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CHAPTER 1: INTRODUCTION

Water is essential for life. People, animals, and plants all need water to live and to grow. But in

many parts of the world people lack enough water to stay healthy. Many people have to travel

long distances to collect water. And often the water that is available is not safe to drink. If

people do not have enough water for their daily needs, they face hardship and serious illnesses,

on the contrary, when a community has a water supply that is accessible and safe, everyone’s

health is improved.

Water is nature’s gift, but there is a limit to what nature can provide. In many places the

amount of water for drinking is becoming dangerously low. Where land has been paved and

trees cut down, rain that once soaked the ground and was stored as ground water now runs off

into the ocean and becomes salt water. Much of the water that is left is too polluted for human

use (Conant 2009).

Benefits of improved water availability

There a number of potential benefits of improved access to water supply, some of which are

listed below:

1. Convenience.

2. The time saved by having a safe water source closer to the household can be very

significant.

3. The physical requirement of having to fetch water from far is eliminated.

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4. Measures that improve the availability of water eliminate vendors from the picture and

thus directly benefiting families.

5. Water becomes available even for agricultural activities.

6. Sanitation is improved with water availability.

1.1 Statement of the problem and problem analysis

Lack of water for personal consumption and farming is the most urgent problem in Ukambani

and is for most a matter of basic survival. Water, water everywhere, but not a drop to drink

may be a famous saying, but it is a reality in Ukambani. A mention of water in this country more

often than not brings to mind the problems facing many Kenyans including people from

Nzeveni. Water in the study area is a rare commodity; this is because though it is essential to

human beings, it is not as equally available to all everywhere as it should be.

Currently, accessibility to water remains a major problem in the Nzeveni. The task of collecting

water from the only available source (private borehole) which is approximately 5km away is left

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to women and girls. The number of people trying to get water from the source creates

congestion and very long queues thus wasting most of their day time. This means that no other

work is done during the day except fetching water. Also in this private borehole, water is sold

and thus making it difficult for the poor families to access enough water.

The chronic water shortage is causing a serious concern to all: water crisis is due not only in the

wave of drought but also poor management and conservation of the water, under investment

and rampant deforestation. Though, the potential water resources available in the study area

are abundant, it is under- exploited. An area that suffers frequent water stress and water

scarcity is said to experience water shortage that causes serious production problems and

economic development.

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*Pictures showing residents of Nzeveni queuing for water

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1.2 Site analysis and inventory

Makueni County is an agricultural zone. Almost all rivers are seasonal. As a result, Makueni

often endures serious droughts, triggering poverty and destabilizing entire communities.

Scarcity of water is the over-arching theme in Makueni. Makueni has a population density of

110.4 people per km2. Temperatures in the county range from a minimum of 12ºC to a

maximum of 28ºC and rainfall ranges from 150 mm to 650 mm per annum, typical of ASALs in

Kenya. The county has two rainy seasons with two peaks in March / April (long rains) and

November/December (short rains). From June to October is a long dry period, while January to

March is a short one. The high temperature experienced in the low-lying areas cause high

evaporation.

Nzeveni is an area within this county. It is located at the west of Makueni, specifically in

Itumbule sub-location, Ngaamba, Kilome constituency. It borders Kajiado County. The major

activity in Nzeveni is subsistence agriculture.

Within Nzeveni is River Kiu, a seasonal river passing through the area. During the rainy season

this river has a lot of water and people use it for domestic purposes. However, this water does

not last for a long time; it dries off about a month after the rains. People therefore go back to

their normal hustle of getting water. This therefore calls for a measure to store this water for

use during the dry season.

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FIGURE 1

1.3 Objectives

1.3.1 Overall objective

The overall objective of this project is design a multipurpose dam to enable communities in

Nzeveni ,a typical ASAL in Kenya, gain access to water for their utilization and also have enough

to practice small scale farming and thus generate income for their families and alleviate poverty

within the community

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1.3.2 Specific objectives

The specific objectives are to:-

1. Estimate the water demand in the study area.

2. Identify the type and suitable location for the dam

3. Establish the amount of water that can be stored in the reservoir

4. Establish the specifications of the dam

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CHAPTER 2: LITERATURE REVIEW

2.1 Sources of water

There are basically three categories of naturally occurring water resources as given by Keast

&Gray (1999).

1. Groundwater: occurs under most of the worlds land surface, but there are great

variations in the depths at which it is found, its mineral quality, the quantities present

and the rates of infiltration (thus yields potential) and the nature of the ground above it

(thus accessibility).

2. Rainwater collection, from roofs or larger catchment areas ,can be utilized as a source of

drinking water, particularly where there are no other safe water sources available (for

example in areas where ground water is polluted or too deep to economically tap). In

extreme situations, small quantities of water can be condensed from the atmosphere

(as dew) on screens or similar devices.

3. Surface water, in streams, lakes and ponds it is readily available in many populated

areas, but it is almost always polluted. It should only be used after an elaborate

treatment process.

2.1.1 Factors to be considered in selecting a suitable water source

While water is a very essential commodity for life, careful considerations are needed in

selecting an appropriate source for community water supply. House, Reed & Shaw (1997)

outlined some of the pertinent considerations as discussed below:

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1) Socio- political and cultural considerations. If the water supply is not culturally

appropriate, and causes security difficulties or restricts access for certain groups the

benefits of the new system will be limited

2) Water committees. Water committees are set up in many areas to manage water supply

systems. Care must be taken to ensure that all groups in the community are represented

and can make their concerns and needs heard and understood

3) Operation and maintenance. Care must be taken when identifying personnel both to

undertake training and to be responsible for operation and maintenance.

4) Yield versus demand. The yield must be adequate. If a more convenient supply is

developed, then consideration must be given to the potential increase in demand and

the possible migration of outsiders into the community, particularly in areas where

water is scarce

5) Water quality. The water quality must also be acceptable and treatment methods suited

to the community concerned

6) Technical requirements. The development of the source must be technically feasible

and the operation and maintenance requirements for the source abstraction and supply

system must be appropriate to the resources available

7) Economic considerations. Care must be taken to ensure that funds are available for both

the construction and the operation and maintenance of the system over the longer term

8) Legal and management requirements. Current ownership of the land and the legal

requirements of obtaining permission to impound or abstract are also factors to

consider when selecting a source. Sources of private land may cause access problems

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for certain groups which may not be apparent at the outset. The consequences of sitting

decisions must be considered carefully

9) Impacts of development. The use of a particular water source will have impacts on the

people who use it, on animals, and on the environment. The impacts on people may be

negative or positive and maybe related amongst other things, to health, economic

status or time. If a surface water source is used there may be impacts on remote users

and, likewise, if waste water enters surface water sources there may be similar impacts.

Impacts on the environment may include loss of vegetation, erosion, or draining of an

aquifer.

2.2 Need for dams

In ancient times, dams were built for the single purpose of water supply or irrigation. As

civilizations developed, there was a greater need for water supply, irrigation, flood control,

navigation, water quality, sediment control and energy. Therefore, dams are constructed for a

specific purpose such as water supply, flood control, irrigation, navigation, sedimentation

control, and hydropower. A dam is the cornerstone in the development and management of

water resources development of a river basin. The multipurpose dam is a very important

project for developing countries, because the population receives domestic and economic

benefits from a single investment.

