MAKERERE
UNIVERSITY
COLLEGE OF ENGINEERING, DESIGN, ART AND TECHNOLOGY (CEDAT)
SCHOOL OF ENGINEERING
DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING
CIV 4100: CIVIL ENGINEERING PROJECT II
COMPREHENSIVE FLOOD RISK ASSESSMENT AND TESTING OF SUSTAINABLE
URBAN DRAINAGE SYSTEMS AS POTENTIAL FLOOD RISK REDUCTION
STRATEGIES IN URBAN AREAS
BY
OWARUGAMBWA Denis
MUGERWA Akram
16/U/1067
16/U/639
Submitted in Partial Fulfilment of the Requirements for the Award of a Degree of Bachelor
of Science in Civil Engineering
……………………………………..
………………………………..
Eng. Dr. SEITH MUGUME
Dr. HILLARY BAKAMWESIGA
MAIN SUPERVISOR
CO-SUPERVISOR
JANUARY 2021
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Table of Contents
List of tables .............................................................................................................................................. 4
List of figures ............................................................................................................................................. 4
ABSTRACT...................................................................................................................................................... 6
CHAPTER ONE: INTRODUCTION .................................................................................................................... 7
1.1 Background ......................................................................................................................................... 7
1.2 Problem statement ............................................................................................................................. 8
1.3 Objectives............................................................................................................................................ 9
1.3.1 Main objective ............................................................................................................................. 9
1.3.2 Specific objectives ........................................................................................................................ 9
1.4 Justification ......................................................................................................................................... 9
1.5 Scope ................................................................................................................................................. 10
CHAPTER TWO: LITERATURE REVIEW ......................................................................................................... 14
2.1 Flooding............................................................................................................................................. 14
2.1.1 Types and causes of flooding ..................................................................................................... 14
2.2 Flood risk ........................................................................................................................................... 15
2.3 Hazard, vulnerability and exposure .................................................................................................. 16
2.4 Flood assessment .............................................................................................................................. 17
2.5 Flood modeling ................................................................................................................................. 18
Hydrologic modeling is classified in several ways that could include but not limited to; .................. 18
2.5.1 Deterministic hydrologic modeling ............................................................................................ 18
2.6 Flood management measures .......................................................................................................... 19
2.6.1 SUDs ............................................................................................................................................... 19
2.6.2 Benefits of SUDs ............................................................................................................................. 19
2.6.3 Types of SUDS ................................................................................................................................ 20
2.6.3.1 Retention Ponds...................................................................................................................... 20
2.6.3.2 Green roofs ............................................................................................................................. 21
2.6.3.3 Pervious surfaces .................................................................................................................... 21
2.6.3.4 Grass swales ............................................................................................................................ 21
2.6.3.5 Infiltration Trenches................................................................................................................ 21
2
2.6.3.6 Soakaways ............................................................................................................................... 22
2.6.3.7 Pervious Hydraulic structures ................................................................................................. 22
CHAPTER THREE: METHODOLOGY .............................................................................................................. 23
3.1 Introduction ...................................................................................................................................... 23
3.2 Flood defense analysis ...................................................................................................................... 23
3.3 Flood management ........................................................................................................................... 23
3.4 Flood modeling ................................................................................................................................. 23
3.4.1 The governing equations ........................................................................................................... 23
3.5 Determining runoff quantity ............................................................................................................. 25
3.5.1 Catchment delineation............................................................................................................... 25
3.5.2 Determining pervious and impervious areas ............................................................................. 25
3.5.3 Rainfall data ............................................................................................................................... 25
3.5.4 Extreme rainfall frequency analysis ........................................................................................... 26
3.5.5 Time of concentration ................................................................................................................ 26
3.5.6 Hydrological modeling ............................................................................................................... 27
3.5.7 Hydraulic modeling .................................................................................................................... 27
CHAPTER FOUR: ANALYSIS AND DISCUSSION ............................................................................................. 28
4.1 Runoff Quantity Results .................................................................................................................... 28
4.1.1 Runoff Coefficient ...................................................................................................................... 28
4.1.2 Rainfall Intensity ........................................................................................................................ 29
4.1.3 Time of concentration ................................................................................................................ 35
4.1.4 The Design Discharge ................................................................................................................. 35
4.2 Hydraulic capacity of existing drainage ............................................................................................ 36
4.2.1 Model Setup ............................................................................................................................... 36
4.2.2 Input Units.................................................................................................................................. 36
4.2.3 Infiltration data .......................................................................................................................... 36
4.2.4 Rainfall ....................................................................................................................................... 37
4.3 Incorporation of Sustainable Urban Drainage Systems .................................................................... 39
4.3.1 Infiltration Trenches................................................................................................................... 39
4.3.2 Retention ponds......................................................................................................................... 40
4.3.3 Rainwater harvesting tanks........................................................................................................ 41
4.3.4 Vegetated channels.................................................................................................................... 42
CHAPTER FIVE: RECOMMENDATIONS AND CONCLUSION .......................................................................... 46
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5.1 Conclusion ......................................................................................................................................... 46
5.2 Recommendations ............................................................................................................................ 46
References .................................................................................................................................................. 48
APPENDIX .................................................................................................................................................... 52
Soil permeability data as obtained from the Laboratory........................................................................ 52
List of tables
Table 1: Table 1: Types and causes of urban flooding (Butler and Davies, 2011; Hankin et al., 2008;
Lancaster et al., 2004; Maksimović et al., 2009; Mugume et al., 2015b; Ryu and Butler, 2008; Ryu, 2008;
Ten Veldhuis, 2010).................................................................................................................................... 14
Table 2: Benefits of SUDs (Bartolini et al., 2016)...................................................................................... 19
Table 3: Runoff coefficients from drainage manual ................................................................................... 28
Table 4: Yearly maximum rainfall values .................................................................................................... 29
Table 5: Rainfall rank values ...................................................................................................................... 30
Table 6: Maximum rainfall depths at return periods ................................................................................... 32
Table 7: Values of maximum 24 hour intensity .......................................................................................... 33
Table 8: Intensities against duration ........................................................................................................... 33
Table 9: Soil Permebility data for the area ................................................................................................. 37
Table 10: The volumes of flood for each subcatchment ............................................................................. 42
Table 11: Hours flooded for each subcatchment......................................................................................... 43
List of figures
Figure 1:Flooding in part of the catchment ................................................................................................. 10
Figure 2:The Mayanja Kaliddubi catchment where the study area lies ...................................................... 11
Figure 3: A map of the study catchment ..................................................................................................... 12
Figure 4:The SPRC concept and the components of risk (modified based on Sayers et al., 2013) ............ 15
Figure 5: The SPRC concept....................................................................................................................... 16
Figure 6: Land use map for the catchment ................................................................................................. 29
Figure 7: Graph of daily rainfall against return period ............................................................................... 32
Figure 8: IDF Curves .................................................................................................................................. 34
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Figure 9: The subcatchments for the study area.......................................................................................... 36
Figure 10: Time Series for the maximum rainfall....................................................................................... 37
Figure 11: Summary of results obtained from running the analysis ........................................................... 38
Figure 12: Subcatchment runoff values obtained in SWMM ..................................................................... 38
Figure 13: Results showing nodal flooding in the analysis of the catchment ............................................. 39
Figure 14: Flooding from catchment when infiltration trenches are used ................................................. 40
Figure 15: Flooding when the flow width is 0.5m....................................................................................... 40
Figure 16: Flooding with retention ponds used as SUDs ............................................................................ 41
Figure 17: Flooding when rainwater tanks are used as SUDs ..................................................................... 41
Figure 18: Amount of flooding when vegetated channels are used as SUDs ............................................. 42
Figure 19: Flood volumes against the SUDs method used in subcatchment 1 ........................................... 43
Figure 20: Flood volumes observed against the SUDs in subcatchment 2 ................................................. 44
Figure 21: Hours for which subcatchment 1 was flooded depeding on the SUDs used ............................. 44
Figure 22: Hours for which subcatchment 2 was flooded depending on the SUDs used ........................... 45
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ABSTRACT
Floods are among the most catastrophic natural disasters around the globe impacting human lives
and infrastructure. The always changing climate and land use especially in under developed
regions that haven’t been planned for adequately have raised challenges associated with increased
runoff and flood management. This, coupled with poor disposal of wastes and little to no
maintenance works being done on the existing drainage systems magnifies the effect of flooding
especially in urban to semi-urban catchments.
