i
PROJECT REPORT
On
“FLOOD CONTROL IN NYABIHU DISTRICT:
CASE STUDY OF NKURI SUB-CATCHMENT”
By
NIYIBIZI Jean Paul
Reg. No: UG10105724
Under guidance of
Eng. Omar MUNYANEZA
(MSc, PhD Fellow)
Submitted to the DEPARTMENT OF Civil Engineering
in the FACULTY OF APPLIED SCIENCES
in partial fulfilment of requirement
for the award of degree of
BACHELOR OF SCIENCE
In Civil Engineering
NATIONAL UNIVERSITY OF RWANDA
FACULTY OF APPLIED SCIENCES
DEPARTMENT OF CIVIL ENGINEERING
October, 2011
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BONAFIDE CERTIFICATE
Certified that this project report titled “FLOOD CONTROL IN NYABIHU DISTRICT:
CASE STUDY OF NKURI SUBCATCHMENT” is the bonafide work of Mr. NIYIBIZI
Jean Paul, who carried out the research under my supervision. Certified further, that to the best
of my knowledge the work reported herein does not form part of any other project report or
dissertation on the basis of which a degree or award was conferred on an earlier occasion on this
or any other candidate.
Signature of the Supervisor
Signature of the H.O.D
Name of the Supervisor
Name of the H.O.D
Eng. Omar MUNYANEZA
Signature of Internal Guide/
Name:
Signature of External Guide/
Name:
Submitted for University examination held in October, 2011 at National University of Rwanda,
University Avenue, Butare, Rwanda.
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DECLALATION
I declare that, the project entitled, “FLOOD CONTROL IN NYABIHU DISTRICT:
CASE STUDY OF NKURI SUBCATCHMENT” is original work and has never been
submitted to any University or other Institutions of Higher Learning. It is my own research
whereby other scholar's writings were cited and references provided. I thus declare this work is
mine and was completed successfully under the supervisor Eng. Omar MUNYANEZA
(MSc, PhD Fellow).
Student signature
Name: NIYIBIZI Jean Paul
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ABSTRACT
The period of heavy rains has particularly been a real threat to flooding in Western province of
Rwanda where people displaced from their homes; and crops and fields were damaged. The
operation of the surface water system for flood control is very important and crucial to
minimizing the impacts of flood during the real-time flood events.
This research aims at analyzing the average annual rainfall and runoff of the flood plain of
NKURI sub-catchment, and computes a water balance of this place to improve flood control to
provide adequate flood risk reduction system and flood control measures that leads to protection
of land and various infrastructures.
To achieve this goal, different analysis was performed. The catchment related data has been
obtained from RS and GIS where a satellite image analysed for extracting useful data such as
sub-catchment delineation. Secondary, slope gradient and land cover/use have been determined.
Finally, peak runoff and proposed flood mitigation are determined to insure an adequate flood
control in Nkuri sub-catchment. The results showed that Nkuri sub-catchment covers an area of
37.3 km2 with the Nkuri river total length of 3.26 km and average slope of 8.3%. The proposed
flood mitigation is to make terraces of uphill mountains within Nkuri sub-catchment, to make
Nkuri river diversion and levee construction on Kinoni river.
This study offers flood causes and consequences within Nkuri sub-catchment, therefore proposes
its flood control system in order to reduce human and economical losses caused by the above
flood.
Keywords: Flood prone area, water balance, rainfall and runoff data, Flood management,
Rwanda.
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DEDICATION
To the Almighty God,
To my parents,
To my brother and sisters,
To my friends and colleagues;
To all who helped to complete this work.
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ACKNOWLEDGEMENT
For a better completion of this work, I have received assistance from various areas. I would like
to give my thanks to all of them briefly.
First of all, I thank the Almighty God for having granted me courage and ability to finish this
work. I thank my parents, my sisters Rose and Yvonne and my brother Aimable for their
everyday support and advices provided to me during my life.
I extend my thanks to NYABIHU District employees, Alphonse MUTABAZI of REMA,
department of disaster of MIDIMAR, for their support and good collaboration which has enabled
me to fulfil this work adequately.
From the deepest of my heart, I would like to thank my supervisor Eng. Omar MUNYANEZA,
MSc, PhD Fellow, for his assistance and bearing over me the success of this work comes from
him.
Further, I would like to give thanks all my friends and colleagues, especially BIZUMUREMYI
Dieudonné and SIBOMANA Jean Baptiste for their kind support and encouragement throughout
my studies.
Thank you all.
vii
TABLES OF CONTENTS
ABSTRACT ................................................................................................................................... ix
DEDICATION…………..............................................................................................................ix
ACKNOWLEDGMENT..............................................................................................................ix
LIST OF TABLES………………………………………………………………………………..x
LIST OF ABBREVIATIONS...................................................................................................... ..x
CHAPTER I. GENERAL INTRODUCTION ................................................................................ 1
1.1 BACKGROUND ...................................................................................................................... 1
1.2. PROBLEM STATEMENT ...................................................................................................... 2
1.3. RESEARCH OBJECTIVE .................................................................................................... 3
1.3.1 Main objective ....................................................................................................................... 3
1.3.2 Specific objective ................................................................................................................... 3
1.4. SCOPE OF THE RESEACH ................................................................................................... 3
1.5 CONTRIBUTION OF THE RESEARCH ................................................................................ 4
1.6 METHODOLOGY ................................................................................................................... 4
1.7 RESEARCH PROJECT STRUCTURE ................................................................................... 5
CHAPTER 2. LITTERATURE REVIEW ...................................................................................... 6
2.1 Introduction ............................................................................................................................... 6
2.2 Catchment delineation .............................................................................................................. 6
2.2.1 GIS technology ...................................................................................................................... 6
2.2.2 Vegetation cover .................................................................................................................... 7
2.2.3 Hydrological soil groups ........................................................................................................ 7
2.2.4 Land use/land cover ............................................................................................................... 8
2.3 Runoff ....................................................................................................................................... 8
This very basic relationship is the basis for most methods used to estimate runoff. ...................... 9
2.3.1 Surface runoff ........................................................................................................................ 9
2.3.2 Subsurface runoff ................................................................................................................... 9
2.3.3 Factors affecting runoff.......................................................................................................... 9
2.4. Methods of flood control ....................................................................................................... 13
2.4.1 Flood Control by levees ....................................................................................................... 14
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2.5.2 Flood control by channel improvement ............................................................................... 14
2.5.3 Soil conservation measures .................................................................................................. 15
2.5 Estimation of rainfall .............................................................................................................. 15