Demand for water is steadily increasing throughout the world. There is no life on earth without

water, our most important resource apart from air and land. During the past three centuries,

the amount of water withdrawn from freshwater resources has increased by a factor of 35,

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world population by a factor of 8. With the present world population of 5.6 billion still growing

at a rate of about 90 million per year, and with their legitimate expectations of higher standards

of living, global water demand is expected to rise by a further 2-3 percent annually in the

decades ahead.

But freshwater resources are limited and unevenly distributed. In the high-consumption

countries with rich resources and a highly developed technical infrastructure, the many ways of

conserving, recycling and re-using water may more or less suffice to curb further growth in

supply. In many other regions, however, water availability is critical to any further development

above the present unsatisfactorily low level, and even to the mere survival of existing

communities or to meet the continuously growing demand originating from the rapid increase

of their population. In these regions man cannot forego the contribution to be made by dams

and reservoirs to the harnessing of water resources.

2.2.1 The purposes of dams

Most of the dams are single-purpose dams, but there is now a growing number of multipurpose

dams. Using the most recent publication of the World Register of Dams, irrigation is by far the

most common purpose of dams. Among the single purpose dams, 48 % are for irrigation, 17%

for hydropower (production of electricity), 13% for water supply , 10% for flood control, 5% for

recreation and less than 1% for navigation and fish farming.

Irrigation:

Presently, irrigated land covers about 277 million hectares i.e. about 18% of world's arable land

but is responsible for around 40% of crop output and employs nearly 30% of population spread

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over rural areas. With the large population growth expected for the next decades, irrigation

must be expanded to increase the food capacity production. It is estimated that 80% of

additional food production by the year 2025 will need to come from irrigated land. Even with

the widespread measures to conserve water by improvements in irrigation technology, the

construction of more reservoir projects will be required

Hydropower:

Hydroelectric power plants generally range in size from several hundred kilowatts to several

hundred megawatts, but a few enormous plants have capacities near 10,000 megawatts in

order to supply electricity to millions of people. World hydroelectric power plants have a

combined capacity of 675,000 megawatts that produces over 2.3 trillion kilowatt-hours of

electricity each year; supplying 24 percent of the world's electricity.

In many countries, hydroelectric power provides nearly all of the electrical power. In 1998, the

hydroelectric plants of Norway and the Democratic Republic of the Congo (formerly Zaire)

provided 99 percent of each country's power; and hydroelectric plants in Brazil provided 91

percent of total used electricity.

Electricity generated from dams is by very far the largest renewable energy source in the world.

More than 90% of the world's renewable electricity comes from dams. Hydropower also offers

unique possibilities to manage the power network by its ability to quickly respond to peak

demands. Pumping-storage plants, using power produced during the night, while the demand is

low, is used to pump water up to the higher reservoir. That water is then used during the peak

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demand period to produce electricity. This system today constitutes the only economic mass

storage available for electricity.

Water supply for domestic and industrial use:

It has been stressed how essential water is for our civilization. It is important to remember that

of the total rainfall falling on the earth, most falls on the sea and a large portion of that which

falls on earth ends up as runoff. Only 2% of the total is infiltrated to replenish the groundwater.

Properly planned, designed and constructed and maintained dams to store water contribute

significantly toward fulfilling our water supply requirements. To accommodate the variations in

the hydrologic cycle, dams and reservoirs are needed to store water and then provide more

consistent supplies during shortages.

Inland navigation:

Natural river conditions, such as changes in the flow rate and river level, ice and changing river

channels due to erosion and sedimentation, create major problems and obstacles for inland

navigation. The advantages of inland navigation, however, when compared with highway and

rail are the large load carrying capacity of each barge, the ability to handle cargo with large-

dimensions and fuel savings. Enhanced inland navigation is a result of comprehensive basin

planning and development utilizing dams, locks and reservoirs which are regulated to provide a

vital role in realizing regional and national economic benefits. In addition to the economic

benefits, a river that has been developed with dams and reservoirs for navigation may also

provide additional benefits of flood control, reduced erosion, stabilized groundwater levels

throughout the system and recreation.

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Flood control:

Natural river conditions, such as changes in the flow rate and river level, ice and changing river

channels due to erosion and sedimentation, create major problems and obstacles for inland

navigation. The advantages of inland navigation, however, when compared with highway and

rail are the large load carrying capacity of each barge, the ability to handle cargo with large-

dimensions and fuel savings. Enhanced inland navigation is a result of comprehensive basin

planning and development utilizing dams, locks and reservoirs which are regulated to provide a

vital role in realizing regional and national economic benefits. In addition to the economic

benefits, a river that has been developed with dams and reservoirs for navigation may also

provide additional benefits of flood control, reduced erosion, stabilized groundwater levels

throughout the system and recreation.

2.3 Reservoirs

A reservoir is a large, artificial lake created by constructing a dam across a river. Broadly

speaking, any water pool or a lake may be termed a reservoir. However, the term reservoir in

water resources engineering is used in a restricted sense for a comparatively large body of

water stored on the upstream of a dam constructed for this purpose. Thus a dam and a

reservoir exist together. The discharge in a river generally varies considerably during different

periods of a year. If a reservoir serves only one purpose, it is called a single-purpose reservoir.

On the other hand, if it serves more than one purpose, it is termed a multipurpose reservoir.

The various purposes served by a multipurpose reservoir include :irrigation, municipal and

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industrial water supply, flood control, hydropower, navigation, recreation, development of fish

and wild life, soil conservation, pollution control and mosquito control.

Types of Reservoirs

Depending upon the purpose served, the reservoirs may be broadly classified into five types:

1. Storage (or conservation) reservoirs

2. Flood control reservoirs

3. Multipurpose reservoirs

4. Distribution reservoirs.

5. Balancing reservoirs

1. Storage reservoirs

Storage reservoirs are also called conservation reservoirs because they are used to conserve

water. Storage reservoirs are constructed to store the water in the rainy season and to release

it later when the river flow is low. Storage reservoirs are usually constructed for irrigation,

municipal water supply and hydropower. Although the storage reservoirs are constructed for

storing water for various purposes, incidentally they also help in moderating the floods and

reducing the flood damage to some extent on the downstream. However, they are not

designed as flood control reservoirs.

2. Flood control reservoirs

A flood control reservoir is constructed for the purpose of flood control it protects the areas

lying on its downstream side from the damages due to flood. However, absolute protection

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from extreme floods is not economically feasible. A flood control reservoir reduces the flood

damage, and it is also known as the flood-mitigation reservoir. Sometimes, it is called flood

protection reservoir. In a flood control reservoir, the floodwater is discharged downstream till

the outflow reaches the safe capacity of the channel downstream. The excess water is stored in

the reservoir. The stored water is subsequently released when the inflow to reservoir

decreases. Care is, however, taken that the discharge in the channel downstream, including

local inflow, does not exceed its safe capacity. A flood control reservoir is designed to moderate

the flood and not to conserve water. However, incidentally some storage is also done during

the period of floods. Flood control reservoirs have relatively large sluice-way capacity to permit

rapid drawdown before or after the occurrence of a flood.

3. Multipurpose Reservoirs

A multipurpose reservoir is designed and constructed to serve two or more purposes. Most of

the reservoirs are designed as multipurpose reservoirs to store water for irrigation and

hydropower, and also to effect flood control.

