MAIN REPORT VOL 4-Drainage Development

KDMP GENERAL INDEX TO MAIN REPORTQ:\H0782-kampala\Final Report July 2002\COVER PAGES final.doc

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)


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


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

KDMP VOLUME 4 MAIN REPORT PART III - CHAPTER 11 Page 11.1Q:\H0782-kampala\Final Report July 2002\CHAPTER 11 VOL 4 final.doc

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.


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


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.


(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


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


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.


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.


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.


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


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.


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


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


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)


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


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.


(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.


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


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


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.


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)


(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.


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.


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.


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.


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.


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:


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.


Figure 11.6 : Kampala Kerb Inlets


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


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.


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.


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


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:


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


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.


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.


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


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


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


(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.


(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).


(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

MAIN REPORT VOL 4-Drainage Development

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