflow facilities will need to be incorporated into such underground storages. The design for the overflow
facilities will need to consider that the invert level of the underground storages will be lower than the lowest
top level of the manhole within the development located upstream of the storage.
The volume of such underground storages is to be determined by applying the criteria that the inflow
hydrograph must be provided and the release rate must be assigned, Storm runoff volumes. In essence, the
approach must be based on the determination of the inflow hydrograph from the development and the
outflow hydrograph from the storage.
As Rational Method is commonly used for sizing of drainage systems in small catchments in Kuwait, two
scenarios are presented to allow for the designer to size the underground storages required for such
developments.

Page 3-3
CASE 1 Time of rainfall duration is equal to Time of Concentration
Where:
QD -Outflow from developed site
QPD -Outflow from pre developed site
Tc -Time of concentration
Td -Time of rainfall duration
The volume of the storage required (Vs) will be calculated as follows:
Vs = ½ * (2 Tc * QD) – ½ Tc * QPD
Vs = Tc * (QD – ½ * QPD)

Page 3-4
CASE 2 Time of rainfall duration is greater than Time of Concentration
Where:
QD - Outflow from developed site
QPD - Outflow from pre developed site
Tc - Time of concentration
Td - Time of rainfall duration
The volume of the storage required (Vs) will be calculated as follows:
Vs = ½ * [(Tc + Td) + (Td - Tc)] * QD – ½ * (Tc * QPD)
Vs = Td * QD – ½ Td * QPD
NOTE: MPW has stipulated that the time of rainfall duration will be generally 30 minutes but is to be
confirmed with MPW for each particular design.
3.2 STREETS AND OPEN CHANNELS
Although the primary function of streets is for traffic movement, streets are also an essential part of a
drainage system, as they convey surface runoff and provide reserve for underground conveyance. This is
especially the case in an urban setting with impervious areas with full street improvements. Rural
roadways also have an impact on drainage by diverting and concentrating runoff.
Drainage design balances the use of available roadway for drainage purposes without interfering with the
traffic carrying capacity of the street. The following sections outline criteria that try to optimize this
balance.
Open channel hydraulics can be evaluated using Manning’s Equation. Open channels can take many forms,
such as triangular (v-ditch), trapezoidal, and also street sections. In an urban setting with full street
improvements, runoff will concentrate at the kerb and gutter. In a rural setting, runoff will concentrate in a
roadside ditch or channel. The depth of sheet flow will essentially be zero at the crown of the road and
increase in the direction of the gutter or drainage ditch. The drainage capacity of the roadway is primarily
governed by the longitudinal slope of the road, assuming similar cross slopes and roughness. A general
rule is that flatter street slopes will generate slower flow velocities which equates to greater flow depths,
and vice versa.

Page 3-5
Pavement drainage occurs in two different ways. The first involves sheet flow across the pavement surface.
The second occurs where kerbs contain and channel the runoff within the gutter until it can be removed via
an inlet.
3.2.1 Surface Drainage of Pavements
Effective surface drainage of the road pavement is essential for effective maintenance of the roadway and
for the safety of vehicular and pedestrian traffic. Water on the pavement can interrupt traffic, reduce skid
resistance, increase potential for hydroplaning, reduce visibility due to spray and cause difficulty in steering.
In addition, accelerating, braking or cornering forces may cause the driver to lose control.
The accumulation of stormwater runoff on the pavement is dependent on the longitudinal slope, crossfall,
width, surface texture, and rainfall intensity. Potentially hazardous locations include curves, superelevation
and associated transitions, wide pavements, bridge decks or anywhere where excess water may
accumulate. For information of pavement surface properties and methods of calculating resultant flow paths
and water depths, refer to Storm Drain Systems, Highway Drainage Guidelines, AASHTO.
The water depth required to produce a sufficient loss of friction to present a major driving hazard is in the
range of 1.5mm to 5.0mm. Unfortunately, it is virtually impossible to prevent water from exceeding these
depths on wide pavements during high intensity rainfall similar to that experienced in Kuwait. It is therefore
considered the driver's responsibility to exercise caution when driving during wet conditions.
3.2.2 Kerbs and Gutters
A limited right-of-way within an urban environment will often preclude the use of roadside ditches to collect
and convey runoff. In these situations, kerbs and gutters are commonly used. Kerbs vary in shapes and
sizes. Gutters begin at the intersection of the pavement surface and base of the kerb. They extend from the
kerb toward the roadway centreline. Gutters are generally between 0.3m and 1.6m wide. Gutters need not
necessarily have the same crossfall or be constructed of the same material as the road pavement. Care
should be taken alongside flush or dropped kerbs, where there may be little or no gutter available for runoff
conveyance. Gutters may also be positioned on inverted crowns, where flush kerbs delineate between a
travelled way and a parking bay, for example.
Runoff from the road and adjacent developments drains down to the kerb and gutter flow line, which acts
as a small, triangular channel. As the runoff accumulates and rises in the gutter, the water surface top
width widens into the parking and traffic lanes of the roadway. This water surface top width is known as
spread. This spread is associated with a flow depth and velocity. The product of this depth times velocity,
known as DxV, is used to evaluate and prevent potentially dangerous flow conditions. Therefore, the
primary drainage considerations for street design are:
 Spread width
 Depth x Velocity product
As the flow progresses downstream and additional areas contribute to the runoff, the spread width will
increase and progressively infringe upon the traffic lanes. Field observations show that vehicles will crowd
adjacent lanes to avoid kerb flow thereby increasing the risk of traffic accidents. As the flow width
increases, the traffic must eventually move through the inundated lanes, progressively reducing traffic
movement as the depth of flow increases. Although some reduction of traffic movement caused by runoff is
acceptable, certain limitations on the depth of flow in the street are required. Runoff must be removed from
the roadway (inlets, kerb openings, etc.) when the acceptable spread width and DxV product cannot be
satisfied. The amount of spread that is acceptable is a function of the type of road, design flow, road width,
and design speed, as shown in Table 3.1 and the figure below:
A modification of Manning’s equation can be used to compute the spread width in the gutter if the rate of
discharge, pavement cross slopes, street grade (longitudinal slope) and Manning’s roughness coefficient are
known. This modification is required because the hydraulic radius does not adequately describe the flow
cross section in such a shallow, wide channel, particularly when the top width of the water surface can be
more than 40 times the depth at the kerb. The modification is accomplished by integrating Manning’s
equation for an increment of width across the section, with the resulting equations shown below:

Page 3-6
Table 3.1: Acceptable Design Frequency
Road Classification
Design Frequency
(year)
Design Spread
High Volume or
Divided or
Bi-Directional
< 70 km/hr
> 70 km/hr
Sag Point
10
10
50
Shoulder + 1 m
Shoulder
Shoulder + 1 m
Collector < 70 km/hr
> 70 km/hr
Sag Point
10
10
10
½ Driving Lane
Shoulder
½ Driving Lane
Local Streets Low ADT
High ADT
Sag Point
5
10
10
½ Driving Lane
½ Driving Lane
½ Driving Lane
* Referenced from AASHTO Drainage Manual.
3.2.3 Inlet Hydraulics
The starting point for storm drain hydraulics is intercepting the surface runoff, whether from a roadway,
roadside ditch, or parking lots. An inlet has two functions, to intercept runoff into the system, and to stop
debris from entering the system. Note that an open inlet may intercept more runoff, but would also allow
more debris into the system. Conversely, a more restrictive inlet may not allow debris into the system, but
will also intercept less flow. The inlet design must balance these opposing functions.
Inlet hydraulics are similar to culvert hydraulics in that as the flow depth increases, there is a transition
from free surface flow to a pressure flow condition. With inlets, shallow flow enters an inlet as a weir, and
as the depth increases there is a shift to orifice flow. In general, as the flow depth increases, the inlet
capacity also increases. Inlet interception capacity is largely dependent on flow depth and velocity. These
equations are shown below:
x
0.375
0.5
1.67
x
2.67
0.5
1.67
TS
d
)]
S
/S
1.443[(Qn)
T
T
S
(0.376/n)S
Q


 x
Where:
Q = rate of discharge, (m /s)
n = Manning’s coefficient of channel roughness
S = longitudinal slope, (m/m)
S x = cross slope, (m/m)
T = top width of water surface, (m)
d = depth of flow at deepest point, (m)
Figure 3.1