4. Distribution Reservoir

A distribution reservoir is a small storage reservoir to tide over the peak demand of water for

municipal water supply or irrigation. The distribution reservoir is helpful in permitting the

pumps to work at a uniform rate. It stores water during the period of lean demand and supplies

the same during the period of high demand. As the storage is limited, it merely helps in

distribution of water as per demand for a day or so and not for storing it for a long period.

Water is pumped from a water source at a uniform rate throughout the day for 24 hours but

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the demand varies from time to time. During the period when the demand of water is less than

the pumping rate, the water is stored in the distribution reservoir. On the other hand, when

the demand of water is more than the pumping rate, the distribution reservoir is used for

supplying water at rates greater than the pumping rate. Distribution reservoirs are rarely used

for the supply of water for irrigation. These are mainly used for municipal water supply.

5. Balancing reservoir

A balancing reservoir is a small reservoir constructed downstream of the main reservoir for

holding water released from the main reservoir.

2.4 Multipurpose Reservoirs

A reservoir is formed when a dam is constructed across a river. A multipurpose reservoir is a

manmade lake which is managed for multiple purposes. The multipurpose nature of these

facilities dictates that the agencies which manage them are responsible for balancing

competing demands. For example, managers responsible for hydroelectric power generation

often want to keep lake levels as high as possible, since the water stored in the reservoir serves

as a kind of "fuel" for their generators. However, managers responsible for flood control often

want to keep lake levels as low as possible to provide the maximum amount of storage capacity

for rainwater runoff.

However, there is a considerable choice of types of hydraulic structures, and deciding which

particular one to adopt will largely depend on the uses it will be put to, and on the overall

conditions of the area where it will be installed. In choosing a structure type, the on-site

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availability of building materials and the skills and experience of local workers should also be

verified, with an eye to future maintenance requirements.

The specific characteristics of each structure type should be taken into account to select a

structure that meets the demands and conditions of the particular site under consideration.

Therefore, it will be useful to provide a general classification of hydraulic works before moving

on to the analysis of the site selection procedure.

2.5 Location of site selection of reservoir

In selecting the site for a reservoir the following points are to be considered:

(i). It is located in the area of minimum percolation and maximum runoff.

(ii). Leakage in the selected area should be minimal to minimize the grouting works.

(iii). It should not be located on highly permeable rocks like shales, slates, gneisses,

granite, etc

(iv). Suitable dam site with water tight rock base should be available in the located area.

(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 location should have adequate capacity without submerging

excessive land and other properties.

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(viii). Located area should provide sufficient water depth with smaller water area. Higher

depth provides lower submerged area/unit capacity and decreases the possibility of

weed growth. Smaller surface causes less evaporation losses.

(ix). Reservoir site should exclude water from tributaries which carry high percentage of silt

and sediment.

(x). Reservoir location should be free from objectionable solution of minerals and salts.

(xi). Construction materials for the dam and other allied works should be locally available.

(xii). Suitable area near the location should be available for construction of staff quarters,

labor colonies, go downs, stack yards, etc.

(xiii). Mitchell emphasized that site should be selected after investigation, so that cost of

construction should be economic.

2.6 Reservoir Levels

1. Full reservoir level (FRL)

The full reservoir level (FRL) is the highest water level to which the water surface will rise during

normal operating conditions. The effective storage of the reservoir is computed up to the full

reservoir level. The FRL is the highest level at which water is intended to be held for various

uses without any passage of water through the spillway. In case of dams without spillway gates,

the FRL is equal to the crest level of the spillway. However, if the spillway is gated, the FRL is

equal to the level of the top of the gates. The full reservoir level is also called the full tank level

(FTL) or the normal pool level

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2. Normal conservation level (NCL)

It is the highest level of the reservoir at which water is intended to be stored for various uses

other than flood. The normal conservation level is different from the FRL as the latter may

include a part of the flood. However, if there is no storage for flood up to FRL, the normal

conservation level and the FRL become identical.

3. Maximum water level (MWL)

The maximum water level is the maximum level to which the water surface will rise when the

design flood passes over the spillway. The maximum water level is higher than the full reservoir

level so that some surcharge storage is available between the two levels to absorb flood. The

maximum water level is also called the maximum pool level (MPL) or maximum flood level

(MFL).

4. Minimum pool level

The minimum pool level is the lowest level up to which the water is withdrawn from the

reservoir under ordinary conditions. The minimum pool level generally corresponds to the

elevation of the lowest outlet (or sluiceway) of the dam. However, in the case of a reservoir for

hydroelectric power, the minimum pool level is fixed after considering the minimum working

head required for the efficient working of turbines. The storage below the minimum pool level

is not useful and is called the dead storage.

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5. Useful storage

The volume of water stored between the full reservoir level (FRL) and the minimum pool level is

called the useful storage. The useful storage is available for various purposes of the reservoir. In

most of the reservoirs, the useful storage is the conservation storage of the reservoir. However,

in the case of multipurpose reservoirs in which the flood control is also a designed function, the

useful storage is subdivided into (a) the conservation storage for other purposes and (b) the

flood control storage for the flood control, in accordance with the adopted plan of operation of

the reservoir. The useful storage is also known as the live storage.

6. Surcharge storage

The surcharge storage is the volume of water stored above the full reservoir level up to the

maximum water level. The surcharge storage is an uncontrolled storage which exists only when

the river is in flood and the flood water is passing over the spillway. This storage is available

only for the absorption of flood and it cannot be used for other purposes.

7. Dead storage

The volume of water held below the minimum pool level is called the dead storage. The dead

storage is not useful, as it cannot be used for any purpose under ordinary operating conditions.

8. Bank storage

If the banks of the reservoir are porous, some water is temporarily stored by them when the

reservoir is full. The stored water in banks later drains into the reservoir when the water level in

the reservoir falls. Thus the banks of the reservoir act like mini reservoirs. The bank storage

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increases the effective capacity of the reservoir above that indicated by the elevation-storage

curve. However, in most of the reservoirs, the bank storage is small because the banks are

usually impervious.

9. Valley storage

The volume of water held by the natural river channel in its valley up to the top of its banks

before the construction of a reservoir is called the valley storage. The valley storage depends

upon the cross section of the river, the length of the river and its water level. The net increase

in the storage capacity after the construction of a reservoir is equal to the total capacity of the

reservoir up to FRL minus the valley storage. However, this distinction between the net storage

capacity and the total storage capacity is not of much significance in a conservation or storage

reservoir where the main concern is the total water available for different purposes. But in the

case of a flood control reservoir, the difference between the net storage capacity and the total

storage capacity is quite important because the effective storage for flood control is reduced

due to the valley storage. The effective storage is equal to the sum of the useful storage and the

surcharge storage minus the valley storage in the case of a flood control reservoir.

10. Yield from a reservoir

Yield is the volume of water which can be withdrawn from a reservoir in a specified period of

time. The time period for the estimation of yield is selected according to the size of the

reservoir. It may be a day for a small reservoir and a month or a year for a large reservoir. The

yield is usually expressed as ha/year or Mm3/year for large reservoirs. As discussed later, the

yield is determined from the storage capacity of the reservoir and the mass inflow curve.