2.5.1 Arithmetic mean method...................................................................................................... 15
2.6 Estimation of peak-runoff into river discharge ....................................................................... 16
2.7 Rational formula method ........................................................................................................ 16
CHAPTER 3. MATERIALS AND METHODS .......................................................................... 18
3.1 Nkuri catchment study area .................................................................................................... 18
3.1.1 Geographical location of Nkuri subcatchment .................................................................... 18
3.1.2 Climate description of Nkuri sub-catchment ....................................................................... 19
3.2 Data collection and processing techniques ............................................................................. 19
3.2.1 Topographical information .................................................................................................. 20
3.2.2 Land cover/land use in Nkuri sub-catchment ...................................................................... 21
3.2.3 Hydrological soil groups in Nkuri subcatchment ................................................................ 22
3.2.4 Determination of runoff coefficient ..................................................................................... 22
3.2.5 Determination of rainfall intensity ....................................................................................... 23
3.2.6 Determination of peak runoff............................................................................................... 24
3.2.7 Flood vulnerability ............................................................................................................... 24
3.2.8 Summary of Natural Disasters in Rwanda from 1974 to 2008 ............................................ 24
3.2.9 Flood hazard identification and mapping ............................................................................ 26
CHAPTER 4. RESULTS AND DISCUSSIONS.......................................................................... 29
4.1. CATCHMENT DELINEATION ........................................................................................... 29
4.2 RESULTS ............................................................................................................................... 31
4.3 RAINFALL INTENSITY AND ESTIMATION .................................................................... 31
4.4 DETERMINATION OF TIME OF CONCENTRATION ..................................................... 32
4.5. PEAK RUNOFF ESTIMATION ........................................................................................... 32
4.6 PROPOSED FLOOD MITIGATION AT NKURI RIVER .................................................... 33
CHAPTER 5. CONCLUSION AND RECOMMENDATIONS ..................................................29
5.1 RESULTS AND DISCUSSIONS ………………………………………….…………….…..32
5.2 RECOMMENDATIONS.........................................................................................................29
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LIST OF TABLES
Table 1: Runoff coefficients for urban watersheds ......................................................................11
Table 2: Table 2: Nkuri land cover description ............................................................................21
Table 3: Summary of methodology for runoff coefficient calculation…………………….…….22
Table 4: Summary of Natural Disasters in Rwanda from 1974 to 2008.......................................24
Table 5: Land cover/use within Nkuri Sub-catchment…………………………………………..30
Table 6: Summary of determination of runoff coefficient.............................................................31
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LIST OF FIGURES
Figure 1.1: Flowchart summarizing processes of this research ..................................................5
Figure 2.1: Triangular irregular networks (TIN) representation..................................................7
Figure 2.2: Mass rainfall curve ................................................................................................12
Figure 2.3: Thiessen polygons......................................................................................................14
Figure 2.4: Levees along a meandering river...............................................................................15
Figure 2.6.3 : Cut-off in a meandering river........................................................................16
Figure 3.1: Localization of Nkuri sub-catchment map ………………………........................18
Figure 3.2.2 : Topographic map ........................................................................................20
Figure 3.3: Nkuri land cover/use.......................................................................................21
Figure 3.3: Meteorological station within Nkuri subcatchment …………………………...........23
Figure 3.4: Number and percentage of people killed............................................................25
Figure 3.5: Total and percentage of people affected ..........................................................25
Figure 3.6: Pictures taken on the site................................................................................27
Figure 4.1: TIN representation of Nkuri catchment..............................................................28
Figure 4.2: Nkuri sub-catchment slope gradient.................................................................29
Figure 4.3: Nkuri subcatchment hydrological soil groups ...................................................31
Figure 4.4 The equation showing the relationship between Kigali city l and Nkuri sub-catchment
rainfall intensity ...........................................................................................................31
Figure 4.5: Nkuri river diversion…………………………………………..............................32
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ABBREVIATIONS
CGIS: Centre for Geographical Information System
DEM: Digital Elevation Model
GIS: Geographical Information System
GPS: Global Positioning System
GWP: Global Water Partnership
HSG: Hydrological Soil Group
MIDIMAR: Ministry of Disaster Management and Refugees
NUR: National University of Rwanda
REMA: Rwanda Environment Management Authority
WMO: World Meteorology Organization
1
CHAPTER I. GENERAL INTRODUCTION
1.1 BACKGROUND
Flooding has caused the most prevalent and costly natural disasters in the world. Floods often
endanger lives and cause financial losses in damage (Kyu-Cheoul Shim, Darrell G. Fontane,
Soon-Bo Shim). Floods also cause impacts to society that go beyond cost and facilities;
including impacts such as family and community disruptions, dislocation, injuries and
unemployment (Mays, 1992). The operation of the surface water system for flood control is very
important and crucial to minimizing the impacts of flood during the real-time flood events.
Historically, flood plains have been a preferred place for human settlement and socio-economic
development because of their proximity to rivers, guaranteeing rich soils, abundant water
supplies and means of transport. They replenish wetlands, recharge groundwater and support
fisheries and agriculture systems thereby supporting livelihoods. At the same time, floods are
also a source of risk when people and their activities are exposed to flooding without factoring
their negative impacts. They can produce severe adverse impacts on the economy and people‘s
safety. Given their beneficial location, people prefer to stay in flood plains even though they are
aware of the flood risks. People in the flood plain have to adapt their life to these conditions.
(WMO/GWP, 2007)
Surface ―drainage‖ problems occur in nearly flat areas, uneven land surfaces with depression or
ridges preventing natural runoff and areas without outlet. Soil with low infiltration rates are
susceptible to surface drainage problems. Surface ‖drainage‖ is intended for safe removal of
excess water from the land surface through land shaping and channel construction. It uses the
head difference that exists due to land elevation to provide necessary hydraulic gradient for the
movement of water (Gosh, 1999).
Even if, it is a country of great lakes and the headwater of two greatest African rivers: The Nile
and Congo, Rwanda is considered as water scarce country (Nahayo, 2007). As Ministry of
Agriculture shows, about 80% of the Rwandan population are farmers and cultivators. However,
they do not produce enough food to satisfy their needs. This is due to inexperienced persons who
must make some improvement on the traditional systems (NIZEYIMANA, 2010).
There should be an advanced flood control system to protect cultivated areas, and provide
effective use of flood waters.