This study was aimed at analyzing the existing flood defenses of Namasuba along KampalaEntebbe highway, determining the runoff from this area and incorporating Sustainable Urban
Drainage Systems as a potential mitigation strategy.
Chapter one comprises of the introduction of the report; The background, objectives and specific
objectives, problem statement and justification for the research.
Chapter two gives a detailed description of the different methods of determining runoff,
stormwater management approaches, hydrologic and hydraulic analysis.
Chapter three involves a detailed description of the methodology. This involved site visits, data
collection, and delineation of catchment using ArcGIS and determining runoff quantity.
Chapter four discusses the results attained in modelling catchment using EPA SWMM software.
A detailed hydrologic analysis was also undertaken as shown.
Chapter five involves the recommendations and conclusions.
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CHAPTER ONE: INTRODUCTION
1.1 Background
In recent decades due to global climate change, floods with return periods of 100 years have been
occurring worldwide with frightening regularity (Zakaria et al., 2017). Flooding is a natural
phenomenon of the environment however floods are known to be the most common natural
disaster. Extreme global flooding with increased frequency affects both the developed and the
developing world with catastrophic results (Mugesh, 2015).
In developing countries due to low levels of flood protection the impact of flooding is especially
harmful. For example, 6,648 flood fatalities were recorded in 2013 in India and Nepal, while the
Philippines has suffered from recurring flooding that has caused more than 100 fatalities every
year between 2011 and 2013, and prolonged flooding in Thailand in 2011 caused serious economic
losses (Guha-Sapir et al., 2015). Previous studies show that there is increased exposure of people
and assets as a result of population growth and economic growth which implies greater damage
resulting from weather related natural disasters, flooding inclusive (Tanoue et al., 2016).
In Uganda, like the rest of the world, there are changes in the frequency of severity of extreme
climate events, such as droughts, floods (Lwasa, 2010).
Kampala, Uganda’s capital city is the country’s largest and most important urban area. The
population of the greater Kampala metropolitan area, covering an area of 1450 km2 of which only
about 196 km2 is under the control of Kampala Capital City Authority (KCCA), was recently
estimated to grow from around 3.15 million in 2011 to as much as 8-10 million by 2040 (KCCA,
2013). The city is within the equatorial region where rain is expected to continue increasing as a
result of climate change. Its pattern of occurrence is also expected to keep changing and thus
becoming even more unpredictable. It is projected that unless appropriate measures are put in place
now, extreme weather events such as droughts and floods will continue to ravage the city
(Namayanja, 2009).
Frequent, high intensity tropical rain storms almost inevitably generate extremely high runoff that
quickly exceeds the capacity of the urban storm water drainage system causing frequent flooding
especially in the low-lying valleys and wetland areas (Sliuzas et al., 2013). Due to urbanization,
forested areas and grasslands are increasingly converted to commercial, residential or industrial
7
uses. The conversion creates a significant increase in impermeable surfaces such as concrete,
asphalts, building roofs and even compacted vegetated sites (US Environmental Protection Agency
(EPA), 2003).
1.2 Problem statement
Areas in Kampala that have suffered floods include Kyambogo, Bwaise, Kalerwe, Nateete,
Nalukolongo, Ndeeba, Katwe, Clock Tower and Namasuba among others (Ssenyonga & Waiswa,
2017). In Bwaise, it becomes almost impassable during heavy rains and many pedestrians who do
not wish to wade in the flood waters are carried by others people (“transporters”) across the flooded
streets (Monitor Online, 2011). On Sunday 26th May 2019, eight people are believed to have lost
their lives in the areas of Kikajjo, Lubowa and Lufuka all located in Masajja and Bunamwaya
divisions (Ministry for Kampala Capital City and Metropolitan Affairs, 2019), where our case
study is located, and a great deal of property was destroyed including displacement of people.
Climate change impacts feature prominently in the lives of the people in Kampala and metropolitan
(Mabasi, 2009). Extreme events such as floods and storms threaten lives and leave people feeling
insecure and thus often terrible experiences for those affected. Construction of unregulated shelters
by the poor has reduced infiltration of rainfall, increasing runoff to six times that which would
occur in natural terrain (Mabasi, 2009). Both the increased storminess due to climate change and
densification aggravate flooding (Action Aid, 2006).
A significant portion of the population in Kampala is not served by solid waste collection services.
KCCA acknowledges that the amount of solid waste generated overwhelms its capacity to collect
and dispose given its enormous collection costs (Water Aid Uganda, 2011). Out of 1,200 – 1,500
tons of garbage generated per day, only 400 – 500 tons are collected giving a collection efficiency
of 40 % leaving 60% of solid waste generated not properly disposed of (Office of Auditor General.
Republic of Uganda, 2010). Therefore, garbage quickly clogs drains leading to localized flooding
with even light rainfall (Mabasi, 2009). Blocked drains that are not able to transport the increasing
volumes of runoff also lead to stagnation of water that serves as a habitat for breeding anopheles’
mosquitoes, which are a threat to human health
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1.3 Objectives
1.3.1 Main objective
The main objective is to carry out an assessment of flood risk and evaluate the effectiveness of
Sustainable Urban Drainage systems (SUDs) as potential flood risk management strategies to
reduce floods in Namasuba-Zzana area.
1.3.2 Specific objectives

To determine the runoff quantity for Namasuba-Zzana area.

To carry out a hydrologic and hydraulic analysis of the flooding area.

To assess the impact of Sustainable Urban Drainage systems (SUDs) on flood reduction.
