of 88
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NAKIVUBO CHANNEL REHABILITATION PROJECT (NCRP)
KAMPALA DRAINAGE MASTER PLAN
GENERAL INDEX TO REPORT
VOLUME 1 : EXECUTIVE REPORT
VOLUME 2 : MAIN REPORT PART IINSTITUTIONAL, ENVIRONMENTAL AND URBAN ASPECTS
VOLUME 3 : MAIN REPORT PART II
ENGINEERING AND ECONOMIC ASPECTS
VOLUME 4 : MAIN REPORT PART IIIDRAINAGE DEVELOPMENT
VOLUME 5 : INVENTORIES
VOLUME 6 : FIGURES AND MAPS
(All A3-size Figures and Maps)
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VOLUME 4
MAIN REPORT PART III
DRAINAGE DEVELOPMENT
PageCHAPTER 11 : DESIGN STANDARDS AND NORMS
11.1 INTRODUCTION 11.111.2 DESIGN RETURN PERIODS 11.211.2.1 CONCEPT OF RISK 11.211.2.2 ECONOMIC CONSIDERATIONS 11.311.2.3 DRAINAGE SYSTEMS 11.311.2.4 STORAGE AND FLOOD ATTENUATION DAMS 11.511.2.5 ROADS AND STREETS 11.611.2.6 BUILDINGS 11.711.3 FLOODLINES 11.811.3.1 GENERAL 11.811.3.2 TOPOGRAPHICAL INFORMATION AND SURVEYS 11.811.3.3 HYDRAULIC ROUGHNESS 11.911.3.4 CROSS-SECTIONS 11.911.4 HYDRAULIC DESIGN 11.1011.4.1 GENERAL 11.1011.4.2 PERMISSIBLE FLOW VELOCITIES 11.1011.4.3 ROUGHNESS COEFFICIENTS 11.1311.4.4 HYDRAULIC SIZING 11.1711.4.5 INLETS AND OUTLETS FOR MAJOR DRAINAGE SYSTEMS 11.1811.4.6 FREEBOARD 11.2011.5 STORMWATER DRAINAGE COMPONENTS 11.2311.5.1 GENERAL 11.2311.5.2 CHANNELS 11.2311.5.3 LARGE CONDUITS 11.2411.5.4 STORMWATER PIPES 11.2511.5.5 KERB INLETS 11.2611.5.6 CULVERTS AND BRIDGES 11.2711.5.7 STORAGE / FLOOD ATTENUATION FACILITIES 11.3011.5.8 EMBANKMENTS AND LEVEES 11.3111.5.9 PARKING AREAS 11.3111.5.10 BUILDINGS 11.3211.5.11 REMOVAL OF URBAN LITTER 11.3211.6 PUBLIC INCONVENIENCE AND EXPOSURE TO DAMAGE 11.3311.7 BIBLIOGRAPHY 11.35
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CHAPTER 12 : DEVELOPMENT PLAN AND LONG-TERM PROGRAMME
12.1 INTRODUCTION 12.112.2 MAJOR SYSTEMS 12.212.2.1 GENERAL 12.212.2.2 DRAINAGE SYSTEM 1 NAKIVUBO 12.412.2.3 DRAINAGE SYSTEM 2 LUBIGI 12.712.2.4 DRAINAGE SYSTEM 3 NALUKOLONGO 12.1112.2.5 DRAINAGE SYSTEMS 4 KANSANGA AND 4A GABA 12.1312.2.6 DRAINAGE SYSTEM 5 MAYANJA/KALIDDUBI 12.1412.2.7 DRAINAGE SYSTEM 6 KINAWATKA 12.1512.2.8 DRAINAGE SYSTEMS 7 NALUBAGA AND 7A NAKALERE 12.1712.2.9 DRAINAGE SYSTEMS 8 WALUFUME AND 8A
MAYANJA NORTH 12.1812.3 MINOR SYSTEMS 12.1812.4 DISTRIBUTION OF CAPITAL COSTS 12.2012.4.1 MAJOR SYSTEMS 12.2012.4.2 MINOR SYSTEMS 12.20
CHAPTER 13 : IDENTIFICATION OF ARRANGEMENTSFOR IMPLEMENTATION
13.1 INTRODUCTION 13.113.2 LEGISLATIVE, REGULATORY AND INSTITUTIONAL
ARRANGEMENTS 13.113.2.1 LEGISLATIVE ARRANGEMENTS 13.113.2.2 REGULATORY ARRANGEMENTS 13.113.2.3 INSTITUTIONAL ARRANGEMENTS 13.213.3 FINANCIAL ARRANGEMENTS 13.213.4 OPERATIONAL ARRANGEMENTS 13.313.5 ENGINEERING AND IMPLEMENTATION ARRANGEMENTS 13.3
CHAPTER 14 : SHORT-TERM ACTION PLAN
14.1 INTRODUCTION 14.114.2 NON-STRUCTURAL MEASURES 14.114.2.1 ORGANIZATIONAL STRUCTURE FOR IMPLEMENTATION
OF KDMP 14.114.2.2 OPERATION AND MAINTENANCE 14.114.2.3 RAINFALL AND RUNOFF DATA COLLECTION 14.214.3 STRUCTURAL MEASURES 14.414.3.1 PRIORITIZED INTERVENTIONS AND PROGRAMMING 14.414.3.2 CONCEPTUAL DESIGNS 14.514.3.3 DESIGN BRIEFS 14.614.3.4 COST ESTIMATES 14.9
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CHAPTER 11
DESIGN STANDARDS AND NORMS
11.1 INTRODUCTION
As described in Chapter 2 (Volume 2), management of stormwater drainage
distinguishes between structural measures and non-structural measures. Structural
measures consist of physical engineering works such as channelization of
watercourses, channel crossing (ie bridges and culverts), temporary storage
facilities, embankments, levees, etc. Non-structural measures include regulation of
floodplain use, regulation of land-use in the catchment and flood forecasting and
warnings, etc. The recommended design standards and norms described in this
chapter are applicable to structural measures or physical works only.
Planning and, in particular, the design of structural flood control measures must
generally comply with a number of criteria, such as:
minimizing the risk of damage to property and infrastructure
minimizing public inconvenience caused by frequent storms
protecting the public from severe floods and/or malfunctioning drainage systems
preventing erosion and siltation
preserving the environment
minimizing costs
The design standards and norms described in this chapter aim to assist in
quantifying the above criteria and to provide a uniform basis for future design of
improvements to the drainage systems in the Kampala District. For instance, the
design standards are essential to quantify stormwater discharges or flood
magnitudes, to size conveyance systems and culvert openings, to determine
acceptable levels of risk against damage and to establish acceptable levels of public
inconvenience, etc.
This chapter, therefore, deals with the general design standards associated with
flood and floodlines, standards applicable to hydraulic design or sizing, specific
standards and norms pertaining to components of stormwater drainage and norms
to evaluate the degree of inconvenience to the public and damage to buildings.
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The methodologies or techniques for the design of stormwater drainage systems are
not addressed, but can be obtained from various text books and other technical
publications. Similarly, standards and norms for the structural stability of stormwater
drainage measures are not considered to be the purpose of this chapter.
It must be noted that the recommended design standards and norms are influenced
by many physical factors and site-specific conditions. The design standards and
norms therefore serve as a guide with respect to minimum requirements, but they
cannot substitute for experience and sound engineering judgement.
11.2 DESIGN RETURN PERIODS
11.2.1 CONCEPT OF RISK
There will always be a risk that the design flood can be exceeded. The risk,
however, decreases with increases in design return period.
The probability or risk (p) that an event having a return period of T years will be
equalled or exceeded at least once during a design life of N years is given by:
p = 1 (1 1/T)N
This interrelationship of probability, return period and design life is illustrated in
Table 11.1 and Figure 11.1. The flood peak ratios in Figure 11.1 are expressed in
terms of the 100-year flood peak and are based on the average ratios of all
estimated flood peaks (or peak stormwater discharges) for different return periods,
as contained in Section 4 of Volume 5.
