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UNIVERSITY OF NAIROBI
SCHOOL OF ENGINEERING
DEPARTMENT OF ENVIRONMENTAL AND BIOSYSTEMS ENGINEERING
PROJECT TITLE: DESIGN OF AN EARTH-DAM IN SIAYA
CANDIDATE NAME: NICHOLAS WAFULA OWADE
CANDIDATE No.: F21/1714/2010
SUPERVISOR’S NAME: MR. ORODI ODHIAMBO
A Report Submitted in Partial Fulfillment for the Requirements of
the Degree of Bachelor of Science in Environmental and Biosystems
Engineering, of the University Of Nairobi
MAY, 2015
FEB 540: ENGINEERING DESIGN PROJECT
2014/2015 ACADEMIC YEAR
F21/1714/2010 Page i
DECLARATION
I declare that this project is my work and has not been submitted for the conferment of a degree
in any other University.
Date: Sign:
OWADE W. NICHOLAS
This project has been submitted to me for examination with approval as a university supervisor.
Date: Sign:
DR. ORODI ODHIAMBO
F21/1714/2010 Page ii
DEDICATION
This project is dedicated to my parents Charles Owade and mother Bernadette Murrey, brothers
and sisters who have been an encouragement to me throughout my stay at the University of
Nairobi.
F21/1714/2010 Page iii
ACKNOWLEDGEMENT
I wish to express my sincere gratitude to my project supervisor Dr. Orodi Odhiambo for his
invaluable help and guidance throughout the project design period.
I would also wish to appreciate Tingare Community Development Group, Kenya Meteorological
Department, Kenya National Bureau of Statistics and the Kenya Agricultural Research institute
for their assistance in data acquisition for the project.
My sincere gratitude goes to my classmates and the entire Department of Environmental and
Bio-systems engineering fraternity for their support, encouragement and contribution towards the
project design.
I Am greatly indebted to my family for their constant encouragement and best wishes.
F21/1714/2010 Page iv
ABSTRACT
Due to non-uniform distribution of rainfall throughout the year characterised by heavy downpour
in a short period followed long dry periods, it has necessitated the design of artificial structures
to store water during heavy downpour to control floods which the same water will be used for
irrigation during dry dry periods.
The overall objective of this project is to design an off-stream earth dam of sufficient capacity
which will be used to supply adequate water for irrigation of 5Ha of Soya beans.
The specific objectives of the design project was to: review geophysical condition of the site
which included obtaining geotechnical, topographical and hydrological data, estimate the
reservoir capacity, design the dam, find the crop water requirement of Soya beans and to
determine the cost benefit ratio.
Climatic data such as mean monthly rainfall were obtained also the crop evapotranspiration rates.
The type of crop to be irrigated and the size of land in acreage were determined in order to find
the volume of water to be used for irrigation.
Based on the information obtained, the dam’s structural specifications such as height of
embankent, slopes, crest width spilling capacity and volume of earthworks were determined. The
system designed was big enough to supply the necessary water for irrigation.
Structurally, the designed dam embankment has a height of 4.5m with a crest width of 4m. The
reservoir capacity is 5500m3 and the volume of the earthworks likely to be used for construction
is 350.83m3.
The BoQ was determined and found to be KShs. 5130490. The Cost benefit analysis was also
done and the Cost-Benefit ratio computed and found to be 2.19. This value is far much less than
1 hence indicates that the enterprise will be profitable if it will be adopted.
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LIST OF ABBREVIATIONS
BM- Bench Mark
LM- Lower Medium
Ha- Hectares
L- Litres
M- Metres
KM- Kilometres
MOWI- Ministry of Water and Irrigation
FAO- Food and Agriculture Organization
F21/1714/2010 Page vi
Table of Contents DECLARATION.......................................................................................................................................... i
DEDICATION............................................................................................................................................. ii
ACKNOWLEDGEMENT ......................................................................................................................... iii
ABSTRACT ................................................................................................................................................ iv
LIST OF ABBREVIATIONS .................................................................................................................... v
LIST OF FIGURES ................................................................................................................................... ix
LIST OF TABLES ...................................................................................................................................... x
1.0 INTRODUCTION ................................................................................................................................. 1
1.1 Background ....................................................................................................................................... 1
1.2 Problem statement and problem analysis ....................................................................................... 1
1.3 Site analysis and inventory ............................................................................................................... 1
1.3.1 Site selection of reservoir ............................................................................................................... 2
1.4 Overall objective ............................................................................................................................... 3
1.4.1 Specific objectives ...................................................................................................................... 3
1.5 Statement of scope ............................................................................................................................. 3
2.0 LITERATURE REVIEW .................................................................................................................... 4
2.1 Legal requirements............................................................................................................................. 4
2.2 Review of dams .................................................................................................................................. 5
2.2.1 Classification based on function and use ................................................................................... 5
2.2.2 Classification based on structural design ................................................................................... 6
2.2.3 Classification based on material of construction ....................................................................... 6
2.2.4 Classification based on shape of cross section ........................................................................... 7
2.2.5 Classification based on hydraulic design .................................................................................... 8
2.3 Modes of failure: Stability requirements ........................................................................................ 8
2.4 Design criteria for earth dams. ...................................................................................................... 10
3.0 METHODOLOGY ............................................................................................................................. 11
3.1 Geotechnical data review. ............................................................................................................... 11
3.1.1 Data on In Situ testing. .............................................................................................................. 11
3.1.2 Laboratory Tests data. .............................................................................................................. 11
3.2 Topographical data review ............................................................................................................. 12
3.3 Hydrological data review ................................................................................................................. 12
3.4 Determination of Crop water requirements ................................................................................. 13
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3.6 Structural components of embankment ........................................................................................ 13
3.6.1 Spillway ..................................................................................................................................... 13
3.6.2 Dam height ................................................................................................................................ 14
3.6.3 Toe and heel .............................................................................................................................. 14
4.0 THEORITICAL FRAMEWORK ..................................................................................................... 15
4.1 Classification of soils ....................................................................................................................... 15
4.2 Unified soil classification .................................................................................................................. 15
4.3 Engineering properties of soils ........................................................................................................ 16
4.4 Topographical data review ............................................................................................................. 19
4.5 Hydrological data review................................................................................................................ 19
4.6 Combination of forces acting on a dam ........................................................................................ 19
4.7 Combination of loading for design ................................................................................................ 21
4.8 Computation of runoff .................................................................................................................... 22
5.0 RESULTS AND ANALYSIS.............................................................................................................. 23
5.1 Soil analysis and suitability ............................................................................................................ 23
5.1.1 Profile texture ........................................................................................................................... 23
5.1.2 Permeability tests ..................................................................................................................... 28
5.2 Topographical data review ............................................................................................................. 29
5.3 Determination of crop water requirement .................................................................................... 30
5.4 Rainfall data analysis ...................................................................................................................... 32
5.5 Components of the water reservoir ............................................................................................... 39
5.5.1 Reservoir capacity .................................................................................................................... 39
5.5.2 Reservoir area .............................................................................................................................. 40
5.5.3 Spillway ..................................................................................................................................... 41
5.5.4 Embankment ............................................................................................................................ 42
5.6 Volume of earth from pan bed ....................................................................................................... 42
5.7 Volume of earth on embankment .................................................................................................. 42
5.8 Impermeable layer for lining ......................................................................................................... 43
5.9 Calculations for determining reservoir parameters .................................................................... 44
5.9.1 Site clearance ............................................................................................................................ 44
5.9.2 Volume of earthworks on embankment ................................................................................. 44
5.9.3 Embankment force analysis .................................................................................................... 45
5.9.4 Sedimentation in reservoirs ..................................................................................................... 46
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5.9.5 Prevention and control of sedimentation ............................................................................... 47
5.9.6 Dam yields ................................................................................................................................. 49
6.0 DESIGN DRAWINGS ........................................................................................................................ 50
7.0 THE BILL OF QUANTITIES AND THE COST BENEFIT RATIO............................................ 52
7.1 The Bill of Quantities ...................................................................................................................... 52
7.1 Cost benefit analysis ....................................................................................................................... 53
7.0 CONCLUSION AND RECOMMENDATION ................................................................................ 54
7.1 Conclusion ....................................................................................................................................... 54
7.2 Recommendations ........................................................................................................................... 54
8.0 REFERENCE ...................................................................................................................................... 55
9.0 APPENDICES ..................................................................................................................................... 56
F21/1714/2010 Page ix
LIST OF FIGURES Figure 1: the proposed site for the dam .................................................................................................... 2
Figure 2: Earth dam ................................................................................................................................... 7
Figure 3: Gravity dam ................................................................................................................................ 7
Figure 4: Sieve analysis graph 1 .............................................................................................................. 24
Figure 5: Sieve analysis graph 2 .............................................................................................................. 25
Figure 6: Sieve analysis 3 ......................................................................................................................... 27
Figure 7: Probability of exceedence versus monthly rainfall ................................................................ 35
Figure 8: Water pan contour map ........................................................................................................... 38
Figure 9: Spillway ..................................................................................................................................... 41
Figure 10: Embankment dimensions ....................................................................................................... 45
F21/1714/2010 Page x
LIST OF TABLES Table 1: Standards for irrigation water use ............................................................................................. 5
Table 2: Classification of soil basing on international system convention ........................................... 15
Table 3: Unified soil classification ........................................................................................................... 15
Table 4: Unified soil classification ........................................................................................................... 16
Table 5 Test Pit one ................................................................................................................................... 23
Table 6: The wet and dry sieve analysis according to BS 1377 ............................................................. 23
Table 7: Test pit two ................................................................................................................................. 24
Table 8: Wet and dry sieve analysis 2 ..................................................................................................... 25
Table 9: Test pit three ............................................................................................................................... 26
Table 10: Wet and dry sieve analysis ...................................................................................................... 26
Table 11: Test pit one analysis ................................................................................................................. 28
Table 12: Test pit two analysis ................................................................................................................. 28
Table 13: Test pit three analysis .............................................................................................................. 29
Table 14: Topographical survey data...................................................................................................... 29
Table 15: River gauging data ................................................................................................................... 31
Table 16: Rainfall data for Siaya region ................................................................................................. 32
Table 17: Mean monthly rainfall for Siaya district ............................................................................... 33
Table 18: Probability of exceedence ........................................................................................................ 34
Table 19: Design rainfall .......................................................................................................................... 36
Table 20: Runoff ....................................................................................................................................... 37
Table 21: Contour elevations ................................................................................................................... 39
Table 22: Summary of reservoir levels ................................................................................................... 43
Table 23: Forest cover and annual sediment yield ................................................................................. 47
F21/1714/2010 Page 1
1.0 INTRODUCTION
1.1 Background
In many tropical, subtropical and Mediterranean climates, dry season agriculture and the pre-
rainy season agriculture and the pre-rainy season establishment of food and cash crops cannot be
undertaken without large quantities of water. To rely upon stream flow at a time when
temperature and evaporation are often at a peak can be unrealistic and risky. It may become
essential for a dam to be constructed on a river or stream to allow off storage of vital water
supplies. Although primarily for irrigation, such structures can be used, either separately or
combined, for fish farming, stock and domestic water purposes, drainage sumps, groundwater
recharge, flood amelioration and conservation storage.