Page 3-7
The equation typically used for a broad-crested weir is:
Q = CBCW L H1.5
Where:
Q = discharge, m³/s
CBCW = broad-crested weir coefficient, 1.35-1.83
L = broad-crested weir length, m
H = head above weir crest, m
The equation typically used for an orifice is:
Q = Co Ao (2gHo) 0.5
Where:
Q = the orifice flow rate, m³/s
Co= discharge coefficient (0.40 – 0.60)
Ao= area of orifice, m²
Ho= effective head on the orifice measured from the centroid of the opening, m
g= gravitational acceleration, 9.81m/s²
3.2.4 Inlet Design
Inlets enable the stormwater to be removed from the roadway area. Inlets must be properly located and
sized so that kerb and gutter drainage is effective. Several types of inlet are available for intercepting water
flow and these include:
 Grate inlets: Type “A2” gullies where there is depressed kerb or no kerbstone
 Kerb inlet gully Type “A1”
 Combination inlets - combined gullies (including Motorway Type “A” gullies)
 Concrete grated trenches
The following information is needed in order to properly locate the inlets:
 Plan and profile of the road
 Topographic plans of the adjacent area
 Typical road cross section
 Superelevation information
There are a number of locations where inlets should be provided regardless of contributing drainage areas.
These include:
 Sag points in the gutter profile
 Immediately upstream of median breaks or merge / diverges noses.
 Immediately upstream of bridges
 Immediately upstream of crossfall reversals
 Immediately upstream of pedestrian crossings
 At the end of channels in cut sections
 On side streets immediately upstream of intersections
 Behind kerbs, shoulders of footpath to drain low areas
In locations where significant ponding may occur, such as on sag curves, it may be necessary to place
flanking inlets on each side of the inlet at the low point of the sag. As a general design rule, the maximum
spacing between inlets should not exceed 50m. For further information on flanking inlets, refer to Storm
Drain Systems, Highway Drainage Guidelines, AASHTO.

Page 3-8
Concentrated runoff from large area adjacent to the road should be intercepted prior to reaching the
pavement. Large volumes of water can be collected more efficiently in channels rather than being allowed
to flow onto pavements and into pavement inlets.
Inlet size and location are interrelated. For example, the use of lower capacity inlets requires more inlets,
the use of higher capacity inlets allows for fewer inlets. It should also be noted that the use of more, lower
capacity inlets enables a decrease in the allowable spread in the gutter.
The inlet capacity is a function of the inlet types, the geometry of the opening, the longitudinal slope and
the crossfall. The MPW has established standard inlet lengths and sizes in order to facilitate design and
control construction costs, as shown on the MPW Standard Detail Drawings.
Carryover represents the portion of the total flow that is allowed to bypass an inlet and flow on to the next.
Inlets sized to allow some carryover optimises the number of inlets by making more efficient use of the
allowable spread in the gutter. However, inlets that are provided regardless of contributing drainage areas,
as listed above, should be sized to intercept 100% of the design flow. All sag inlets should be sized to
accommodate the total design flow. Calculations for inlet spacing should be submitted to MPW for approval.
Further information on different types of inlet, such as grate inlets, kerb opening inlets, combination inlets,
slotted drain inlets and bridge deck inlets, can be found in Storm Drain Systems, Highway Drainage
Guidelines, AASHTO.
MPW prefers the use of Motorway Types A, A1 and A2. However, other types of inlets may be used if
appropriate.
3.3 STORM DRAINS
The storm drain is part of the highway drainage system that receives water through inlets and conveys the
water through conduits to an outfall. The storm drain is made up of pipes, boxes and other closed conduits,
inlet structures, manholes and other miscellaneous structures.
It is important to understand the hydraulics of storm drains in order to select the correct criteria, to develop
a sound design process and to design the appurtenant structures. The hydraulics of storm drains is further
explained in Storm Drain Systems, Highway Drainage Guidelines, AASHTO.
3.3.1 Design Criteria
Design criteria describe the limiting factors that produce an acceptable design. These factors include:
 Flood frequency (see Chapter 2 of this manual)
 Allowable high water at inlets and manholes
 Minimum flow velocities to prevent deposition
 Clashes with other utilities
Soil conditions
 Future expansion of the system
 Future land development
Maximum high water is the maximum allowable elevation of the water surface (hydraulic gradient) in the
storm drain. This is especially relevant at inlet and manholes where there is access from the storm drain to
the ground surface. Maximum high water should not interfere with the functioning of an inlet or reach a
manhole cover. Therefore, the maximum high water should not be less than 0.5m below manhole.
A generally accepted self-cleansing velocity is 0.9m/s. The absolute minimum actual velocity of the design
flow shall be 0.75m/s. MPW prefers a maximum design flow velocity of 3m/s for unreinforced concrete
pipes and 5m/s for reinforced concrete pipes.
MPW requires that minimum pipe diameter for single inlet connections shall be 300mm and the minimum
slope shall be 1%. The minimum pipe diameter for double inlet connections shall be 300mm, the minimum
slope shall be 1% and the maximum slope shall be 12%. The maximum length for an inlet connection shall
be 25m.