1.4 Justification
Efforts to improve drainage and curb flooding in the Namasuba area relates to Sustainable
Development Goal number 11 – “to make cities and human settlements more inclusive, safe,
resilient and sustainable.” (Sustainable Development Solutions Network (SDSN), 2014). Drainage
improvement initiative draws from the Kampala Drainage Master Plan which is a 40-year plan that
was developed to address the drainage challenges in the city and the flood risk assessment,
strategies and Actions report by the UN Habitat under cities and the climate change initiative
(KCCA., 2014).
For a long time, urban drainage systems have existed as a vital city infrastructure to collect and
convey storm water and wastewater away from urban areas. Despite development over the years,
it remains a significant challenge to design an effective functioning system. Sustainable drainage
systems have been addressed long-term sustainability in the design of the system (Zhou, 2014).
The project is being undertaken to identify and assess sustainable solutions to flooding using
Sustainable Urban Drainage systems (SUDs) that enhance drainage and make urban drainage
systems more compatible with components of the natural water cycle. The motivation to explore
the potential of SUDs are that the associated techniques provide multiple benefits beyond the
conveyance of storm water into the nearest watercourse and thus minimizing flooding (Ihuhwa,
2016).
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Concerning conventional sewerage, sewers are designed either to convey both wastewater and
storm water in a combined system or to convey storm water separately from the wastewater. Upon
the use of such SUDs, reduction of surface runoff volumes into these sewers implies conveyance
of a less hydraulic load (wastewater and reduced volumes of storm water) and with continued
application there would be no need for storm sewers (Butler & Davies, 2011).
1.5 Scope
The intended research was limited to settlements of Namasuba-Zzana a city suburb off Entebbe
road. This area is targeted because it is one of the flood prone areas that are affected by heavy rains
on top of being a low-lying area.
The study area is 16km2 and lies in the Mayanja Kaliddubi catchment which is approximately
41km2.
Figure 1:Flooding in part of the catchment
10
Figure 2:The Mayanja Kaliddubi catchment where the study area lies
11
Figure 3: A map of the study catchment
The study scope is to involve the assessment using Sustainable Urban Drainage systems (SUDs)
particularly infiltration galleries and retention ponds only as the identified technology. The
technical scope on the other hand is to involve infiltration implying the need to obtain soil
permeability. In addition, for the design of the infiltration structure (French drains), the area
proportions of pervious and impervious land are to be obtained as well as surface runoff.
Presently, there are no Sustainable Urban Drainage Systems in place in the study area, however
there is a trapezoidal shaped drainage channel on the roadside that is approximately 1.5m deep,
and 5m wide that has proven to be insufficient to drain the study area thus flooding.
12
The impact on flood reduction was obtained by comparing the total amount of estimated runoff
and the amount that can be retained and allowed to infiltrate by the designed infiltration structure.
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CHAPTER TWO: LITERATURE REVIEW
2.1 Flooding
Floods can be defined as a temporary covering of land by water outside its normal confines
(FLOODsite-Consortium, 2005; cf. Munich Re, 1997). It is a relatively high flow of water, which
overflows the natural channel provided for runoff (Afeku, 2005).
2.1.1 Types and causes of flooding
Flood is classified based upon its source and special characteristics of the flood event (Schanze,
2006; MWO, 2007; The World Bank, 2012) as shown in the table below (Mugume, 2015).
Table 1: Table 1: Types and causes of urban flooding (Butler and Davies, 2011; Hankin et al., 2008; Lancaster et al., 2004;
Maksimović et al., 2009; Mugume et al., 2015b; Ryu and Butler, 2008; Ryu, 2008; Ten Veldhuis, 2010)
Type of flooding
Description of cause (threat)
Pluvial flooding
Caused by both external threats such as high intensity, short duration
extreme rainfall, high urban imperviousness levels coupled with
insufficient urban drainage network capacity (including inlets) which
leads to hydraulic overloading, overflow operation, surcharging and
surface flooding
UDS (sewer) flooding
Caused by internal system threats (other causes) such equipment
malfunction, sewer collapse and blockages that also lead to flooding.
UDS flooding may occur in either dry or wet weather flow conditions
Fluvial flooding
Exceedance of the flow capacity of the channel of a river, stream or
other natural water course, typically associated with heavy rainfall
events in which the excess water spills on the flood plain.
Coastal
flooding
and
tidal Caused by either or a combination of high tides, storm surges and
wave
action.
14
Estuarial flooding
Estuarial and water courses affected by tide locking. This often
involves high tidal levels and high fluvial flows in combination.
Ground water flooding
Caused by raised ground water levels, typically following prolonged
rain and may result into increased overland flow flooding.
Overland flow flooding
Caused by water flowing over the ground surface before reaching a
natural or artificial drainage channel due to extreme rainfall that
exceeds the infiltration capacity of the ground, or when the ground is
highly saturated with water.
Infrastructure failure
Caused by structural, hydraulic or geotechnical failure of
infrastructure that retains, transmits or controls the flow of water e.g.
dam failure.
Leakages and external Drinking water flows on the surface due to a pipe failure, leaking
overflows
hydrants or values, discharge of water from other sources e.g.
construction sites or emptying of swimming pools.
2.2 Flood risk
Klijn et al., (2009) defines flood risk in two ways; first, it is defined as hazard (exposure) and
vulnerability of the society. Second. It is defined as probability of the flood consequences. Each
flood event can be characterized by features such as water depth, flow velocity, matter fluxes, and
temporal and spatial dynamics. (Schanze et. al, 2006).
The Source-Pathway-Receptor-Consequence (SPRC) concept, forces a systematic understanding
the sources, pathways, and receptors of risk, considering of all aspects of the flooding system (ICE,
2001; Evans et al., 2004; Schanze 2006; Sayers et al., 2013)
15
Source of the flood
(e.g rainfall, waves)
Susceptibility
(harm that
results from
Resilience
(ability to
recovery)
Value
(quantified
flooding harm)
Pathway between
Source and
Receptor (e.g
overtopping,
overflow, floodplain
Exposure of
Receptor (e.g
people, property,
Vulnerability of
Receptor (that is
exposed to given
velocity depth,
velocity, duration
Consequences of
flood (e.g loss of
life, material
damage,
environmental
Risk (described
for a single flood
event or
expected risk
over a given
timeframe)
Figure 5: The SPRC concept
2.3 Hazard, vulnerability and exposure
The probability of the occurrence of potentially damaging flood events is called flood hazard (cf
ITC, 2004). Potentially damaging means that there are elements exposed to floods which could,
but need not necessarily, be harmed (FLOODsite-Consortium, 2005). The UNISDR (2009: 17)
defines a hazard as a dangerous phenomenon, substance, human activity or condition that may
cause loss of life, injury or other health impacts, property damage, loss of livelihoods and services,
social and economic disruption, or environmental damage.