Table 11.1 : Interrelationship of probability, return period and design life
Probability (p) that an event will be equalled or exceeded during adesign life of N yearsReturn period of event(T years)
N=1 N=10 N=20 N=50 N=1005 0,20 0,89 0,99 1,00 1,0010 0,10 0,65 0,88 0,99 1,0020 0,05 0,40 0,64 0,92 0,9950 0,02 0,18 0,33 0,64 0,87100 0,01 0,10 0,18 0,39 0,63
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Figure 11.1 : Flood peak ratios and risks of being exceeded
For example, it can be noted that although the risk of the 10-year event being
equalled or exceeded in any one year is only 10% (or 0,1), there is almost a 100%
probability that it will be equalled or exceeded at least once in the next 50 years.
11.2.2 ECONOMIC CONSIDERATIONS
The selection of design return periods should be based on economic considerations
(ie cost-benefit analyses of the capital and maintenance costs of improvements
compared to the benefits achieved with improved drainage) as described in
Chapter 9 (Volume 3).
The following sections contain recommended minimum design return periods. The
risk associated with these minimum return periods must be considered as the
baseline for comparison purposes. Cost-benefit analyses should be carried out for
longer return periods to establish whether it would be economically beneficial to
design for a longer return period than the recommended minimum.
11.2.3 DRAINAGE SYSTEMS
(a) General
The design of any stormwater drainage system is based on a specific
discharge capacity, or flood peak, associated with a pre-selected return
period. The selection of a design return period is affected by the particular
stormwater drainage system under consideration, namely major or minor
systems as described in the following sub-sections.
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(b) Major Systems
A major system consists of all natural watercourses, which collect and
convey surface stormwater in a definite direction and includes all natural
channels, streams and rivers, whether or not its conformation has been
changed by artificial means such as channelization. All the primary and
secondary channels shown in Figures 3 to 10 in Volume 6, are thus, by
definition, major systems.
The major system should be designed to accommodate less frequent
storms, to also take account of the downstream impacts of unusually high
flood events.
Although economic considerations, as described in Chapter 9 (Volume 3),
usually result in optimum return periods of between 10 and 20 years, many
countries have legal requirements stipulating longer return periods. By due
consideration of the inadequate existing discharge capacities of the major
systems in Kampala and the cost and affordability of upgradings, a design
return period of 10 years as a minimum is recommended for the major
systems.
An economic analysis should still be a prerequisite for the detail design of
any major system to establish whether it would be economically beneficial
to design for return periods longer than 10 years.
In addition, it is essential that the behaviour of an upgraded major system
also be verified for stormwater discharges with return periods of up to 100
years to ensure that all affected or possibly affected persons have access
to information regarding potential flooding.
(c) Minor Systems
The purpose of minor systems is to convey stormwater to the major
systems in such a way that inconvenience to pedestrian and vehicular
traffic is minimized and properties are protected from flood damage from
frequent storms of lower intensity. The minor systems conveniently consist
of pipes or small open drains to avoid frequent nuisance, which results from
overland flow. By definition, the minor system therefore corresponds to pipe
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and open drain systems, which have traditionally been provided in Kampala
to convey stormwater to the major systems (or primary and secondary
channels).
The roadside drainage referred to in the previous chapters consisting of
kerbs, kerb inlets or catchpits, underground pipes and small open drains
are all components of the minor systems.
Minor systems are designed for stormwater discharges with shorter return
periods to minimize inconvenience in the areas outside the primary and
secondary channels and floodplains.
It is recommended that design return periods for minor systems be based
on the return periods listed in Table 11.2. These are commonly used in
South Africa and other countries and have been abstracted from the
Guidelines for Human Settlement Planning and Design of the RSA
Department of Housing (2000). These must be considered as the
minimum design return periods and the onus is still on the designer to
consider longer return periods in cases where the risk of inconvenience and
monetary losses due to regular damage are unacceptably high.
Table 11.2 : Design return periods for minor systems
Land-use Recommended Design Return Period
Residential 2 to 5 years
Institutional (eg schools) 2 to 5 years
General commercial and industrial 5 years
High value central business districts 5 to 10 years
11.2.4 STORAGE AND FLOOD ATTENUATION DAMS
Storage on major systems (ie on primary and secondary channels) is provided by
dams for recreational and water supply purposes or for temporary storage to
attenuate or retard the peak stormwater discharges.
Breaching or failure of a dam will result in catastrophic damages and will affect
public safety downstream of the dam. The designer should, therefore, not rely on
generalized design standards, but cater for site-specific conditions also taking the
storage volume of the dam and the population density downstream of the dam into
account. The norms for selecting a return period for the design of the spillway or
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outlet, as given below, are based on international current practice and merely serve
to assist the designer in determining an appropriate flood magnitude (ICOLD 1987
and SANCOLD 1991).
Two scenarios are usually considered to determine acceptable levels of spillway
performance, namely:
Design flood conditions : The spillway operates without damage to any of its
components or to the associated dam structure. For this scenario it is
recommended that the 50-year flood hydrograph, routed through the dam
reservoir with appropriate freeboard, be used for sizing of the spillway.
Extreme flood conditions : Spillway operation may result in substantial
damage to its components and/or parts of the dam structure, but will not result
in catastrophic failure of the dam. For this scenario it is recommended that the
100-year or 200-year flood hydrograph, routed through the dam reservoir
without overtopping of the dam wall, be used to verify the safety against
catastrophic failure of the dam.
Kabakas Lake is the only existing dam in Kampala District of any significance, but
still has a relatively small storage capacity. It will not be feasible to construct storage
dams for flood peak attenuation on the floodplains of the lower lying primary
channels. Such dams will only be feasible in the upper reaches of the primary
channels and on the secondary channels. Space limitations and the steep channel
slopes mean that the storage volumes will be limited and catastrophic failure or
breaching may, therefore, have an insignificant effect on the downstream floodlines
or areas that would have been inundated without the dam. It is, therefore, also
recommended that the designer perform dam break analyses for floods with
different return periods and compare its effect to the without dam condition, to
assist in selecting a return period for the above extreme flood conditions.
11.2.5 ROADS AND STREETS
The selection of a return period for roads and streets forming part of minor systems
should be based on the recommendations given in Table 11.2. Where roads pass
through areas of different land-use, consideration should be given to designing the
entire route for the longest return period (see Table 11.2) associated with the
various land-uses occurring along the route.
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The return periods associated with the design of road crossings (ie bridges and
culverts) of major systems must at least be similar to the return period applicable to
upgradings of the primary and secondary channels, namely 10 years. It is, however,
essential that longer return periods also be considered in the following cases:
high potential damage to the road and high associated cost of repairs
long time needed for repairs to make the route usable for traffic again
detours not available
long period of flooding
high traffic density
deep flow depth and high flow velocity of floodwaters
high strategic importance (military, police, fire brigade, medical services, etc.)
high economic importance.
11.2.6 BUILDINGS
Selection of a return period for design of stormwater from and around large buildings
or a complex of buildings, is governed by site-specific conditions and economic
considerations. Site-specific conditions determine whether drainage forms part of
the major or the minor systems.
For buildings where only (unconcentrated) overland stormwater runoff needs to be
accommodated, the drainage forms part of the minor system and return periods
recommended in Table 11.2 would be applicable.
Where stormwater drainage at buildings is classified as forming part of the major
system, a return period of at least 10 years must be considered for design. In
addition, an economic or risk analysis is essential to determine whether a longer
return period should be used for design. Typical examples are:
buildings adjacent to primary and secondary channels where floodlines should
also be taken into account
large buildings or a complex of buildings situated across land depressions
diversions of a natural watercourse to suit the building layout.
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11.3 FLOODLINES
11.3.1 GENERAL
Floodlines are required on township layout plans to indicate the strip or area along
the watercourse that will be prone to inundation by stormwater discharges or floods.
Floodlines are hydraulically analyzed or determined for a specific return period and
the areas outside the floodlines on both sides of the watercourse are still subject to
inundation during floods with longer return periods than those on which the
floodlines are based. It is therefore good practice, and recommended, that floodlines
associated with longer return periods (at least up to 100 years) also be shown on
the layout plans and made available to all interested and affected parties to ensure
that they are aware of the risk of inundation along the watercourse.
Floodlines are applicable to the major systems only (ie the primary and secondary
channels) as analysed in Chapter 7 (Volume 3).
11.3.2 TOPOGRAPHICAL INFORMATION AND SURVEYS
The morphology of a watercourse (ie cross-sectional shape and area as well as bed
slope) plays a major role in the location of floodlines. The floodlines determined in
Chapter 7 (Volume 3) are based on the available digitised spot heights, which were
also used by the Department of Surveys and Mapping to prepare the maps with 2m
contours contained in Volume 6. Although this topographical information is
considered adequate for master planning (as used in Chapter 7 : Volume 3), better
topographical information that is more accurate will be essential for detail design
purposes.