Dams serve a primary purpose of retaining water which can be used for several purposes such as
generating electricity or obtaining water which can be used for irrigation. Dams maybe of
various types such as earth and rock fill dams, concrete and masonry dams. Dams are supposed
to satisfy two main requirements: Imperviousness and stability under all conditions of operation.
1.2 Problem statement and problem analysis
According to the National Bureau of Statistics, the population growth rate in Kenya in the year
2010 was 2.7% and there was an increase factor of 2.5% over the last three decades. This has led
to a significant increase in the Kenyan population. Given that a significant population of
Kenyans live in the rural areas, this has led to a substantial increase in the rural population.
Feeding this population has paused a serious challenge to all parties concerned. This is mainly
caused due to lack of water management structures which can help in realization of optimal
usage of water. Taking the case of Tingare, during rainy season there is normally heavy
downpour which causes flooding and during the dry season there is scarcity of water to sustain
agricultural activities. This leads to losses during the heavy downpour and water is also wasted in
large quantities. By designing an earth dam in Tingare, excess water during rainy season will be
stored and used during the dry season to sustain agricultural activities.
1.3 Site analysis and inventory
Tingare is a village in Siaya county. Siaya county is one of the counties in the former Nyanza
Province in the Southwest Part of Kenya. The total area of the county is approximately 2496.1
Km2. The county lies between latitude 0º 26´ to 0º 18´ north and Longitude 33º 58´ East and 34º
33´ west. According to the Kenya National Bureau of Statistics (2009), the population of Siaya
County was 842304.
The area is about 1225m above the sea level with temperature range 25º C to 30º C and average
wind speed of 3m/s. The area generally has a mean annual duration of bright sunshine 7-8 hours
a day. The long rains are in March to May and short rains in September to December. The mean
annual rainfall is 1000mm with 60% of the annual total falling during long rains with 60%
reliability.
In Siaya, Agricultural productivity is low because of poor soil and hot and dry climate.
F21/1714/2010 Page 2
River Yala passes in Siaya which can be used for irrigation purposes. In this project, I intend to
design a dam across River Yala to impound water to be used for irrigation purposes.
Figure 1: the proposed site for the dam
An aerial view of proposed earth-dam site in Siaya. (Courtesy of Google earth)
1.3.1 Site selection of reservoir If a suitable site can be found, constructing a small earth dam at a valley site is a cost effective
way to create a water storage reservoir, this is because it has a high water storage capacity per
cubic meter of soil removed. Nevertheless, the impact of a small earth dam being washed away
in a flood could be very serious and endanger lives and property. This is particularly so for valley
dams where a large quantity of water suddenly released would be channeled down the valley. For
this reason, experienced technical always be sought for the design and construction for any dam
which might present a threat to a downstream households or communities.
In construction of small dams, the cost of construction should be lower per gallon of water
stored. This is the reason that a dam can store water behind the dam as well as in the excavated
portion of the reservoir where the earth fill is obtained for its construction.
The following is considered when selecting site for a reservoir.
i) It should be an area of minimum percolation and maximum runoff.
ii) Leakage should be minimum to minimize the grouting works.
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iii) The area should be of not highly permeable rocks like shales, slates, gneisses and
granite.
iv) Water tight base rock should be available in the selected location.
v) To reduce the length of the dam, narrow opening of the basin is essential.
vi) Site should be easily accessible by road and railway and if required to construct them,
cost of construction should be minimum.
vii) Topography of the area should have adequate capacity without submerging excessive
land and other properties.
viii) Located area should provide sufficient depth with smaller water area. Higher depth
provides lower submerged area/ unity capaicity and decreases the possibility of weed
growth.
ix) Exclude water from tributaries which carry a lot silt and sediment.
x) Free from objectionable solution of minerals and salts.
xi) Construction materials and other allied works should be locally available.
xii) Suitable area near the location should be available for construction of staff quarters,
labor colonies, godowns and stack yard.
1.4 Overall objective
The overall objective of the project is to design an earth-dam that will aid in providing water for
irrigation.
1.4.1 Specific objectives
I) To review geophysical condition of the place which will include obtaining
geotechnical, topographical and hydrological data.
II) To estimate the reservoir capacity.
III) To design the dam.
IV) To find the Crop water requirement of Soya beans.
V) To determine the cost benefit ratio.
1.5 Statement of scope
The scarcity of water in Tingare (Siaya County) during the dry periods hinders Agricultural
production thereby leading to food insecurity. This project seeks to lay down a design of a small,
replicable, relatively affordable dam using locally available materials.
The project will encompass the following tasks for the development of earth dam design:
i) Reviewing geophysical condition of the site including obtaining geotechnical,
topographical and hydrological data.
ii) Delineation of dam site and Catchment area.
iii) Determination of the reservoir capacity.
iv) Design of the earth dam.
v) Determining the Crop water requirement for Soya beans.
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2.0 LITERATURE REVIEW
Currently, the conventional way of irrigation in Tingare is that of pumping water straight from
the river to the farms without any means of storage. This method has proved to be expensive and
inefficient. It has proved to be problematic since when the water flow in the river reduces or goes
down, the irrigation process becomes practically impossible. This has necessitated the design of
dam to ensure continuous supply of water. Dams are a critical and essential part of a nation’s
infrastructure for storage and management of water in watersheds. The two principal types of
embankments dams are earth and rock-fill dams, depending on the predominant fill material
used. An earth dam is relatively cheaper to construct if the materials are available hence mostly
preferred for small projects.
An earth dam is composed of suitable soils obtained from borrow areas or required excavation
and compacted in layers by mechanical means. Following preparation of foundation, earth from
borrow areas and required excavation is transported to the site, dumped and spread in layers of
required depth. The soil layers are then compacted by tamping rollers, sheepsfoot rollers, heavy
pneumatic tired rollers, vibratory rollers, tractors or Erath filling equipment.
2.1 Legal requirements When working on a dam, it is essential to check on the government regulations controlling dam
establishment and use of water. Different areas have different requirements for the control of
water conservation works. The regulations are aimed at protecting the community from hazards
of poorly constructed dams and to ensure there is a fair distribution of limited water resources.
The water Act No. 8 of 2002 of Kenya Gazette supplement and the Environment Management
and Coordination Regulation of Kenya Gazette Supplement govern the use and development of
water resources in Kenya.
The Environmental Management and Coordination (Water Quality) Regulations of 2006 part IV-
Water for Agriculture use, sets out the quality standards for irrigation water. They are set out as
follows:
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Table 1: Standards for irrigation water use
PARAMETER PERMISSIBLE LEVEL (mg/l)
Ph. 6.8-8.5
Aluminium 5
Arsenic 0.1
Boron 0.1
Cadmium 0.5
Chloride 0.01
Chromium 1.5
Cobalt 0.1
Copper 0.05
E. coli Nil/100 ml
Fluoride 1.0
Iron 1.0
Lead 5
Selenium 0.19
Sodium Absorption rate 6
Total Dissolved solids 1200
Zinc 2
Source: Kenya Gazette Supplement No. 68
2.2 Review of dams
Dams are classified on the basis of the following:
i) Function and use.
ii) Structural design.
iii) Materials of construction.
iv) Shape of cross section.
v) Hydraulic design.
2.2.1 Classification based on function and use
Here, dams are classified either as storage or conservation, diversion or detention.
Storage or conservation dams store excess flood water which is used during the period of
deficient rainfall. Upstream of this dam, a reservoir is formed. Water stored in the dam may be
used for multipurpose projects.