Page 3-9
Pipes used in stormwater drain systems can be concrete, HDPE or any other material approved by MPW.
The minimum pipe diameter shall be 300mm (400mm for the Special Road Network). The following pipe
diameters are available in Kuwait: 300, 350, 400, 450, 500, 600, 700, 800, 900, 1000, 1200, 1400, 1600,
1800 and 2000mm. Pipe details are shown on MPW Standard Detail Drawings.
A minimum cover depth of 1.5m to all conduits (except gulley branch pipes) is recommended by MPW.
Gulley lines are to be concrete encased and designed / constructed in accordance with MPW Standard
Drawings No. SD-11 and SD-12.
3.3.2 Design Process
The design process is the compilation of all the activities discussed in this manual. There is no fixed order in
which these activities must be performed, nor are all the activities required on every project. The designer
must select the steps that are most appropriate. The design process is one of trial and error. The design will
continue to change and adjustments will be required as the design progresses.
Data acquisition is the first step, followed by the pavement drainage design. This yields information such as
road crossfalls and longitudinal slopes, inlet locations and catchment areas. Having accumulated this
information, the designer then prepares a storm drain system plan, which delineates main and lateral
drainage runs, whilst ensuring that all inlets are efficiently connected and directed to an outfall. The
following aspects should be addressed.
 Minimum and maximum conduit sizes should be determined
 Conflicts with utilities should be avoided
 Deep trenches should be minimised
 Alternate main and lateral runs should be compared for cost and efficiency
 The outfall should be low enough to provide an efficient conduit slope
 The layout should enable construction with minimum disruption to traffic
 Alteration to existing drainage patterns should be minimised
The storm drain network must be arranged so that all lines have gravity access to an outfall, otherwise a
pumping station will be required. The designer then sizes the conduits and checks the hydraulic adequacy
and efficiency of the system. This is done by computing the hydraulic gradient for all the main and lateral
lines in the system. Computing the hydraulic gradient for more onerous flood frequencies will also enable
the designer to evaluate the risks associated with larger floods.
The designer should then review the entire design before final acceptance and inclusion in the highway
project. At this stage, the following items should be considered:
 Elimination of over design
 Improving compatibility with other construction processes
 Opportunities for better satisfying the drainage needs
The following equations shall be used in the calculation of pipe capacities. Average velocity shall be
calculated using the Manning's equation:
n
S
R
V
1/2
2/3

R = A / P
Where:
V = Average velocity (m/s)
n = Manning's coefficient of roughness (for concrete pipes n = 0.013)
R = Hydraulic radius (m)
S = Pipe slope (m/m)
A = Cross-sectional area of flow (m2
)

Page 3-10
P = Wetted perimeter (m)
Flow rates shall be calculated using the continuity equation:
Q = V A
Where:
Q = Flow rate (m3
/s)
)
Where:
n = Manning's coefficient of roughness (for concrete pipes n = 0.013)
R = Hydraulic radius (m)
S = Pipe slope (m/m)
)
P = Wetted perimeter (m)
Flow rates shall be calculated using the continuity equation:
Q = V A
Where:
Q = Flow rate (m3
/s)
)
3.3.3 Appurtenant Structures
Drain runs are connected by appurtenant structures such as inlets and manholes. Manholes enable access
for inspection and maintenance purposes. Junction chambers within manholes join two or more drain runs
together or connect conduits of different types, sizes or shapes.
Manholes are positioned at changes in direction, slope and storm drain size changes, as well as at conduit
intersections or where inlet connections are made. Manholes should also be placed at intervals along
lengthy sections of conduit. The maximum length of conduit between manholes shall be 50-70 m for pipes
of 1400 mm diameter or less and 100-200 m for pipes of greater diameter.
Manholes in Kuwait are usually constructed of reinforced concrete and should be designed to withstand both
live and dead loads that may be imposed on them. Manholes should be provided with corrosion resistant
access steps. Details of various types of typical manholes are shown on the MPW Standard Detail Drawings.
Typical manholes are considered to have pipes entering and leaving at either angles of 0o
(±5o
) or 90o
(±5o
).
Manholes losses should be considered as part of the design process and conduits passing through manholes
should have good hydraulic properties in order to minimise these losses. If possible, the slope of storm
drain should be continued through the manhole. If the conduit size is increased on the downstream side of a
manhole, then the soffit levels of the inlet and outlet conduits shall be matched. If this is not possible, a
minimum drop of 50mm is recommended. If, for some reason, the height of the conduit on the downstream
size is lower than that of the upstream conduit, the invert levels of the conduits should be matched.
Manhole necks are required where the depth from ground level to invert level exceeds the following values
for the corresponding pipe diameters.