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NYABIHU District is situated in western province of Rwanda, and is the place in the whole
country that faces most consequences of flooding and erosion in the rainy season; there is a need
of a better flood management system. In mid March till April 2010 there has been flood events
that caused population to run away from home. On 12th September 2007 flood occurred in
Bigogwe sector. More than 10 deaths, 562 displaced families in BIGOGWE and 458 in
KANZENZE, thus more than 1000 families were victim of heavy rain (NDAYISABA and
MUNYANEZA, 2009). On 9th March 2011 flood destroyed 38 houses, 67 surrounded by flood
waters, 3 classes of Nkerima Protestant Primary school, 63 hectares of field completely flooded,
etc ( Nyabihu District, 2011).
Flood events have been a threat for the human health and socio-economic development; still they
result from periodic hydrological process which has to be analyzed. The determination of flood
generation requires a perfect hydrological process analysis. Nkuri sub-catchment is surrounded
by hills and RURENGELI cell is a flat and lowlying area suitable for human settlement,
precisely exposed at high risk of flooding.
1.2. PROBLEM STATEMENT
In countries where life of the people is based on the economy of agricultural production,
improvement of flood management resources and water conservation may be one of the most
important solutions that may lead to increase food security, implies poverty reduction and
environmental protection.
Majority of Rwandan population, especially in the rural area, depends on agricultural production
for their livelihoods. However, there are some places in Rwanda which faced with many
problems such as erosion, flooding and poor maintenance of existing irrigation systems. The lack
of competent irrigation support services, lack of detailed knowledge about water management in
fragile valley agro-ecosystems contribute to flooding.
Agricultural Engineers are also facing the problem of knowing the periodical flood occurrences
(water balance studies) in order to help them to select the type of crops to cultivate in selected
land exposed to flood risk to meet their plan of Land consolidation through STA (Agriculture
Transformation Strategies).
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If those problems are not treated well, they can cause the decreasing of crop production and
increase in land degradation in the country. Rwanda government needs to take measures for
avoiding those problems in order to get the sufficient crop production.
There are many diseases that are transmitted in water flooding may cause diseases, such as
bilharziasis and malaria. Urban development can also create obstructions to runoff, such as
sanitary landfills, bridges, inadequate drainage, obstructions of runoff and conduits, and clogging
(Tucci, 2007).
Particularly, on this selected place one may find that the flooding waters are related to ineffective
drainage systems, where water is directly conveyed to households and fields. Huge damages
happen on this place every year.
It is in this regards that we selected this topic as BSc final year research project to answer some
of these problems by giving our contribution to flood management system and mitigation
measures in one of the most flooded place in Rwanda.
1.3. RESEARCH OBJECTIVE
1.3.1 Main objective
The main objective of this research is to establish flood management system in Nyabihu District,
case study of NKURI sub-catchment located in Western Province of Rwanda.
1.3.2 Specific objective
The specific objectives of the study are:
 To delineate the flood prone area in Nkuri sub-catchment;
 Determine the values of runoff coefficient within Nkuri sub-catchment using SCS
method and peak runoff; and
 To provide flood mitigation to reduce flood risk in Nkuri sub-catchment.
1.4. SCOPE OF THE RESEACH
The main objective of this research was achieved by determining the total flood prone area in
NKURI subcatchment. The flood risk reduction system and flood control measures have been
4
provided. There are obstacles that prevent the effectiveness of this study. Among them we can
state insufficient timeframe and budget limitation.
1.5 CONTRIBUTION OF THE RESEARCH
The output of this research is to provide an appropriate flood management system. This should
be a tool that may prevent and minimize losses caused by flooding in NYABIHU District. It
should contribute to the improvement of livelihood of surrounding population, for poverty
reduction and sustainable environmental development.
Methods and techniques used in this research should be used in other similar flood plains in
Rwanda. Shortly, this research should make a crucial contribution allowing early decisions for
flood management in Rwanda.
1.6 METHODOLOGY
In order to achieve the objective of this research, the following methodology is to be taken into
consideration:
- Extensive literature review on flood management
- Catchment delineation was determined using Digital Elevation Model map by extracting useful
data such as slope; sub-catchments identification, soil classes, land use and land cover with
satellite image.
- Rainfall amount of the catchment is developed using Excel, for calculating the average daily
rainfall within catchment and to compute runoff depths in Nkuri sub-catchment.
- Catchment data has been computed using computer software Excel and Geographic
Information System (GIS) software.
- Global Positioning System (GPS) and GIS were used to develop flood mitigation within Nkuri
sub-catchment.
The methodology for the research work consists of three phases: Data collection, data preprocessing and data analysis.
5
The data collection phase constitutes literature review on water balance both using library and
internet, hydrological and meteorological data collection from hydro meteorological stations
installed in Nkuri sub-catchment and satellite images retrieval.
In the pre-processing phase, minor analysis work such as calculation of average rainfall over
Nkuri sub-catchment using arithmetic mean method, flood control and drainage system.
The final analysis mainly focuses on achievement of the research objectives.
Figure 1.1: Flowchart summarizing the processes of this research
1.7 RESEARCH PROJECT STRUCTURE
This research project is containing four chapters:
 The first chapter fosters on the importance of the selected problem and gives a clear
statement of the problem. Moreover, it describes shortly objectives of this research and
gives the way to perform them,
 The second chapter contains reviews, explanations of different expressions used in this
research project,
 The third chapter briefly describes the study area and the methodology in use to achieve
the goal of this research project,
 The fourth chapter presents the obtained results, their analysis and discussion.
Eventually, conclusion and recommendations, after gaining results of this research project and
make a discussion about them, will be summarised before the end of this project presentation.
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CHAPTER 2. LITTERATURE REVIEW
2.1 Introduction
Flooding is caused by the meteorological and hydrological conditions. Our knowledge of longterm weather patterns is very limited owing to the many factors involved in meteorological
phenomena and the interdependence of the physical processes affecting the earth‘s atmosphere.
The hydrological conditions leading to flooding may be natural or artificial. The natural
conditions are those occurring as a result of the watershed in its natural state. These conditions
include: relief, precipitation type, vegetation cover, and drainage capacity (Tucci, 2007).
Chapter one offers general overview of hydrologic parameters to be considered in computations
to determine volumes and peak rates of stormwater runoff as well as the hydraulic calculations
for sizing floods conveyance systems. When analyzing an area for design purposes, urbanization
of the full watershed should be considered. Zoning maps, land use plans, and master plans
should be used as aids in establishing the anticipated surface character of the ultimate
development.
The selection of design runoff coefficients and/or percent impervious cover
factors are explained in the following discussions.
2.2 Catchment delineation
A catchment area (watershed or drainage basin) is a unit of land from which runoff flows to
common outlet (Anyemedu, 2008). Accurate delineation of a catchment/watershed plays an
extremely important role in the management of the watershed (G.Savant,2002).