The following norms should be adhered to when collating topographical and other
associated information or surveys.
Topographical surveys should be based on the geodetic datum level with all
details of manmade structures (eg buildings, roads and bridges, dams,
channels, etc.) shown on the plans to facilitate transformation to GIS
(Geographic Information System). Transposition of floodlines from the
topographical map used to determine the floodlines to other topographical
maps must be done with care, taking discrepancies in contours, datum levels
and differences in coordinate systems into account.
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The required contour intervals (or spacing of spot heights) depend on the
topography, density and complexity of manmade structures, and is left to the
engineering judgement of the designer.
Details of existing bridges and culverts (ie number and sizes of openings,
transition dimensions, invert levels, road surface levels, etc.) should be shown
on the plans.
Depending on the topographical complexity and density of manmade structures, the
survey and use of cross-sections only (see Section 11.3.4) could also suffice.
11.3.3 HYDRAULIC ROUGHNESS
The greatest difficulty in determining floodlines is in assessing applicable roughness
coefficients and their variation along a watercourse. There are no exact norms for
selecting the roughness coefficient and this is usually based on engineering
judgement and experience.
Guidelines for selecting the roughness coefficient (n) in the Manning formula are
available from various text books, and reference can be made in this regard to Ven
te Chows Open-Channel Hydraulics (1959). (Also see Section 11.4.3).
It is good to investigate the sensitivity of variations in the roughness coefficient as
part of the floodline analysis. This process is facilitated if the model can be
calibrated on the basis of recorded flood peaks and flood marks.
11.3.4 CROSS-SECTIONS
The distance between cross-sections at which the water surface levels are modelled
to determine the floodlines depends on the uniformity of the watercourse and on the
desired level of accuracy. As a norm, cross-sections should be selected with
spacings of less than 25 times the flow depth and at changes in cross-sectional area
and shape, so that average velocities will not vary by more than 10 - 20% between
successive cross-sections. Localized irregularities can be ignored. It should be
noted that in cases where a channel meanders along a floodplain, the distance
between cross-sections, and thus also the slope, will differ for the channel flow and
overbank flow. It is usually also necessary to subdivide the cross-sections into
segments according to variations in the roughness coefficient and the occurrence of
stationary or dead water, as the case may be for overbank flow or flow on
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floodplains (refer to the Handbook of Applied Hydraulics (1969) by Davis and
Sorensen).
11.4 HYDRAULIC DESIGN
11.4.1 GENERAL
The behaviour of flow is influenced by many physical factors and site conditions.
The standards and norms applicable to hydraulic design of stormwater drainage
systems and flood protection measures as described in this section can, therefore,
only serve as a guide and cannot substitute for experience and sound engineering
judgement.
11.4.2 PERMISSIBLE FLOW VELOCITIES
(a) Maximum Flow Velocities
The maximum permissible flow velocity is the highest velocity that will not
cause significant erosion or scour and will not cause structural damage. The
norms, serving as a guide to establish these limiting velocities, are different for
various types of surfaces as described below.
(i) Unprotected soil surfaces
Unlined channels and drains, as well as natural channels in unprotected
soil, are considered erodible, with erodibility dependant on the type of
soil. In accordance with a soils map for Kampala, made available by the
Department of Surveys and Mapping, the soil type in the Kampala
District is defined as clay loams or loams, except along the wetlands of
Drainage Systems 2 (Lubigi) and 3 (Nalukolongo), where humous clays
are found and along the wetland of Drainage System 5
(Mayanja/Kaliddubi) where humous sands are found. Based on
information abstracted from the RSA Roads Drainage Manual (1997)1,
the recommended maximum permissible average velocities or non-
erodible velocities at different flow depths for these soil types are shown
in Figure 11.2. Hydraulic calculations are necessary to determine the
flow depth and velocity for a given discharge, channel gradient and
roughness coefficient. These recommended velocities apply to straight
reaches and need to be reduced for sinuous sections to reduce scour
1 A copy of the RSA Roads Drainage Manual has been handed to KCC for reference purposes.
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around bends. The maximum permissible average velocity is variable
and can be estimated only by experience and engineering judgement.
Figure 11.2 : Permissible average flow velocities
(ii) Lined surfaces
The surfaces of channels, drains and dam spillways are lined inter alia to
accommodate higher flow velocities. Lining materials usually consist of
cast in-situ concrete, precast concrete blocks or slabs, stone pitching
and gabions.
The maximum permissible velocity is not critical, but is still governed by
water carrying sand, gravel and stones and the tendency for fast-flowing
water to lift the lining material and displace it. This applies, in particular,
to concrete blocks, gabions and stone pitching.
(iii) Grassed surfaces
Grass provides effective protection against erosion if the surface to be
protected is subject to occasional or intermittent flow of water only, as is
the case with stormwater conveyance. It can be used successfully in
Kampala on the upper side slopes of channels, auxiliary spillways on
dams and along embankments. However, although grass provides a
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low-cost, environmentally acceptable solution, it requires regular and
continuous maintenance to remain effective in the long-term.
The use of reinforcement in a grassed waterway enhances the
engineering functions of plain grass, while retaining its environmental
attributes. Reinforced grass is used where flow velocity is high enough
to cause erosion that grass on its own might not withstand. Guidelines
for the design of grassed waterways can be found in CIRIAs Design of
Reinforced Grassed Waterways (1987) and in US Department of
Agricultures Stability Design of Grass-lined Open Channels (1990).
These guidelines show that the limiting flow velocity on plain grass
should also be considered in terms of duration of flow, as shown in
Figure 11.3. Effectiveness in preventing erosion in a grassed waterway
depends on:
full and intimate cover of the subsoil surface
no seepage flow in the direction of the slope
good integration of the soil/root mat with the underlying subsoil
avoidance of surface irregularities.
The above requires a high standard of construction and maintenance,
which is not always achievable. A high standard of maintenance
demands that the grass be cut regularly to increase its density.
The recommended flow velocities given in Figure 11.3 can be
increased if reinforcement of the grass surface is provided.
Figure 11.3 : Recommended limiting values for erosion resistance of plain grass
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The flow velocities along the steeper slopes in the upper catchments will
usually be higher than the above maximum permissible flow velocities for
unprotected soil and grassed surfaces. Erosion can be avoided by lining as
described above or by reducing the flow velocity, which requires flatter
gradients or slopes. This can be achieved by small concrete or masonry weirs
or drop structures. A stilling basin or pool will be required at the drop
structures to dissipate energy and avoid erosion immediately downstream of
the structure.
(b) Minimum Flow Velocities
The minimum permissible flow velocity, or non-silting velocity, is the lowest
velocity that will not cause sedimentation or siltation. This velocity is uncertain
and its exact value cannot be easily determined. The minimum recommended
average velocity to avoid siltation or deposition of fine material is shown in
Figure 11.2.
The minimum permissible flow velocities given in Figure 11.2 may be too low
for lined surfaces. Generally, a mean velocity of 0,6 to 0,9m/s on straight
sections may be used when the percentage of silt is low (Ven te Chow
1959). In the case of channels, siltation usually occurs on the inside of bends.
This can be minimized by tilting the bottom, or superelevating the canal
bottom, to ensure that reasonable velocity is maintained on the inside of the
bend.
The minimum permissible velocity to prevent deposition of material will
depend largely on the particle sizes of the materials being transported during
flood flows. Deposition of material usually occurs during flows lower than the
design flow. This makes it essential to verify the accepted minimum
permissible velocity for flood flows with lower return periods than the design
return period.
11.4.3 ROUGHNESS COEFFICIENTS
(a) Flow Formulae
The formulae used most often to determine the velocity and depth of steady
uniform flow in an open channel for a given discharge are:
V = (1/n) R2/3 S (Manning formula)
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V = C (RS) (Chezy formula)
in which V is the velocity (m/s)
R is the hydraulic radius (m)
S is the bed slope or hydraulic gradient (m/m)
n is the Manning roughness coefficient
C is the Chezy roughness coefficient.
Selection of values for the Manning roughness coefficient (n) or Chezy
roughness coefficient (C) applicable to open-channel or free-flow conditions is
more complex and usually requires some judgement based on experience.