Page 3-11
Depth (mm)
Pipe Diameter
(mm)
2800 300 - 1600
3000 1800
3230 2000
The manhole frame and cover should be as per MPW typical drawing and must be designed to support the
expected loads. It is preferable to locate manholes away from traffic. Surface topography shall also be
considered when locating manholes.
Junction chambers of different types are also shown on the MPW Standard Detail Drawings. For further
information on methods of calculating head losses in manholes or junction chambers, refer to Design of
Urban Highway Drainage, FHWA.
3.3.4 Roadside Channels
Roadside drainage channels are used for highways and in rural settings to collect surface runoff from the
roadway and are also designed to protect the roadway from offsite flows. Roadside drainage channels are
commonly designed as v-ditch (triangular), or trapezoidal channels for larger flows.
The design of roadside channels is influenced by design flow, local terrain, and available roadside reserve.
Preliminary channel designs can be started based on the following steps and assumptions shown below:
 Step 1: Estimate design flow
 Step 2: Determine available roadway reserve to accommodate channel top width
 Step 3: Propose channel slope. If feasible, and has positive drainage, matchadjacent roadway slope.
 Step 4: Initial V-ditch channel shape, assuming unlined with 4:1 side slopes
 Step 5: If flow depth is greater than 1 m, consider a trapezoidal channel.
 Step 6: Evaluate channel velocities and adjust roughness values for appropriate erosion protection as
shown below.
 Step 7: Consider some amount of freeboard depending on each situation.
Appropriate Manning’s roughness values and maximum allowable velocities for different channel lining types
are summarized in the table below:
Channel Lining Manning’s “n”
V max
(m/s)
Unlined, earthen 0.025 1.5
Vegetation 0.030 1.5
Rock 0.035 3.0
Gabion 0.035 4.5
Concrete 0.015 6.0
Channels may also be used to direct flow to or from a storm drain conduit. Further details can be found in
Hydraulic Analysis and Design of Open Channels, Highway Drainage Guidelines, AASHTO.
3.3.5 Storage Facilities
Temporary storage or detention of excess stormwater runoff may be required to prevent the overloading of
existing downstream storm drain systems. The storage and regulated release of stormwater can reduce the
frequency and extent of downstream flooding, soil erosion, sedimentation and water pollution. For further
design details, refer to Storm Drain Systems, Highway Drainage Guidelines, AASHTO and Section 3.1 of this
manual.

Page 3-12
3.3.6 Pumping Stations
Stormwater pumping stations remove water from highway facilities that cannot be drained by gravity.
Because of the costs and potential maintenance problems, a pumping station should only be used when no
other system is feasible. Efforts should be taken in the design stage to reduce the burden on the pump
system by minimizing the amount of runoff it will receive. For example, this can be done by minimizing the
drainage area or adjusting the surface cover to increase infiltration.
While designing the pump station to pump the peak flow is not unreasonable, an alternative approach is to
pump at a lower flow than the peak incoming flow rate and hence allow volume to build up in the wet well
until such a time as the incoming flow rate reduces. This approach reduces the required pumping capacity
and also the peak power consumption and the diameter of the required rising main, which can reduce both
capital and running costs.
Consideration needs to be given to handling the flow should a single pump system fail. Providing storage
for the total 100-year volume reaching the pump station may not be the most cost effective way of dealing
with this possibility. Instead, minimizing the likelihood of pump station failure by including additional pump
sets (standby pump sets), and ensuring that the pump station has a secure power supply by either dual
independent power supplies or by having a standby diesel generator. Adding these safeguards may allow
for a significantly smaller storage volume.
A multiple pump system solution could be achieved by using three identical pumps, each with a rated flow
rate. This type of configuration is commonly referred to as Duty, Assist, and Standby. The pumps would be
controlled by level indication within the wet well and as the level initially increases the duty pump would
start. Following a significant storm event, as the first pump (Duty) reaches capacity, the second pump
(Assist) would start. The third pump (Standby) would only start upon failure of either of the other pumps.
This configuration gives the advantage of no loss of pumping capacity given 1 pump failure and only 50%
loss of capacity should two pumps fail.
An example of an appropriate, conceptual-level pump system design is summarized below:
 Pump station type : Wet well with submersible pumps.
 Pumping configuration : Duty / Assist / Standby.
 Pump type : High capacity / low head pumps.
 Type of motor starter : Variable frequency drive.
 Incoming power supply : Dual independent supplies or a standby generator(s).
 Building requirements : Control building complete with pump & motor controls / monitoring
and telemetry.
For further information, refer to Storm Drain Systems, Highway Details and supporting documentation
Guidelines, AASHTO and the Manual for Highway Stormwater Pumping Stations, FHWA.
3.3.7 Outfalls
Outfalls transfer collected water to an acceptable point of release, usually referred to as the "receiving
waters". In Kuwait, these may include the sea, overland flow (wadis), percolation areas or other storm
drains. An outfall may be an open channel or a closed conduit. It is the most downstream element of the
storm drain system.
Outfalls can range from a few meters to several kilometres in length, and as such may extend well beyond
the limits of the highway corridor. Additional right-of-way requirements may have a significant bearing on
the location of the outfall.
Open channel outfalls are less expensive than closed conduit outfalls and provide a safety factor against
storms in excess of the design storm. However they require more maintenance and are often used as
dumping grounds.
Closed conduits should be used where the right of way is too narrow for an open channel or there is a risk
of flooding to adjacent property. Closed conduits are difficult and expensive to enlarge therefore provision
for the future should be accommodated in the design.

