PROPOSED STORMWATER MANAGEMENT, FLOODPLAIN

ANALYSIS
ON PLOTS .51 ALONG UPPER KOLOLO MULAGO HILL ROAD

HYDROLOGICAL STUDY AND HYDRAULIC DESIGN REPORT

Client:

UGANDA CANCER INSTITUTE

Prepared by:

PLOT 76 NAALYA

TEL.0777716398,0704339100, EMAIL.SANTOBER

AUGUST 2024
1 List of figures
Figure 1: Project Site Location; 0.34038589420677834, 32.57907742451419 Bukoto, Sserwadda Cl ... 2

Figure 2: Existing Road with 900mm culvert as the road side drain ........................................................ 5

Figure 3: Downstream of tank house and eco park site, with no outlet point ........................................... 6

Figure 4: Flow chart for the Assignment .................................................................................................. 7

Figure 5: IDF Curves from the analysis of the results ............................................................................ 10

Figure 6: Delineated catchment ............................................................................................................. 15

Figure 7: Land use map for the catchment area .................................................................................... 16

Figure 8: Proposed Concrete section ................................................................................................... 23

Figure 9: Design Computation for Proposed Primary drain ................................................................... 24

Figure 10: Design cross-section for Proposed Primary drain ................................................................. 24

Figure 11: Design Computation for proposed site secondary drain ....................................................... 25

Figure 12: Design cross-section for Proposed site secondary drain ...................................................... 25

Figure 13: Design Computation for proposed site closed drain ............................................................. 26

Figure 14: Design cross-section for Proposed site closed drain ............................................................ 26

i
2 List of tables
Table 1: Typical values of rational coefficients for urban areas [Extract from MOWT drainage design
manual] ......................................................................................................................................... 11

Table 2: Velocity Limits .......................................................................................................................... 13

Table 3: Peak run of computation from the catchment using TRRL....................................................... 18

Table 4: Peak run of computation from the catchment using Rational method ...................................... 18

Table 5: Selected Peak flows from the catchment ................................................................................. 18

Table 6: Run off computation using Rational method (on-site) .............................................................. 19

Table 7 : Stormwater distribution between the parking and the roof ...................................................... 19

Table 3: Peak run of computation from the catchment using TRRL (Catchment A)............................... 20

Table 4: Peak run of computation from the catchment using Rational method (Catchment A) .............. 20

Table 5: Selected Peak flows from the catchment ................................................................................. 20

Table 3: Peak run of computation from the catchment using TRRL (Catchment B)............................... 21

Table 4: Peak run of computation from the catchment using Rational method (Catchment B) .............. 21

Table 5: Selected Peak flows from the catchment ................................................................................. 21

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Table of Contents

1 List of figures ............................................................................................................. i

2 List of tables ............................................................................................................. ii

3 Introduction .............................................................................................................. 1

3.1 Objective ..................................................................................................................... 1

3.1.1 General Objective.............................................................................................................. 1

3.1.2 Specific Objectives of the assignment............................................................................... 1

3.2 Site location................................................................................................................. 2

3.3 Project brief ................................................................................................................. 2

3.4 Literature review ......................................................................................................... 3

3.4.1 Design manuals and guidelines ......................................................................................... 3

3.5 Geotechnical data survey ............................................................................................. 3

3.5.1 General .............................................................................................................................. 3

3.5.2 Water table ....................................................................................................................... 4

3.5.3 Stability of Channel Sides .................................................................................................. 4

3.5.4 Channel Inverts ................................................................................................................. 4

3.6 Site reconnaissance...................................................................................................... 5

4 Methodology ............................................................................................................ 7

4.1 Hydrological analysis.................................................................................................... 8

4.1.1 Collected data ................................................................................................................... 8

4.2 Catchment delineation and parameter estimation ........................................................ 8

4.2.1 Pre-processing of terrain................................................................................................... 8

4.3 Catchment delineation ................................................................................................. 8

4.4 Catchment parameter estimation ................................................................................. 8

4.5 Rainfall frequency analysis ........................................................................................... 9