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Diversion dams raise the water level and divert water from upstream to canal under conveyance
system like canal or ditches to the place it may be used as runoff river hydroelectric scheme,
water supply, irrigation or some other purposes.
Detention dams store and detain flood water temporarily and release it when flood subsides.
2.2.2 Classification based on structural design
Dams are classified with respect to their structure e.g. their foundations, which in turn affect the
different forces acting in them. Dams can be gravity dams, arch dams, embankment dams, earth
dams, rockfill dams or combined earth and rockfill dams.
Gravity dams are solid concrete and masonry dams in which all the pressures like water, wave
silt, uplift and others are resisted by the dam’s weight or gravity forces.
Arch dams are curve in plan, arching between two abutments of the river which convex faces
towards the flow of water.
Embankment dams are2.2.2 suitable for sites in which considerable excavation can be made to
reach a foundation which is capable of supporting the heavy stress imposed by the dam.
Rockfill dams are preferred when plenty of rocks are available from adjacent quaries.
Combined earth and rockfill dams are composite embankment dams whose downstream portion
is filled with rock and upstream with soil with riprap on the slope in reservoir side with cement
grouted core wall against seepage.
2.2.3 Classification based on material of construction
Dams are constructed by reinforced cement concrete, masonry, steel, timber, rubber, soil, rock
etc. Upstream water cannot flow through rigid dams such as gravity dam, arch dams, buttress
dam, steel dam, timber dam and rubber dam. On the other hand, earth dam, rockfill dam,
combined earth and rock dam have the problem of seepage through the body of the dam. Hence,
they are called non rigid dams.
Rigid dam is made of masonry, concrete, RCC i.e. gravity, steel, buttress, rubber, timber and
arch dams.
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Non rigid dams are earth dam, rockfill dam, combined earth and rockfill dam.
Figure 2: Earth dam
(Source: Wikipedia)
Figure 3: Gravity dam
(Source: Wikipedia)
2.2.4 Classification based on shape of cross section
Dams can be trapezoidal or arch in shape.
Trapezoidal dams are earth dam, rockfill dam, combined earth and rockfill dam and to some
extent gravity dam is trapezoidal in shape.
Arch dams are curve in plan, arching between two abutments of the river with convex faces
towards the flow of water.
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2.2.5 Classification based on hydraulic design
Dams are designed as overflow or non-overflow for the disposal of reservoir excess flood water.
Overflow dams are usually made of masonry or concrete, they are gravity dams. Excess flood
water is allowed to escape from reservoir above the top of the dam as overflow spillway.
From the above mentioned types of dams, I chose to design an earth-dam over the others due to
the following reasons:
Earth dams are made of locally available materials like clay, gravel, sand and silt.
Earthen dams are much cheaper compared to gravity dams which require cement and
steel and are getting costlier day by day.
They are also favorable to a variety of sites unlike gravity dams.
2.3 Modes of failure: Stability requirements
Depending on the size of the dam, dam failure can lead to destruction of property or even loss of
lives. Routine deformation monitoring of seepage from drains in and around larger dams is
necessary to anticipate any problems and action taken before structural failure occurs. Most dam
designs incorporate mechanisms to permit the reservoir to be lowered or even drained in case of
such problems. Another solution can be rock grouting.
The main causes of dam failure include dam design error, geological instability caused by
changes in water levels during filling or poor surveying, poor maintenance especially of outlet
pipes, extreme rain volumes and human design errors. Some of the modes of failure caused by
hydrological conditions include:
Overturning.
The overturning of a dam section takes place when the resultant force at nay section cuts the base
of the dam downstream of the toe. On the other hand, if the resultant cuts the base within the
body of the dam, there will be no overturning. For stability, the dam must be safe against
overturning. The factor of safety against overturning is given by;
𝐹. 𝑆 =∑𝑅𝐼𝐺𝐻𝑇𝐼𝑁𝐺 𝑀𝑂𝑀𝐸𝑁𝑇𝑆
∑𝑂𝑉𝐸𝑅𝑇𝑈𝑅𝑁𝐼𝑁𝐺 𝑀𝑂𝑀𝐸𝑁𝑇𝑆=
𝑀𝑝
𝑀𝑜
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Overtopping.
Overtopping occurs when the water level in the reservoir exceeds the height of the dam and
flows over the crest. Overtopping will not necessarily result in failure. Failure depends on the
type, composition and condition of the dam and the depth and duration of the flow of the dam.
During overtopping, the abutments of concrete dams also can be eroded leading to a loss of
support and failure from sliding and overturning.
To avoid sliding, the factor of safety should be designed such it is adequate. The factor of safety
is calculated from the formula:
∑ (𝑐∆𝐿 + 𝑊𝑛𝐶𝑂𝑆 𝛼𝑛 + tan ∅)𝑛=𝑝𝑛=1
∑ 𝑊𝑛𝑆𝑖𝑛𝛼𝑛𝑛=𝑝𝑛=1
Where:
Fs-Factor of safety with respect to strength .
d-developed shear stress.
f-shear strength of soil.
L-Length of slice at its base.
C-cohesion
Cd-Developed Cohesion.
Overstressing of structural components.
As a flood flow enters the reservoir, the reservoir will rise to a higher elevation. Even though a
dam may not be overtopped, the reservoir surcharge will result in higher loading conditions. If
the dam is not properly designed for this flood surcharge condition either the entire dam or
structural components may become overstretched resulting in overturning failure, sliding failure
or failure of specific structural components.
Erosion of earth spillways.
High or large flows through spillways adjacent to dams threatens the stability of dams. Erosion
can also cause head-cutting that progresses towards the spillway crest and eventually leads to a
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breach. Flood depths that exceed the safe design parameters can produce erosive forces that may
cause serious erosion of the in the spillway.
2.4 Design criteria for earth dams.
The essential requirements of design of an earth dam are safe and stable structure at a minimum
construction and maintenance cost.
The essential design criteria are:
i) Safe against overtopping during design flood by providing adequate spillway and oulet
capacity.
ii) Spillway be of sufficient capacity to pass peak flood.
iii) Safe against overtopping by wave action by providing adequate freeboard.
iv) Side slopes stable during construction and all conditions of reservoir operation. Upstream
slope is safe against rapid drawdown condition while the downstream slope is safe against
sloughing.
v) Side slopes upstream and downstream are flat enough so that the shear stress induced in
the foundation is within the shear strength of the material of the material comprising the
foundation with a suitable factor of safety.
vi) Upstream slope is protected against erosion by wave action while the crest and downstream
slope is protected against erosion due to wind and rain. Horizontal berns at suitable
intervals in upstream and downstream faces may be provided for this purpose.
vii) Downstream slope is safe during steady seepage under full reservoir condition.
viii) Portion of the dam downstream of the impervious core is properly drained by the provision
of suitable drainage system.
ix) Seepage flow through the dam and foundation is so controlled that there is no danger of
fine particles getting washed out from the downstream by the efflux seepage. Moreover,
quantity of seepage loss is restricted to the minimum.
x) There is no possibility of free flow of water from the upstream through either the dam or
the foundation.
xi) Adequate impervious core to act as water barrier. Top of impervious core is maintained
higher than the maximum reservoir level.
xii) Seepage line (phreatic line) is well within the downstream face so that no sloughing of the
slope takes place.
xiii) Dam as a whole is earthquake resistant. The seismic condition of the region are investigated
with reference to geological map of vicinity.
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3.0 METHODOLOGY
3.1 Geotechnical data review. Two types of geotechnical data were obtained. There is data on In Situ testing and data on
laboratory testing.
3.1.1 Data on In Situ testing.
Standard penetration Test. (STP)
An STP test is performed during the advancement of soil boring to obtain an approximate
measure of the dynamic soil resistance. The STP test is carried out according to the AASHTO T-
206.
Cone Penetration Test. (CPT)
CPT provides continuous profiling of geostratigraphy and soil properties evaluation.
The test is performed according to ASTM-3441 (Mechanical systems) and ASTMD 5778
(electric and electronic systems) and consists of pushing a cylindrical steel probe into the ground
at a constant rate of 20mm/s and measuring the resistance to penetration.
Pie zone Penetration Testing.
Pie Zone are cone penetrometers with added transducers to measure penetration porewater
pressures during the advancement of the properties.
Vane Shear Test
The Vane Shear test (VST) or field vane is used to evaluate the in place undrained Shear
Strength (Suv) of soft to stiff clays and silts at regular depth of 1m (3.28).
3.1.2 Laboratory Tests data.
Sieve analysis
This is to determine the percentage of various grain sizes. The grain size distribution is used to
determine the textural classification of soils i.e. gravel, sand, silt, clay etc. which in turn is useful
in evaluating the engineering characteristics such as permeability, strength, swelling potential
and susceptibility to frost action. It is done according to AASHTO D422.
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Atterberg Limits.
To describe the consistency and plasticity of fine-grained soils with varying degrees of moisture.
It is done according to AASHTO T89, T90.
Triaxial Testing
To determine strength characteristics of soils including detailed information on effects of lateral
confinement, powerwater pressure, drainage and consolidation. Triaxial tests provide a reliable
means to determine the friction angle of natural clays and silts as well as reconstituted sands. The
stiffness (modulus) at intermediate to large strains can also be evaluated.