The main delineated catchment characteristics are land use, land change, soil types, geology and
river flows are analyzed from which appropriate conclusions are drawn.
2.2.1 GIS technology
A Geographic Information System (GIS) is a computer-based system for capturing and
processing spatially distributed data of geographic nature (Mohan, 1991).
It is a computer-assisted system for the acquisition, storage, analysis and display of geographic
data( topography, climatology, land-use, soil type, etc) (Eastman, 1997). The core of the system
is the database, a collection of maps and associated information in digital format. Since the
database is concerned with earth‘s surface features, two elements can be distinguished i.e the
coordinates and the attribute of each point. GIS is a perfect tool for management of spatially
7
distributed data and the need to solve complex natural resources problems through modelling
techniques. TIN (Triangular Irregular Networks) is the modelling made in a three dimension
analysis. TINs are a form of vector based digital geographic data and are constructed by
triangulating a set of vertices (points). The vertices are connected with a series of edges to form a
network of triangles (ArcGIS 9 help).
Figure 2.1: Triangular irregular networks (TIN) representation
2.2.2 Vegetation cover
Vegetation cover has the effect of intercepting part of the precipitation that can generate runoff
and protecting the soil against erosion. Loss of that cover through farming use has led to more
frequent flooding owing to precipitation not being intercepted and clogging of the rivers(C.E.M
Tucci, 2007).
2.2.3 Hydrological soil groups
(-HSG-) reflects the infiltration rate of the soil, the permeability of any restrictive layer(s), and
the moisture-holding capacity of the soil profile to a depth of 60 inches. The infiltration rate of
the soil affects runoff. Generally, the higher the rate of infiltration, the lower the quantity of
stormwater runoff. Fine textured soils such as clay produce a greater rate of runoff than coarsegrained soils such as sand. The hydrologic soils groups are defined as follows (Source: NEH-4)
Group A: (Low runoff potential): Soils having a low runoff potential and high infiltration rates
even when thoroughly wetted and consisting chiefly of deep, well to excessively drained sands
or gravels and having a high rate of water transmission.
Group B: Soils having moderate infiltration rates when thoroughly wetted and consisting chiefly
of moderately deep to deep, moderately well to well drained soils with moderately fine to
moderately coarse textures. These soils have a moderate rate of water transmission.
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Group C: Soils having slow infiltration rates when thoroughly wetted and consisting chiefly of
soils with a layer that impedes downward movement of water, or soils with moderately fine to
fine textures. These soils have a slow rate of water transmission.
Group D: (High runoff potential): Soils having very slow infiltration rates when thoroughly
wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent
high water table, soils with a clay pan or clay layer at or near the surface, and shallow soils over
nearly impervious material. These soils have a very slow rate of water transmission.
The best data for identifying the Soil Hydrologic Group are the field-derived sub-soil parameters
of texture, structure and colour. The Antecedent Moisture Condition is an index of basin
wetness. The rainfall-runoff transformation is a non linear process. The most important cause of
non linearity is represented by the effect of antecedent conditions; consequently the runoff
coefficient depends on the initial conditions (Musoni, 2009).
2.2.4 Land use/land cover
―Land Use‖ refers to human activities that take place on the earth‘s surface (how the land is
being used) such as residential housing or agricultural cropping.
―Land Cover‖ refers to natural or man-made physical properties of the land surface (what the
land is covered with) such as coniferous forest or impervious surfaces. Both systems utilize
multiple and varying classification schema with distinct overlaps and important differences.
2.3 Runoff
When rain falls on the earth‘s surface, some of that rain is intercepted by the surfaces of
vegetation located in its path (interception). Depending on soil characteristics and amount of
rainfall, some or all of the remaining rainfall will enter the ground through pores in the surface
soils (infiltration).
As the remaining water, if any, flows overland, irregularities in the surface of the land trap some
of this water as depression storage. The portion of this overland flow that reaches the watershed
outlet is called direct runoff, or stormwater runoff.
This relationship can be expressed as a storm event water balance, by the following equation:
Runoff = Precipitation - Interception - Infiltration - Depression Storage
(2.1)
9
This very basic relationship is the basis for most methods used to estimate runoff.
2.3.1 Surface runoff
Surface runoff or overland runoff is that part of runoff that travels over the surface of the ground
to reach a stream channel and through the channel to the basin outlet. Surface runoff includes the
precipitation directly falling over the channel reach (Anyemedu, 2008). Storm water runoff can
lead to flooding and impacts in urban areas by means of two processes, separately or in
combination.
Flooding of riverside areas: natural flooding that occurs in the flood plains of rivers owing to
temporal and spatial variations in precipitation and runoff in the catchment area;
Flooding due to urbanization: flooding from the urban drainage system due to the effect of soil
impermeabilization, canalization or obstruction of water flow (Tucci, 2007).
2.3.2 Subsurface runoff
Subsurface runoff is that part of the runoff that travels under the ground to reach a stream
channel and to the basin outlet. It consists of two parts: one part moves laterally through the
upper soil horizons within the unsaturated zone or through the shallow perched saturated zone
towards the stream channel. This is known as the subsurface flow, interflow or through flow.
Another part infiltrates deeper to the saturated zone to form the groundwater flow. This flow
discharges into the stream channel as the base flow (Anyemedu, 2008). In case of lack of
adequate drainage, subsurface runoff can be a source of floods.
2.3.3 Factors affecting runoff
2.3.3.1 Soil type
Soil is a complex, multi-phase, heterogeneous system of various gases, liquids and solids and as
a result, the concept of soil can be interpreted in many different ways. In surface water hydrology
analysis, the most important soil properties are the amount and rate of passage of infiltrated
rainfall through the surface layers of soil. In particular, the soil storage capacity requires a
detailed look at the relationship of water content to the soil void space.
As it was said by Baird et al. (1997) the runoff generation in the area is also associated with the
porosity of a soil such as the peat soil layering as the deeper layers may be an important overall
contributor to runoff. The highest infiltration capacities are observed in loose, sandy soils while
10
heavy clay or loamy soils have considerable smaller infiltration capacities. For example, a
change in land use by surface vegetation increases the infiltration capacity and reduces surface
runoff (RAGHUNATH, 2006).
2.3.3.2 Runoff Coefficient
The runoff coefficient is a function of the ground cover and a host of other hydrologic
abstractions. It relates the estimated peak discharge to a theoretical maximum.
If the catchment is occupied by varying amounts of different land cover due to land use, a
composite coefficient may be used. It is estimated as the ratio between quick flow and rainfall
volume. This coefficient varies with:
 Topography,
 Land use,
 Vegetal cover,
 Soil type,
 And moisture content of the soil.