Flood vulnerability can refer to the inherent characteristics of a flood plain that make it susceptible
to flood hazards (Schanze, 2006; Blanco-Vogt and Schanze, 2014). It is further understood as the
combination of susceptibility, societal value or function and coping capacity of elements exposed
to flood hazard (Schanze, 2006; Amman, 2010; Blanco-Vangt & Schanze, 2014; IPCC, 2014;
Rufat et al., 2015). Schanze (2006) adds that three basic areas of vulnerability can be distinguished
according to the principle of sustainability: social and cultural vulnerability which refers to loss of
16
life, health impacts, loss of vitality, social impacts and loss of cultural heritage; economic
vulnerability which alludes to direct indirect financial losses by damage to property assets, basic
material and goods, reduced productivity and relief efforts; and ecological vulnerability comprises
anthropogenic pollution of waters, soils and ecological systems with their biota (cf Messner and
Meyer).
Flood exposure is the proneness of vulnerable elements to flood hazard such as people, property,
goods that can be lost, injured or damaged during flood event (UNDRO, 1984; Franzi, 2010:239).
GIS based spatial analysis and visual elements are used frequently in the recent years for detection
of flood hazard areas and preparation of maps (Ozhan & Tarhan, 2015).
2.4 Flood assessment
Risk assessment altogether covers the perception of the causal interrelations and the weighing of
the tolerability of risks (Schanze, 2006). The purpose of the flood risk assessment is to determine
the level of flood risk so as to identify measures necessary to secure the community. According to
FLOODsite, a risk assessment is supposed to include an individual point of view where it is
important to determine if floods are life-threatening or just a nuisance; whether they cause huge
material losses and immense stress or are just a matter of life people are used to “live with” which
is largely dependent on the type of flood.
Generally, flood risk assessment can be divided into supra-national, macro-, maso-, and microscale; the supra-national scale refers to assessments of the entire globe or continent, the macroscale concerns the assessments of the entire country, which may cover multiple watersheds,
requiring consistent or national data while the maso-scale is generally a sub national, e.g. cities on
a regional scale while the micro-scale, based on an object-oriented approach, relates to a town or
specific river stretch (Yang, 2017).
City scale flood risk assessment is perceived as local or micro level. They allow planners and
decision makers to identify the most at-risk areas, to assess how risk may change in the future, and
to achieve flood risk reduction effectively by various adaptation measures (Ward et al. 2011a, b;
Budiyono et al., 2016).
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2.5 Flood modeling
A hydrologic system flood model is an approximation of the actual system; its inputs and outputs
are measurable hydrologic variables and its structure is a set of equations linking the inputs and
outputs. It allows prediction of the hydrologic response to various watershed management
practices and obtain understanding of the impacts of these practices (Úr, 2017). The objective of
the hydrologic system analysis is prediction of the output from studying the system operation.
Hydrologic modeling is classified in several ways that could include but not limited to;
2.5.1 Deterministic hydrologic modeling
This model doesn’t consider randomness. There are three categories under this form of modeling,
namely; Lumped models, Semi distributed and distributed modeling. (Eldho, 2011)
2.5.1.1 Lumped models
In this type of modeling, spatial variability of processes, inputs or boundary characteristics is not
taken into account. Lumped models are not applicable to event-based processes and the parameters
used do not represent features of hydrologic processes. Examples of these models are SCS-CN
based models, IHACRES, WATBAL (Eldho, 2011).
2.5.1.2 Semi- distributed models
Here, the basin is divided into a number of smaller basins.
The main types are Kinematic wave theory for example SWMM model and probability distributed
models (spatial resolution is accounted for by using probability distributions of input parameters
across the basin. Other examples include; HEC-HMS, TOPMODEL, SWAT (Eldho, 2011).
2.5.1.3 Distributed models
These are models where parameters are fully allowed to vary in space at a resolution chosen by
the user. It requires large amounts of data and attains the highest accuracy in the rainfall-runoff
modeling. Example include; HYDROTEL; MIKE11/SHE, WATFLOOD (Eldho, 2011).
2.5.1.4 SWMM model (Storm Water Management Model)
SWMM is a physically based discrete time hydrological and hydrodynamic model that can be used
as a single event and continuous simulation of run-off quantity and quality and is primarily built
for urban areas (Rossman, 2010).
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Surface flooding was modeled using the ponding option built in SWMM which allows exceedance
flows to be stored (Rossman, 2010).
2.6 Flood management measures
Flood management measures refer to coordination of efforts of all organizations that tackle
flooding concentrating on those areas where the risk of flooding is greatest (Scottish Environment
Protection Agency (SEPA), 2015).
Flood management measures can be grouped into either structural or nonstructural measures;
Structural flood management measures are measures which are undertaken to protect people and
property that counteracts the flood event in order to reduce the hazard or to influence the course
or probability of occurrence of the event.
Nonstructural measures are a set of mitigation and/or adaptation measures that do not make use of
traditional structural flood defense measures but reduce damage without influencing the current of
the flood event.
2.6.1 SUDs
The term SUDS stands for Sustainable Urban Drainage Systems or Sustainable Drainage Systems.
They are a range of drainage techniques and devices allowing for runoff attenuation and mitigation,
pollutants reduction and amenity construction (Zhou, 2014). These systems drain surface water in
a manner that is more sustainable than conventional solutions (Fletcher, et al., 2014). SUDs also
mimic natural drainage processes to reduce the effect of urbanization on the quality and quantity
of storm water runoff.
2.6.2 Benefits of SUDs
Table 2: Benefits of SUDs (Bartolini et al., 2016)
Benefit
What it covers
Floodrisk management
Impact on people and property
Water quality management
Surface
water
quality improvements
to
aesthetics, health, biodiversity etc
Groundwater recharge
Improved water availability
19
Pumping wastewater
Reduced flow of wastewater to treatment
works
Rainwater harvesting
Reduced
flows
in
sewer,
pollution
or
dependence on potable water
Treating wastewater
Reduced volume of wastewater to treat from
combined drainage systems
Climate change adaptation
Improved ability to make incremental changes
and adapt infrastructure (no regrets)
Biodiversity and ecology
Sites of ecological value
Amenity
Attractiveness and desirability of an area
Health and wellbeing
Physical, emotional, mental health benefits
from recreation and aesthetics
Air quality
Impact on health from air pollution control
Carbon reduction and sequestration
Operational and embodied carbon reduction
together with sequestration
Economic growth
Business, jobs and productivity
Tourism
Attractiveness of touristic sites
Education
Enhanced education opportunities
2.6.3 Types of SUDS
2.6.3.1 Retention Ponds
These are areas of land, which are excavated to shallow depths, filled with earth, rock or gravel
and then saturated with water. The substrate is planted with aquatic vegetation, which acts as a
surface flow retention system. They can be classified as wet or dry depending on whether or not
they have a permanent volume of water.
20
They store excess storm runoff associated with watershed imperviousness. During major events,
there is a filling period when inflow rate is greater than outflow and a depletion period when
outflow is greater than inflow rate; the higher the peak outflow, the lesser the storage volume
(James, 2017).
2.6.3.2 Green roofs
They are roofs, which consist of a planted area that has significant storage potential, encourages
evapotranspiration and provides the added benefit of water quality improvement as storm water
travels through the soil.