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11. Safe yield (Firm yield)

Safe yield is the maximum quantity of water which can be supplied from a reservoir in a

specified period of time during a critical dry year. Generally, the lowest recorded natural flow of

the river for a number of years is taken as the critical dry period for determining the safe yield.

However, there is a possibility that a still drier period may occur in future and the yield available

may be even less than that determined on the basis of past records. This factor should be kept

in mind while fixing the safe yield. There is generally a firm commitment by the organization to

the consumers that the safe yield will be available to them. It is therefore also called the firm

yield or the guaranteed yield.

12. Secondary yield

Secondary yield is the quantity of water which is available during the period of high flow in the

rivers when the yield is more than the safe yield. There is no firm commitment (or guarantee)

to supply the secondary yield. It is supplied on as and when basis at the lower rates. The

hydropower developed from secondary yield is sold to industries at cheaper rates. However,

the power commitment for domestic supply should be based on the firm yield.

13. Average yield

The average yield is the arithmetic average of the firm yield and the secondary yield over a long

period of time.

14. Design yield

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The design yield is the yield adopted in the design of a reservoir. The design yield is usually fixed

after considering the urgency of the water needs and the amount of risk involved. The design

yield should be such that the demands of the consumers reasonably met with, and at the same

time, the storage required is not unduly large. Generally, a reservoir for the domestic water

supply is planned on the basis of firm yield. On the other hand, a reservoir for irrigation may be

planned with a value of design yield equal to 1 - 2 times the firm yield because more risk can be

taken for the irrigation water supply than for domestic water supply.

2.7 Dams

2.7.1 Types of dams

Structures that are created as obstructions across rivers with an intention to store some of the

water for future use are called storage dams; they are functionally slightly different from

structures used for flow diversion. IIT, Kharagpur (2010) has broadly classified dams according

to construction materials as follows

1) Embankment dams

This are dams constructed of natural materials excavated or obtained from the vicinity of the

dam site. Two main types of Embankment dams that are commonly constructed include:

i. Earth-fill dams- these are dams that use compacted soil for constructing the

bulk of the dam volume. An earth-fill dam is constructed primarily of selected

engineering soil compacted uniformly and intensively in the relatively thin

layers and controlled moisture content

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ii. Rock fill dams- in these types of dams there is an impervious core of

compacted earth fill or a slender concrete or bituminous membrane but the

bulk of the dam volume is made of coarse grained gravels, crashed rocks or

boulders

2) Concrete dams

The use of mass concrete in dam construction started from about 1900 for reasons of ease of

the construction and to suite complex designs like having a spillway within the dam body. From

about 1950 onwards mass concrete came to be strengthened by the use of additives like slag or

pulverized fuel ash in order to reduce temperature induced problems or avoid undesirable

cracking or reduce the total cost of the project. Amongst concrete dams there are many

varieties the principal types of which are described below:

i. Gravity dams- a gravity dam is one which depends entirely on its own weight for

stability it may be constructed of either masonry or concrete

ii. Buttress dams- these types of dams consist of a continuous upstream face supported at

regular intervals by buttress walls and the downstream side

iii. Arch dam- these types of dams have considerable upstream curvature in plan and rely

on an arching action on the abutments through which of the water loads is passed onto

the walls of the river valley

2.7.2 Choice of site and type of dam

During the early stages of planning and design the selection of site and type of dam should be

carefully considered. Emiroglu (2008) states that the selection of the best type of the dam for a

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particular site calls for thorough considerations of the characteristic of each type, as related to

physical features if the site and the adoption to the purposes the dam is supposed to serve as

well as safety, economy and other pertinent limitations. The final choice of type of dam is

made after consideration of these factors. Some of the factors are discussed below as given by

Pierre, degoutte, & lautrin (n.d).

1) Topography and inflow in the catchment area

Topographic considerations include the surface configurations of the dam site and the reservoir

are and accessibility to construction materials. Topography in large measure dictates the first

choice of the dam. The accompanying task will then be to check whether conditions in the

catchment area are such that the reservoir will be filled and to calculate the risk of shortfall

2) Morphology of the river valley

A dam is by nature linked to an environment. The morphology of the river valley therefore plays

a vital role in the choice of a dam site and the most suitable type of dam. As a first approach a

wide valley is more suitable for the construction of a fill dam a narrow site for a gravity dam

and a very narrow site for an arch provided that the foundation is acceptable.

3) Geology and foundation conditions

Foundation conditions depend upon the geology character and the thickness of the structure

which are to carry the weight of the dam, their inclination, permeability and relation to

underlying strata, existing faults and fissures. The foundation will limit the choice of type to a

certain extent although such limitations can be frequently be modified considering the height

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of the proposed dam. If a high dam is being considered the compressive of the foundation is an

important consideration the selection of the type of dam

4) Available materials

Availability, on the site or near it, of suitable materials to build the dam has a considerable

influence and one that is often decisive in choosing the type of dam:

• Soil that can be used for earth fill

• Rock for rock fill or slope protection(rip-rap)

• Concrete aggregate(alluvial or crushed materials)

• Cementitious materials(cement, fly ash etc)

If it is possible to extract the materials from the reservoir itself, reservoir storage can be

increased. These also usually keeps the cost of transport and restoring borrow areas to a

minimum

5) Floods and flood discharge structures

The cost of flood discharge structures depend on the hydrological characteristics of the

catchment area. When the catchment area is large and floods are likely to be high, it may be

advantageous to combine the dam and spillway functions and build an overspill dam. On the

other hand if the spillway can be kept small a fill will be preferred if all other conditions are

equal. When construction of the spillway would require significant excavation, the possibility of

using the excavated materials is also a factor in favor of building a fill dam

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6) Economic criteria

After safety and fulfillment of the purpose of which the project is designed, cost is usually the

most important factor in the selection of the dam type.

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CHAPTER 3: THEORETICAL FRAMEWORK

3.1 Determination of design population and demand

3.1.1 Population Estimates and Projections

Standards of service provide important assessments of existing public facilities and programs in

light of desired objectives and the population/client groups to be served. Estimates of future

population, including demographic and geographic distribution, are required to translate these

standards into future capital improvement needs.

Demographic Techniques

Until very recently demographic projections (frequently known as "conditional forecasts") have

had no predictive pretensions. Demographers often apply a range of projections in lieu of more

definitive estimates of future population characteristics. Such parameters usually are

extrapolated from current data with insufficient detail to be of much utility to the capital

facilities planner.

The basic demographic equation is P2 = P1 + B - D + I - O, which indicates that the population at

any given point in time (P2) is a function of the population at a previous point in time (P1) plus

the amount of natural increase (births minus deaths) and the net migration (in-migration minus

out-migration) during the interim.

Population estimates are used to update population data gathered from the last census to

approximate the current situation. Population projections refer to future population levels and

indicate what changes might occur, given assumptions inherent in the projection method and

30

data. Analysts typically develop more than one set of projections, each set embodying different

assumptions. A population forecast is the set of projections deemed most likely to occur.

Projections do not necessarily lead to forecasts.

• Sets of projections often are prepared, ranging from slow growth to rapid growth, so

that users may select the forecast that most closely approximate their needs.

• Alternative projections may be based on the same method, differing only in their

designated growth rates, birth rates, population densities, and so forth.

Population change involves three separate components: births, deaths, and migration.

• Component models consider the separate effects of each of these factors.