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 4.5.1 IDF Curves.......................................................................................................................... 9

4.5.2 Lognormal distribution .................................................................................................... 10

4.6 Hydrological analysis computation ............................................................................. 10

4.6.1 The Rational method ....................................................................................................... 11

4.6.2 The TRRL East African Flood Model Methodology .......................................................... 12

4.7 Hydraulic analysis ...................................................................................................... 13

5 Results .................................................................................................................... 15

5.1 Catchment parameters .............................................................................................. 15

5.1.1 Channel A catchment extent ........................................................................................... 15

5.1.2 Soil characteristics........................................................................................................... 15

5.1.3 Land use .......................................................................................................................... 16

5.1.4 Soil Type .......................................................................................................................... 17

5.2 Runoff computation ................................................................................................... 18

5.2.1 Run off computation for Tank House. ............................................................................. 18

5.2.2 Run off computation for Eco-Park................................................................................... 19

5.2.3 Run off computation for Pet House. ............................................................................... 20

5.3 Hydraulic Analysis ...................................................................................................... 22

5.3.1 Concrete channels sections............................................................................................. 22

5.3.2 Proposed Primary Channel.............................................................................................. 24

5.3.3 Proposed On site Secondary drain .................................................................................. 25

5.3.4 Proposed On site Closed drain ........................................................................................ 26

5.3.5 Soak away pit .................................................................................................................. 27

iv
3 Introduction
The report details flood analysis and detailed storm water management plan and for The Proposed
Drainage System on Plots .51 Along Upper Kololo Mulago Hill Road-Kampala District. The site
under development is situated in middle of developed plots leaving the site with no possible
conveyance of stormwater generated on site

This prompted the need to carryout hydrological analysis to determine the impact of the catchment
runoff on the drainage of the site. A hydraulic study was then carried out to propose the drainage
network that can safely convey all the storm water safely away from the site.

3.1 Objective

3.1.1 General Objective

The main objective of the assignment is to assess the potential risk of flooding on the site and
surrounding area and thus formulate a site drainage master plan to mitigate this risk. The scope of work
will involve checking the current natural drainage system’s adequacy to convey the simulated runoff
and propose improvements to be made on the network so as to safely conduct future flows hence
averting the possible risk of flooding on the site.

3.1.2 Specific Objectives of the assignment

The specific objectives of the assignment include;

 To deduce the contributing area of the site to the drainage structures and compute the peak
flow discharge generated for a ten-year return period
 Carry out flood simulation for a 5,10, 25 and 50 return period and deduce the extent of the
natural flow extents
 To minimize the impact of the plot drainage system’s collection and conveyance of storm water
on the surrounding areas.
 Propose drainage structures and integrating them within the development plan

1
3.2 Site location

The plot is situated within Bukoto Sserwadda close, located in the Kampala district. It is surrounded by
neighboring plots that have already been developed, as depicted in the accompanying figure.

Figure 1: Project Site Location; 0.34038589420677834, 32.57907742451419 Bukoto, Sserwadda Cl

3.3 Project brief

The objective of the project illustrated in Figure 1 is to assess the effectiveness of the existing natural
earth drains in handling anticipated flood volumes and to suggest an improved drainage design to
mitigate the flooding problems in the vicinity of the area. Additionally, the project entails modifying the
routes of certain existing drains and reinforcing them with better-suited materials along the sides and
base.

2
3.4 Literature review

3.4.1 Design manuals and guidelines

Currently, Uganda does not have specific engineering manuals and specifications that govern the
design and construction of urban stormwater management systems. However, Volume 2 of the Revised
Road Manual on drainage design (MoWT, 2010) provides a general overview of stormwater
management and focuses on designing drainage systems for roads, including information on rainfall
and methods for determining design flow. It does not cover aspects of open channel design such as
slope alignment, channel grades, lining materials, and trickle flow.