The Manning flow formula is used extensively for open-channel flow, and the
remainder of this section provides general norms for selecting the Manning
roughness coefficient, also known as the retardance coefficient.
In closed conduits with a circular cross-section (eg pipes), the following
formulae are most often used to determine head losses for full flow conditions:
hf = f(L/D) V2/2g (Darcy - Weisbach formula)
V = 0,849 C R 0,63 (h f / L) 0,54 (Hazen-Williams formula)
in which h f is the head loss (m)
L is the length of conduit (m)
D is the internal diameter of conduit (m)
V is the velocity (m/s)
g is the acceleration of gravity (9,81m/s2)
R is the hydraulic radius (m)
f is the Darcy-Weisbach friction factor
C is the Hazen-Williams roughness coefficient.
The values for the Darcy-Weisbach friction factor (f) and for the Hazen-
Williams roughness coefficient (C) are affected by age, type and size of pipe
or conduit and, to a lesser extent, by the properties of the water. Guidelines or
norms for a reasonably accurate assessment of these friction factors and
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roughness coefficients are provided by suppliers of the various types of pipes
or conduits, and can also be verified in various text books, such as the
handbook of Applied Hydraulics by Davis and Sorensen (1969) and Pipeline
Design for Water Engineers by Stephenson (1979).
(b) Factors Affecting Mannings Roughness Coefficient
The value of Mannings roughness coefficient (n) is highly variable and
depends on a number of factors. The factors listed below exert the greatest
influence on the value of n and reference should be made to Ven te Chows
Open-Channel Hydraulics (1959) for a detailed description of the influence of:
surface roughness
vegetation
irregularities
alignment
siltation and scouring
obstructions
discharge or flow depth
seasonal changes
suspended material and bed load.
The above factors should all be evaluated with respect to conditions regarding
the type of channel, state of flow, degree of maintenance, and other related
considerations. As a general norm, conditions tending to induce turbulence
and retardance will increase the n-value while those tending to reduce
turbulence and retardance will decrease the n-value.
Various technical publications and text books (see Bibliography) can be
consulted when assessing Mannings roughness coefficient, but engineering
judgement and experience will ultimately be required. It is always good
practice to investigate the sensitivity of flow depth or discharge due to
variations in the roughness coefficient.
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(c) Grassed Surfaces
Protection against scouring or erosion by means of grassed surfaces is often
used because of its low establishment cost and pleasing environmental
appearance. Assessment of Mannings roughness coefficient for plain grassed
surfaces varies considerably between designers. It is considered appropriate,
therefore, to provide norms or guidance for the selection of a roughness
coefficient.
The hydraulic roughness of a grassed surface depends on its physical
characteristics, such as the height, stiffness and density of the grass and its
interaction with the flow. This interaction is divided into the following three
basic regimes according to hydraulic loading (CIRCA 1987):
The flow depth is significantly less than the height of the vegetation,
which is not deflected, and velocity at the soil surface is low due to
interference by the vegetation.
The combined effect of increasing flow velocity and depth causes the
vegetation to deflect and oscillate in the flow.
The velocity is high enough to push the vegetation down and a relatively
smooth, stationary surface is presented to the flow, with the effective
height of the vegetation being considerably lower than its natural height.
The roughness coefficients associated with these regimes, as prepared by
CIRCA (1987), are shown in Figure 11.4(a). The so-called VR method is
recommended for channels with slopes flatter than 1:10. For slopes steeper
than 1:10, the grass tends to be pushed down by the flow throughout the
normal range of discharges, and the hydraulic roughness appears to be
independent of the flow parameter (VR) and grass length, but to vary with the
waterway slope, as shown in Figure 11.4(b). These values are not applicable
to hydraulic loadings (or flow parameters) of less than about 0,01m2/s.
Presently there appears to be insufficient justification to warrant adopting
different values of hydraulic roughness for reinforced grass systems versus
those used for plain grass.
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It is advisable to adopt a lower roughness coefficient to determine the flow
velocity when selecting the required (grass) reinforcement, and an upper
roughness coefficient to determinate flow depth for allocation of freeboard or
for determination of overall shear stress imposed by flow on the waterway.
These lower and upper limits should be based on site-specific conditions and
engineering judgement. A nominal variation of at least 10% should be used.
Figure 11.4 : Roughness coefficient for grassed surfaces
11.4.4 HYDRAULIC SIZING
(a) Free Surface Flow
Free surface flow conditions normally apply to open channels and drains and
to unpressurized conduits or pipes.
Hydraulically, a channel section having the least wetted perimeter for a given
area has the maximum conveyance (Ven te Chow, 1959). In the case of a
rectangular cross-section, the best hydraulic section is achieved with a bottom
width of twice the water depth. In the case of a trapezoidal cross-section, the
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best hydraulic section is achieved with half a hexagon. This implies side
slopes of 3 vertically and 1 horizontally (or 60 measured from the horizontal)
with the bottom width equal to 2/3 times the water depth. In the case of a V-
shaped cross-section, the best hydraulic section is achieved with side slopes
of 1 in 1 or 45. These optimum trapezoidal and V-shaped sections are usually
impractical due to difficulties in construction and type of lining material, which
affects the stability of the side slopes; but are often used for concrete-lined
side drains along roads.
These optimum hydraulic sections may not be the most economical option if
the cost of excavation, allowance for freeboard, and type of lining material are
taken into account. Preference is usually given to a trapezoidal section with a
side slope of 1 vertically to 3 horizontally (minimum) for lining materials other
than concrete or gabions. However, confined spaces may necessitate a
rectangular, concrete-lined section.
Sizing of conduits and pipes should be based on free-flow conditions for the
design flood peak. In any case, for circular conduits and pipes, the maximum
free-flow conveyance is achieved with a flow depth equal to about 93% of the
internal diameter.
(b) Full-flow Conditions
When the conduit or pipe becomes pressurized (ie full-flow conditions under a
surcharge head at the inlet), higher flows can be discharged. Head losses at
inlets, outlets, junction boxes, bends and changes in diameter or size have a
significant effect on the sizing of the pipe. Full-flow conditions, which result
from outlet control (ie submergence at the outlet), should also be taken into
account to determine the reduction in flow capacity under such conditions,
which are also associated with floods larger than the design flood.
Pipe diameters smaller than 450mm should not be used. Pipe sizes should
generally not be reduced on steep gradients or blockage may occur.
11.4.5 INLETS AND OUTLETS FOR MAJOR DRAINAGE SYSTEMS
Inlets for major drainage systems, including inlet transitions to bridges and culverts,
and intakes of outlet works for dams require careful attention during planning and
design. The main problem with inlets, inlet transitions and intakes is blockage by
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tree stumps and urban litter (see Section 11.5.11). Standards for the planning and
design of inlets and intakes cannot be laid down, but the following principles should
be taken into account:
Abrupt inlet transition sections are unacceptable and should be avoided. Inlet
transitions should allow for a gradual change in cross-sectional flow area to
minimize the formation of standing and cross waves. For super-critical flow,
the inward deflection of the side walls should in plan be less than 1 in 3 times
the Froude number.
Damming up at inlet transitions, which can be caused by inadequate
discharge and blockage, will only be acceptable if the upstream water levels
can be accommodated and slower flow velocities do not result in deposition of
sediment. (See also Section 11.5.11).
Protection against scouring immediately downstream of a rigid (concrete)
invert may be required for super-critical flow conditions.
The need for additional freeboard over and above that proposed in
Section 4.6.
Public safety measures (eg headwalls, handrails, etc).
Outlets for channels, large conduits and dams also require special attention,
particularly as far as energy dissipation is concerned. The design of energy
dissipators is dependent on many variables, which are site-specific and should,
therefore, be designed by an experienced engineer. The norms for planning and
design of energy dissipators and protection against erosion are covered extensively
in text books (Ven te Chow, 1959) and other technical publications (US Bureau of
Reclamation, 1963 : Hydraulic Design of Stilling Basins), and fall beyond the scope
of this chapter. The basic norm should be that resulting flow velocities comply with
the norms for permissible flow velocities downstream (see Figure 11.2). Therefore,
an outlet transition (concrete or stone pitching) could also be used in cases with
appreciable flow depth downstream of the outlet. The aim should be to enlarge the
flow area by means of the transition to ensure a reduced flow velocity at the
downstream end of the transition.