C
RunoffVolume
Ra inf allVolume
R
P
(2.2)
Table I: Runoff coefficients for urban watersheds (Source: Hagemana, 2006)
Runoff coefficient for urban watersheds
Type of drainage area
Range
of
Runoff coefficient for urban watersheds
runoff Type of drainage area
coefficient
Business:
Range
coefficient
Residential:
-Downtown areas
0.70-0.95
-Single-family areas
0.30-0.50
-Neighbourhood areas
0.30-0.70
-Multi-units, detached
0.40-0.60
-Multi-units, attached
0.60-0.75
-Suburban
0.35-0.40
Industrial:
-light areas
0.30-0.80
of
runoff
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-Heavy areas
0.60-0.90
-Apartment dwelling 0.30-0.70
area
-Parks, cemeteries
0.10-0.25
Lawns:
-Playgrounds
0.30-0.40
-Sandy soil, flat 2%
-Railroad yards
0.30-0.40
-Sandy soil, average 0.10-0.15
0.05-0.10
2-7%
Unimproved areas:
-Sandy soil, steep 7%
0.15-0.20
-Sand or sandy loam 0.15-0.20
-Heavy soil, flat 2%
0.13-0.17
soil, 0-3%
-Sand or sandy loam 0.20-0.25
-Heavy soil, average 0.18-0.22
soil, 3-5%
2-7%
-Black soil, 0-3%
0.18-0.25
-Heavy soil, steep 7%
0.25-0.35
-Black soil, 3-5%
0.25-0.30
Streets
-Black soil, >5%
0.70-0.80
-Asphaltic
0.85-0.95
-Deep sand area
0.05-0.15
-Concrete
0.90-0.95
-Steep grassed slopes
0.70
-Brick
0.70-0.85
-Pastures
0.10-0.60
-Drives and walks
0.75-0.95
-Arable land
0.30-0.80
-Roofs
0.75-0.95
2.3.3.3 Rainfall Intensity
A rainfall is a part of precipitation on liquid state; forming drops over 0.5mm to about 6mm
diameter. Rain is considered light when rate of fall is less than 2.5mm/hr; moderate 2.5 to
7.5mm/hr and heavy when rate is over 7.5mm/hr (Anyemedu, 2008). The Rainfall measurement
is made with an instrument called rain gauge (pluviometer) and the aim of measurement is to
intercept precipitation over a known carefully defined area bounded by the rain gauge rim.
12
Rainfall is measured in millimetres forming the vertical depth of water accumulated on level
surface during an interval of time, if all the rainfall remained where it fell.
One of most convenient method used to estimate the intensity and sequence of rainfall during
past storms at several rain gauge stations within a catchment is ―mass rainfall curves‖. A mass
rainfall curve is a plot of accumulated total rainfall up to known time intervals against this time
interval (Fig. 3)(GOSH, 1999).
Figure 2.2 : Mass rainfall curve
Generally, there may be several raingauge stations within a catchment. Having obtained directly
and by interpolation, the mass rainfall of all raingauge stations available in a catchment, the next
step is to determine the influence area of each, that is to say the area over which the mass rainfall
curve obtained at a particular station may considered to be applicable. A very good method of
doing so is to divide up the area into what are known as Thiessen polygons (Fig. 4). Each
polygon represents the influence area of the station which is located within that polygon(GOSH,
1999).
Figure 2.3 : Thiessen polygons
Bellow is a represented an expression of rainfall intensity. It is expressed in depth per unit time,
usually mm per hour.
13
(2.3)
Where ∆h: depth of precipitation (mm)
∆t: rainfall duration (h)
2.4. Methods of flood control
A flood is an unusual high stage of a river overflowing its banks and inundating the marginal
lands.
This is due to severe storm of unusual meteorological combination, sometimes combined with
melting of accumulated snow on the catchment. This may also be due to shifting of the course of
the river, earthquake causing bank erosion, or blocking of river, or breaching of the river flood
banks.
The damages due to the devastating floods can be minimized by the following flood control
measures, singly or in combination.
(i) by confining the flow between high banks by constructing levees (flood banks) , dykes, or
flood walls.
(ii) by channel improvement by cutting, straightening or deepening and following river training
works.
(iii) by diversion of a portion of the flood through bypasses or flood ways. In some cases a fuse
plug levee is provided. It is a low section of levee, which when once over topped, will wash out
rapidly and develop full discharge capacity into the flood-way. In other locations, a concrete sill,
weir or spillway controlled by stop logs or needles may be provided so that the overflow occurs
at a definite river stage. Sometimes dynamiting a section of levee is resorted to bypass the flood.
(iv) by providing a temporary storage of the peak floods by constructing upstream reservoirs and
retarding basins (detention basins).
(v) by adopting soil conservation measures (land management) in the catchment area.
(vi) by temporary and permanent evacuation of the flood plain, and flood plain zoning
by enacting legislation.
(vii) by flood proofing of specific properties by constructing a ring levee or flood wall around the
property.
(viii) by setting up flood forecasting—short term, long term, rhythm signals and radar, and
warning centres at vulnerable areas.(RAGHUNATH,2006)
14
2.4.1 Flood Control by levees
The design and construction of levees are similar to those of an earth dam. The levees are
constructed beyond the meander belt of a river, and they tame a river not to change its course. As
far as possible, there should be very few curves in their alignment. They require constant watch
and after the floods recede, repairs and restoration of levees should be resorted too.
A height is assumed and the discharge through the proper channel is computed for the assumed
high water flow, which is the level of the top of bank less the free board. This flow subtracted
from the estimated probable maximum flood discharge gives the discharge to be passed over the
flood ways between the proper channel and the levees. Area of the flood ways is then obtained
by dividing it by the velocity of flow.
The spacing of levees thus obtained, should give a minimum value for the cost of levees and the
value of the submerged land in the flood way (RAGHUNATH, 2006).
The effects of levees on flood flow are:
(i) increase in the rate of flood flow
(ii) increase in the flood water elevation
(iii) increase in the carrying capacity of the channel
(iv) increase in the scouring action
(v) decrease of surface slope of stream above the leveed section
Figure 2.4 : Levees along a meandering river
Figure 2.5 : Ring levees to protect the city
2.5.2 Flood control by channel improvement
Channel improvement increases the discharging capacity of the stream thereby decreasing
the height and duration of the flood. Flood carrying capacity can be increased either by
increasing the cross-sectional area or by increasing the velocity along the river.
15
Deepening is preferred to widening since the hydraulic mean radius increases more with depth
(for the same increase in the sectional area) thus increasing the velocity. The channel velocity
(given by Manning‘s or Chezy‘s formulae) is affected by hydraulic mean radius, slope of river
bed and roughness of the bed and sides.