Green roofs are source control measures aimed at detaining and attenuating excess water runoff
upstream thus preventing and reducing flood hazard impacts on recipient sustainability (Zhou,
2014)
2.6.3.3 Pervious surfaces
Pervious pavements are a well renowned example of such surfaces. These are built with concrete
blocks, crushed rock or porous asphalt. Depending on the soil conditions, water may infiltrate into
the sub soil or be stored in an underground reservoir. Infiltration action promotes pollutant removal
both at the surface or sub-base of the material itself.
Pervious surfaces reduce peak flow and improve water quality in extreme rainfall (Zhou, 2014).
2.6.3.4 Grass swales
They are grass-lined channels used for the conveyance, storage, infiltration and treatment of
stormwater. Runoff enters directly from adjoining buildings or pavements and it is stored either
until infiltration takes place or until the filtered runoff is conveyed elsewhere.
Swales delay runoff peaks and provide a reduction in runoff volume due to infiltration and
Evapotranspiration, as they slow runoff and spread it out thus the water infiltrating into the soil
and evaporating in the air implying less runoff reaches the endpoint which reduces flooding
(Laurie et al., 2018)
2.6.3.5 Infiltration Trenches
An infiltration trench is a long narrow, shallow excavation located over porous soils and backfilled
with stone to form a sub-surface reservoir into which water is directed for initial storage and from
21
which it infiltrates. To increase the infiltration rate, it is advised to install a shallow, large and flat
porous bed (James, 2017). They do not consume a lot of space, reduce the risk of floods by
attenuating runoff peaks, and integrate well with urban areas. Infiltration trenches are only efficient
in areas with a low water table to allow a free flow of storm water into the subsoil.
They remove pollutants from their tributary areas and also reduce runoff volumes (James, 2017).
2.6.3.6 Soakaways
A soakaway is an underground structure which can be stone filled, formed with plastic mesh boxes,
dry wall lined or built with precast concrete ring units (Butler & Davies, 2004). Similar to
infiltration trenches, soakaways are only suitable in places with a low water table. The main
difficulty is that the spaces between the stones get clogged by the fine material entering the
soakaway. It is therefore recommended to use a filter geotextile material.
2.6.3.7 Pervious Hydraulic structures
Tucci & Bertoni (2007) introduce the concept of pervious hydraulic structures for the first time
used to drain runoff and allow infiltration. These structures are also called infiltration galleries but
they have not been applied widely as SUDs. For example, pervious trenches which are a special
type of infiltration trench consisting of a chamber with gravel and a filter with a porous or
perforated pipe passing through pervious inlets to the drainage system
They encourage the process of groundwater recharge by allowing water to infiltrate the bed
material, and the water is harvested by a network of collection pipes installed under or beside the
bed (Fraser, 2014)
22
CHAPTER THREE: METHODOLOGY
3.1 Introduction
This chapter gives elaborate information on the tools used, procedures and steps that are to be
taken to achieve the specific objectives for evaluating the effectiveness of various potential flood
risk management strategies. The methodology covers the steps taken in the assessment of flood
risk in urban areas using infiltration structures and retention ponds as the potential SUDS for the
project.
3.2 Flood defense analysis
The reliability and the strength of the flood defenses is was determined for various failure modes.
This was done by assessing the ability of the existing drainage systems to convey runoff at peak
rainfall.
3.3 Flood management
Flood hazard according to Klijn et al., (2009) is reduced by managing hazard characteristics such
as flood probability or extent, while in other instances reducing exposure and vulnerability of
people and property is considered the better option. For our research, we concentrated on
management of hazard characteristics.
3.4 Flood modeling
The flood modeling environment that was used is SWMM which analyzes runoff and erosion
problems in small to medium catchments. It uses five main input datasets i.e., a DEM, a highresolution satellite image, a digital drainage channel system map, soil hydrological and infiltration
data and detailed rainfall data (5-minute storm intensities) (Sliuzas et al., 2013).
A model of the existing drainage was built in SWMM where the data needed to build the model
was obtained from a digital elevation model for the area. A satellite image and a drainage channel
system map were also used to build the model.
3.4.1 The governing equations
The study area was modeled as a Low Impact Development (LID) unit. The hydrologic
performance of this LID unit was modeled by solving simple mass balance equations that express
23
the change in water volume in each layer over time as the difference between the inflow water flux
rate and the outflow flux rate (Rossman., 2010). These equations for the surface layer (Equation
1), the soil layer (Equation 2), and the storage layer (Equation 3) are;
∂d1
dt
= i + q0 – e1 – f1 – q1
∂θ
L2 dt = f1 - e2 - f2
φ
∂d3
dt
= f2 – f3 – q1
(Equation 1)
(Equation 2)
(Equation 3)
where:
d1 = depth of ponded surface water (m),
θ = soil layer moisture content (m3/m3),
d3 = depth of water in storage layer (m),
i = externally supplied rate of precipitation (m/s),
q0 = externally supplied surface runon flow rate (m/s),
q1 = surface runoff flow rate (m/s),
q3 = underdrain outflow rate (m/s),
e1 = surface ET rate (m/s),
e2 = soil zone ET rate (m/s),
f1 = surface infiltration rate (m/s),
f2 = soil percolation rate (m/s),
f3 = native soil infiltration rate (m/s),
L2 = known depth of the soil layer (m), and
φ = known void ratio of the storage layer (m3/m3)
24
3.5 Determining runoff quantity
This section involves obtaining the catchment area by delineation using ArcGIS, rainfall intensity
from IDF curves and coefficient of runoff from land use and soil data.
3.5.1 Catchment delineation
GIS based spatial analysis and visual elements are used frequently in the recent years for detecting
of flood hazard areas and preparation of maps (Ozhan & Tarhan, 2015) and to delineate the entire
catchment into sub catchments that represent areas where runoff flows towards a single outlet
(Mugume, 2015). It was done following these steps;
The work environment of the DEM for the Namasuba-Zzana area was added and analyzed using
the ArcHydro extension tool of ArcGIS.
The depressions were filled thus creating a depression less DEM and the directions of the steepest
descent defined.
Upstream cells draining to a given cell were determined using flow accumulation where those cells
with accumulation greater than the user defined threshold were classified and segmented using the
stream segmentation process.
For every stream segment, a sub basin was delineated by clicking the catchment grid delineation
option.
The widths of the sub catchments were determined and the slope analysis was done. Based on the
results the minimum, average and maximum slope values of each of the delineated sub catchments
were also determined.
3.5.2 Determining pervious and impervious areas
A satellite image was obtained and spatial analysis was used to determine the imperviousness of
the area under study. Pervious and impervious areas were determined for each sub catchment and
the appropriate coefficients attached as obtained from the MoWT drainage design manual, to
approximate the amount of runoff (that which doesn’t infiltrate the soil).
3.5.3 Rainfall data
Meteorological data was obtained from the Uganda National Meteorological Authority that is data
on daily rainfall from 2005-2019. This dataset was used for generation of the intensity duration
25
frequency that helped in obtaining different rainfall intensities for different return Periods which
were later used in determining peak discharge by using the Rational method.