To address culvert crossings and stormwater quantification for the channels, we followed the guidelines
outlined in the Drainage Design Manual (MoWT, 2010). For other specifications related to the design of
stormwater conveyance systems that are not covered by the MoWT (2010) guidelines, we relied on the
manual published by the American Society of Civil Engineers (ASCE) on the design and construction of
urban stormwater management systems (ASCE, 1992).

Additionally, we incorporated the design standards set forth in the Kampala Drainage Master Plan 2016
(T2 - design standards for stormwater facilities), specifically in Section 4, Design Standard for Drainage
System, from subsections 4.2.1 to 4.2.6. All of this information has been taken into account in the
design of the system.

3.5 Geotechnical data survey

3.5.1 General

A prerequisite for detailed design is an assessment of the geotechnical conditions, particularly the
founding conditions in order to design a stable foundation for drainage concrete or masonry lined
channels. The geotechnical conditions at selected locations were investigated by means of hand dug
trial holes and the consistency of the in-situ material was determined by means of Dynamic Cone
Penetrometer (DCP) tests. These were used to determine the shear stress for the bank angle of the
channel side slope and set the box culvert conditions

3
3.5.2 Water table

In general, the water table in the floodplain is at the same level as the water surface levels in the
existing low flow drainage channels along the floodplain. There appears to be no sealing effect on the
sides of the exiting flow channels and the water can be assumed to be free flowing between the
channels at maximum base flows. The average ground water depth was 0.8m. However, this reduces
to 0.2m towards the exit of the premises.

Dewatering will be essential during the upgrading operations (eg excavations) and may be carried out
by means of excavating trenches for free drainage and pumping from sumps, or by the installation of
well points. The capacities of the pumps will be determined under the prevailing site conditions.

3.5.3 Stability of Channel Sides

The sides of the channels can be excavated to a slope of 1 (vertically) to 2 (horizontally) when
protected with grass or lined with stone pitching with pore pressure allowances. If the sides are to be
vertical or nearly vertical due to space constraints, gabions will be used especially at the discharge
point and collection point. Masonry walls can be used. In the mid-section of the drainage. Stability
analyses using the provided geotechnical data showed that masonry walls need to be constructed with
a batter of 1,0 (vertically) to 0,4 (horizontally) on the water face.

Whatever system is employed; it is imperative that adequate weepholes are provided to prevent a
build-up of water pressure behind the side wall. In the case of Reno matresses or “Armorflex” precast
blocks, these must be laid on geotextile or geofabric, such as “Kaytech A4”, to avoid the leaching of fine
material out of the surrounding soil into the channel. Two horizontal rows of weepholes should be
provided. s

3.5.4 Channel Inverts

Wherever possible, the invert of the channel should be lined in order to avoid erosion and scour, except
in cases of very low flow velocities where grass can be used for protection. The invert lining may
comprise either of “Armorflex” precast blocks, Reno matresses or a concrete slab. A geofabric or
geotextile layer will also be required for lining by means of “Armorflex” precast blocks or Reno
mattresses, to avoid the leaching of fine material out of the subsoil. This will be a choice that is specific
to the employer and choice of material does not have any design consequences since all the options
have been analyzed and have no effect on the design parameters.

4
3.6 Site reconnaissance

The site is situated in a very hilly area and is divided into three distinct sections: the tank house, the pet
house, and the eco park. For the pet house, stormwater can be channeled into the existing 900mm
culvert roadside drain along Mulago Hill Road. However, for the tank house and eco park, there is no
existing outlet for stormwater. Consequently, the client decided to construct a stormwater soak away pit
to temporarily hold the water, allowing it to gradually percolate into the soil.

Figure 2: Existing Road with 900mm culvert as the road side drain

5
Figure 3: Downstream of tank house and eco park site, with no outlet point

6
4 Methodology
The study proceeded in a systematic manner, commencing with a site reconnaissance to evaluate the
existing drainage conditions, followed by an extensive review of available literature such as topographic
and geotechnical survey reports.

Additional data were collected to augment the analysis. The subsequent engineering design phase
encompassed hydrological analysis, hydraulic design, and structural engineering considerations for the
proposed conveyance structures. Preliminary cost estimates were then derived, indicating the potential
expenses associated with the project implementation. The step-by-step progression of the study is
visually depicted in the provided flow chart.