• Non component models use the net effects of the three components.

Non component models may be based on past patterns of net population growth or may relate

net growth to some indicator, such as changes in housing or the economic base of the

community. Symptomatic data are used to determine a correlation between population size

and various other events, such as tax returns, voter registration, school enrollments, utility

connections, telephone installations, occupancy permits issued, and motor vehicle licenses.

Non component models lack detailed age-sex breakdowns which are useful in planning for

schools, community services, and different housing types.

Most models that project population below the state scale are usually of the non component

variety because of data limitations (and demographic skills).Births and deaths are referred to

as vital statistics, usually available on an annual basis.

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• Natural increase (or decrease) is numerical difference between births and deaths.

• A crude death rate is a gross statistic which indicates the number of deaths per 1,000 of

population; it provides no age-sex detail.

• A crude birth rate indicates births per 1,000 population, but provides no age-sex

information.

• General fertility rate is the ratio of births to women of child-bearing age (15 to 44 years

of age).

• Age specific fertility rate provides a greater level of specification by calculating fertility

rates for each 5-year age cohort of women.

• Birth rates and fertility rates change fairly slowly, and are subject to regional, racial, and

ethnic differences.

• Birth rates used in population projections often are determined empirically for the area

under analysis.

Migration is subject to relatively rapid fluctuations and is influenced by the location, size, shape,

and economic base of the locality. A large county or city will have a lower proportion of

migrants than will a small one, since many moves cover a relatively short distance.

The intercensal component method of estimating migration makes use of the population

balancing equation which rewrites the basic population equation as follows: I - O = P2 - P1 - B

+D

The reverse survival rate method may be used to produce net migration estimates by age, race,

and sex groups.

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• Estimates of net migration are produced by applying 10-year survival rates to the

number of individuals recorded in a particular cohort in the earlier census in order to

predict members in that cohort who should have survived to the current census.

• The difference between the actual number of individuals in the cohort that has been

"aged" by 10 years and the estimated number based on the survival rate is assumed to

be the estimated migration.

Types of Population Models

In choosing a population model, it is important to consider its relative accuracy, the type of

data available, the quality of available data, the scale of the analysis, the length of the

projection period, the purpose of the projections, and the budget and time frame implications

of the study.

Trend extrapolation is involved, to some extent, in nearly all projection methods.

• This model uses historical growth patterns to project the future pattern, dealing with

the net effects of births, deaths, and migration rather than with individual components.

• The primary disadvantage is the lack of detail regarding the components of population.

Comparative forecasting examines the locality's past growth pattern in conjunction with growth

patterns of older, larger, civil divisions, the assumption being that the locality's pattern will

match that of communities more advanced in their stage of growth.

33

Ratio trend or step-down techniques assume that the relationship of a locality to some larger

geographic entity--county or state--will prevail in the future and that population projections at

the larger scale represent degrees of reliability and component detail that are not possible to

achieve at the smaller scale of analysis.

Density ceiling models use capacity constraints by assuming that when a given density is

reached, population will either stabilize or decline.

• The density model may utilize linear, exponential, or logistic curves to express

population density growth rates.

• Maximum population levels typically determined via zoning and land use development

patterns that affect population density.

The ratio correlation method is similar to the ratio trend method except that population is

treated as a function of some other independent variables--employment, housing units, motor

vehicles registered, or other symptomatic data. Multiple regression may be used to determine

the population's historic relationship to the independent variables.

The housing unit method establishes a relationship between the number of dwelling units and

population via a family-size multiplier.

• Net changes in dwelling units presumed to indicate net changes in population.

• Dwelling units can be estimated by utility or telephone connections, building permit

data, land use surveys, vacancy rates, home interviews, and other local records.

34

Market force methods include: deterministic regression models, holding capacity, multiplier

studies, and mathematical programming.

• Linear regression may be used to formulate equations that will relate population

distribution to such factors as vacant land, the presence of minority group populations,

accessibility to work, land values, and other important variables.

• Employment forecasts made by shift and share, economic base, and input-output

techniques may be converted by the use of multipliers to population forecasts.

• The future population distributions may be modeled to represent improved conditions

in which a specific objective is sought--such as minimizing travel time to work--subject

to equations representing constraints on supply and demand for developable land,

avail-ability of services, and other factors.

The Greenberg-Kruckeberg-Mautner (GKM) model combines historical extrapolation, ratio

trend, and density ceiling alternatives at the local scale, constrained by federal-state-county

projections developed by component and market force techniques, and provides the option of

five separate sub models to project local population.

The residual method--through an examination of the records of births and deaths, the known

population (based on the last census) is adjusted accordingly to produce an estimate of current

population--the difference between this anticipated population and the actual population is

assumed to be the result of net migration.

The vital rates method is a ratio technique that relates total population to births and deaths.

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• Ratios between state and local births and deaths are derived from the historical records

and are then used to develop estimated populations based on births and deaths.

• Estimates based on the ratio are averaged to reduce errors involved in each of the

projections.

• Rapid migration, which affects the age structure, will impact the vital statistics, resulting

in inaccurate estimates.

Cohort-survival models project future population based on growth due to natural increase.

• The population is disaggregated into male and female age five-year cohorts, and age-

specific death rates (or survival rates) are developed and applied to each cohort.

• Age-specific fertility rates are applied to female cohorts between the ages of 15 and 44.

• Each cohort group is then "aged forward" towards the final projection year, with

mortality and fertility rates applied to the survivors at five year intervals.

• Births are added to the bottom of the pyramid and aged forward accordingly.

Various cohort-component methods have been developed by the Bureau of the Census.

• Method I uses school enrollment data to estimate the migration component--net

migration is assumed to be the difference between the growth rate of school-age

cohorts at the national level and the growth rate of school-age population at the scale

of analysis.

• Method II assumes that the migration component is the difference between the

anticipated school-age population, based on natural increase, and the actual population

36

of school age. A variation of Method II is the grade progression method that breaks

down school enrollment by grades.

The composite model applies various techniques to different segments of the total population.

• Instead of analyzing the components of change, the population is projected for different

age groups using different methods and then summed for a total population figure.

• It takes advantage of the fact that different methods are better focused for estimating

population of different groups.

The choice of the methods employed to develop population estimates or projections usually

involves a trade-off of some sort with the level of accuracy required, the availability of data,

and the composition of the final product as the chief determining factors.

3.1.2 Population projection

A population projection is an extrapolation of historical data into the future. It is an attempt to

describe what is likely to happen under certain explicit assumptions about the future as related

to the immediate past. A set of calculations, which show the future course of fertility, mortality

and migration depending on the assumptions used

Projection – Linear growth

It implies that there is a constant amount of increase per unit of time. A straight line is used to

project population growth. It is expressed as

Pt = P0 + bt

37

where

P0 = initial population

Pt = population t years later

b = annual amount of population change

Assumptions:

� Growth rate is constant

� Change is only experienced at the end of unit time

� Resultant change (i.e. interest) does not yield any change

Projection – Geometric growth

This growth assumes a geometric series. It is expressed as

Pt = P0 (1+ r)t

where

P0 = initial population

Pt = population t years later

Assumptions:

� Growth rate is constant

� Change is only experienced at the end of unit time

38

� Compounding takes place at specified intervals

Projection – Exponential growth

This is the equivalent to the growth of an investment with compound interest. Growth is

constant, but compounding is continuous. It is expressed as

Pt = P0(ert)

where

P0 = initial population

Pt = population t years later

r = annual rate of growth

e = base of the natural logarithm

3.2 Design Period

Projection Years

Water demand projections should normally, be made for the “initial” the “future” and the

“ultimate” year. The “initial” year is the year when the supply is expected to be taken into

operation that may be assumed to be 0-5 years from the date of the commencement of the

preliminary design. The “future” is 10 years and the “ultimate” year 20 years from the initial

year. Once the initial, future and ultimate

years have been determined for a project they should not normally be changed during the

design period.