Roughness can be reduced by
(i) removing sand bars.
(ii) prevention of cropping on river beds near banks.
(iii) removal of fallen trees and other snags.
(iv) elimination of sharp bends of meanders by providing cutoffs (Fig. 7).
In a stream, deepening results in the loss of slope as its outlet can not usually be lowered
(H.M.RAGHUNATH, 2006).
Figure 2.6.3 : Cut-off in a meandering river
2.5.3 Soil conservation measures
The best way to prevent silt deposition is to arrest silt at the place of its origin, i.e., by
undertaking soil conservation measures in the catchment area. Soil conservation measures for the
entire catchment like contour bunds, check dams, terraces, gully plugging, vegetative cover (strip
croping), afforestation, land management, stream bank protection, etc. are very necessary to
retard the velocity of runoff, to control soil erosion, to absorb more water in the soil and to
protect the dams and reservoirs from being silted up (RAGHUNATH,2006).
2.5 Estimation of rainfall
2.5.1 Arithmetic mean method
It is a method that consists of computing the arithmetic average of n values of the precipitation
for all stations within the area irrespective of the area covered by each rain gauge. It provides
16
results of reasonable range in case when the rain gauges are evenly spaced over the area and the
individual gauge value does not vary much from the mean value.
(2.4)
Where: P= Precipitation in mm
n= total number of rain gauges
2.6 Estimation of peak-runoff into river discharge
The discharge (flow rate of a river/stream) is a volume of water that passes a section of the river
in a unit of time (Encarta, 2009). Despite different methods of measurement in situ that are used
to find a hydrograph curve and analyze it to extract baseflow and runoff contribution from
stream discharge, other theoretical method has been detailed for peak runoff computation into
river discharge.(Ufiteyezu, 2010).
2.7 Rational formula method
An important formula for determining the peak runoff rate is the Rational formula. The Irish
engineer, Mulvaney (1850), was probably the first to publish the principles on which the method
is based on. Valid criticisms have been raised about the adequacy of this formula; however it
continues to be used because of its simplicity (Munyaneza, 1010). The Rational Formula reads:
(2.5)
Where:
Qp = Peak runoff rate [m³/sec], C = Runoff coefficient [-], I = Rainfall intensity [mm/hr],
A = Drainage area [km²]
The Rational Formula follows these assumptions:
 Consideration of the entire drainage area as a single unit,
 Estimation of flow at the most downstream point only,
 Rainfall is uniformly distributed over the drainage area.
17
 The predicted peak discharge has the same probability of occurrence (return period) as
the used rainfall intensity (I),
 The runoff coefficient (C) is constant during the rain storm, and
 The recession time is equal to the time of rise.
The maximum runoff rate in a catchment is reached when all parts of the watershed are
contributing to the outflow. This happens when the time of concentration, the time after which
the runoff rate equals the excess rainfall rate, is reached. In addition, the Kirpich/Ramser formula
is mostly used to calculate the time of concentration (Dawod and Koshak, 2011):
(2.6)
Where: tc = Time of concentration [min], L = Length of main river [m], S = Slope of main stream
[m/m]
18
CHAPTER 3. MATERIALS AND METHODS
3.1 Nkuri catchment study area
3.1.1 Geographical location of Nkuri subcatchment
Figure 3.1: Localization of Nkuri sub-catchment map (Source: CGIS-NUR)
Globally, this study has been focusing on Nkuri catchment located in Western Province of
Rwanda in Nyabihu district. It is a district which has recently faced various disasters. Rwanda is
a small and mountainous country in Central Africa, bordered by Burundi in the South,
Democratic Republic of Congo in the West, Uganda in the North and Tanzania in the East. Its
total area is about 26 338 km2 out of which about 24,948 km2 is land and 1,390 km2 is covered
19
by water (5.3%). In 2007 the population of Rwanda was estimated to be 9.3 million (NISR.,
2008). This gives an estimated population density of about 342 persons/km2 which is the highest
in Africa (NELSAP, 2006).
Nkuri catchment is located in southern part of Rwanda, its area is around 38.413039 km2 and its
coordinates ranging from 1o23‘ to 1o33‘ latitude South and from 29o43‘ to 29o51‘ longitude East.
3.1.2 Climate description of Nkuri sub-catchment
Nkuri catchment is located in Rwanda which has a moderate climate with relatively high rainfall;
an annual average temperature of 15oC and an annual cycle of four seasons:
A short rainy season, locally known as ―Umuhindo‖ runs from September to November, with
November characterized by heavy precipitation;
A short dry season, locally known as ―Urugaryi‖ runs from December to February;
A long rainy season, locally known as ―Itumba‖ runs from March to May, this bringing about 14
to 61% of the total annual precipitation; and
A long dry season, locally known as ―Icyi‖ which runs from June to August.
The mean annual rainfall in Rwanda is about 1100 mm and varies from 700 mm in the SouthWest to about 1600 mm/year in the North-West.
In Nkuri catchment, the mean annual rainfall and temperature are approximately 1231 mm/a and
15oC, respectively. The annual average of the relative soil moisture, calculated over the 30 years.
3.2 Data collection and processing techniques
Data collection has been made in three ways, using Global Positioning System (GPS),
meteorological precipitation data from Rwanda Meteorology and various data from NUR_GIS.
Today, the Rwanda Meteorological Service doesn't have enough capacity to deliver sufficient
data, information and advisories due to the lack of inadequate observing station network and
sufficient data processing equipment. Moreover, the role of climate information and products
into disaster risk reduction is not very well established in Rwanda. Still, there is a problem of
acquiring complete historical meteorological data.
20
3.2.1 Topographical information
The topography of Nkuri sub-catchment area represents the configuration of its physical surface,
including natural and human-made features. It is an important element of this catchment study; it
has been described in terms of slope, aspect, and elevation. These three characteristics affect the
movement, storage and quality of water in Nkuri catchment.
Figure 3.2 : Topographic map (Source: CGIS_NUR)
In this study, Nkuri catchment delineation has been performed by use of GIS tools and DEM
map collected from CGIS_NUR. To have an idea of 3D topographic configuration, TIN
representation has been designed from contour lines map.
21
3.2.2 Land cover/land use in Nkuri sub-catchment
Land cover and land use are sometimes defined as the same. Generally, land cover is presented
as vegetation and water body but when these covers are used by the human being in their
activities; they are now considered as land use, for instance: agriculture, forest and urban.
Vegetation contributes to a healthy catchment by intercepting runoff, allowing some or all of the
runoff to filter gradually into soils or return to atmosphere.