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The outlets of channelized primary and secondary channels into a wetland also
require special attention, particularly as far as deposition of sediment is concerned
when the flow velocities are reduced. The gradients along the wetlands are always
much flatter than the gradients of the upstream channelized reaches. Deposition of
sediment over time therefore cannot be avoided and will have to be manually
removed on a regular basis. Sediment originates from the upper steeper slopes of
the catchments and special measures should also be employed to minimize erosion
in the upper reaches as described in Section 11.4.2(a).
11.4.6 FREEBOARD
(a) Channels, Embankments and Levees
Freeboard is the vertical distance from the design water surface level to the
top of the above structures. This distance should be great enough to prevent
waves or disturbances in the water surface from overtopping the channel
sides, embankments and levees.
The following norms are proposed to determine the required freeboard:
(i) Straight sections
Minimum values for freeboard on straight channel reaches, as extracted
from the RSA Road Drainage Manual are given in Table11.3.
Table 11.3 : Recommended freeboard on straight channel reaches
Freeboard for :Canal Section
Sub-critical flow Super-critical flow
Rectangular 0,15E 0,25y
Trapezoidal 0,20E 0,30y
E = specific energy = y + V2 / 2g
y = depth of flow at deepest point
V = average velocity
The minimum values for freeboard given in Table 11.3 should also apply
to flow along embankments and levees. For stationary or dead water
along embankments and levees, the freeboard component
recommended for wave action and surges on dams can be used (see (c)
below)
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(ii) Super-elevation at bend sections
In addition to the freeboard given in Table 11.3, additional freeboard as
shown in Table 11.4 is required at bends or curved sections to allow for
super-elevation of the water surface and wave action.
Table 11.4 : Additional freeboard for super-elevation
Canal Section Additional Freeboard
Rectangular v2 b_____
gr
Trapezoidal v2 (b + 2Ky)__________
(gr - 2Kv2)
v = average velocity in straight portion of channelb = bottom widthg = acceleration of gravity (9,81m/s2)r = centre-line radius of channel (should not be smaller than three times the width at the water surface)K = cotangent of side slope angle measured from the horizontal (equal to zero for rectangular channel)y = flow depth in straight portion of channel
Note : The above additional freeboard can be reduced for sub-critical flow andincreased for super-critical flow depending on site specific conditions.
For sub-critical flow, the additional freeboard given in Table 11.4 is required
only on the outside of the bend, but for super-critical flow it is required on both
the outside and the inside and for some distance downstream of the bend due
to the propagation of shock or cross waves down the canal.
(b) Large Conduits
Large stormwater conduits are not designed for full-flow conditions, and the
freeboard (ie the vertical distance from the water surface to the soffit of the
conduit) to be allowed can be based on Table 11.3, although this is not critical.
(c) Dams
The total freeboard for a dam is the vertical distance from the full supply level
(FSL) to the non-overspill crest of the dam and consists of two components,
namely the flood surcharge rise above FSL as the primary component, and a
secondary component, allowing for wind wave and surge effects.
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To assess the total freeboard required for dams, reference must be made to
SANCOLDs Guidelines on Freeboard for Dams (1991). This provides a
comprehensive overview of all factors affecting freeboard for various types of
dams.
(d) Culverts
Two freeboard scenarios are applicable to culverts : free flow conditions and
submerged or inlet control conditions.
Free flow conditions are applicable to culvert crossings of channels which are
channelized. The flow conditions in the upstream channelized reach should
not be disturbed by the culvert, requiring freeboard from the water surface to
the soffit of the culvert as given in Table 11.3. A box culvert will be the best
option to ensure that the flow conditions are not disturbed.
Submerged or inlet control conditions occur when the culvert causes
damming-up of the flow. Pipe culverts usually cause disturbance and
damming-up of the flow, unless very large pipes are used. In the case of
crossings over wetlands and other natural channels, damming-up upstream
may be acceptable. In these cases the freeboard is measured from the
upstream (dammed-up) water surface to the top of the road, which must also
conform to the freeboard requirements given in Table 11.3, to prevent
splashing onto the road which can be caused by wave action and other
disturbances.
(e) Bridges
Freeboard for bridge structures is the vertical distance from the upstream
design water level (taking the rise in water level due to damming-up into
account) to the soffit or underside of the bridge. This distance should be
selected to prevent water or disturbances in water level from overtopping the
bridge and approach embankments, and to avoid splashing onto the road.
The recommended minimum freeboard for bridge openings, as extracted from
the RSA Road Drainage Manual (1997), is given in Table 11.5.
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Table 11.5 : Minimum freeboard for bridge openings
Design Discharge(m3/s)
Minimum Freeboard (m)(Interpolate for values in between)
0 - 100 200 400 1 000>1 000
0,30,50,71,0
0,6 + d/15 (minimum 1,0)d = flow depth (m)
Again, the above minimum values for freeboard should be considered in light
of site-specific conditions. Shock waves, which can be caused by abutments
and bridge piers, should also be taken into account, particularly at skew
crossings and for super-critical flow conditions.
11.5 STORMWATER DRAINAGE COMPONENTS
11.5.1 GENERAL
This section contains specific design norms applicable to various stormwater
drainage components not covered in the preceding sections.
11.5.2 CHANNELS
The meandering of a channel is usually reduced by channelization, resulting in an
increase in longitudinal bottom slope and flow velocities. When flow velocities
increase above the permissible maximum (see Section 11.4.2) special protection
measures must be provided against scouring and erosion on the bottom and
particularly the banks or sides.
The side slopes on the banks depend mainly on the type of material. For softer
material, the slopes should preferably be grassed and not made steeper than
1 (vertical) to 3 (horizontal). In harder material, such as rock or other less erodible
materials (eg stiff clay), nearly vertical or steeper slopes can be considered provided
measures are taken to ensure public safety. Levees such as those constructed from
concrete and gabions can also be used with founding levels well below the potential
scouring depths.
The norm for selecting the lining material for channelization depends mainly on the
maximum flow velocities, the availability and cost of the lining material, method of
construction (eg labour-intensive methods) and duration of the design flood.
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The longitudinal bottom slope of a stormwater channel is generally governed by the
topography, which affects the flow velocity and consequently affects the selection of
lining material.
Trapezoidal and rectangular sections are mostly used for stormwater channels.
Rectangular sections are more expensive than trapezoidal sections and are only
used to overcome certain problems, such as confined spaces, and to facilitate road
crossings. The side slopes can vary from vertical, for concrete-lined canals, to any
slope depending on the stability of the lining under design flow conditions. Handrails
or barriers are essential along the top edges of the channel for public safety. It is
also essential that access into and out of channels be provided at least every 200m
to facilitate maintenance operations and to serve as escapes for people who may
have fallen into the channel.
Any improved or upgraded channel will be subject to maintenance on a regular
basis. Access to the channels is therefore essential and it is recommended that a
minimum 3m wide right-of-way be provided on both sides of the channel. This is
particularly important in built-up areas where houses and other buildings are
situated on the banks close to the channel.
11.5.3 LARGE CONDUITS
Large conduits are usually constructed of reinforced concrete, including precast
concrete sections. For conveyance of stormwater, large conduits should generally
not flow full at the design condition. Factors to be considered in the design are,
therefore, similar to those for a lined canal except that freeboard can be reduced.
When precast concrete sections are used the roughness coefficients need to be
increased to allow for irregularities at the joints, which will depend on the length and
type of precast section used.
The minimum permissible velocity, or non-silting velocity recommended for lined
canals (Section 11.4.2) would also apply to conduits. The maximum permissible
velocity in conduits is not critical, provided the structural stability of the precast
sections is not affected and adequate energy dissipation is provided at the outlet.
Access into large conduits is required at least every 200 - 350m to facilitate
maintenance operations and to rescue people and animals that may have been
drawn into the conduit.
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11.5.4 STORMWATER PIPES
Stormwater pipes are installed underground, usually in areas unsuitable for open
side drains (on the sides of streets) and when the flow in the side drains approaches
critical flow conditions.
The layout planning of stormwater pipes is controlled by site-specific conditions and
specific norms cannot be laid down. They are usually provided along roads and
streets, but also across stands or plots (by means of a servitude) to shorten the
distance to a suitable discharge point into the major system depending on the
topography.