3.2 Topographical data review
A topographical survey of the proposed dam site was conducted to establish the different natural
features of land around the Tingare dam site. The survey was used to produce a feasibility report
and a contour map which captured the land in general and included the catchment area, trees and
ravine ecosystem. The topographical survey was done with a Total Station equipment. The result
of the survey was presented in the form of contour lines on a map of the land. The topographical
survey was also used to formulate an optimal plan for drainage of the water from the dam.
3.3 Hydrological data review Stream gauging data review
Hydrologic measurements on streams were conducted at carefully selected locations which are
capable of recording accurate data. The site is preferably located on a rigid surface and is kept
approachable to operators. These sites are called stream gauging stations.
Stream stage is the elevation of water surface in a stream with reference to a fixed datum.
The other kinds of hydrological measurements were collected from the weather station such as
the rainfall pattern of the place.
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3.4 Determination of Crop water requirements
The dam is estimated to irrigate 5 ha of Soya beans. The water consumption of the Soya beans
was estimated to aid in the sizing of the reservoir.
Since the dam is designed to supply water for irrigation during the dry season which at times
arises before the crops have matured, the calculation was based on deficit water requirements of
the crops.
Evapotranspiration (consumptive use) of the crops is either estimated from climatological data or
found by conducting measurements in the field. Some of the popularly used methods of
estimation are:
Blaney-Criddle
Thornwaite
Penman
Christiansen
Some of the methods used in the field are:
Lysimeters
Field plots soil moisture depletion studies
Water balance methods
The dam will then be designed. In dam design, the forces acting on the dam will be determined
to ensure stability under all service conditions. The main forces acting on the dam are:
i) Water pressure.
ii) Weight of the dam.
iii) Ice pressure.
iv) Uplift pressure.
v) Wave pressure.
vi) Silt pressure.
3.6 Structural components of embankment
3.6.1 Spillway
Spillway are provided to release surplus or floodwater from the reservoir in order to prevent
overtopping and possible failure of the dam.
The selection of the spillway was done by a combination of the maximum reservoir storage
capacity, maximum discharge capacity of the outlet, type of the dam, topography and the
hydrological conditions of the site.
This type will convey discharge from the reservoir to the downstream level through a steep open
channel placed along the dam abutment.
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For the spillway section, the Manning’s formula is used to calculate the outflow assuming that a
uniform flow has developed after the crest:
𝑄 =1
𝑛𝑅
2
3𝑆1
2
Where;
Q – Discharge in m3 for the adopted return flood
n – Manning’s coefficient taken as 0.025 (MOWI, 1986)
R – Hydraulic gradient (m)
S – Slope in decimals
A – Wet surface in m2
Spillway dimension is given a minimum of 10m according to the Ministry of Water and
Irrigation guidelines manual. The reservoir does not get water from a catchment but from a
spring of 1.65l/s flow which would give small dimensioned spillway. Spillway crest is at level
1225.50 with excess water depth above crest fixed at 0.5m.
3.6.2 Dam height
Structural height
It is the height between the top of the dam and the lowest foundation level, exclusive of narrow
faul zones, if any.
Hydraulic height
It is the height between the highest controlled water surface and the lowest point in the original
river bed level at the axis of the dam.
3.6.3 Toe and heel
The toe of the dam is the downstream edge of the base and the heel is the upstream edge of the
base.
3.6.4 Length of the dam
It is the distance from one abutment to the other, measured along the axis of the dam at the level
of the top of the dam.
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4.0 THEORITICAL FRAMEWORK
4.1 Classification of soils
Soils can be divided into four main groups:
i) Gravel.
ii) Sands.
iii) Silts.
iv) Clays.
The above classification is based on particle size.
Using the International System Convention, the classification can be as follows:
Table 2: Classification of soil basing on international system convention
Coarse 2-0.21mm
Fine sand 0.2-0.02mm
Silt 0.02-0.002mm
Clay Less than 0.002mm
Gravels and sands can be identified by appearance and feel. Silts and clay are indistinguishable
when dry. While clay is one of the most useful in dam building, silt when wet is the most
troublesome. It is unstable in the presence of water and tends to become unstable when saturated.
4.2 Unified soil classification The Unified soil classification has been widely adopted for use in various stages of
investigations, design and construction. Soil types are designed by various symbols such as
Table 3: Unified soil classification
Soil type Symbol
Gravel G
Sands S
Silts M
Clays C
Organic soils O
Peats Pt
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Gravels and sands are further subdivided into Well graded (W), poorly graded (P). Silts and
clays are divided into those with Low (L) and high (H) liquid limits.
Table 4: Unified soil classification
Symbols Description
GW Well graded gravels
GP Poorly graded gravels
GM Silty gravel
GC Clayey gravel
SW Well graded sands
SP Poorly graded sands
SM Silty sands
SC Clayey sands
ML Inorganic silts with low liquid limits
CL Inorganic clays with low liquid limits
OL Inorganic silts with low liquid limits
CH Inorganic silts with high liquid limits.
OH Inorganic clays with high liquid limits
Pt Peats with highly organic soils
4.3 Engineering properties of soils
I) Permeability
The voids in soils and most rocks are interconnected forming irregular but continuous conduits
with branches in all directions. The average diameter of the conduit is roughly 1/5D10.
Theoretical analysis shows that the flow through voids is laminar and is governed by Darcy’s
Law i.e.
𝑄 =𝛥ℎ
𝐿𝐴
𝑄 = 𝐾∆ℎ
𝐿𝐴
But 𝑖 = ∆ℎ/𝐿
𝑄 = 𝐾𝑖𝐴
Where:
L-Length of soil sample.
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Cross sectional area of soil.
Δh- difference in head.
Head loss per unit length (energy gradient)
K- Darcy’s coefficient i.e. coefficient of permeability.
Capillarity.
In partially saturated soil, the air water boundary is subjected to unbalanced stresses caused by
the difference between the intermolecular attraction forces resulting into surface tension.
The effect of surface tension is the rise of water above the groundwater i.e. capillary rise. The
maximum height can be gotten from the formula:
ℎ𝑐 =4𝑇𝑜
𝛶𝑑
Where:
To- surface tension.
Υw- density of water.
d- tube diameter.
Mixing dry soil with wet soil increases surface tension. This will induce seepage towards a dry
zone and eventual increase in water content. This occurs when we try to build embankments out
of excessively wet soil by blending it with drier material.
II) Compressibility
Soils undergo reduction in volume under load causing a change in void ratio. The
compressibility test is done using a consolidimeter where a series of increasing stresses are
applied to a soil sample and the results expressed in stress void ratio curves. The compressibility
of a soil depends on its composition, structure and its history of deposition and stresses.
The compression index is calculated from the following formula:
𝐶𝑐 = 0.009(𝐿𝐿 − 10)
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This is for clays of low- moderate plasticity.
Where: LL- Liquid limit
For soils of low plasticity,
𝐶𝑐 = 0.75(𝑒 − 0.6)
Where:
e- void ratio.
III) Swelling and shrinkage
Soils undergo volume changes that are not produced by changes in the external load. Instead,
they are produced by changes in their water content brought about by their internal forces.
Shrinkage is caused by capillary tension. When a saturated soil dries, a meniscus develops at the
surface of each void. This produces a tension (-U) in the soil water. The water tension produces
an equal compression (Δσ) in the soil structure if the external load O remains constant.
𝜎 = 𝜎′ + 𝑈
𝜎 = 𝜎′ (before shrinkage)
𝜎 = 𝜎′ + 𝛥𝜎′ + 𝑈(−)
𝛥𝜎 = 𝑈
IV) Combined stresses and failure
The soil strength is a cardinal factor in design of both the foundation and embankment of the
earth dam. Because of its 3-Phase composition, the strength of the soil is far more complex than
that of simple materials. The stress applied to a plane surface can be resolved into two
components i.e. normal stress and shear stress.
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The stress experienced on a cube and oblique plane can be represented graphically on Mohr
circle.
4.4 Topographical data review
Topography is the shape, configuration, relief, roughness or 3-Dimensional characteristic of the
earth’s surface. Topographic surveys are surveys made to determine the configuration of the
earth’s surface and to locate natural and cultural features on it. The most common use of
topographic maps is in the planning stages of projects to help in the layouts buildings, roads,
dams, pipelines, landscapes, fire control routes and many more.
4.5 Hydrological data review
Hydrology has to do with the movement, distribution and quality of water on earth and other
planets. It includes the hydrologic cycle, water resources and environmental watershed
susceptibility. Hydrology is subdivided into surface water hydrology, groundwater hydrology
and marine hydrology. Domains of hydrology include hydrometeorology, surface hydrology,
hydrogeology, drainage basin management and water quality, where water plays the control role.
Dam design.
4.6 Combination of forces acting on a dam
The following are the forces acting a dam.
Water pressure. (Reservoir and tail water loads)
Water pressure on the upstream face is the main destabilizing (or overturning) force acting on a
dam. Although the weight of water varies slightly with temperature, the variation is usually
ignored. Unit mass of water is taken as 1000 kg/m3 and specific weight= 10 kN/m3 instead of
9.81 kN/m3.
The water pressure intensity P(kN/m2) varies linearly with the depth of the water measured
below the free surface y(m).
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Weight of the dam
It is the main stabilizing force in a gravity dam. Dead load= Weight of concrete or both + weight
of such appurtenances as piers, gates and bridges. Weight of the dam per unit length is equal to
the product of the area of cross section of the dam and the specific weight of the material.