39

Design Demand

A water supply should normally be designed for the ultimate demand. However phasing of the

implementation will often become a financial necessity and the possibilities of phasing should

therefore be examined using the initial and future demand projections. Mechanical equipment

is often designed for shorter periods.

Water demand formula

���������� = ������ ����� × ������� �

3.3 Rainfall analysis

3.3.1 Rainfall characteristics

Precipitation in arid and semi-arid zones results largely from convective cloud mechanisms

producing storms typically of short duration, relatively high intensity and limited areal extent.

Rainfall intensity is defined as the ratio of the total amount of rain (rainfall depth) falling during

a given period to the duration of the period It is expressed in depth units per unit time, usually

as mm per hour (mm/h).

The statistical characteristics of high-intensity, short-duration, convective rainfall are essentially

independent of locations within a region and are similar in many parts of the world. Analysis of

short-term rainfall data suggests that there is a reasonably stable relationship governing the

intensity characteristics of this type of rainfall.

40

3.3.2 Variability of annual rainfall

Water harvesting planning and management in arid and semi-arid zones present difficulties

which are due less to the limited amount of rainfall than to the inherent degree of variability

associated with it. In temperate climates, the standard deviation of annual rainfall is about 10-

20 percent and in 13 years out of 20, annual amounts are between 75 and 125 percent of the

mean. In arid and semi-arid climates the ratio of maximum to minimum annual amounts is

much greater and the annual rainfall distribution becomes increasingly skewed with increasing

aridity. With mean annual rainfalls of 200-300 mm the rainfall in 19 years out of 20 typically

ranges from 40 to 200 percent of the mean and for 100 mm/year, 30 to 350 percent of the

mean. At more arid locations it is not uncommon to experience several consecutive years with

no rainfall.

For a water harvesting planner, the most difficult task is therefore to select the appropriate

"design" rainfall according to which the ratio of catchment to cultivated area will be

determined.

3.3.3 Design rainfall

Design rainfall is defined as the total amount of rain during the cropping season at which or

above which the catchment area will provide sufficient runoff to satisfy the crop water

requirements. If the actual rainfall in the cropping season is below the design rainfall, there will

be moisture stress in the plants; if the actual rainfall exceeds the design rainfall, there will be

surplus runoff which may result in a damage to the structures.

41

The design rainfall is usually assigned to a certain probability of occurrence or exceedance. If,

for example, the design rainfall with a 67 percent probability of exceedance is selected, this

means that on average this value will be reached or exceeded in two years out of three and

therefore the crop water requirements would also be met in two years out of three.

The design rainfall is determined by means of a statistical probability analysis.

Probability analysis

The first step is to obtain annual rainfall totals for the cropping season from the area of

concern. In locations where rainfall records do not exist, figures from stations nearby may be

used with caution. It is important to obtain long-term records. As explained above, the

variability of rainfall in arid and semi-arid areas is considerable. An analysis of only 5 or 6 years

of observations is inadequate as these 5 or 6 values may belong to a particularly dry or wet

period and hence may not be representative for the long term rainfall pattern.

The next step is to rank the annual totals and to rearrange the data accordingly.

The probability of occurrence P (%) for each of the ranked observations can be calculated from

the equation:

��%� = � − 0.375� + 0.25 × 100

where:

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� = probability in % of the observation of the rank m

� = the rank of the observation

� = total number of observations used

The above equation is recommended for N = 10 to 100 (Reining et al. 1989). There are several

other, but similar, equations known to compute experimental probabilities.

The next step is to plot the ranked observations against the corresponding probabilities. For

this purpose normal probability paper must be used or a program (CumFreq)

Finally a curve is fitted to the plotted observations in such a way that the distance of

observations above or below the curve should be as close as possible to the curve .The curve

may be a straight line.

From this curve it is now possible to obtain the probability of occurrence or exceedance of a

rainfall value of a specific magnitude. Inversely, it is also possible to obtain the magnitude of

the rain corresponding to a given probability.

The return period T (in years) can easily be derived once the exceedance probability P (%) is

known from the equations.

! = 100� �"�����

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3.3.4 Rainfall-runoff relationship

When rain falls, the first drops of water are intercepted by the leaves and stems of the

vegetation. This is usually referred to as interception storage. As the rain continues, water

reaching the ground surface infiltrates into the soil until it reaches a stage where the rate of

rainfall (intensity) exceeds the infiltration capacity of the soil. Thereafter, surface puddles,

ditches, and other depressions are filled (depression storage), after which runoff is generated.

The infiltration capacity of the soil depends on its texture and structure, as well as on the

antecedent soil moisture content (previous rainfall or dry season). The initial capacity (of a dry

soil) is high but, as the storm continues, it decreases until it reaches a steady value termed as

final infiltration .The process of runoff generation continues as long as the rainfall intensity

exceeds the actual infiltration capacity of the soil but it stops as soon as the rate of rainfall

drops below the actual rate of infiltration.

The rainfall runoff process is well described in the literature. Numerous papers on the subject

have been published and many computer simulation models have been developed. All these

models, however, require detailed knowledge of a number of factors and initial boundary

conditions in a catchment area which in most cases are not readily available.

For a better understanding of the difficulties of accurately predicting the amount of runoff

resulting from a rainfall event, the major factors which influence the rainfall-runoff process are

described below.

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Factors affecting runoff

Apart from rainfall characteristics such as intensity, duration and distribution, there are a

number of site (or catchment) specific factors which have a direct bearing on the occurrence

and volume of runoff.

i. Soil type

The infiltration capacity is among others dependent on the porosity of a soil which determines

the water storage capacity and affects the resistance of water to flow into deeper layers.

Porosity differs from one soil type to the other. The highest infiltration capacities are observed

in loose, sandy soils while heavy clay or loamy soils have considerable smaller infiltration

capacities.

The FIGURE 2 below illustrates the difference in infiltration capacities measured in different

soil types.

45

The infiltration capacity depends furthermore on the moisture content prevailing in a soil at the

onset of a rainstorm. The initial high capacity decreases with time (provided the rain does not

stop) until it reaches a constant value as the soil profile becomes saturated. This however, is

only valid when the soil surface remains undisturbed.

It is well known that the average size of raindrops increases with the intensity of a rainstorm. In

a high intensity storm the kinetic energy of raindrops is considerable when hitting the soil

surface. This causes a breakdown of the soil aggregate as well as soil dispersion with the

consequence of driving fine soil particles into the upper soil pores. This results in clogging of the

pores, formation of a thin but dense and compacted layer at the surface which highly reduces

the infiltration capacity.

ii. Vegetation

The amount of rain lost to interception storage on the foliage depends on the kind of

vegetation and its growth stage. Values of interception are between 1 and 4 mm. A cereal crop,

for example, has a smaller storage capacity than a dense grass cover. More significant is the

effect the vegetation has on the infiltration capacity of the soil. A dense vegetation cover

shields the soil from the raindrop impact and reduces the crusting effect.