Page 3-13
In general, drainage outfalls within Kuwait can be classified into 3 categories as follows:
 Type A: This type of culvert outfall is to be used in areas which have wide flat tidal plains. This type of
outfall is found in the Jahra Governorate and western Capital Governorate. The culvert discharges into an
open channel which “daylights” at the point where the open channel meets the sea bed. The culvert
outfall is usually located at or near the high tide level
 Type B: This type of culvert outfall is to be used in areas where the culvert discharges through a
revetment wall or similar. Revetment walls are usually associated with coastal developments;
 Type C: This type of culvert outfall is to be used in areas where the culvert discharges on typical beach
profiles.
For details on each of these outfall types, refer to MPW Standard Drawings. For existing drainage outfalls
throughout the governorates, invert levels for existing governorates in the table below:
Governorate
Invert level
(m, mean sea level)
Jahra 2.00
Capital & Farwania 1.10
Hawalli 1.00
Mubarak Al-Kabeer 0.65
Ahmadi 1.00
The establishment of the invert level for the drainage outfall is critical. The invert level of the drainage
outfall needs to be considered on a site by site basis.
Specific considerations should be given to environmentally sensitive receiving waters. Contamination due to
the discharge of untreated stormwater can be mitigated by incorporating interceptors into the system or by
relocating the outfall.
3.3.8 Subsurface Drainage
Subsurface water in generally collected in a separate system that is connected to the main storm drain
system. Water trapped beneath the pavement surface but within the roadbed structure or foundation can
cause a rapid deterioration of the pavement. Subsurface drainage systems are designed to remove or
prevent water from reaching the roadbed. There are several sources of water that can enter sub grades or
pavement layers, these are:
 Surface water infiltrating through porous or cracked pavements or unsealed joints
 Lateral seepage into the edges from a saturated median or shoulders
 Upward seepage from groundwater
 Capillary action from underlying groundwater
 Accumulated water vapour from temperature variations and humidity
Water can be removed from embankment slopes, pavement layers and sub grades using a number of
different systems. These include:
 Horizontal drains
 Pipe underdrains
 Vertical wells
 Sub grade drainage systems
 Edge drain collector systems
For further design details, refer to Storm Drain Systems, Highway Drainage Guidelines, AASHTO.

Page 3-14
3.4 CULVERTS AND CROSS DRAINAGE
The function of a culvert is to convey surface water across or from the highway right-of-way. In addition to
its hydraulic functions, it must also carry construction and highway traffic, and earth loads. Culvert design
involves both hydraulic and structural design. For information on the hydraulic aspects of culvert design,
refer to Hydraulic Design of Culverts, Highway Drainage Guidelines, AASHTO. For information on the
structural aspects of culvert design, refer to the Kuwait Bridges and Highway Structures Design Manual.
3.4.1 Culvert Hydraulics
Culvert hydraulics can be classified as either inlet control or outlet control as shown in Figure 3.3.
Figure 3.3: Inlet Control compared with Outlet Control (Referenced from AASHTO Drainage
Manual)
Inlet control occurs when the culvert barrel is capable of carrying more flow than the inlet will accept.
Factors affecting inlet control are limited to the culvert entrance geometry and headwater depth. Culverts
under inlet control are not affected (up to the point where control shifts to downstream) by changes
downstream of the inlet, including culvert slope and roughness.
Outlet control occurs when the culvert barrel is not capable of conveying as much flow as the inlet will
accept. Factors affecting outlet control include those same factors governing inlet control, but also include
culvert slope, length, and roughness.
The flow is usually non-uniform with regions of both gradually varying and rapidly varying flow. As the flow
rate and tailwater elevations change, the flow type within the barrel can also change. A detailed hydraulic
analysis therefore involves backwater and drawdown calculations, energy and momentum balance. This
complex level of analysis is rarely warranted in culvert design.
Culvert hydraulics has been studied extensively in the past, with years of practical field case studies to add
confidence to these methods. Numerous charts, nomographs, and empirical equations have been
developed that simplify this analysis. Software programs are available to aid in the analysis and design of
culverts, allowing for rapid comparison of design alternatives.