Ice pressure
Ice expands and contracts with changes in temperatures. In a reservoir completely frozen over, a
drop in the air temperature or in the level of the reservoir water may cause the opening of the
cracks which subsequently fill with water and frozen solid. When the next rise in temperature
occurs, the ice expands and if restrained, it exerts pressure on the dam.
Uplift pressure
Water has a tendency to seep through the pores and fissures of the material in the body of the
dam and foundation material and through the joints between the body of the dam and its
foundation at the base. The seeping water exerts pressure. The uplift pressure is the upward
pressure of water as it flows or seeps through the body of dam or its foundation.
According to the Indian Standards (IS: 6512-1984), there are two constituent elements in uplift
pressure: the area factor or the percentage of area on which uplift acts and the intensity factor or
the ratio which the actual intensity of uplift pressure bears to the intensity gradient extending
from headwater to tailwater at various points.
The total area should be considered as effective to account for uplift.
Wave pressure
Waves are generated on the reservoir surface because of the wind blowing over it. Wave pressure
depends on the height of the wave developed.
Wave height may be calculated using the formulae below:
ℎ𝑤 = 0.0322√𝑉. 𝐹 + 0.763 − 0.271𝐹0.25
For F˂32 km
ℎ𝑤 = 0.0322√𝑉. 𝐹
For F˃32 km
hw= height of waves in metres, between crest and trough.
V= Wind velocity in Km per hour.
F= Fetch in Km.
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Silt pressure
A river brings silt and debris along with it. The silt load gets deposited to an appreciable extent
when the dam is constructed. The dam is therefore subjected to silt pressure in addition to the
water pressure. If ys is the submerged unit weight of silt and θ is the angle of internal friction
taken as 30º for sand, gravel, clay, silt and h, is the height to which silt can be deposited, the
horizontal silt pressure is given by:
𝑝𝑠 =1
2𝑦𝑠ℎ𝑠
2 1−sin 𝜃
1+𝑠𝑖𝑛𝜃
Ps is assumed to be equivalent to the pressure of a fluid weighing 1360 Kg/m3. Vertical silt and
water pressure is determined as if silt and together have a density of 1925 Kg/m3. In a dam with
an inclined wet-stream face, the vertical weight of silt resting against the slope also acts as a
vertical force (Dandekor and Sharma, 1979).
4.7 Combination of loading for design
According to Indian standards recommendations, (IS6512-1984) given by Pande, Lal and Punmit
(2009), dam design shall be based on the most adverse load conditions A,B,C,D,E,F ang G given
below using the prescribed safety factors:
Load combination A (construction condition)
Dam completed but no water in the reservoir and no tail water.
Load combination B (Normal operating condition).
Full reservoir elevation, normal dry weather tailwater, normal tailwater, normal uplift, ice and
silt (if applicable).
Load combination C (Flood discharge condition).
Reservoir of maximum flood pool elevation, normal uplift and silt (if applicable).
Load combination D. Combination A with earthquake.
Load combination E. Combination B with earthquake but no ice.
Load combination F. Combination C, but with extreme uplift (drainage inoperative).
Load combination G. Combination E, but with extreme uplift (drainage inoperative).
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4.8 Computation of runoff
In order to determine the design criteria for hydrological structures, it is necessary to compute
the probable maximum flood that may be expected from the catchment site of the dam. This is
significant since it influences the other design parameters of the dam for example, the spillway
size and freeboard.
The flood/ runoff volume expected from a particular catchment for a certain return period
depends on the following factors;
i) The mean rainfall intensity and its distribution.
ii) The antecedent rainfall.
iii) The type and configuration of soil type.
iv) The catchment size, shape and slope.
v) The vegetation cover of the catchment area.
The formulas which are commonly used to peak flow from a catchment area are:
i) Rational formula method.
ii) Curve number method.
Empirical methods include;
i) Richard’s method.
ii) Cook method.
iii) Dicken’s method.
iv) Ryves formula.
v) Khoslas formula.
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5.0 RESULTS AND ANALYSIS
5.1 Soil analysis and suitability
Soil samples were taken from ten different points within the proposed construction site, these
samples were carried to the lab and the following tests were performed:
5.1.1 Profile texture
This was done to help determine the proportion of silt, clay, sand and gravel of the soil on the
proposed site. Sieve analysis was done on the samples collected from four different pits within
the site and a grading curve was drawn. The test results are as tabulated below:
Table 5 Test Pit one
Initial dry sample
mass
100gm Fine mass 92.5gm
Fine percentage 92.5%
Washed dry sample
mass
7.5gm
Table 6: The wet and dry sieve analysis according to BS 1377
Sieve size (mm) Retained mass (gm) % retained Cumulative passed
percentage
20 0 0.0 100.0
10 0 0.0 100.0
5 0 0.0 100.0
2.36 0.1 0.1 99.9
1.18 0.3 0.3 99.6
0.6 0.4 0.4 99.2
0.425 0.8 0.8 98.4
0.3 1.4 1.4 97.0
0.40.15 2.8 2.8 94.2
0.075 2.7 2.7 91.5
˂ 0.075 91.5 91.5
total 100
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The above results were represented graphically as shown below:
Figure 4: Sieve analysis graph 1
Table 7: Test pit two
Initial dry sample
mass
100gm Fine mass 88.1 gm
Fine percentage 88.1%
Washed dry sample
mass
11.9gm
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Table 8: Wet and dry sieve analysis 2
Sieve size (mm) Retained mass (gm) % retained Cumulative passed
percentage
20 0 0.0 100.0
10 0 0.0 100.0
5 0.9 0.9 99.1
2.36 0.8 0.8 98.3
1.18 0.7 0.7 97.6
0.6 0.6 0.6 97.0
0.425 0.9 0.9 96.1
0.3 1.7 1.7 94.4
0.40.15 2.6 2.6 91.8
0.075 2.7 2.7 89.1
˂ 0.075 89.1 89.1
total 100
Figure 5: Sieve analysis graph 2
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Table 9: Test pit three
Initial dry sample
mass
100gm Fine mass 83.8 gm
Fine percentage 83.8%
Washed dry sample
mass
14gm
Table 10: Wet and dry sieve analysis
Sieve size (mm) Retained mass (gm) % retained Cumulative passed
percentage
20 0 0.0 100.0
10 0 0.0 100.0
5 0 0.0 100.0
2.36 0.1 0.1 99.9
1.18 0.3 0.3 99.6
0.6 1.4 1.4 98.2
0.425 1.6 1.6 96.6
0.3 2 2 94..6
0.15 4.2 4.2 90.4
0.075 4.6 4.6 85.8
˂ 0.075 85.8 85.8
total 100
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Figure 6: Sieve analysis 3
Results of each grain sieve analysis of the soil are reported in the form of a grain distribution
curve. As an alternative, the analysis may be reported in tabular form giving percentages passing
various sieve sizes or percentages found within found within various particle ranges.
Coefficient of gradation:
𝐶𝑔 =𝑑30
2
𝑑60 × 𝑑10=
0.0082
0.001 × 0.04= 1.6
Uniformity coefficient:
𝐶𝑢 =𝑑60
𝑑10=
0.04
0.001= 25
Since the value of 25 is greater than 4 and the value of Cg of 1.6 is between 1 and 3, then the
sample meets both the criteria for sandy clay with gravel pebbles.
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5.1.2 Permeability tests
This tests the moisture holding capacity of the soil. This test will determine the rate of
permeability of moisture per centimeter per minute.
It was done according to BS 1377:1990
Table 11: Test pit one analysis
Dry density
(Kg/m3)
1424 Moisture
content (%)
24.90% Area of
sample
(mm2)
8171
Height of
mould
100 Diameter of
mould
102 Area of
Burette
95
Time, t (min) Head, H (cm) Log10 H
0 95.8 1.980
3 95.5 1.980
6 95.2 1.979
9 94.9 1.977
Coefficient of permeability, K= 1.323×10-03
Table 12: Test pit two analysis
Dry density
(Kg/m3)
1448 Moisture
content (%)
21.90% Area of
sample
(mm2)
7854
Height of
mould
107 Diameter of
mould
100 Area of
Burette
95
Time, t (min) Head, H (cm) Log10 H
0 96.0 1.982
4 95.7 1.981
8 95.4 1.980
12 95.1 1.978
Coefficient of permeability, K= 1.561×10-5 mm/s
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Table 13: Test pit three analysis
Dry density
(Kg/m3)
1490 Moisture
content (%)
21.30% Area of
sample
(mm2)
8495
Height of
mould
107 Diameter of
mould
104 Area of
Burette
95
Time, t (min) Head, H (cm) Log10 H
0 96.2 1.983
1 91.6 1.962
2 87.0 1.940
3 92.4 1.916
Coefficient of permeability = 1.02×10-5 mm/s
5.2 Topographical data review
The results of the rise and fall method of topographical survey is as shown below:
Table 14: Topographical survey data
B.S I.S F.S RISE FALL R.L REMARKS
1200 1880
1.725 0.705 1879.295
2.130 0.405 1878.890
2.975 0.845 1878.045
0.695 3.860 0.885 1877.160 Change of
station
1.500 0.805 1876.355
2.880 1.38 1874.975
1.150 3.870 0.99 1873.985 Change of
station
1.945 0.795 1873.190
2.595 0.65 1872.540
0.015 3.870 1.275 1871.265 Change of
station
0.645 2.700 2.685 1868.580 Change of
station
3.150 2.505 1866.075
ΣFS=17.450 Σ=0 Σ
F=13.925
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5.3 Determination of crop water requirement
The crop water requirement is denoted by the following formula:
𝐸𝑇𝐶 = 𝐸𝑇𝑂 ∗ 𝐾𝐶
Where ETC is crop evapotranspiration in mm/day
ETO is the reference evapotranspiration in mm/day which is obtained from reference
evapotranspiration atlas for Kenya by FAO 1998a as 5.2 mm/day
KC is the crop coefficient which is obtained from the same manual as 1.15 when the crop has
maximum foliage.