In addition, the root system as well as organic matter in the soil increases the soil porosity thus

allowing more water to infiltrate. Vegetation also retards the surface flow particularly on gentle

slopes, giving the water more time to infiltrate and to evaporate.

In conclusion, an area densely covered with vegetation, yields less runoff than bare ground.

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iii. Slope and catchment size

Investigations on experimental runoff plots (Sharma et al. 1986) have shown that steep slope

plots yield more runoff than those with gentle slopes. In addition, it was observed that the

quantity of runoff decreases with increasing slope length.

This is mainly due to lower flow velocities and subsequently a longer time of concentration

(defined as the time needed for a drop of water to reach the outlet of a catchment from the

most remote location in the catchment). This means that the water is exposed for a longer

duration to infiltration and evaporation before it reaches the measuring point. The same

applies when catchment areas of different sizes are compared. The runoff efficiency (volume of

runoff per unit of area) increases with the decreasing size of the catchment i.e. the larger the

size of the catchment the larger the time of concentration and the smaller the runoff efficiency.

Figure below clearly illustrates this relationship.

FIGURE 3-Runoff efficiency as a function of catchment size (Ben Asher 1988)

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3.3.5 Runoff coefficients

Apart from the above-mentioned site-specific factors which strongly influence the rainfall-

runoff process, it should also be considered that the physical conditions of a catchment area

are not homogenous. Even at the micro level there are a variety of different slopes, soil types,

vegetation covers etc. Each catchment has therefore its own runoff response and will respond

differently to different rainstorm events.

The design of water harvesting schemes requires the knowledge of the quantity of runoff to be

produced by rainstorms in a given catchment area. It is commonly assumed that the quantity

(volume) of runoff is a proportion (percentage) of the rainfall depth.

Runoff [mm] = K x Rainfall depth [mm]

In rural catchments where no or only small parts of the area are impervious, the coefficient K,

which describes the percentage of runoff resulting from a rainstorm, is however not a constant

factor. Instead its value is highly variable and depends on the above described catchment-

specific factors and on the rainstorm characteristics.

For example, in a specific catchment area with the same initial boundary condition (e.g.

antecedent soil moisture), a rainstorm of 40 minutes duration with an average intensity of 30

mm/h would produce a smaller percentage of runoff than a rainstorm of only 20 minutes

duration but with an average intensity of 60 mm/h although the total rainfall depth of both

events were equal.

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Determination of runoff coefficients

For reasons explained before, the use of runoff coefficients which have been derived for

watersheds in other geographical locations should be avoided for the design of a water

harvesting scheme. Also runoff coefficients for large watersheds should not be applied to small

catchment areas.

An analysis of the rainfall-runoff relationship and subsequently an assessment of relevant

runoff coefficients should best be based on actual, simultaneous measurements of both rainfall

and runoff in the project area.

The runoff coefficient from an individual rainstorm is defined as runoff divided by the

corresponding rainfall both expressed as depth over catchment area (mm):

# = $��%%����$� %�������

Actual measurements should be carried out until a representative range is obtained. Shanan

and Tadmor recommend that at least 2 years should be spent to measure rainfall and runoff

data before any larger construction programme starts. Such a time span would in any case be

justified bearing in mind the negative demonstration effect a water harvesting project would

have if the structures were seriously damaged or destroyed already during the first rainstorm

because the design was based on erroneous runoff coefficients.

When plotting the runoff coefficients against the relevant rainfall depths a satisfactory

correlation is usually observed

49

FIGURE 4-Rainfall-runoff relationships, Baringo, Kenya (Source: Finkel 1987)

A much better relationship would be obtained if in addition to rainfall depth the corresponding

rainstorm intensity, the rainstorm duration and the antecedent soil moisture were also

measured. This would allow rainstorm events to be grouped according to their average

intensity and their antecedent soil moisture and to plot the runoff coefficients against the

relevant rainfall durations separately for different intensities .Rainfall intensities can be

accurately measured by means of a continuously recording autographic rain gauge. It is also

possible to time the length of individual rainstorms and to calculate the average intensities by

dividing the measured rainfall depths by the corresponding duration of the storms.

When analyzing the measured data it will be noted that a certain amount of rainfall is always

required before any runoff occurs. This amount, usually referred to as threshold rainfall,

represents the initial losses due to interception and depression storage as well as to meet the

initially high infiltration losses.

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The threshold rainfall depends on the physical characteristics of the area and varies from

catchment to catchment. In areas with only sparse vegetation and where the land is very

regularly shaped, the threshold rainfall may be only in the range of 3 mm while in other

catchments this value can easily exceed 12 mm, particularly where the prevailing soils have a

high infiltration capacity. The fact that the threshold rainfall has first to be surpassed explains

why not every rainstorm produces runoff. This is important to know when assessing the annual

runoff-coefficient of a catchment area.

Assessment of annual or seasonal runoff

The knowledge of runoff from individual storms as described before is essential to assess the

runoff behavior of a catchment area and to obtain an indication both of runoff-peaks which the

structure of a water harvesting scheme must withstand and of the needed capacity for

temporary surface storage of runoff, for example the size of an infiltration pit in a micro

catchment system.

However, to determine the ratio of catchment to cultivated area, it is necessary to assess either

the annual (for perennial crops) or the seasonal runoff coefficient. This is defined as the total

runoff observed in a year (or season) divided by the total rainfall in the same year (or season).

# = &����"���������!�������%%����&����"���������!������ %�������

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The annual (seasonal) runoff coefficient differs from the runoff coefficients derived from

individual storms as it takes into account also those rainfall events which did not produce any

runoff. The annual (seasonal) runoff-coefficient is therefore always smaller than the arithmetic

mean of runoff coefficients derived from individual runoff-producing storms.

Runoff plots

Runoff plots are used to measure surface runoff under controlled conditions. The plots should

be established directly in the project area. Their physical characteristics, such as soil type, slope

and vegetation must be representative of the sites where water harvesting schemes are

planned.

The size of a plot should ideally be as large as the estimated size of the catchment planned for

the water harvesting project. This is not always possible mainly due to the problem of storing

the accumulated runoff. A minimum size of 3-4 m in width and 10-12 m in length is

recommended. Smaller dimensions should be avoided, since the results obtained from very

small plots are rather misleading.

Care must be taken to avoid sites with special problems such as rills, cracks or gullies crossing

the plot. These would drastically affect the results which would not be representative for the

whole area. The gradient along the plot should be regular and free of local depressions. During

construction of the plot, care must be taken not to disturb or change the natural conditions of

the plot such as destroying the vegetation or compacting the soil. It is advisable to construct

52

several plots in series in the project area which would permit comparison of the measured

runoff volumes and to judge on the representative character of the selected plot sites.