Access, via manholes or junction boxes, to the stormwater pipes is required at every
junction and every point where there is a change in pipe size, grade and direction of
flow. In addition, manholes must be provided at least every 200 - 350m for
diameters exceeding 1 200mm, and at least every 100 - 200m for diameters of less
than 1 200mm to facilitate maintenance operations.
The flow velocities are controlled by the gradient or slope of the stormwater pipe and
the depth of flow in the pipe. The minimum permissible gradient should be based on
achieving non-silting velocities on a regular basis. The non-silting velocity is
uncertain and its exact value cannot be easily determined. Generally, velocities
higher than 0,6m/s are generally accepted as non-silting velocity. Non-silting velocity
must be achieved regularly to remove accumulated silt from the pipeline. It is thus
proposed that the minimum grade be determined on the basis of a non-silting
velocity of 0,6m/s for regular floods. In cases where this cannot be achieved, it is
essential to provide access manholes at closer spacings than given above to
facilitate regular cleaning of deposits.
The minimum gradients for various size pipes to achieve a non-silting velocity of
0,6 m/s are shown in Figure 11.5. These curves are based on Mannings flow
formula with a roughness coefficient of 0,015. It is essential that the discharge
associated with the non-silting velocity be verified to ensure that it is achieved on a
regular basis.
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The maximum permissible velocity is not critical, but is still governed by water
carrying sand, gravel and stones, which could cause damage to the pipes.
Figure 11.5 : Minimum gradients for pipes to ensure a non-silting flow velocity of 0,6 m/s
Outlets require special attention because of the usual relatively high flow velocities
under design conditions. The outlet velocities should comply with the norms for
permissible flow velocities downstream (see Figure 11.2).
11.5.5 KERB INLETS
Surface water is collected along the kerbs and discharged into stormwater pipes by
means of kerb inlets (or catchpits) positioned at specific points in a controlled
manner for maximum traffic safety.
Flow should be transferred along a kerb into a piped system when the surface flow
is still sub-critical at Froude numbers of less than 0,8. However, in most cases this is
not possible and special attention is required in the design of kerb inlets for critical or
super-critical surface flow conditions. The norms for positioning of kerb inlets based
on the guidelines of the RSA Road Drainage Manual (1997) are:
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adequate freeboard is allowed along the kerb
intermediate kerb inlets should intercept at least 80% of the flow occurring at
their positions with the lowest kerb inlet accommodating all the remaining flow
unnecessary concentration of water is prevented; as a rule a maximum spacing
of 200m may be used.
Kerb inlets should be designed so that ponding does not occur upstream of the inlet
for design conditions unless specific provision is made for it not to cause
unnecessary inconvenience to the public. Ponding in roads and streets is not
permissible for the design discharge.
Details of the kerb inlets being used along the roads in Kampala are shown in
Figure 11.6. These are not considered very effective in withdrawing stormwater from
the roadway, mainly because of the small openings of the kerb inlets, but also
because of the lack of crossfall or camber on the roads. The twin inlet system shown
in Figure 11.6 can intercept only about 0,8m3/s when the water surface level along
the kerb reaches the top edge of the kerb. The number and spacing of these kerb
inlets can be determined on the basis of this inlet capacity and the related
stormwater discharge to be accommodated. The small inlet openings assist in
preventing litter being drawn into the catchpit. However, it is recommended that
attention being given to improve the capacity of these kerb inlets during detail
design. Perspective views of different types of kerb inlets are shown in Figure 8.11
at the end of Chapter 8 in Volume 3.
The stormwater tends to flow over the full width of the road and is thus affecting the
efficiency of the kerb inlets. Proper improvement of road drainage is thus only
possible if a crossfall or camber of at least 2% is provided on the roads, which may
also necessitate upgrading of the road surface. Any future planning of road
upgradings should therefore take this issue into acount.
11.5.6 CULVERTS AND BRIDGES
(a) Culverts
Reference should be made to text books for guidance on the hydraulic design
of culverts. Only the more pertinent planning and design norms to be
considered are given in this sub-section.
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Figure 11.6 : Kampala Kerb Inlets
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Every natural watercourse reflects the prevailing pattern of equilibrium
between flow and erosion processes. This balance should be disturbed as
little as possible, which means that flow should be concentrated as little as
possible, the direction of flow disturbed as little as possible and flow velocities
altered as little as possible.
Flow velocities should comply with the norms for permissible flow velocities
given in Section 11.4.2, except that the minimum velocity should not be lower
than 0,6m/s to ensure that deposition of sediments inside the culvert is
prevented. This usually requires a minimum slope of between 0,2% and 1%
depending on the size of the culvert and the discharge considered.
The norms given in Section 11.4.5 for inlets and outlets also apply to culverts,
particularly energy dissipation at the outlets for highly erosive velocities.
Special measures may be required when approaching super-critical flow.
For inlet control, a ratio between upstream total head and height of culvert of
1,2 yields approximately the optimum hydraulic section. This can also be used
as a practical guide for preventing inlet erosion and determining the height of
the embankment over the culvert, also taking the norms for minimum
freeboard as given in Section 11.4.6 into account. However, the effect on the
upstream floodlines may be the overriding factor in the sizing of a culvert with
inlet control.
For maintenance purposes, the minimum acceptable size for a culvert up to
30m long is 600mm diameter, or 750mm wide x 450mm high, and for culverts
longer than 30m, a diameter of 900mm, or 900mm wide x 450mm high.
(b) Bridges
Reference can be made to the US Department of Transportations Hydraulics
of Bridge Waterways (1970) for guidance on the hydraulic design of bridges.
Only the more pertinent planning and design norms are given in this sub-
section.
The most important considerations in the hydraulic design of a bridge crossing
are the backwater effect caused by constriction of flow due to the bridge
structure and approach embankments, increased velocities through openings
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and turbulence causing scouring at the abutments, the piers and immediately
downstream of the bridge.
Urban development often limits permissible backwater or damming-up and
therefore controls the sizing of the bridge openings. Norms cannot be
prescribed for maximum permissible flow velocities through bridge openings
because this may not prevent severe local scour (eg along piers and
abutments). Potential scour should be investigated by experts and should be
based on site-specific conditions.
For super-critical flow conditions, the flow should preferably not be constricted
and adequate freeboard should be provided to ensure that the superstructure
will not come into contact with the fast-flowing water. Norms for freeboard are
covered in Section 11.4.6.
11.5.7 STORAGE / FLOOD ATTENUATION FACILITIES
Storage facilities, as far as stormwater management is concerned, are used to
attenuate or retard the flood peak. Where the downstream existing stormwater
system is clearly inadequate and its upgrading becomes uneconomical, storage
facilities can be used to attenuate and retard the flood flow to suit the discharge
capacity of the downstream stormwater system.
Significant attenuation/retardation on a major drainage system is achievable only by
the provision of a dam designed only for temporary storage. The storage capacity
determines the degree of attenuation/retardation. In the planning and design of
stormwater systems, the secondary effect of parking areas, sports fields, flat roofs
attenuating/retarding runoff as part of the minor drainage systems should also be
taken into account.
The storage capacity of a flood attenuation dam in urban areas is usually controlled
by the available space and/or area that can be expropriated. The behaviour of a
flood attenuation dam can best be illustrated with the aid of triangular hydrographs
as shown in Figure 11.7. The temporary storage available determines the outflow
peak discharge. The outlet of the storage dam is then sized to discharge a
maximum equal to this outlet peak with the water in the dam at the full supply level
or at the crest level of the emergency spillway.
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Figure 11.7 : Illustration of flood peak attenuation
11.5.8 EMBANKMENTS AND LEVEES
Embankments or levees, usually provided along the banks of river and channels,
protect lower-lying areas on floodplains against regular flooding. In many cases, it is
also used to make land subject to flooding available for development by means of
infill on the floodplains behind the embankment or levee.
The positioning of embankments or levees should not have an unacceptable effect
on the floodlines or flood levels either upstream or downstream of the area under
consideration and should also not result in significant variations in flow velocities
which could cause erosion or siltation. Careful attention should be given to the
drainage of stormwater behind an embankment or levee.
Embankments and levees can be protected against erosion by grass, gabions or
other suitable commercially-available materials. The design guidelines applicable to
the side slopes of channels and grassed waterways are, therefore, also applicable
to embankments and levees. The side slopes of grass embankments should be
flatter than 1 (vertical) to 3 (horizontally) for maintenance purposes.
