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

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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

F21/1714/2010 Page 42

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

F21/1714/2010 Page 44

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:

F21/1714/2010 Page 45

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.

F21/1714/2010 Page 46

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

F21/1714/2010 Page 48

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)

F21/1714/2010 Page 49

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.

F21/1714/2010 Page 54

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