Thus, water requirement for Soya bean when grown in open space= 5.5 mm/day * 1.15 = 5.98
mm/day.
According to the same FAO 1998a manual, Soya bean crop takes 85 days to grow to maturity in
the tropics.
Gross water requirement for Soya beans= 5.98 mm/day*85= 508.3
The farm area intended for irrigating the crop is 5ha= 50,000 m2
The volume of water needed for irrigation = gross water requirement in mm* area of the farm in
ha.
= 0.5083×50000= 25,400m3
Evaporation loss
This refers to loss of water through evaporation from the reservoir surface during the growing
period of the crop.
Evaporation of the area is 5mm/day from Kenya atlas of the evaporation by FAO 1998a and the
growing period of the crop is 85 days =425mm
Using surface area of the reservoir at full water level as 3656.25m2
We get 3656.25×0.425= 1553.9m2 as evaporation loss.
Water demand analysis
Water requirement for irrigation per day= 5.98 mm/day
Area to be irrigated per day= 5ha/day
Volume of water per day= area to be irrigated per day= 5 ha/day
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Volume of water per day= area to be irrigated per day× water requirement for irrigation required
per day.
=0.00598m/day×50000m2 =299m3
Evaporation loss per day= 5mm/day
Surface area of the reservoir at full water level =3656.25m2
Volume of water loss through evaporation per day= 0.005×3656.25= 18.3m3
Designed reservoir capacity is 5500m3 less daily extraction of 299m3 and 18.3m3 gives balance of
5182.7m3. The reservoir shall be filled by a spring which has a discharge of 1.651L/sec
(determined during survey) and translates to 142.65 m3 per day.
Irrigation is not done every day, good practice is that it be done in blocks and for frequency of 6
days with one rest day. This means that only 0.83 Ha per block (5ha/6 days) shall be irrigated per
day giving abstraction discharge of 49.83 m3/ day. There is sufficient water for irrigation.
The following data was collected from the stream gauging station:
Width= 40cm
10cm, depth 8cm, v= 53, 50
20cm, depth 8cm, v=33, 34
30cm, depth 6cm, v= 0
Table 15: River gauging data
Distance from the
start (m)
Subsection parameters
Width (m) Depth (m) Velocity
0 - - -
0.1 0.1 0.08 53
0.2 0.1 0.08 33
0.3 0.1 0.06 0
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5.4 Rainfall data analysis
The mean monthly rainfall data and the average annual data for 17 years are tabulated below:
Table 16: Rainfall data for Siaya region
(Courtesy of Water Resources Management Authority)
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Table 17: Mean monthly rainfall for Siaya district
Year 2014 2013 2012 2011 2010 2009 2008 2007 2006
Precipitation
(mm)
1570 1567 1524 1587 1476 1418 1343 1202 1265
2005 2004 2003 2001 2000 1999 1999 1998 1997
1065.6 1355 1252 1618 1419 1214 1511 1185 1610
The recurrence interval assigned to the highest rainfall amount (1610) in the 18 years is obtained
in the formula.
𝑇𝑟 =𝑁 + 1
𝑚=
18 + 1
1= 19 𝑦𝑒𝑎𝑟𝑠
The corresponding probability of exceedence is obtained from the formula.
𝑃 =𝑀
𝑁+1=
1
𝑇𝑟=
1
19= 0.05 = 5%
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Table 18: Probability of exceedence
Rank Tr P Jan rain (mm)
1 22 0.05 315
2 11 0.09 299
3 7.3 0.14 218
4 5.5 0.18 192
5 4.4 0.23 164
6 3.7 0.27 133
7 3.1 0.32 128
8 2.8 0.36 114
9 2.4 0.42 91
10 2.2 0.46 77
11 2 0.5 68
12 1.8 0.56 55
13 1.7 0.59 47
14 1.6 0.63 40
15 1.5 0.67 36
16 1.4 0.71 33
17 1.3 0.77 31
The prevailing convention in rain water harvesting systems is to take a probability of exceedance
of 67% and the corresponding rainfall as the design rainfall amount. This value is obtained by
plotting by the probability of exceedance, P against the corresponding monthly rainfall.
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Figure 7: Probability of exceedence versus monthly rainfall
The design rainfall amount is obtained with probability of exceedence of 0.67. This is obtained
from the average i.e 36mm for January. The following design rainfall were then obtained for all
the months. The following design rainfall were then obtained for all the months.
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Table 19: Design rainfall
Month Design rainfall (mm)
Jan 36
Feb 43
March 122
|April 138
May 143
June 95
July 71
Aug 53
Sept 121
Oct 86
Nov 167
Dec 157
Estimating Runoff
Runoff volumes depends on: Mean rainfall intensity and its distribution, antecedent rainfall, type
and configuration of soil, catchment size, shape and slope and the vegetation cover of the
catchment area.
From the soil analysis done, the catchment area is composed of sandy clay with gravel pebbles
and a lot of vegetation using the Curve Number method:
𝑄 =(𝑃 − 0.2𝑆𝑀)2
𝑃 + 0.8𝑆𝑀
𝐶𝑁 =25400
254 + 𝑆𝑀
Jan runoff
For sandy clay with land covered fallow row crops, the Curve Number is 78 and for antecedent
Moisture (USSCS 1964).
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78 =25400
254 + 𝑆𝑀
Sm=71
𝑄 =(36 − 0.2 × 71)2
36 + (0.8 × 71)= 5𝑚𝑚
This was done for every month and the results were tabulated as below:
Table 20: Runoff
Month Design Rainfall Runoff (mm) Volume of runoff
(m3)
Jan 36 5 250
Feb 43 8 400
March 122 65 3250
|April 138 79 3950
May 143 83 4150
June 95 43 2150
July 71 25 1250
Aug 53 14 700
Sept 121 64 3200
Oct 86 36 1800
Nov 167 104 5200
Dec 157 95 4750
Total runoff per year is 31050m3
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Figure 8: Water pan contour map
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5.5 Components of the water reservoir
I) Reservoir capacity
II) Reservoir area
II) Watering point
III) Cut off drain
5.5.1 Reservoir capacity
This was determined using a contour map of the entire reservoir area attached. A triangulation
survey of the area was done using survey equipment: total station and GPS, a contour map was
then drawn from which the volume was computed. The sectional areas were obtained using a
planimeter. The work is summarized in the table below.
Table 21: Contour elevations
Contour
elevation
capacity
Contour
interval
Contour
elevation
difference
Water depth Average area
(m2)
Estimated area
1221.40 0.5 0 0 0 0
1221.80 0.5 0.4 0.40 14 52
1222.00 0.5 1.0 1.40 56 141
1222.24 0.5 0.24 1.64 222 372
1222.80 0.5 0.56 2.2 256 411
1223.00 0.5 0.2 2.4 542 507
1223.60 0.5 0.6 3.0 977 1308
1224.20 0.5 0.6 3.6 878 1404
1224.80 0.5 0.2 3.8 950 1581
1225.40 0.5 0.6 4.4 1089 1599
1225.80 0.5 0.4 4.8 1116 1603
1226.40 0.5 0.6 5.4 1140 2317
1226.80 0.5 0.4 5.8 1254 2714
From the calculation above, the reservoir capacity at 3.6m was found to be 5301 m3
F21/1714/2010 Page 40
5.5.2 Reservoir area
According to the Ministry of water and Irrigation guidelines manual, the depth should not be less
than 2.5m and should not exceed 5.00m, average depth of 3.50m, side slope of 1: 2.5 on the
upstream shoulder top width of the embankment as 4m to allow for machine operations, has been
adopted.
The bed of the reservoir dimensions is derived from:
b = √3𝑉 − 𝑑3 𝑧2
3𝑑− 𝑑. 𝑧
where, b= width of reservoir bottom (m)
v= volume of reservoir (m3)
d= depth of reservoir
z= side slope ratio (Horizontal: vertical)
b = √(3 × 5500) − (3.53 × 2.52)
(3 × 3.5)− 3.5 × 2.5
b = √16500 − 267.67)
10.5− 8.75 = 30.57
Taking b as 30m, if the bottom shape of pan a square then the area would be
Area= b2 = 30m×30m =900m2
But taking a rectangular shape assume dimensions that produce the same area and applicable to
site conditions,
Thus, 45m× 20m= 900m2
Reservoir storage capacity is generated using Primoidal formula FAO Irrigation manual module
7
𝑉 =𝑑
6(𝐴0 + 441 + 𝐴2)
Where V is the volume of the reservoir (m3)
F21/1714/2010 Page 41
d is the depth of the reservoir (m)
A0 is the area of bottom of reservoir (m2)
A1 is the area of half depth of reservoir from top (d/2) m2
A0 =45*20=900m2
Top of pan length = Length of bottom of reservoir+ 2*depth*2.5 (from Trapezoidal formula)
L2= L0 + 2D*2= 80+ 2*3.5*2.5= 97.5m
Top of reservoir width= Width of bottom of reservoir+ 2*depth*2.5
W2 = 20m+2*3.5*2.5= 37.5
Calculating for pan at half depth
i.e. d= 1.75
Length of A1 = 80m+ 2*1.75*2.5= 88.75
Width of A1 = 20m+ 2*1.75*2.5= 28.75m
Am = 88.75m*28.75m= 2551.3m2
A2 = 97.5m*37.5m= 3656.25m2 (Are the top reservoir dimensions)
𝑉 =3.5
6(900+4×2551.3+3656.25)
=7735.84m3
From contour drawings, the inlet level to the reservoir shall be at 1223.00m, the depth is fixed at
3.5m and bottom bed level is at 1219.00m with full water level at 1222.50m.