The Nkuri land cover map used in this study was extracted from Rwanda land cover map.
Moreover, it has been georeferenced and digitalized to manipulate it easily by use of GIS
technology. In Nkuri catchment, land cover contains different cover types which may be grouped
into:

 Agriculture: small grain, good practice
 Forest: woodland, mature, good
 Building: urban areas
The figure 3.3 shows Nkuri sub-catchment land cover/use summarized in Table 2.
Figure 3.3: Nkuri sub-catchment land cover/use
22
Table 2: Nkuri land cover description (ArcGIS, 2011)
Land cover
Land cover name
ID
Land use/
Land cover
AG-5
Rain fed Herbaceous Crop-Two crop year
AG-8
Tea plantation
AG-5/9
Combination of Rain fed Herbaceous Crop-Two crop per year and Small
Shrub Plantation
AG-4C
Isolated Rainfed Herbaceous Crop
AG-9/5
Combination of Shrub Plantation and Rain fed Herbaceous Crop-
Agriculture:
grain,
good
practice
Two crop per year
gfhgkjhkh
AG-9/5/6
Combination of shrub plantation and Rain fed herbaceous CropTwo crop per year, remaining forest plantation (Eucalyptus Forest:
or Pinus and Cypress)
AG-5/6
Forest Plantation - (Eucalyptus or Pinus and Cypress)
Woodland,
mature,
good
3.2.3 Hydrological soil groups in Nkuri subcatchment
The type of soil in a catchment plays an important role in the catchment‘s hydrology. The
composition and texture of the soil determines whether rainfall or irrigation water will be
retained in the soil and released gradually, percolating downward to the groundwater, or if these
inputs of water instead contribute to surface runoff, leading to increased erosion.
The hydrological soil group (HSG) map of the study area was extracted from a map called ‗Soil
texture classes of the dominant soil units in Rwanda (Verdoodt A, et al., 2003) by use of GIS
technology. The results are shown in the figure 16.
3.2.4 Determination of runoff coefficient
The runoff coefficient is defined as either the ratio of total depth of rainfall, or as the ratio of
peak rate of runoff to rainfall intensity for the time of concentration (Wanielista and Yousef.,
1993).
In this study, a runoff coefficient of Nkuri subcatchment have been estimated after making an
overlay of land cover map and hydrological soil group map by use of ArcMap processing tools.
23
An average of runoff coefficient have been obtained by making a summation of products of
partial areas (Ai) and partial rainfall coefficients (Ci) under total area. The summary of these
processes has been represented in an appropriate Table 2.
Table 3: Summary of methodology for runoff coefficient calculation
Land cover
Hydrological
soil Area (Ai)
group
Runoff coefficient (Ai)× (Ci)
(Ci)
3.2.5 Determination of rainfall intensity
In purpose of gathering meteorological data on Nkuri subcatchment, Rwankeri station records
meteorological data (rainfall, temperature, relative humidity, soil moisture, wind speed and
direction) in Nkuri subcatchment.
Average of rainfall intensity on Nkuri catchment has been determined after calculation of
average rainfall collected from Rwankeri meteorological station by use of arithmetic mean
method and divide it with duration of that rainfall on the whole catchment. In this research, the
rainfall duration was considered as equal to the time of concentration, formula (2.6).
Figure 3.4: Meteorological station within Nkuri subcatchment.
24
3.2.6 Determination of peak runoff
After getting rainfall intensity on Nkuri subcatchment, information on Nkuri subcatchment
describing topographical configuration and runoff coefficient from land cover and HSG; peak
surface runoff is calculated by use of Rational formula (2.5).
3.2.7 Flood vulnerability
From December 2010-August12,2011, floods have taken life of 14 people, damaged 3 houses
and demolished 106 houses and a land area of 162.5 ha in this district (MIDIMAR,2011).
In 1974 floods affected 1,900,000 people in Rwanda 1,900,000. It is the most disastrous flood in
Rwanda during past 30years (MUTABAZI, 2008). From 1974-2008 floods happened in six
events where 34,516 people were affected and 111 people died.
3.2.8 Summary of Natural Disasters in Rwanda from 1974 to 2008
The total number of disasters (Droughts, earthquake, epidemic, flood, landslides),
registered since 1974 is 29, they are shown in the table 4. These disasters killed 823
people and affected 6,124,424 people;
Epidemics are more frequent with 41% followed by floods: 28%, Drought: 21% and
earthquake: 7%.
Epidemics kill more people: 39% followed by drought: 29%, floods: 19% and
Earthquake 10%;
Climate related disasters are almost responsible to affect more people with 68% by
Drought and 32% by floods.
The following table shows clearly the natural distasters situation within Rwanda. It shows that
floods occupy an important role in disasters that Rwanda has faced throughout 34 years.
25
Table 4: Summary of Natural Disasters in Rwanda from 1974 to 2008
People killed
Number
Disaster
Category
of Events
People affected
Av. per
Av.
Total
event
Total
per
event
Drought
Drought
6
237
39.5
4,156,545 692,757.5
Earthquake
Earthquake
2
81
40.5
2,286
1,143
Enteric
7
126
18
6,037
862.4
Meningitis
5
196
39.2
1,362
272.4
Unspecified
6
111
18.5
34,516
5,752.7
Flood
2
48
24
1,921,678 960,839
Landslide
1
24
24
2,000
29
823
Diarrhoeal/
Epidemic
Flood
Slides
Total
2,000
6,124,424
24; 3%
159; 19%
237; 29%
Drought
Earthquake
Epidemic (Diarrhoeal, Enteric and
Meningitis)
Flood including unspecified
Slides
81; 10%
322; 39%
Figure 3.5: Number and percentage of people killed
26
Data source: http://www.emdat.be/Database/CountryProfile/countryprofile.php
2000; 0%
1956194; 32%
Drought
Earthquake
Epidemic (Diarrhoeal, Enteric and
Meningitis)
Flood including unspecified
7399; 0%
2286; 0%
Slides
4156545; 68%
Figure 3.6: Total and percentage of people affected
Data source: http://www.emdat.be/Database/CountryProfile/countryprofile.php
3.2.9 Flood hazard identification and mapping
Nkuri subcatchment flood zones have been identified within Nkuri subcatchment using GPS
(Global Positioning System), a satellite receiving system which permits to identify position of
any point on the earth surface. The GPS used for this study is GPS Map30x model. After getting
these points, they have been used to locate the flood zones. The flood map shows zones in which
flood waters were still present from April 2011 till August 2011.