Figure 4: Flow chart for the Assignment

7
4.1 Hydrological analysis

4.1.1 Collected data

In the execution of hydrological analysis, the following data was used;

 Digital elevation model: A 5m resolution DEM was obtained from KCCA

 Meteorological data IDF curves were used to obtain the rainfall data for the various return
periods. These were extracted from the Kampala Drainage Master Plan 2016.
 Soil data: The soil map was obtained from Land and Water Resource, FAO soil database.
 Land use map: The land use map was obtained from KCCA.

4.2 Catchment delineation and parameter estimation

The catchment was defined using ArcGIS 10.8's ARC-Hydro tool. This was done in order to obtain
catchment characteristics such as slope, longest flow path, area, and catchment extent contributing to
flow at the outlet. The following is how the delineation and parameter generation were carried out:

4.2.1 Pre-processing of terrain

A hydrologically corrected terrain model that is, one which had been filled was created using pre-
processing tool. The DEM was analyzed by applying the 8-point pour model, where water flows across
the landscape from cell to cell based on the direction of the greatest elevation gradient. Steps in the
analysis included; filling depressions or pits, calculating flow direction and flow accumulation,
delineating streams with an accumulation threshold, defining streams, segmenting streams, delineating
watersheds, processing watershed polygons, processing streams and aggregating watersheds.

4.3 Catchment delineation

The catchment outlet was defined using project setup tool and catchment whose parameters were to be
determined generated. Some of the generated sub-catchments were merged together using basin
merge tool in basin processing.

4.4 Catchment parameter estimation

The basin parameters such as river length, river slope, centroid, centroidal elevation, centroidal longest
flow path, longest flow path lengths were determined using characteristics tool. The tool also generated
the slopes and lengths of the respective longest flow paths.
8
The utility tool was use to generate basin curve numbers. A land use-soil type polygon was developed
for the catchment using the union tool and a curve number lookup table made in ArcGIS.

A curve number grid raster file was then generated from the above data. The zonal statistics tool was
used to determine the weighted curve numbers for the catchment.

Rainfall data from 2003-2014 was used to develop Intensity Duration Frequency Curves for return
periods of 5, 10, 25 and 50 years respectively. The Watkins and Fiddes procedure was adopted as
shown below;

4.5 Rainfall frequency analysis

4.5.1 IDF Curves

The daily maximum rainfall depth in each month was considered from which the maximum in every
year was selected together with the duration for the period between 2003 and 2014 and these values
were ranked in a decreasing order. The corresponding return periods were estimated for each data set
using Weibull’s plotting position formula, T= (N+1)/r.

A plot of maximum daily rainfall against return periods was made and a line of best fit obtained and its
corresponding equation obtained also. Maximum daily rainfall depths at desired return periods that is 5,
10, 25 and 50 years were obtained using the equation from the graph.

A sequence of durations(minutes) was selected that is 10, 20, 30, 40, 60, 70, 80, 90, 100, 120, 140,
160. for each data set and used to calculate the rainfall intensity from;

𝑎
𝐼=
(𝑡 + 𝑏)

where a, b and n are constants, t is the duration (min) and I is the rainfall intensity in mm/hr take
constant b= 1/3 for East Africa and teff= 1.5 hrs

14.4
ln ( )
𝑡
𝑛=
𝑏 + 24
ln ( )
𝑏+𝑡

from calculation, n= 0.8747

a was calculated for each return period using;

aT= iT24x (b+24)n

9
Intensities were then plotted against duration for each return period to give the IDF curves.

190
180
170
160
150
140
130
Intensity (mm/h)

120
110
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Duration (minutes)

5Y 10Y 25Y 50Y

Poly. (5Y) Poly. (10Y) Poly. (25Y) Poly. (50Y)

Figure 5: IDF Curves from the analysis of the results

4.5.2 Lognormal distribution

Design rainfall intensity represents the average rainfall intensity of a duration equal to the time of
concentration for the catchment. The procedure for estimating design rainfall intensity are detailed in
Section 4.3 of the MOWT Drainage Design Manual (2010). the lognormal distribution was selected to
estimate the 24-hr rainfall corresponding to different return periods.