3.5.4 Extreme rainfall frequency analysis
Intensity-duration-frequency (IDF) relationships provide a widely used form of conveying the
rainfall information for a given location (Butler & Davies, 2011). The IDF curves were constructed
by performing statistical analysis on Annual Maxima Series (AMS) or Partial Duration Series
(POT) by fitting curves to empirical quantiles or fitting probability distributions for several
preselected rainfall durations (Bougadis & Adamowski, 2016)
In this method, annual maximum daily rainfall depths were abstracted from observed rainfall time
series and ranked in decreasing order of magnitude. To estimate the rainfall return periods,
Weibull’s plotting position is applied (Butler & Davis, 2011)
𝑥+1
T=
𝑚
Where, T is the return period
The intensity duration frequency (IDF) curves were generated using the Watkins and
Fiddles method. The maximum rainfall depths were plotted against return periods on a semi-log
paper and a line of best fit plotted through the points. From the generated IDF curves, rainfall
intensity of a particular return period was obtained.
3.5.5 Time of concentration
The commonly used equation to calculate time of concentration is Kirpich’s formula given by:
Tc = 0.01947L0.77 S-0.385
Where:
TC: Time of concentration (in minutes)
L: Maximum length of travel of water (meters)
S: Slope of the drainage basin = (H/L)
H: Difference in elevation between the most remote place in the basin and the outlet.
26
The maximum length of run off travel, the elevations of the remotest point and the elevations of
the points considered in the catchment were determined. This was determined by the flow length
tool in ArcGIS.
3.5.6 Hydrological modeling
After developing the basin model component in SWMM, populating the meteorological model and
defining the control specifications, the model was run while calibrating the parameters. The model
output results were quantified runoff floods that result from input rainfall data.
3.5.7 Hydraulic modeling
Still with SWMM as the hydraulic model it was calibrated and then used to simulate the 25 year
floods to aid in determination of maximum channel flood depths for all cross sections along the
drainage system.
Considering the rainfall data, the first step in modeling was setup and calibrate the baseline model
that is the current drainage system considering peak rainfall. A series of model runs was then done
on possible drainage systems with specific focus on SUDS particularly infiltration galleries and
retention ponds. Assessment of the effectiveness of undertaken measures was judged basing on
the achieved effect in comparison to the intended effect.
27
CHAPTER FOUR: ANALYSIS AND DISCUSSION
4.1 Runoff Quantity Results
The peak runoff rate 𝑄𝑃 was calculated using the rational method for the site area under
consideration (A). The rainfall intensity (I) and the surface coverage represented by the runoff
coefficient (C) were also obtained as seen in sections below.
𝑄𝑃 = CIA
Where;
𝑄𝑃 = Peak runoff rate (m3/s)
C = runoff coefficient
I = rainfall intensity (m/s)
A = total area (m2)
4.1.1 Runoff Coefficient
The runoff coefficients to be used were obtained from The Ministry of Works and Transport
Drainage Design Manual, Table 4.7 (a)
Table 3: Runoff coefficients from drainage manual
Land Use
Area (ha)
Coefficient
Of
Runoff Range
Built up area
1191.74
0.9
Unimproved Grass
373.25
0.25
Total
1564.99
From ArcGIS, the areas occupied by the various land uses were obtained and that information,
together with the table of coefficients given below, the weighted coefficient of runoff was obtained
and used to determine the runoff.
Weighted Coefficient =
∑(𝐶𝑥 𝐴𝑥 )
𝐴
(1191.74)(0.9) + (373.25)(0.25)
Weighted C
=
Weighted C
= 0.74
1564.99
28
Weighted C
= 0.74
Figure 6: Land use map for the catchment
4.1.2 Rainfall Intensity
The procedure used in estimating the rainfall intensity utilized rainfall data extracted and procedure
for obtaining the critical intensity was that according to the Watkins and Fiddes equation (Ministry
of Works and Transport, 2010)
Yearly maximum daily rainfall
Table 4: Yearly maximum rainfall values
Year
Maximum daily
rainfall
2005
73.6
2006
67.3
2007
65.9
2008
37.2
29
2009
51.4
2010
64.8
2011
71.2
2012
65.0
2013
57.9
2014
56.2
2015
91.2
2016
73.9
2017
63.8
2018
77.9
2019
54.6
The Watkins and Fiddes procedure (Watkins and Fiddes, 1984) was adopted for obtaining the
rainfall intensity, and is elaborated below:
Daily maximum rainfall values together with the duration for the available period were read and
ranked from 1 to N (the number of years of record) in a decreasing order.
The corresponding return period was estimated for each data set, using Weibull’s plotting position
formula;
T=
𝑁+1
𝑀
Where M is the event rank number (1, 2… N)
Table 5: Rainfall rank values
Year
Maximum
daily Rank
Return Period
rainfall
2015
91.2
1
16.00
2018
77.9
2
8.00
30
2016
73.9
3
5.33
2005
73.6
4
4.00
2011
71.2
5
3.20
2006
67.3
6
2.67
2007
65.9
7
2.29
2012
65.0
8
2.00
2010
64.8
9
1.78
2017
63.8
10
1.60
2013
57.9
11
1.45
2014
56.2
12
1.33
2019
54.6
13
1.23
2009
51.4
14
1.14
2008
37.2
15
1.07
The maximum rainfall depths were plotted against return periods on a semi-log graph and a line
of best fit plotted through the points as shown below.
31
Daily Maximum Rainfall vs Return Period
100
90
Daily Maximum Rainfall (mm)
80
70
60
50
40
30
20
10
0
1,00
10,00
Return Period
100,00
Figure 7: Graph of daily rainfall against return period
Using the equation generated from the line of best fit R = 14.915ln(T) + 51.181, where R represents
the maximum daily rainfall and T the return period, the maximum daily rainfall for the return
periods 1, 2, 5, 10, 25, 50 and 100 years was computed as shown below;
Table 6: Maximum rainfall depths at return periods
Return Period
14.915In(T)+51.181
R(mm)
iT24
1
0
51.181
2.133
2
0.693
61.517
2.563
5
1.609
75.179
3.132
10
2.303
85.530
3.564
25
3.219
99.192
4.133
From the graphs, maximum rainfall depths at desired return periods were read off. That is for 2, 5
and 10 years.
32
The value of n in the equation was obtained from table 4.5 of the Drainage Design Manual, 2010,
for Kampala as 0.97 and b = 1/3 for East Africa.
From the graph of Daily Maximum Rainfall against return period, the Maximum 24-hour rainfall
(𝑅 𝑇 24 ) was obtained by reading off at the desired return period. Thereafter the corresponding
maximum 24-hour intensity (𝑖 𝑇 24 ) was obtained.