Page 3-15
3.4.2 Culvert Design
Alignment
Culverts should be located and aligned as closely as possible to the natural flow path to minimize
disturbances to historical flow patterns, which can eventually result in erosion and/ or siltation. Where
location in the natural channel would require an unusually long culvert, some stream modification may be
required, as shown in Figure 3.4.
Figure 3.4: Drainage Alignment (Referenced from AASHTO Drainage Manual)
Maximum Headwater
The maximum headwater for the design storm flow for culverts greater than 1000 mm diameter shall be 1.5
times the culvert height. The maximum headwater for culverts with a height of 1m or less shall be 1.5m if
adjacent properties are not adversely affected. If the design flow exceeds 15 m3
/s in an urban area, the
maximum headwater shall not exceed the height of the culvert for an ultimate condition.
Flow Velocities
The culvert entrance will be affected by the approach velocity in the upstream ditch or channel. The
approach velocity can be calculated assuming normal depth (Manning’s Equation) using the total design
flow. In reality, this is a conservative estimate since the total design flow would not arrive at the culvert
entrance until some ponding has already occurred. However, since the integrity of the roadway is at stake,
this increased factor of safety may be warranted.
Flow velocities inside the culvert should also be evaluated. A non-erosive (velocity less than 4.5 m/s), self-
cleaning (velocity greater than 0.6 m/s) is the target. If flow velocities are too low, suspended sediment
will settle. Over time, this deposition will gradually reduce the hydraulic capacity of the culvert if not

Page 3-16
properly maintained. Alternatively, excessively high exit velocities may lead to erosion and eventually
jeopardizing the integrity of the culvert and roadway.
Culvert Size, Geometry, Material, Strength
Culverts are available in many different shapes, sizes, and materials. Common culvert shapes include
circular, arch, horizontal ellipse, vertical ellipse, and rectangular (box). Culverts sizes can vary from
300 mm to 2850 mm. Culvert materials include but are not limited to: concrete, metal, masonry, asbestos
cement, vitrified clay pipe, and plastic. Specifically, reinforced concrete, metal, and plastic are most
common in designs today.
Given that over 95 percent of all culverts and storm drain lines in Kuwait are concrete box or pipe,
information provided will emphasize design considerations that use concrete.
Structural Consideration of Culverts
It is necessary to consider both minimum and maximum earth cover over culverts. All culverts shall be
designed in accordance with AASHTO standards.
Preliminary Design
At an early conceptual stage of a project, required roadway crossings can be identified by overlaying
roadway alignments onto existing topography. Preliminary culvert designs can be started based on the
assumptions and taking the steps outlined below:
 Step 1: Assume inlet control which is acceptable for most roadway crossings.
 Step 2: Assume worst case design flow, possibly an ultimate, full build-out design condition.
 Step 3: Determine available headwater, measuring vertical distance from ground at the proposed
culvert inlet to the finished grade elevation of the roadway shoulder (assuming that no
adjacent ground point is lower).
 Step 4: Assuming circular concrete pipe, minimum diameter of 300 mm and maximum diameter based
on available headwater depth, Dmax = 0.67 x HW
 Step 5: If either (i) a larger diameter is needed to satisfy the headwater constraint, or (ii) the
diameter is greater than 1800 mm, then use multiple barrels, dividing the total flow evenly
between the barrels.
 Step 6: Check exit velocity. For velocities greater than 1.5 m/s, some level of erosion protection will
be needed.
Once an initial assessment has been conducted, a more detailed design can continue that will consider
precise location and alignment, design flow, culvert(s) size and material, entrance structure layout, outlet
structure layout, erosion protection, headwall, wingwall and possible emergency overflow path.
Ponding and Cross Flow
Street designs should attempt to minimize temporary ponding and cross flow. Ponded runoff can be due to
grade changes or intersection street crowns. Although the velocity of ponding water is negligible, it
adversely impacts traffic movement and creates potential hazards. If possible, ponding areas should be
minimized by over sizing inlets or providing emergency overflow paths away from critical infrastructure.
The cross flow may be caused by super elevation of a curve, by the intersection of two streets, or by
exceeding the capacity of the higher gutter on a street with cross fall. Cross flow should also be evaluated
in parking areas or other open areas where pedestrians have access. Cross flow should be minimized, but
is acceptable if the DxV criteria are met as discussed above.