T(years) 2 5 10 25 50 100 200 500

Lognormal 71.0 87.8 98.1 110.5 119.2 127.7 136.0 146.8

4.6 Hydrological analysis computation

The runoff generated from the catchment in the event of a rainfall storm was computed using the curve
number and rational method.

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4.6.1 The Rational method

The Rational Method developed by Mulvaney in 19th century is used to calculate the peak surface
runoff rate for design of storm water management structures, like storm drains, storm sewers, and
storm water detention facilities (Butlert, and Davies, 2000). The rational method equation is given by:

Q = CIA;

Where Q: Peak surface runoff rate

C: coefficient of runoff

I: Rainfall intensity

A: Area of the watershed best for areas less than 100 acres,

The following assumption are considered in the computation of discharge using the rational method;

1. Peak flow occurs when the entire watershed is contributing to the flow which implies that all the
water generated will end up in the channels and losses as infiltration evaporation have been
ignored
2. Rainfall intensity is the same over the entire drainage area.
3. Rainfall intensity will be assumed spread over uniformly across the plot
4. Frequency of the computed peak flow is the same as that of the rainfall intensity, therefore
rainfall intensity we are going to consider is assumed to produce the 5-year peak flow.
5. Coefficient of runoff is the same for all storms of all recurrence probabilities. Which implies that
since we what to minimize risk we shall assume the coefficient flow as
6. The calculated runoff is directly proportional to the rainfall intensity.
7. The frequency of occurrence for the peak discharge is the same as the frequency of the rainfall
producing this event

The runoff coefficient used in the computation were selected from the table below.

Table 1: Typical values of rational coefficients for urban areas [Extract from MOWT drainage design manual]

11
4.6.2 The TRRL East African Flood Model Methodology

The steps involved in applying the TRRL East African Flood Model are detailed in Section 3 of the
Drainage Design Manual (MoWT 2010) and Watkins and Fiddes (1984) and updated as follows:

a) Selection of design return period, T.

b) Estimation of catchment area (A) and land slope (SL) from the DEM. Culvert and channel
slopes were determined from the topographic maps of the area.
c) Establishment of the catchment type from site inspection which is used to estimate the surface
cover flow time (Ts).
d) Determination of soil type by both geotechnical investigations and available soil maps, the soil
permeability class (I) and slope class (S). From the soil classification, a soil class 3 was
adopted. From the topography map, catchment slope was determined and the slope
classification was selected. Using the soil class and the slope class, the basic runoff coefficient
was estimated using equation (1);
Cs  53  12I  8S
(1)
e) Determination of land use factor (CL) and catchment wetness factor (CW)
f) Estimation of the runoff coefficient from equation (2);
CA  CS CW CL
(2)
g) Estimation of the base time from equation (3)
CA 1 2
TB   Ts
S2 (3)
h) Where; TB = base time (hr)

12
 A = catchment area (km2)
S = Slope class
C = 30 for humid catchments
i) Computation of the design storm rainfall for each recurrence interval, to be allowed for during
base time. The determination
j) Analysis of available rainfall records to determine the rainfall intensity for various recurrence
intervals.
k) Determination of area reduction factor using equation 4
1 3
ARF  1  0 . 04 T A1 2 (4)
l) where T is selected as 8 hours since daily rainfall is used
m) Determination of rainfall ratio from equation (5)
n
T  b  24
RRt  b  
24  b  Tb 
(5)
n) Where; Tb is the base time
b = 0.3
n = 0.97
o) Determination of the design rainfall intensity using equation (6)
PT  P2  rT:2  ARF RRt
(6)
p) Calculation of the average flow during base time from equation (7)
C A PT A
Q 
360TB (7)

q) Estimation of the design peak using a peak factor of 1.7 which is applicable to study
catchments (Watkins and Fiddes, 1984)