3.2.9.1 Flood hazard and explosure analysis:
The purpose of flood hazard and explosure analysis is to describe the process for identification of
flood-prone areas and to identify the elements that are exposed to flooding.
Nkuri settlement is situated in a place where the major national road goes through this
settlement. After passing through the culvert of this road this road, water is directly flushed in
people‘s homes, water coming from the Nkuri river stagnate in various places and create ponds
(Fig. 3.7). Another part of the Nkuri river passes through temporary canals made by local people.
These canals do not offer appropriate drainage. Water destroys various structures and fields.
27
Water enters in houses and people are obliged to flee their homes. During this year 2011 from
January till August, 89 houses have been affected by recent floods, 10 houses destroyed and
2people dead. In order to fight against flooding damages, local people have tried to take local
measures among them we can say:
-Creating river drainage -Using sandbags for flood protection. These measures do not offer
adequate flood control as can be seen in the Figure 3.7.
1. Nkuri river canal from road culvert.
2. Sandbag usage for routing river water.
3. Sandbag usage for routing flood waters.
4. Nkuri river waters destroying road.
28
5. Waters in buildings and population fields
Figure 3.7: Pictures taken on the site in September 2011 by Niyibizi Jean Paul.
The exposed buildings are 78 in 260 houses. This proportion represents 29.23% of all buildings
in the study area. This proportion is a basic tool for the future flood mitigation measures
especially when relocating the concerned population by the Government of Rwanda. However it
is not easy to expect that relocation will be economically reasonable regarding compensation,
expropriation and infrastructure provision for resettlement. The alternative available is relocating
only those buildings that are in floods zones, or making diversion of Nkuri river.
29
CHAPTER 4. RESULTS AND DISCUSSIONS
4.1. CATCHMENT DELINEATION
The purpose of TIN in this study is to provide basic tool from which topographic data should be
extracted. TIN map for 3D configuration (Fig 14) and slope gradient map of Nkuri Subcatchment (Fig 15).
Figure 4.1: TIN representation of Nkuri catchment
The folowing figure illustrates the Slope gradient in Nkuri Sub-catchment.
30
Figure 4.2: Nkuri sub-catchment slope gradient (in degrees)
From GIS technology offers topographic conditions of this site as mountainous with elevation
ranging from 2199 at its junction with KINONI river 2278 m at its source.The longitudinal
slopes of the valleys varies from 6 to 13% upstream and from 1 to 18% downstream (average
slope is between 4 and 6%). The slope of main stream from a profile in the figure 8 is around
8.3% and the total area which is around 37.28km2 has been calculated by use of Arc map
geometry analysis of Nkuri polygon boundary. The main stream length is around 3256.79m.
The value of Ci was obtained by use of Tables 2 and 3.
31
4.2 Hydrological soil groups of Nkuri sub-catchment
Figure 4.3: Nkuri sub-catchment hydrological soil groups
4.3 RAINFALL INTENSITY AND ESTIMATION
The rainfall intensity is 94, estimated in litres per second per hectare, It has been found according
to the equation described in the following figure, where the rainfall of Nkuri sub-catchment is
estimated according to rainfall intensity of Kigali city (72).
Figure 4.4 The equation showing the relationship between Kigali city and Nkuri sub-catchment
rainfall intensity.
32
4.4 DETERMINATION OF TIME OF CONCENTRATION
The Kirpich/Ramser formula (2.3) has been used to calculate the time of concentration:
With L=3.257 km and S=12.3%
tc=0.0195*3256.791610.77*0.083-0.385=25.76527265minutes
The following table shows the determination of runoff coefficient.
Table 6: Summary of determination of runoff coefficient
Land cover
Agriculture:
Small grain,
good practice
Forest:
Woodland,
mature, good
Buildings
∑
Hydrological Area (Ai)
Runoff
soil group
coefficient (Ci)
km2
(Ai)× (Ci)
A
2.931626823
0.21x1.28=0.2688 0.78802129
Ac
8.37083175
0.21x1.3=0.273
2.285237068
C
6.808089
0.21x1.7=0.357
2.430487773
E
3.84990615
0.21x1.16=0.2436 0.937837138
A
5.863253646
0.1x1.25=0.127
Ac
2.79027725
0.1x1.7=0.17
D
1.274027
1
37.283645
0.744633213
0.474347132
1.274027
8.934590614
4.5. PEAK RUNOFF ESTIMATION
Peak runoff of the catchment calculated using the equation (2.5) is the following:
Qp=0.28*0.23968335*94*37.28=235.49 l.s ha
33
4.6 PROPOSED FLOOD MITIGATION AT NKURI RIVER
Diversion point
Figure 4.5: Nkuri river diversion
4.7 DISCUSSION
There should be a water diversion for two main purpose: The first one is that water can pass to
an alternate path by gravity which can reduce damages caused by the present Nkuri river flow
passage. Secondary by meeting the high pick runoff found at Nkuri river 6.32 l/s.
The delienation of Nkuri subcatchment has been determined where the subcatchment area of has
been calculated using ArcGIS. The proposed mitigation is Nkuri river diversion so that water
flow by gravity safely to people. The second is making levee along Kinoni river so that water
should not cause flooding into the population fields. The third is establishing terraces on the up
hills in order to reduce surface runoff which highly contribute to flood generation in the
Rurengeli cell. The calculated peak runoff should be a tool for hydraulic stucture design.
34
CHAPTER 5 CONCLUSION AND RECOMMENDATIONS
5.1. CONCLUSION
The objective of this study has been achieved; because the study shows clearly the flood
delieation where the floods surfaces are calculated using ArcGIS. The delienation of Nkuri
subcatchment has been determined where the subcatchment area of has been calculated using
ArcGIS. The Nkuri Sub-catchment area is 37.28 km2 with the Nkuri river total length of
3256.79161m and average slope of 8.3%. The calculated peak runoff of 6.32 l/s is a value on
which hydraulic structures can be design from. It is a great value this shows how much rainfall
waters are important on the catchment surface. This is the major cause of erosion and floods
within this catchment.
The flood are causes by poor drainage system which conveys water to fields and home of the
population, river diversion and levee construction have been proposed to meet an adequate flood
control within Nkuri Sub-catchment.
The proposed flood mitigation also consists of making terraces of uphill mountains within Nkuri
sub-catchment, in order to reduce runoff.
5.2. RECOMMENDATIONS
Further research in flood management should be promoted. Rwanda as a small country with a
high rate of population growth is mostly facing a problem of the lack of land. Basic studies on
hydrological waters in flood plain should be promoted.
There is a gap for the hydrological data, especially historical data. There should be new measures
for perfect hydrological data banking.
35
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38
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