Part 3 Kuwait Highway Drainage Design Manual Other Factors Affecting Design
Page 4-1
4 OTHER FACTORS AFFECTING DESIGN
The drainage aspects of highway design are affected by many factors that are only peripherally related to
hydrology and hydraulics. Some of these are discussed in the following section.
4.1 UTILITIES
Since highways and utilities often share the same right-of-way, coordination with the service authorities is
necessary to accommodate their current and future needs.
Widening an existing highway may incur problems due to a restricted right-of way and numerous existing
utilities that lie within it. When storm drains and utilities are in conflict, there are three options:
 Relocate the utility
 Relocate the storm drain
 Provide a structure to accommodate both the storm drain and the utility
Approval for relocation of utilities should be sought from the relevant utility authority.
Placing the storm drain under the roadway in order to avoid utility conflicts is not recommended for the
following reasons:
 Manholes may be required in the roadway
 Maintenance operations will interfere with traffic flow
 Settlement problems can result from poor backfilling, infiltration or pipe failure
4.2 RIGHT-OF-WAY
It may be necessary to buy additional land to accommodate drainage features.
Approval for purchase of additional right-of-way should be sought from MPW and Kuwait Municipality early
on in the design process.
4.3 SERVICE LIFE
Service life is generally defined as the number of years of relatively rnaintenance free life of the conduit
material. The service design life should be based on:
 Service life of the facility
 Importance of the facility
 Economics
 Difficulties associated with repair or replacement
 Future demands on the facility
Service design life for the different elements of the storm drain system should be agreed with MPW early on
in the design process.
4.4 ENVIRONMENTAL ISSUES
Erosion and sedimentation can be very visible, particularly on urban projects. Erosion control features
should be carefully designed, installed and maintained in sensitive surroundings. Refer to Erosion and
Sediment Control in Highway Construction, Highway Drainage Guidelines, AASHTO.
Hazardous spills occurring on the highway may be transported through the storm drain system to the
outfall, Interceptors may be required if the receiving waters are deemed sensitive to pollutants.
At several locations in Kuwait, contaminated groundwater has been encountered. Dewatering of excavations
during construction may lead to the release of harmful gases into the atmosphere. Water removed from the
ground during dewatering - procedures should be treated in accordance with EPA requirements.

Part 3 Kuwait Highway Drainage Design Manual Documentation, Construction and Maintenance
Page 5-1
5 DOCUMENTATION, CONSTRUCTION AND MAINTENANCE
Documentation is an important feature of design and facilitates the incorporation of proposed systems into
existing systems. Knowledge of the methodology and criteria used in past designs is necessary for the
accurate interpretation of design data.
A design drainage manual should accompany the design drawings and should include the following:
 Design criteria
 Photographs
 Contoured survey plans showing catchment boundaries and flow directions
 Records of existing drainage systems
 Drawings from previous projects
 Proposed drainage details
Documentation of hydrologic and hydraulic data in the form of drawings, notes, correspondence and
calculations should be included in the appendices.
 As-built records offer the best documentation of drainage features. These should include:
 Plan and profile drawings showing drainage structure sizes
 Invert levels
 Detail drawings of structure types
 Design drainage plans showing flow quantities, flow directions and catchment boundaries
 Drainage calculations
5.1 CONSTRUCTION
Different personnel may perform the design and construction functions. Adequate communication between
these two groups of personnel should be maintained throughout the construction period. The designer
should always be consulted when construction changes occur as these changes may affect the performance
of the drainage facilities.
The designer should aim to achieve a proper balance between material and construction costs. For example,
the least expensive material may not be the proper choice because the constructions costs may be greater
than for a more expensive material.
Changes in land use or utilities added after the design is complete may bring about an extensive redesign of
a storm drain system. This reinforces the need for full cooperation with MPW, Kuwait Municipality, service
providers, developers and any other interested parties during the design phase.
Temporary traffic detours should include provision for temporary drainage.
5.2 MAINTENANCE
Different personnel will more than likely perform the design and maintenance functions. Design personnel
should be aware of maintenance-related design considerations and vice versa.
A relatively maintenance free drainage facility may have a greater construction cost, but the life cycle cost
may be much lower. Maintenance personnel can advise the designer as to which drainage features require
considerable annual maintenance. The designer should seek the best balance between construction and
maintenance costs.

Part 3 Kuwait Highway Drainage Design Manual References
Page 6-1
6 REFERENCES
(1) American Association of State Highway and Transportation Officials, AASHTO Highway Drainage
Guidelines, 44th th edition, 2006.
(2) United States Department of Agriculture, National Resources Conservation Service, Urban
Hydrology for Small Watersheds, Technical Releases No. 55, Second Edition, June 1986.
(3) United States Department of Transportation, Federal Highway Administration, Hydraulic Design of
Highway Culverts, Hydraulic Design series No. 5, HDS 5,. 2005.
(4) United States Department of Transportation, Federal Highway Administration, Urban Drainage
Design Manual, Hydraulic Engineering Circular (HEC) No. 22, HEC 22, 3rd
edition, 2009.

Kuwait Highway Drainage Design Manual Summary

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