4.7 Hydraulic analysis

For the open channels the flow master Connection Edition Bently was utilized in the analysis with
velocity used as the critical parameter whose limits are extracted from the Ministry of Works and
Transport design Manuals and the Kampala Drainage Master Plan 2016-Under drainage guidelines
extracted in the table 4 below

Table 2: Velocity Limits

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Velocity: Minimum design velocity Allowable velocities

Maximum design velocity

Stone Pitched channels 1.5-3.0 m/s

concrete 3.0 – 4.5 m/s

14
5 Results

5.1 Catchment parameters

5.1.1 Channel A catchment extent

From ArcGIS, the catchment below was obtained from the delineation process of the watershed. From
the delineation, it was observed that the catchment areas were 0.01km2 on average for all the different
site as shown Figure 6 below.

Figure 6: Delineated catchment

5.1.2 Soil characteristics

In order to select curve numbers, it was necessary to identify the percentage composition of the various
hydrological soil groups in the catchment. According to the Harmonized World Soil Database, the soils
in the catchment were identified by the Soil Mapping Unit Symbol Fo44-2b.

15
These soils are made up of 50% hydrological soil group A, 20% hydrological soil group B, 20%
hydrological soil group C and 10% hydrological soil group D.

5.1.3 Land use

The site under development is situated in an urban area. The catchment is comprised of mainly built-up
land (residential development) as shown in the land use.

Figure 7: Land use map for the catchment area

The catchment is under middle income and small scale & informal businesses use. It also has
significant vacant land which is the site itself.

16
5.1.4 Soil Type

17
5.2 Runoff computation

5.2.1 Run off computation for Tank House.

Table 3: Peak run of computation from the catchment using TRRL
TRRL East African Flood Model
Return period (T) - years 5 10 25 50
Area drained (km2) 0.01 0.01 0.01 0.01
Catchmt slope (%) 21.6% 21.6% 21.6% 21.6%
Slope class (S) 6 6 6 6
Surface cover flow time (Ts) - hrs 1.15 1.15 1.15 1.15
Soil class (I) 4 4 4 4
Land-use factor (CL) 1.00 1.00 1.00 1.00
Catchmt. wetness factor (Cw) 1.0 1.0 1.0 1.0
Basic runoff coeff. (Cs) 53% 53% 53% 53%
Runoff coeff (Ca) 53% 53% 53% 53%
Base Time (TB) - hrs 1.2 1.2 1.2 1.2
T-yr- 24hr storm depth (mm) - area averaged 101.0 112.9 127.0 137.1
Av. flow during base time (m3/s) 0.1 0.2 0.2 0.2
Peak flow factor 2.3 2.3 2.3 2.3
T-yr peak flow (m3/s) 0.32 0.35 0.40 0.43

Table 4: Peak run of computation from the catchment using Rational method
Return Period Area Time of Runoff Rainfall Rainfall Peak
drained Concentration coefficient(C) - total - 24 intensity Flow, Q
(km2) (Bransby Williams) Urban hour - (mm/hr) [m3/s]
(hrs) incl. CC
factor
5 0.0115 0.53 0.75 71.00 38.75 0.09
10 0.0115 0.53 0.75 87.80 47.92 0.11
25 0.0115 0.53 0.75 127.70 69.70 0.17