𝑅𝑇
𝑖 𝑇 24 = 24
The values for aT for each return period were calculated using the formula: aT=iT24(b+24)n
Table 7: Values of maximum 24 hour intensity
Return
Period
iT24
aT
1
2.133
47.15341
2
2.563
56.67612
5
3.132
69.26315
10
3.564
78.7996
25
4.133
91.38663
The desired sequence of duration (e.g. 10, 15, 30 minutes) was chosen for each data set and used
to calculate the rainfall intensity using the basic mathematical form of the intensity-duration
frequency curve represented by the rectangular hyperbola as equation below:
𝑎
i= (𝑡+𝑏)𝑛
Where i is the intensity (mm/hr), t the duration (hours) and a, b and n are constants developed for
each IDF curve.
Table 8: Intensities against duration
Return Period
1
2
5
10
25
10
92.366
111.019
135.675
154.356
179.012
20
69.875
83.986
102.639
116.770
135.423
30
56.275
67.640
82.662
94.044
109.066
40
47.153
56.676
69.263
78.800
91.387
50
40.605
48.805
59.644
67.856
78.694
Duration(mins)
33
60
35.672
42.876
52.398
59.612
69.134
70
31.820
38.246
46.741
53.176
61.670
80
28.729
34.531
42.200
48.010
55.679
90
26.192
31.482
38.473
43.770
50.762
100
24.072
28.934
35.359
40.228
46.653
110
22.274
26.772
32.718
37.223
43.168
120
20.729
24.915
30.448
34.641
40.174
130
19.387
23.302
28.477
32.398
37.573
140
18.211
21.888
26.749
30.432
35.293
150
17.171
20.638
25.222
28.694
33.278
The intensities were plotted against duration for each desired return period to give the IDF curves.
IDF Curves for the data
180
160
Intensity (mm/hr)
140
120
100
1 year
2 years
80
5 years
60
10 years
40
20
0
10
20
30
40
50
60
70
80
90
100 110 120 130 140 150
Duration (min)
Figure 8: IDF Curves
The following procedure was adopted for the determination of coefficients a, b and n in the
formulae used.
34
4.1.3 Time of concentration
In order to determine the design storm, the time of concentration of the catchment should be
established. The commonly used equation to calculate time of concentration is Kirpich’s formula
given by:
Tc = 0.0194L0.77S-0.385
Where: Tc is Time of concentration (in minutes)
L: Maximum length of travel of water (meters)
S: Slope of the drainage basin = (H/L)
H: Difference in elevation between the most remote place in the basin and the outlet.
Whereby, the slope,
HO – Elevation at the highest point in the catchment. (From ARC GIS, it was found to be
1304m)
HP – Elevation at lowest point which was found as 1144m
L – Maximum length of travel which is 3835.7m
S=
1304−1144
3835.7
S= 0.041
Tc = 0.0194L0.77S-0.385
Tc = 0.0194*(3835.7)0.77 *(0.041)-0.385
Tc = 38.14 minutes
Critical rainfall intensity
Critical rainfall intensity is rainfall that causes a catchment to operate at a steady state. The critical
intensity for the 10-year return period was read off from the IDF curve using the respective time
of concentration of 38.14 minutes. The critical rainfall intensity was read off as 86mm/hr.
4.1.4 The Design Discharge
Having determined all the necessary parameters in the equation for the design flows, peak
runoff using the Rational Method
Q = CIA
35
0.086
Q = 0.74* 3600 *15649900
=276.7m3/s
4.2 Hydraulic capacity of existing drainage
4.2.1 Model Setup
The drainage system of the Entebbe International Airport was modeled for a total catchment area
1564.99 ha. The catchment was divided into 2 sub-catchments of sizes 939.8ha and 626.8ha.
Simulation was done to determine the response of the study area catchment to a time series rainfall
storm extracted from the maximum daily rainfall.
Figure 9: The subcatchments for the study area
4.2.2 Input Units
The metric units were used as measurement units for the model. This set up the units of different
parameters as follows: Sub catchment Areas in hectares, storage area units in square meters, depth,
elevation, head and diameter in meters, rainfall intensity in millimeters per hour, rainfall volume
in millimeters while flow in Cubic meters per second.
4.2.3 Infiltration data
Infiltration data as collected from the field was input into the model
36
Table 9: Soil Permebility data for the area
Soil Sample
Permeability coefficient (m/s)
Sample 1
2.3699*10-11
Sample 2
2.13585*10-11
Sample 3
2.66585*10-11
The overall coefficient of permeability used in the design was the average of the 3 values which
equates to 2.3905*10-11.
The infiltration model adopted was the Green-Ampt and this was used to guide the values for
suction head, conductivity and initial deficit.
4.2.4 Rainfall
A time series rainfall was used to as input rainfall for the model. The response of the catchment to
24-hour rainfall intensity was generated. The graph of the time series is shown in the figure below;
Figure 10: Time Series for the maximum rainfall
37
Figure 11: Summary of results obtained from running the analysis
From the analysis using EPA-SWMM, the results showed that the volume of water into the
drainage system is a peak of 225.562 hectare-m which is similar to 2255620m3
Figure 12: Subcatchment runoff values obtained in SWMM
38
Figure 13: Results showing nodal flooding in the analysis of the catchment
It can be concluded from the analysis that the nodes are flooded for about three hours as the
drainage system is overwhelmed by the runoff from the subcatchments. This is why there needs to
be a more efficient drainage system.
4.3 Incorporation of Sustainable Urban Drainage Systems
While modeling the catchment, Sustainable Urban Drainage Systems were incorporated and then
simulations were run and observations of the outcomes were noted and recorded;
From the land use maps that were drawn before, it was observed that the catchment averagely has
76.15% impervious area and 23.85% pervious surface. The purpose here was to ensure that the
SUDs modeling was done in the pervious areas of the catchment so the coverage of the SUDs was
limited to less than 24% of the entire catchment area.
In addition, the two subcatchments that make up the entire catchment have the following lanf use
properties; Subcatchment 1 – 86.9% impervious area and 13.1% pervious area. Subcatchment 2 –
68.6% impervious area and 31.4% pervious area so the SUDs had to occupy the pervious area in
the model implying that their percentage couldn’t exceed 13.1% of subcatchment 1 and 31.4% of
subcatchment 2.
4.3.1 Infiltration Trenches
In the first simulation, the SUD implemented was the Infiltration Trenches and the model was
done such that they occupied only 13.1% of the area in subcatchment 1 and 30.9% of the area in
subcatchment 2.
The SUDs were modeled to treat 30% of the pervious and 70% of the impervious area in
subcatchment 1 with a flow width of 1m. In subcatchment 2, with the SUDs occupying 31.4% of
the entire catchment, 70% of the pervious area was treated by the SUD as compared to the 30%
39
impervious which yielded a flood volume of 24,225 m3. As the percentages of the pervious and
impervious treated were increased, the volume of flood and consequently the time of nodal
flooding decreased as well.
Figure 14: Flooding from catchment when infiltration trenches are used
When the flow width was further reduced to 0.5m, the amount of flood observed decreased.
Figure 15: Flooding when the flow width is 0.5m
When the percentage of impervious area treated by the SUDs was increased to 50%, there was
eventually no volume of flood observed.