11.5.9 PARKING AREAS
Parking areas or large paved areas have a significant effect on increasing the
volume of runoff, but do not necessarily result in an increase in the flood peak due to
retardation achieved through ponding. Parking area drainage should be sized in
such a way as to minimize inconvenience to the public under design storm
conditions, but also to create ponding and flood peak retardation for rainfall storms
with longer return periods than the design return period associated with the minor
system.
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11.5.10 BUILDINGS
Stormwater drainage for buildings can form part of the minor or the major systems
described in Section 11.2.6. It is therefore proposed that the norms listed below be
considered in the planning and design of stormwater drainage for buildings in
addition to the norms covered in Section 11.2.6.
Overland flow corridors (flooding easements) should be provided among
residential buildings to accommodate floodwaters that cannot be accommodated
by the street and minor system in the street reserve. Such drainage corridors
generally follow along topographic land depressions and in fact, become major
systems, which should be designed for at least the 10 year return period and
thus require the determination of floodlines.
The floor levels of buildings along and adjacent to overland flow corridors and
other major systems should have a freeboard of at least 0,3m measured from
the design flood level or floodline.
The floor levels of large building complexes along and adjacent to major
systems should have a freeboard of at least 0,5m measured from the design
flood level or floodline.
Attenuation/retardation, caused by large roof areas and parking areas, should be
taken into account.
11.5.11 REMOVAL OF URBAN LITTER
The strategy for the removal of litter from the stormwater systems should be two-
fold; firstly, to reduce the quantity of litter that finds its way into the drainage systems
(which falls beyond the scope of this study) and secondly, to remove the balance as
efficiently as possible. Armitage et al (1998) carried out a study on the most
appropriate and cost effective methods of removing litter from drainage systems. A
copy of their comprehensive report was handed to KCC and could be used for
reference purposes in future. It is important for designers to be able to estimate the
amount of litter that is washed off catchments, because this determines the volume
of material that the trap must hold and the required frequency of cleaning. The traps
that can be considered are described in the study referred to above. Fences,
screens, trash racks, and baffles or bollards may also be successfully used to
intercept litter provided the flow velocities are not too high. To facilitate
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maintenance, these traps should be designed to allow for easy cleaning. The
efficiency of any trap is dependent on regular and continuous maintenance (cleaning
and removal of litter to solid waste disposal areas). The designer should therefore
also outline appropriate operation and maintenance requirements associated with
the particular trap being used. The through-flow area of screens and trash racks
should allow for partial blockage of at least 50%, which implies that the channel
cross-sectional area will need to be doubled where screens are installed.
Alternatively, collapsible screens and trash racks should be used. The provision of
bollards some distance upstream of an inlet can also be considered to ensure that
litter passing through the spacing between the bollards will be small enough to wash
through the culvert openings.
Norms for the location or siting of traps can not be prescribed. As a general norm
easy access to the traps will be essential for cleaning purposes. Litter traps can be
located upstream of pipe inlets, downstream of pipe outlets and across channels.
The flow velocity usually varies along the length of a channel and it would thus be
advisable to place the trap in the area with the lowest flow velocity. Flow velocities
increase as the stormwater passes through bridges and culverts and making use of
bridge/culvert structures for placing a trap must always be avoided. The best option
will be to increase the cross-sectional area of the channel for location of a litter trap.
11.6 PUBLIC INCONVENIENCE AND EXPOSURE TO DAMAGE
The aspect of inconvenience to the public is primarily associated with minor
systems, which provide for efficient drainage of floodwater resulting from more
frequent minor floods.
Figure 11.8, reproduced from the New South Wales Governments Floodplain
Development Manual (1986), can serve as a guideline to evaluate the degree of
public exposure to danger and damage to light structures. The major systems must
be designed to prevent these flood hazards.
The following measures should always be considered as far as public inconvenience
and exposure to damage are concerned:
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flood warnings
flood information and education
preventing the public from approaching hazardous situations or areas
making the onset of flood hazards as gradual as possible.
Figure 11.8 : Public safety : Permissible flow velocities and depths
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11.7 BIBLIOGRAPHY
Armitage N, Rooseboom A, Nel C and Townshend P (1998). The removal of
urban litter from stormwater conduits and streams. Prepared for South African Water
Research Commission (WRC). Report No TT 95/98. PO Box 824, Pretoria, South
Africa.
Davis CV and Sorensen KE (1969). Handbook of Applied Hydraulics. Third Edition,
McGraw-Hill Book Company.
ICOLD (1987). Dam Safety Guidelines. Bulletin 59, International Commission on
Large Dams, 151 BD Haussmann, 75008 Paris.
SANCOLD (1991). Safety Evaluation of Dams Report No. 4 : Guidelines on Safety
in Relation to Floods. South African National Committee on Large Dams.
PO Box 3404, Pretoria, South Africa.
South African National Road Agency (1997). Road Drainage Manual. Fourth
Print. Chief Directorate : Roads, South African Roads Board, Pretoria, South Africa.
RSA Department of Housing (2000). Guidelines for Human Settlement Planning
and Design. Published by CSIR Building and Construction Technology, PO Box 395,
Pretoria, 0001, South Africa.
Stephenson D (1981). Stormwater Hydrology and Drainage. Developments in
Water Science, Elsevier Scientific Publishing Company.
US Bureau of Reclamation (1963). Hydraulic Design of Stilling Basins and Energy
dissipators. A Water Resources Technical Publication, Engineering Monograph No.
25, prepared by A J Peterka.
US Department of Transportation (1970). Hydraulics of Bridge Waterways.
Hydraulic Design Series No. 1, Hydraulic Branch, Bridge Division, Federal Highway
Administration, Bureau of Public Roads.
Ven te Chow (1959). Open-Channel Hydraulics. Published by McGraw-Hill Book
Company.
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CHAPTER 12
DEVELOPMENT PLAN AND LONG-TERM PROGRAMME
12.1 INTRODUCTION
The whole purpose of drainage master planning is to facilitate the accomplishment
of sustainable future development through pre-emptive management of flooding
events. As described in Chapter 2 (Volume 2), options for pre-emptive management
of flooding events are conveniently classed as structural measures and non-
structural measures to provide protection or to reduce the risk of flooding. Structural
flood control measures consist of physical works or upgradings, such as
channelization of watercourses for improving the hydraulic characteristics of the
drainage systems and bridge and culvert crossings over channels, flood attenuation
dams, and levees and embankments for keeping floodwaters out of flood-prone
areas. Non-structural measures include regulation of floodplain use, building
ordinances, regulation of land-use in the catchment area, flood forecastings and
flood warnings, etc.
As described in Chapter 1 (Volume 2), each main drainage system is divided into a
major system and numerous minor systems. The major system includes all the
primary and secondary channels shown in Figures 3 through 11 (Volume 6) and
should be capable of accommodating stormwater discharges of higher intensity. The
minor systems correspond to stormwater flow from properties and along roads to
discharge points into the primary and secondary channels, and usually
accommodate stormwater discharges of lower intensity, mainly to avoid frequent
inconvenience.
This chapter focuses on the structural measures (upgradings) required to
accomplish sustainable drainage development, as well as a long-term
implementation programme. Master planning of structural measures is
predominantly concerned with the major systems, but attention is also given to
typical examples of structural measures for the minor systems. Non-structural
measures, however, involve both the major and minor systems as described in
Chapter 14.
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12.2 MAJOR SYSTEMS
12.2.1 GENERAL
The recommended development plan of structural measures required to accomplish
sustainable drainage development and the long-term implementation programme as
described hereafter have been based on evaluating and integrating the following
aspects, which were dealt with in the previous chapters.
(i) Existing drainage
The inventory of the existing drainage channels and road crossings
(Section 3 Volume 5) provides basic information on the extent and
locations of required upgradings. The existing discharge capacities and
related return periods, also included in the inventory, highlight the relative
urgency of improving the flow conditions at the various culverts and different
channel reaches.
The inventory of identified black spots associated with the major systems
(Section 5.2 Volume 5) provides information on the immediate needs for
upgrading.
The floodlines described in Chapter 7 (Volume 3) provide details on the
extent of flooding along the various channels.
(ii) Upgradings planned and under construction
The only upgradings planned and currently under construction are those
along the primary and selected secondary channels of the Nakivubo
Drainage System.
It is also known that the drainage problems in the Bwaise II area (Kawempe)
are being addressed in a separate study by others, but the details or findings
of that study were not available for incorporation into the compilation of a
Kampala Drainage Master Plan.