Table 5: Selected Peak flows from the catchment

Computed Qt

T Rational Qt TRRL Qt Selected Qt

5 0.09 0.32 0.32

10 0.11 0.35 0.35
25 0.17 0.40 0.40

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5.2.2 Run off computation for Eco-Park
Table 6: Run off computation using Rational method (on-site)
TRRL East African Flood Model
Return period (T) - years 5 10 25 50
Area drained (km2) 0.00 0.00 0.00 0.00
Catchmt slope (%) 25.9% 25.9% 25.9% 25.9%
Slope class (S) 6 6 6 6
Surface cover flow time (Ts) - hrs 1.15 1.15 1.15 1.15
Soil class (I) 4 4 4 4
Land-use factor (CL) 1.00 1.00 1.00 1.00
Catchmt. wetness factor (Cw) 1.0 1.0 1.0 1.0
Basic runoff coeff. (Cs) 53% 53% 53% 53%
Runoff coeff (Ca) 53% 53% 53% 53%
Base Time (TB) - hrs 1.2 1.2 1.2 1.2
T-yr- 24hr storm depth (mm) - area averaged 101.0 112.9 127.0 137.1
Av. flow during base time (m3/s) 0.1 0.1 0.1 0.1
Peak flow factor 2.3 2.3 2.3 2.3
T-yr peak flow (m3/s) 0.14 0.16 0.18 0.19

Table 7 : Stormwater distribution between the parking and the roof

Return Period Area Time of Runoff Rainfall Rainfall Peak Flow,
drained Concentration coefficient(C) total - 24 intensity Q [m3/s]
(km2) (Bransby - Urban hour - incl. (mm/hr)
Williams) CC factor
(hrs)
5 0.0115 0.53 0.75 71.00 38.75 0.09
10 0.0115 0.53 0.75 87.80 47.92 0.11
25 0.0115 0.53 0.75 127.70 69.70 0.17

Computed Qt
T Rational Qt TRRL Qt Selected Qt

5 0.09 0.32 0.32

10 0.11 0.35 0.35
25 0.17 0.40 0.40

19
5.2.3 Run off computation for Pet House.
Table 8: Peak run of computation from the catchment using TRRL (Catchment A)
TRRL East African Flood Model
Return period (T) - years 5 10 25 50
Area drained (km2) 0.00 0.00 0.00 0.00
Catchmt slope (%) 13.2% 13.2% 13.2% 13.2%
Slope class (S) 5 5 5 5
Surface cover flow time (Ts) - hrs 1.15 1.15 1.15 1.15
Soil class (I) 4 4 4 4
Land-use factor (CL) 1.00 1.00 1.00 1.00
Catchmt. wetness factor (Cw) 1.0 1.0 1.0 1.0
Basic runoff coeff. (Cs) 45% 45% 45% 45%
Runoff coeff (Ca) 45% 45% 45% 45%
Base Time (TB) - hrs 1.2 1.2 1.2 1.2
T-yr- 24hr storm depth (mm) - area averaged 101.0 112.9 127.0 137.1
Av. flow during base time (m3/s) 0.0 0.0 0.0 0.1
Peak flow factor 2.3 2.3 2.3 2.3
T-yr peak flow (m3/s) 0.09 0.10 0.11 0.12

Table 9: Peak run of computation from the catchment using Rational method (Catchment A)
Return Period Area Time of Runoff Rainfall Rainfall Peak Flow,
drained Concentration coefficient(C) total - 24 intensity Q [m3/s]
(km2) (Bransby - Urban hour - incl. (mm/hr)
Williams) CC factor
(hrs)
5 0.0039 0.62 0.75 71.00 41.23 0.03
10 0.0039 0.62 0.75 87.80 50.99 0.04
25 0.0039 0.62 0.75 127.70 74.16 0.06

Table 10: Selected Peak flows from the catchment

Computed Qt
T Rational Qt TRRL Qt Selected Qt

5 0.03 0.09 0.09

10 0.04 0.10 0.10
25 0.06 0.11 0.11

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Table 11: Peak run of computation from the catchment using TRRL (Catchment B)
TRRL East African Flood Model
Return period (T) - years 5 10 25 50
Area drained (km2) 0.01 0.01 0.01 0.01
Catchmt slope (%) 17.8% 17.8% 17.8% 17.8%
Slope class (S) 5 5 5 5
Surface cover flow time (Ts) - hrs 1.15 1.15 1.15 1.15
Soil class (I) 4 4 4 4
Land-use factor (CL) 1.00 1.00 1.00 1.00
Catchmt. wetness factor (Cw) 1.0 1.0 1.0 1.0
Basic runoff coeff. (Cs) 45% 45% 45% 45%
Runoff coeff (Ca) 45% 45% 45% 45%
Base Time (TB) - hrs 1.3 1.3 1.3 1.3
T-yr- 24hr storm depth (mm) - area averaged 101.0 112.9 127.0 137.1
Av. flow during base time (m3/s) 0.1 0.1 0.1 0.1
Peak flow factor 2.3 2.3 2.3 2.3
T-yr peak flow (m3/s) 0.20 0.23 0.26 0.28