5.5.3 Spillway
Spillway dimensions is given a minimum of 1.5m as per the Ministry of water and Irrigation
guidelines Manual, the pan does not water from a catchment but from a spring calculated to be
1.65 L/s which would give small dimensional spillway. Spillway crest is at a level 1222.50 with
excess water depth above crest fixed at 0.5m.
Figure 9: Spillway
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Spillway, all dimensions given in meters.
5.5.4 Embankment
The slope of the upstream is fixed at 1:2.5 and downstream part is fixed at 1: 1.5 as per the soils
of the site and the Kenya Belgium Water development Program guidelines manual for small
dams in Kenya. The height of the embankment is calculated 1223.50m (bed level
1219.00+normal water depth 3.5m+flood flow depth 0.5m+ freeboard 0.5m with a top width of
4m and a length of 207.30m.
Draw off inlet
Inside the reservoir, the draw off inlet at the pan bed shall be at level of 1219.00m. Water shall
be drawn from the pan with pipe G.I of 100mm diameter with screen sieves filter provided at
slope for a length of 42m. Maximum water level in the reservoir shall be at 1223.00m giving a
head of 2m.
Draw off outlet
The draw off outlet is set at level 1219.50m.
5.6 Volume of earth from pan bed
The surface area of the pan at full water level is 3656.25 m2 with a depth of 3.5m the site bearing
being inclined. Below is the formula from the ministry of water guidelines manual shall be used
for calculation of the earth works.
𝑉 =𝐿 × 𝐵 × 𝐻
6
Where V is the volume earth in m3 L is crest length of pan in m, B is top reach of water in m, H
is the depth of excavation in m.
𝑉 =37.5𝑚 × 110𝑚 × 3.5𝑚
6= 2406.25𝑚3
5.7 Volume of earth on embankment
The volume of earthworks is determined by using the following formula for site with
characteristics of a dam from FAO manual (Paper page 64) on design of small earth dam.
V=0.216HL (2C+HS)
Where V= Volume of earthworks in m3
H= Crest height of the dam in m= 4.5m
L= Length of the dam, at crest height in m (including the spillway) = (207.4+10)= 217.4m
C= is the crest width in m= 4m
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S is the combined slope value i.e. slopes of the embankment are 1: 2.5 and 1: 1.5 = (2.5+1.5)
V=0.216×4.5×217.4(2×4+4.5×4) = 5494.13m2
The spillway length shall be 76m and that is the natural waterway. The spillway dimensions have
been determined using the MOWI guidelines for water pan construction.
The spillway width of 10m at the bottom and depth of 0.45m with sill of 0.5m×15m×1m at the
entrance for maintaining a constant water level at the reservoir.
Excavation volume is 4.703m2 × 76= 357.43m2
Concrete for sill, volume 0.5m×15m×1m = 7.5 m2
Concrete scour checks or drop structures 1 in number to reduce the slope from 5.6% to 1%
0.2m×12m×1m×1 in number = 2.4m3
5.8 Impermeable layer for lining
The site is located at a place which when augured to a depth of 1m had indications of gravel
which has high percolation rates.
It is a must that clay soils be borrowed for lining the bed.
S.A = Perimeter × Length
= 37.5m × 124m = 4650m2
The lining shall be compacted to a thickness of 0.25m giving a volume of 4650 × 0.25 = 1162.5m2
Table 22: Summary of reservoir levels
Name Level (m) Dimensions
Pan bed at deepest level 1219.00 45m×20m
Pan at normal water level 1222.50 53.75m×28.75m
Draw off intake 1219.50
Top embankment 1224.00 57.5m×37.5m
Spillway crest 1222.50
Flood water level 1223.00
BM 1 1227.94
BM 2 1226.87
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5.9 Calculations for determining reservoir parameters
5.9.1 Site clearance
Surface area to be cleared is 6000m2
Excavation of reservoir bed to a depth of 3.5m
The surface area of the pan at full water level is 3656.25m2 with a depth of 3.5m the site being a
depression pyramid formula
𝑉 =𝐿𝐵𝐻
6
Where V is the volume earth in m3 , L is the crest length of pan in m, B is top reach of water in
m, H is depth of excavation in m.
𝑉 =37.5×110×3.5
6= 2406.25𝑚2
5.9.2 Volume of earthworks on embankment
The volume of earthworks is determined by using the following formula for site with
characteristics of a dam from FAO manual on design of small earth dams paper 64:
Where:
V= the volume of earthworks in m3
H= Crest height of dam in m= 4.8m
L= length of the dam, at crest height in m (including the spillway).
Volume of earthworks diagram:
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Figure 10: Embankment dimensions
The dimensions are given in metres.
The volume of the earthworks is obtained by Queensland’s method using the formula:
Ve= 1.05 × K× B × H × (H×1)
K being the coefficient of Cross section of the dam site which is 1.5.
The length of the dam B is (4+1.8+3.0) m= 8.8m = 9.0m
Maximum height of the embankment is 4.5m
Ve= 1.05 × 1.5× 9 × 4.5 × (4.5+1) = 350.83m3
5.9.3 Embankment force analysis
There mainly two forces acting on the earth-dam embankment: vertical force due to self-weight
of the dam and the horizontal force due to the pressure exerted by the water.
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Forces in embankment
Horizontal force (FH)
Pressure= rgh.
= 4.5*1000*9.81= 44.145KN/m2
Force= Pressure *Area
Area=b*d*z*d2
Area= (8.84×4.5) + (4×4.52)
=120.78m2
Therefore, FH =44.145×120.78
=5331.83KN
=5.332MN
Vertical force (FV)
The bulk density of the material was tested and found to be 1.63g/cm3.
This is also equal to 16.30 KN/m3
The volume of the earthworks is 350.83 m3
Total vertical force due to weight of the dam is:
= 350.83 × 16.30= 5718.53 KN/m2
FV =5.712MN/m2
The vertical force is higher than the horizontal hence the dam is safe against overtopping.
5.9.4 Sedimentation in reservoirs
All streams carry sediments and when flow becomes stationery, as in a dam, the sediments settle
at the bottom. This only becomes a problem when excess sedimentation occurs since limited
sedimentation is beneficial to the dam as it can form a thin watertight seal on the reservoir floor.
Excessive sedimentation reduces the capacity of the reservoir and blocks the outlet from the
dam.
Estimation of sediment load
Upland forests with mineral soils fully protected by a cover of forest litter and humus contribute
little or no sediments to streams. Annual runoff from small-forested catchment carries sediment
loads of up to 2-4 tonnes/mi2. Stream channels erode during periods of high velocity of
floodwaters. Annual sediment discharge from larger catchments involving stream channels range
from 22-400 tonnes/mi, depending on the amount of forest cover depending on the table below:
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Table 23: Forest cover and annual sediment yield
Forest cover in catchment (%) Sediment yield from catchment (Tonnes/mi2)
20 400
40 200
60 90
80 45
100 22
Source: Harold et al (1976)
5.9.5 Prevention and control of sedimentation
Sediment trap
Sediment traps are small impoundments that allow sediments to settle out of run-off water. They
are usually installed in a drainage area or other point of discharge form a disturbed area.
Temporary diversions can be used to direct the runoff from sediment traps. Sediment traps are
used to detain sediments in storm water runoff and trap the sediment to protect the receiving
lakes, drainage systems and the surrounding area. Sediment traps are formed by excavating an
area or by placing an earthen embankment across a low area or drainage swathe.
Sediment traps should be installed as early as possible in the construction process. Natural
drainage patterns should be observed and sites where runoff from potential erosion can be
directed into the traps should be selected. Sediment traps should not be located in areas where
their failure due to storm water runoff can lead to further erosive damage of the landscape.
Alternative diversion pathways should be designed to accommodate potential overflows. A
sediment trap should be designed to maximize surface area for infiltration and sediment settling.
This will increase the effectiveness of the sediment trap and reduce the chances of backup during
and after periods of high runoff intensity. Although site conditions will dictate specific design
criteria, appropriate storage capacity of each trap should be at least 1800ft3 per acre of total
drainage area (Smolen et al, 1998).