Also, it was observed that as the surface width over which each unit of the system acts was
decreased, the efficiency of the overall system to curb flooding was increased.
4.3.2 Retention ponds
A simulation was run with retention ponds only as the flood mitigation measure in each
subcatchment.
With the entire pervious area covered by the retention ponds (13.1% in subcatchment 1 and 31.6%
in subcatchment 2) with the SUD treating 100% of the entire pervious area, it was observed that
flooding occurred at the nodes from both subcatchments. A flood volume of 84,579m3 was
observed from subcatchment 1 against the 36,206m3 for the second subcatchment.
40
Figure 16: Flooding with retention ponds used as SUDs
4.3.3 Rainwater harvesting tanks
For the rainwater tanks, the difference was that they were allowed to occupy more than the
pervious area of the catchment since they basically collect water from roofs of houses or if they’re
open, they collect precipitation too. The dimensions chosen for the rainwater tanks were 10,000
liters (10m3) and 2m deep.
The first simulation was run with rainwater tanks occupying 3.182% of subcatchment 1 and 21.6%
of the area of subcatchment 2. For each simulation, the rainwater barrels treated more of the
impervious area than the pervious in the catchments.
With subcatchment 1, the volume of flooding recorded was 8,662m3 of which corresponded with
20.4 minutes of flooding. For subcatchment 2, the amount of flood was 3,249m3 with a flood time
of 23.4 minutes.
Figure 17: Flooding when rainwater tanks are used as SUDs
When the coverage of rainwater tanks was increased to 4%, it was observed that there was no
flooding recorded from subcatchment 1 while the same result was observed at 30% coverage for
subcatchment 2.
41
4.3.4 Vegetated channels
A simulation was run with the vegetated channels covering the entire pervious area in both
catchments (13.1% in subcatchment 1 and 31.4% in subcatchment 2).
It was observed that flooding occurred in both subcatchments at a volume of 218,948m3 and
370,556m3 in both catchment 1 and 2 respectively. Of all the Sustainable Systems tested, this
seemed to be the least efficient of all the types when used individually.
Figure 18: Amount of flooding when vegetated channels are used as SUDs
After running the simulations, different results were arrived at and these were tabulated and
organized graphically;
Table 10: The volumes of flood for each subcatchment
Flooding volumes (m3)
Subcatchment 1
Subcatchment 2
Existing system (no SUDs)
274,011
564,842
Infiltration Galleries
65,630
45,007
Retention Ponds
84,579
36,206
Rainwater Tanks
8,662
3,249
218,948
370,556
Vegetated Channels
42
Table 11: Hours flooded for each subcatchment
Hours Flooded
Subcatchment 1
Subcatchment 2
Existing system (no SUDs)
2.76
3.08
Infiltration Galleries
1.86
1.88
Retention Ponds
1.74
1.11
Rainwater Tanks
0.34
0.39
Vegetated Channels
2.02
2.18
The volume of flood observed was plotted against the different methods that were used for treating
the subcatchments
FLOOD VOLUMES FOR SUBCATCHMENT 1
Flood volume (m3)
300 000
250 000
200 000
150 000
100 000
50 000
0
Existing system
(no SUDs)
Infiltration
Galleries
Retention
Ponds
Rainwater
Tanks
Vegetated
Channels
Method of treatment
Figure 19: Flood volumes against the SUDs method used in subcatchment 1
43
FLOOD VOLUMES FOR SUCATCHMENT 2
Flood volume (m3)
600 000
500 000
400 000
300 000
200 000
100 000
0
Existing system
(no SUDs)
Infiltration
Galleries
Retention
Ponds
Rainwater
Tanks
Vegetated
Channels
Method of treatment
Figure 20: Flood volumes observed against the SUDs in subcatchment 2
The time for which the nodes were flooded was also tabulated and graphs plotted for each
subcatchment
HOURS FLOODED (SUBCATCHMENT 1)
Flood time (hours)
3
2,5
2
1,5
1
0,5
0
Existing system
(no SUDs)
Infiltration
Galleries
Retention
Ponds
Rainwater
Tanks
Vegetated
Channels
Method of treatment
Figure 21: Hours for which subcatchment 1 was flooded depeding on the SUDs used
44
HOURS FLOODED (SUBCATCHMENT 2)
3,5
Flood time (hours)
3
2,5
2
1,5
1
0,5
0
Existing system
(no SUDs)
Infiltration
Galleries
Retention
Ponds
Rainwater
Tanks
Vegetated
Channels
Method of treatment
Figure 22: Hours for which subcatchment 2 was flooded depending on the SUDs used
45
CHAPTER FIVE: RECOMMENDATIONS AND CONCLUSION
5.1 Conclusion
Flooding is major problem being faced by both developed and undeveloped countries that it has
resulted in destruction of property, Loss of lives and disruption of economic activities. This is most
evident in low lying areas like Namasuba in which economic activities and settlements in these
areas are exposed to great risks of floods.
The first specific objective of this study was to determine the design discharges for Namasuba
area. Using the rational method, the runoff quantity was obtained as 276.7m3/s.
The second objective was to model the catchment hydrologically and assess the hydraulic capacity
of the existing drainage system. EPA-SWMM was used to simulate the runoff and a runoff quantity
was obtained. The use of SWMM enabled subdivision of the study area into number of irregular
sub-catchments to best capture the effect that spatial variability in topography, drainage pathways,
land cover, and soil characteristics have on runoff generation to examine the hydraulic competence
of the existing drainage system for the area model the drainage system to mitigate flooding
impacts.
The third objective was to assess the impact of Sustainable Urban Drainage Systems on flood
reduction. SWMM was used for this and Rainwater harvesting tanks, Infiltration Trenches and
Retention Ponds were added to the system as Low Impact Developments covering the pervious
areas of the catchment except in the occasion of rainwater tanks and the amount of runoff from the
subcatchments greatly decreased as a result of addition of the SUDs.
5.2 Recommendations
The following recommendations have been drawn so as to improve the drainage predicaments in
the area:
I.
Routine maintenance through dredging and desiltation especially after heavy downfalls
should be carried out. Proper solid waste management must be encouraged from source to
sink to avoid various solid wastes ending into storm water channels that will render them
hydraulically incompetent thus leading to flood in the area.
46
II.
Within the catchment, various Sustainable Urban Development (SuDs) techniques should
be adopted particularly Rainwater harvesting tanks, Infiltration Trenches or Retention
ponds. These will reduce on the amount of water that ends up into the artificial major and
minor drainage systems, increase the groundwater resource through infiltration trenches
and a great trick to tackle domestic water scarcity at house hold level though rain and road
water harvesting procedures. This will ultimately reduce on the potential of flooding in
Namasuba and boost water availability as an economic good and a natural resource as well.
III.
Due to the fact that the largest part of the study area is likely to experience flooding, we
recommend the government to engage specialists to engage in the design intervention
measures to prevent the flooding cases
IV.
For Research based project, we recommend that the rainfall data and any other related
information should be entirely free.
47
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APPENDIX
Soil permeability data as obtained from the Laboratory
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53
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