Table 12: Peak run of computation from the catchment using Rational method (Catchment B)
Return Period Area Time of Runoff Rainfall Rainfall Peak Flow,
drained Concentration coefficient(C) total - 24 intensity Q [m3/s]
(km2) (Bransby - Urban hour - incl. (mm/hr)
Williams) CC factor
(hrs)
5 0.0089 0.56 0.75 71.00 39.55 0.07
10 0.0089 0.56 0.75 87.80 48.91 0.09
25 0.0089 0.56 0.75 127.70 71.13 0.13

Table 13: Selected Peak flows from the catchment

T-Year Rational Qt TRRL Qt Selected Qt
5 0.07 0.20 0.20
10 0.09 0.23 0.23
25 0.13 0.26 0.26

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5.3 Hydraulic Analysis

In this section, the critical parameters of flow were assessed, and the proposed channel sections were
designed to handle the flow. Checks were performed to determine appropriate channel sizes, including
depth, base width, shape, and slopes, in order to establish a well-functioning drainage system capable
of accommodating expected flows. The objective was to design channels that would facilitate efficient
and effective water conveyance, ensuring optimal drainage for various flow conditions. By considering
these factors, a well-designed and functional drainage system was established to manage water flow
and minimize the risk of flooding.

In addition to the arithmetic computation the following factors were also considered to aid the decision.

 Practicability and safety of the users that will utilize the design.
 Maintenance requirements: ease of maintenance, safety of maintenance staff, costs associated
with maintenance.
 Landscaping requirements of the site
 Site constraints in consideration of space and off-site conditions
 Planned future expansions of the development
 Probable development of the surrounding areas
 Conditions at the exit point and assume a single outlet.

The Hydraulic tool box served as a valuable resource for creating a design that aimed to identify the
most suitable and optimal depth and base width for the drain systems, while adhering to specific
hydraulic criteria. The primary objective was to maintain velocities below 3 m/s in composite drain
systems, which incorporated both concrete and stone pitching elements. Additionally, careful attention
was given to ensure that shear stresses remained below the advised limit of 450KN, as outlined in the
design manual. Furthermore, the design process emphasized the use of the least achievable slope
while ensuring subcritical flow conditions were maintained. By employing these hydraulic principles and
criteria, a comprehensive and well-informed design was developed, promoting efficient and effective
drainage performance while addressing key hydraulic considerations

5.3.1 Concrete channels sections

Given the site's steep slopes, high flow velocities were anticipated. Consequently, reinforced concrete
was selected for all drainage structures, including primary, secondary, and closed pipe drains, due to its
durability and the high cost-effectiveness of alternative materials in such conditions.

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 Figure 8: Proposed Concrete section

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5.3.2 Proposed Primary Channel
Flow rate; 0.4, for a 25-year return period
Material; concrete
Slope; 5%

Figure 9: Design Computation for Proposed Primary drain

Figure 10: Design cross-section for Proposed Primary drain

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5.3.3 Proposed On site Secondary drain
Design Flow rate; 0.2 (for a 25-year return period)
Material; Concrete
Slope; 5%

Figure 11: Design Computation for proposed site secondary drain

Figure 12: Design cross-section for Proposed site secondary drain

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5.3.4 Proposed On site Closed drain
Design Flow rate; 0.2 (for a 25-year return period)
Material; Concrete
Slope; 5%

Figure 13: Design Computation for proposed site closed drain

Figure 14: Design cross-section for Proposed site closed drain

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5.3.5 Soak away pit

Design soak pit (cylindrical)

Design flow 0.580 Diameter (m) 17.000
Volume in one event (60min) in m3 2088 depth (m) 10

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