The reservoir trap efficiency is a measure of the proportion of the total volume of sediment that
is deposited to that which enters the reservoir. It can be expressed as:
𝑇𝑟𝑎𝑝 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =𝑆𝑒𝑑𝑖𝑚𝑒𝑛𝑡 𝑑𝑒𝑝𝑜𝑠𝑖𝑡𝑒𝑑
𝑠𝑒𝑑𝑖𝑚𝑒𝑛𝑡 𝑖𝑛𝑓𝑙𝑜𝑤
Trap efficiency is related to the Gross storage ratio, which is expressed as:
𝑆𝑅𝑔 =𝐷𝐶
𝑀𝐴𝐼
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Where:
SRg is the gross storage ratio
DC is the dam capacity (m3)
MAI is the mean annual inflow into the reservoir (m3)
For large dams with a gross storage ratio of at least 0.10, the trap efficiency is 100% as it is
assumed that all the sediment will be settled. For very small dams, there will be almost
continuous spilling and only the bed load will settle, thus the trap efficiency will be less than
100% (Ondieki, 2014)
Trap efficiency is a function of surface area, inflow rate and the sediment properties. The total
mass of sediment entering the reservoir each year is obtained using the formula:
SM= MAI*CS
Where:
SM is the sediment mass entering the reservoir annually (Kg)
MAI is the mean annual inflow into the reservoir (m3)
SC river sediment concentration (Kg/m3)
The mean annual inflow is calculated (MAI) is calculated using the formula:
MAI= CA*MA
Where:
CA is the catchment area behind the dam (m2)
MAR is the Mean Annual Runoff (mm)
Therefore, the volume of sediments depositing in the reservoir every year can be calculated using
the following formula:
𝑆𝑉 =𝑆𝑀
𝛿
Where
SV is the sediment volume deposited in the reservoir annually (m3)
SM is the sediment mass entering the reservoir annually (m3)
Δ is the density of deposited sediments (Kg/m3)
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5.9.6 Dam yields
The dam yield (Q) is the volume of water in m3 that can be drawn from a reservoir behind a dam
for use each year at the designated risk level.
The following parameters are used in the estimation of dam yield:
Dam catchment area, CA (Km2)
Mean annual runoff, MAR (mm)
Gross mean annual inflow into the reservoir, MAI (m3): the product of CA and MAR
Evaporation, E (mm), which is the annual net water loss from a free water surface
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6.0 DESIGN DRAWINGS
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7.0 THE BILL OF QUANTITIES AND THE COST BENEFIT RATIO
7.1 The Bill of Quantities
BILL OF QUANTITIES
Construction of an earth dam in Siaya
General item
Item
number
Description Unit Quantity Rate Amount
1.01 Mobilization and
demobilization. To
mobilize to the site and
to demobilize from the
site all materials,
equipment, machines
and staff camping.
Lumpsum 200000.00
Site Clearance
2.01 Clearance and grub area
of the works in
accordance with the
specifications.
ha 0.25 50000 25000.00
Materials
3.01 Clay for lining the water
pan
m3 930 500 465000.00
3.02 Hardcore for
construction of the heel
m3 20 10000 200000.00
3.03 Cement for construction
of the heel
m3 8 1000 288000.00
3.04 Ballast for construction
of the heel
m3 20 15000 300000.00
Dewatering
4.01 Dewatering of the dam
site
ha 0.25 200000 50000.00
Earthworks
5.01 Dam excavation M3 350.83 3000 1052490.00
5.02 Excavation for
spillway(checkstructure),
foundation including
stilling basin and
retaining wall in any soil
as per drawings.
m3 150 4000 600000.00
Dam embankment
6.01 Construction of the
earthen embankment.
M3 390 5000 1950000.00
TOTAL= KShs. 5130490
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7.1 Cost benefit analysis
The dam is expected to supply water for irrigation to the residents of Tingare in Siaya County.
Drip irrigated commercially contracted Soya bean banana poultry and fish value chain systems
on 5 will be installed.
Average yield per season of Soya bean per Ha per year is projected to be around 500 bags of 90
ha in 3 growing seasons.
For the 5ha, the total yield is expected to be 2500 bags of 90 Kg.
Total yield per year per ha is 225000 Kg.
On a good sales period, a Kg of Soyabeans goes at KShs. 50.
Total returns per year from Sales of Soya beans will be Shs. 11250000.
Hence, the Cost- Benefit ration will be determined as follows:
𝐶𝐵𝑅 =𝑇𝑂𝑇𝐴𝐿 𝐶𝑂𝑆𝑇 𝐼𝑁𝐶𝑈𝑅𝑅𝐸𝐷
𝐵𝐸𝑁𝐸𝐹𝐼𝑇𝑆 𝐸𝑋𝑃𝐸𝐶𝑇𝐸𝐷=
11250000
5130490= 2.19
The CBR is far much more than 1 hence the enterprise is worth being adopted.
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7.0 CONCLUSION AND RECOMMENDATION
7.1 Conclusion
Food security is a major challenge facing communities in Kenya. Another serious problem facing
some parts of the country is flood during heavy downpour. Flooding causes wealth destruction
and loss of lives if appropriate control measures are not taken. This twin problem can be solved
by designing water storage facility which can store water during heavy downpour and the same
can be used for irrigation.
The main objective of the design was to design an off stream earth dam with an adequate
reservoir which can supply water for small scale irrigation in Tingare Siaya County. This was
achieved. During the study, it was found out that indeed, agricultural production suffers due to
inadequate rainfall yet during heavy downpour flooding occurs leading to loss of wealth and
even life.
The specific objective to determine the crop water requirement was achieved by considering the
crop water requirement for Soya beans for the 85 days it is expected to grow to maturity in the
tropics. The third specific objective for designing the embankment was also achieved as the dam
was designed for safety against overturning or slip failure. This was done by considering the
volume of the earthworks in the embankment viz-a-viz the volume of water in the reservoir.
The Cost-Benefit ratio was determined and found to be 2.19. This value is far much more than 1
which shows that the project is worth being adopted as wealth will be created due to the much
benefits that will be realized.
7.2 Recommendations
During soil study, it was discovered that soil at the proposed site has gravel which has high
percolation rates. Therefore, it is a must that clay soil be borrowed for lining the bed.
During the design of the project, one of the most important factors is community consultation
and participation. This is important for the long term project maintenance, operation and overall
acceptance.
The operation of the project is very important. Some of the issues which should be looked at
include: the dam site should be secured by fencing to avoid accidents by drowning and to avoid
damage to the embankment by animals. To reduce evapotranspiration rates, trees with low
evapotranspiration rates should be planted near the site.
F21/1714/2010 Page 55
8.0 REFERENCE
1) Nelson K.D (1921). Design and construction of small dams. Melbourne: Inkata.
2) Watts, G (1978). Design of dams and reservoirs
3) Walstrom, E.E (1974). Dams, dam foundations and reservoir sites. Amsterdam:
Elsiever.
4) Pieck, C. (1985). Catchment and storage of rainwater.
5) Nelson K. D. (1985). Design and construction of small earth dams. Melbourne Inkata.
6) Doreenbos, J and Kassam A. H (1986). Yield response of water.
7) Department of soil conservation and extension, G. O (1986). Handbook on basic
Instructions of dam construction. Harare: Longman.
8) Jansen R.B ed 1988. Advanced dam engineering for design , construction and
reharbilitation, Van Nostrand Reinhold, New York.
9) Fowler J. P (1989). The design and construction of small earth dams, Appropriate
technology. London I. T Publication.
10) USEPA. (1993) Dam design. New-York: US Government.
11) Subramanya K. (1994). Engineering Hydrology.
12) Kenya Gazette (2002). The water Act. Nairobi: The government printer.
13) Sharma I.K and Sharma T.K (2003). A textbook on waterpower engineering. New
Delhi: S Chand and Co. Ltd.
14) MOWI. (2005). Practice manual for water supply systems. Nairobi. Ministry of Water
and Irrigation.
15) General design and construction considerations for earth and rockfill dams. US Army
Corps of engineers. 30th July 2004.
16) Pumnia, B.C and Lal P. B. (2009). Irrigation and water power engineering. New
Delhi: laxmi Publications (P) Ltd.
17) Manual on small earth dams. A guide to siting, design and construction. FAO
Irrigation and drainage paper 2010.
18) Khurmi, R.S (2010). A textbook of Hydraulics, Fluid mechanics and hydraulic
machines. New delhi: S Chand and Co. Ltd.
19) Groot Letaba river water development project. Technical study module: preliminary
design of raising Tzaneen dam. May 2010.
20) Ondieki S. C (2014). Hydrological design. S. C. Ondieki Nairobi.
F21/1714/2010 Page 56
9.0 APPENDICES
Appendix I
(Source Wikipedia)
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Appendix II
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Appendix III
Nyamakoye pan plan and Cross-Sections
A
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Appendix IV
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Appendix V
Minimum Crest widths
Height of dam (m) Crest width (m)
Up to 2 2.5
2.1 to 3 2.8
3.1 to 4 3.0
4.1 to 5 3.3
5.1 to 6 3.5
6.1 to 7 3.7
7.1 to 8 3.9
8.1 to 9 4.0
9.1 to 10 4.2
Source: Nelson, 1985
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Appendix VI
Some typical slopes for homogeneous dams
Height of a
dam (m)
Slope
GC SC CL CH
Up to 3 Upstream 2.5:1 2.5:1 2.5:1 3.0:1
Downstream 2.0:1 2.0:1 2.0:1 2.5:1
3.1-6 Upstream 2.5:1 2.5:1 2.5:1 3.0:1
Downstream 2.5:1 2.5:1 2.5:1 3.0:1
6.10-10 Upstream 3.0:1 3.0:1 3.0:1 3.5:1
Downstream 2.5:1 3.0:1 3.0:1 3.0:1