(iii) Expected urban development
Projections of urban development are described in Chapter 4 (Volume 2).
The expected future spread of population densities and development trends
play a major role in establishing the relative importance of upgradings along
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the primary and secondary channels for the compilation of a long-term
implementation programme.
(iv) Environmental considerations
The issues identified in the environmental assessment described in
Chapter 3 (Volume 2) provide details of the preservation status of the various
channels (particularly the wetlands) that must be taken into account while
planning channel upgradings.
(v) Economic considerations
The economic analyses described in Chapter 9 (Volume 3) provide
information regarding prioritization of the various systems (see also
Chapter 10 in Volume 3).
(vi) Required structural measures (upgradings)
The extent of upgradings required for the different channels serves as a
basis to determine construction periods for the various upgradings. These
details are described in Chapter 8 (Volume 3).
(vii) Traffic impacts
The volume of traffic flow, particularly on the main roads, as well as the
availability of alternative routes should also be taken into account when
compiling a development plan and programme.
The main objectives of the study, as described in Chapter 1 (Volume 2), specifically
require that the long-term programme for development covers the period up to 2040.
This is a long period and means that any implementation programming of structural
measures would be subject to regular revision and updating. In compiling a long-
term implementation programme it is beneficial to distinguish between three distinct
13-year periods based on the level of priority as follows:
2002 2014 : high priority period
2015 2027 : medium priority period
2028 2040 : low priority period
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Programming for the first 13-year period can be done with much more certainty than
for the second and third 13-year periods due to circumstances and conditions that
will definitely change with time. The first 13-year period includes the short-term
action plan for the first five years as described in Chapter 14. In this regard it may be
in the interest of KCC to revise and update the long-term programme every five
years on the basis of this study to define a short-term action plan at the beginning of
every five-year period.
12.2.2 DRAINAGE SYSTEM 1 NAKIVUBO
The catchment area of Drainage System 1 (Nakivubo) with its primary and
secondary channels is shown in Figure 3 (Volume 6). The recommended
development and long-term implementation programme are shown in Table 12.1 at
the end of this chapter and are described below.
(a) Primary Channel 1 (Nakivubo)
Rehabilitation of the Nakivubo Channel is nearing completion and no further
upgradings are foreseen.
(b) Secondary Channel 1 (Kintinale)
The lower portion of Kintinale Channel forms part of the Nakivubo wetland
(refer to Figure 3 in Volume 6). General upgradings will thus only be feasible
in the upper reaches. Kintinale Channel downstream of Port Bell Road with its
tributary (Silver Spring) has been identified as a black spot (No. 1A in Section
5.2.1 of Volume 5) requiring widening and lining with high priority. Similarly,
the Port Bell Road culvert and the culvert on Silver Spring have been given a
high priority to ensure that traffic flow is not disrupted along these routes.
(c) Secondary Channel 2 (Kibira)
Kibira Channel drains the area from the Coffee Marketing Board (CMB) to the
Nakivubo swamp. This channel has been identified as a black spot (No. 1B in
Section 5.2.1 of Volume 5). The channel and Kibira Road culvert have thus
been assigned a high priority.
(d) Secondary Channel 3 (Lugogo)
Regular flooding is being experienced along Lugogo Channel and the lower
reach downstream of Naguru Road has been identified as a black spot
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(No. 1C in Section 5.2.1 of Volume 5). The flooding problems can only be
minimized by means of channelization and enlargement of all the road and rail
culverts. This channel reach, together with three railway crossings and Old
Bell Road crossing, has been assigned a high priority.
The channel reach upstream of Naguru Road crossing will also have to be
channelized (including the Nagura Road crossing) and has also been
assigned a (late) high priority.
(e) Secondary Channel 4 (Kitante)
The Kitante Channel downstream of the golf course has been identified as a
black spot (No. 1D in Section 5.2.1 of Volume 5). The options to minimize
flooding are channelization or a flood attenuation dam on the golf course, or a
combination of the two options. Any detail design should take these options
into account to determine the least cost solution.
Upgrading of the channel, together with the two railway crossings, has been
given a high priority. The Jinja Road culvert seems capable of discharging a 7-
year flood and has thus been given a medium priority.
Flooding of the upper channel reach along the golf course is not critical and
there is presently no need to upgrade this channel reach. A low priority has
thus been assigned to this channel reach.
(f) Secondary Channel 5
A black spot has been identified in the lower reach from Kibuli Road to the
Nakivubo Channel (No. 1E in Section 5.2.1 of Volume 5). Upgrading of this
short channel reach by means of channelization and enlargement of the two
culverts in the lower reach at Press House and Kibuli Roads (with inadequate
discharge capacities) has been given a high priority.
Problems are also being experienced at Nsambya Central (near Jack and Jill
Nursery School), which also requires upgrading with a high priority.
The remainder of the channel has been given a low priority.
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(g) Secondary Channel 6
This channel can only be improved by widening and enlargement of the rail
and road culverts. A high priority has been assigned to the channel reach from
Nakivubo to the railway culvert, including the railway culvert and Nsambya
Road crossing, which has been identified as a black spot (No. 1F in
Section 5.2.1 of Volume 5).
The channel reach upstream of the railway line is also critical and has been
assigned a (late) high priority.
(h) Secondary Channel 7 (Katwe)
The lower reach of this channel, from the confluence with Nakivubo up to and
including Katwe Road, has partly been upgraded. The Katwe Road crossing
and downstream channel together with a small tributary also crossing Katwe
Road have been identified as a black spot (No. 1G in Section 5.2.1 of
Volume 5) and assigned a high priority.
The channel reach immediately upstream of Katwe Road is capable of
discharging the 10-year flood peak, but a short reach upstream and
downstream of Mutebi Road can only accommodate a 2-year flood peak,
which has also been identified as a black spot (No. 1H in Section 5.2.1 of
Volume 5). This short reach has also been assigned a high priority, excluding
the Mutebi Road culvert.
(i) Secondary Channel 8
This channel passes through a high-density area and has been identified as a
black spot (No. 1J in Section 5.2.1 of Volume 5). Channelization is the only
feasible solution. Upgradings of the channel and road crossings have been
assigned a high priority.
(j) Secondary Channel 9 (Jugula)
The culvert at Kisenyi Lane is currently being upgraded. The entire channel,
together with four road crossings needs urgent attention and is considered a
high priority. This area has also been identified as a black spot (1K in
Section 5.2.1 of Volume 5).
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(k) Secondary Channel 10 (Kakajjo)
The Makere Road crossing is currently being upgraded. The entire channel
has been identified as a black spot (No. 1L in Section 5.2.1 of Volume 5) and
is assigned a high priority.
12.2.3 DRAINAGE SYSTEM 2 - LUBIGI
The catchment area of Drainage System 2 (Lubigi) with its primary and secondary
channels is shown in Figure 4 (Volume 6). The recommended development and
long-term implementation programme are shown in Table 12.2 at the end of this
chapter and are described below.
(a) Primary Channel 2 (Lubigi)
The route of the planned Northern Bypass (see Figure 4 in Volume 6)
traverses from Mityana Road near the confluence of Lubigi and Nalukolongo
Channels along the left bank of Lubigi floodplain until it crosses the Lubigi
Channel and floodplain near the confluence with Secondary Channel 8 to the
right bank of Nsooba Wetland. It then crosses Secondary Channel 10 twice in
the upper catchment of Lubigi. The culverts for the road crossings of the
affected secondary channels need to be designed for at least the 10-year
flood peaks, such that no damming-up of floodwaters is created upstream in
these secondary channels.
Lubigi Wetland has a low to medium preservation status and it is
recommended in Chapter 3 (Volume 2) that the lower reach downstream of
Hoima Road crossing near the confluence of Lubigi with Secondary Channel 5
(see Figure 4 in Volume 6) be left in its natural state. The culverts at Mityana,
Sentema and Hoima Road crossings along this lower reach of Lubigi can only
discharge flood peaks with return periods of less than 2 years, and have been
assigned a medium priority for upgrading.
The entire length of Lubigi Channel upstream of Hoima Road crossing near
the confluence with Secondary Channel 5 has been identified as a black spot
(Nos. 2A and 2Bin Section 5.2.2 of Volume 5). The options available for
minimizing flooding along this reach of the Lubigi are channelization or a
combination of