Around the plots metal sheets or wooden planks must be driven into the soil with at least 15

cm of height above ground to stop water flowing from outside into the plot and vice versa. A

rain gauge must be installed near to the plot. At the lower end of the plot a gutter is required to

collect the runoff. The gutter should have a gradient of 1% towards the collection tank. The soil

around the gutter should be backfilled and compacted. The joint between the gutter and the

lower side of the plot may be cemented to form an apron in order to allow a smooth flow of

water from the plot into the gutter. The collection tank may be constructed from stone

masonry, brick or concrete blocks, but a buried barrel will also meet the requirements. The tank

should be covered and thus be protected against evaporation and rainfall. The storage capacity

of the tank depends on the size of the plot but should be large enough to collect water also

from extreme rain storms. Following every storm (or every day at a specific time), the volume

of water collected in the rain gauge and in the runoff tank must be measured. Thereafter the

gauge and tank must be completely emptied. Any silt which may have deposited in the tank and

in the gutter must be cleared.

3.4 Catchment Area Delineation Using GIS technique

DEMs are used in water resources projects to identify drainage features such as ridges, valley

bottoms, channel networks, surface drainage patterns, and to quantify sub catchment and

channel properties such as size, length, and slope. The accuracy of this topographic information

is a function both of the quality and resolution of the DEM, and of the DEM processing

53

algorithms used to extract this information. Watershed delineation is one of the most

commonly performed activities in hydrologic analyses. Digital elevation models (DEMs) provide

good terrain representation from which watersheds can be derived automatically using GIS

technology. The techniques for automated watershed delineation have been implemented in

various GIS systems and custom applications

Terrain Processing

Terrain processing uses DEM to satisfy the surface drainage pattern. Once preprocessed, the

DEM and its derivatives can be used for efficient watershed delineation and stream network

generation. All the steps in the Terrain Preprocessing menu should be performed in sequential

order, from top to bottom. All of the preprocessing must be completed before watershed

processing functions can be used. DEM reconditioning and filling sinks might not be required

depending on the quality of the initial DEM. DEM reconditioning involves modifying the

elevation data to be more consistent with the input vector stream. By doing the DEM

reconditioning we can increase the degree of agreement between stream networks delineated

from the DEM and the input vector stream.

3.5 Determination of available reservoir capacity

Whatever may be the use of a reservoir, its most important function is to store water during

floods and to release it later. The storage capacity of a reservoir is, therefore, its most

important characteristic. The available storage capacity of a reservoir depends upon the

topography of the site and the height of dam. To determine the available storage capacity of a

reservoir up to a certain level of water, engineering surveys are usually conducted.

54

For accurate determination of the capacity, a topographic survey of the reservoir area is usually

conducted, and a contour map of the area is prepared. A contour plan of the area is prepared

to a scale of 1 cm = 100 m or 150 m with a contour interval of 1 to 3 m, depending upon the

size of the reservoir. The storage capacity and the water spread area at different elevations can

be determined from the contour map, as explained below.

(a) Area-Elevation Curve

From the contour plan, the water spread area of the reservoir at any elevation is determined by

measuring the area enclosed by the corresponding contour. Generally, a planimeter is used for

measuring the area. An elevation-area curve is then drawn between the surface area as

abscissa and the elevation as ordinate.

(b) Elevation-Capacity Curve

The storage capacity of the reservoir at any elevation is determined from the water spread area

at various elevations. The following formulae are commonly used to determine the storage

capacity (i.e. storage volumes).

1. Trapezoidal formula

According to the trapezoidal formula, the storage volume between two successive contours of

areas A1 and A2 is given by:

∆( = ℎ2 �*+ + *,�

Where ℎ is the contour interval.

Therefore, the total volume V of the storage is given by :

55

( = ∆(+ + ∆(, + ∆(-… .… =/∆(

or

( = ℎ2 �*+ + 2*, + 2*- +⋯2*12+ + *1�

Where is the total number of areas.

2. Cone formula

According to the cone formula, the storage volume between two successive contours of areas

A1 and A2 is given by:

( = ℎ3 �*+ + *, + 3*+. *,�

The total volume V is given by:

( = ∆(+ + ∆(, + ∆(-… .… =/∆(

3. Prismoidal formula

According to the prismoidal formula, the storage volume between 3 successive contours is

given by:

∆( = ℎ3 �*+ + 4*, + *-�

The total volume is given by

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( = ℎ3 5�*+ + *1� + 4�*, + *6 + *7 +⋯� + 2�*- + *8 +⋯�9

Where A3, A5, etc are the areas with odd numbers: A2, A4, A6, etc are the areas with even

numbers, A1 and An are respectively, the first and the last area. The prismoidal formula is

applicable only when there are odd numbers of areas (i.e. n should be an odd number). In the

case of even number of areas, the volume up to the last but one area is determined by the

prismoidal formula, and that of the last segment is determined by the trapezoidal formula.

3.6 Determination of the Required Capacity

The capacity required for a reservoir depends upon the inflow available and demand. If the

available inflow in the river is always greater than the demand, there is no storage required. On

the other hand, if the inflow in the river is small but the demand is high, a large reservoir

capacity is required. The required capacity for a reservoir can be determined by the following

methods:

i. Graphical method, using mass curves.

ii. Analytical method

iii. Flow-duration curves method

Graphical method

(a) Storage required for uniform demand.

The following procedure is used when the mass demand curve is a straight line.

57

1. Prepare a mass inflow curve from the flow hydrograph of the site for a number of

consecutive years including the most critical years (or the driest years) when the

discharge is low.

2. Prepare the mass demand curve corresponding to the given rate of demand. If the rate

of demand is constant, the mass demand curve is a straight line. The scale of the mass

demand curve should be the same as that of the mass inflow curve.

Analytical Method for Determination of Storage Capacity

Mass inflow, the storage capacity and the yield are interdependent. Because the inflow to a

reservoir is variable and at times less than the demand, the storage reservoir is required. The

storage capacity should be adequate to supply the water equal to the demand during the

critical period. The greater the demand, the larger will be the storage required. However, for a

long period, the total outflow volume from the reservoir must be equal to the total inflow

volume minus the volume of water lost and wasted during the period. In other words, the

reservoir does not produce water. It is a sort of water bank in which the total credit and total

debit during the period are equal. The capacity of the reservoir is determined from the net

inflow and demand. The storage is required when the demand exceeds the net inflow. The total

storage required is equal to the sum of the storage required during the various periods. The

following procedure is used for the determination of storage capacity.

1. Collect the stream flow data at the reservoir site during the critical dry period.

Generally, the monthly inflow rates are required. However, for very large reservoirs, the

annual inflow rates may be used.

58

2. Ascertain the discharge to be released downstream to satisfy water rights.

3. Determine the direct precipitation volume falling on the reservoir during the month.

4. Estimate the evaporation losses which would occur from the reservoir. The pan

evaporation data are normally used for the estimation of evaporation losses during the

month.

5. Ascertain the demand during various months.

6. Determine the adjusted inflow during different months as follows:

*�:����� %���= ������ %��� + ���� � ��� � − �;������ �− ���������� ��ℎ��<�

7. Compute the storage capacity for each months.

=����<���>� ��� = *�:�����?%��� − @����

The storage would be required only in those months in which the demand is greater

than the adjusted inflow

8. Determine the total storage capacity of the reservoir by adding the storages required

found in Step 7.

3.7 Investigations for Reservoir

The following investigations are usually conducted for reservoir planning.

1. Engineering surveys

2. Geological investigation

59

3. Hydrologic investigations

3.7.1 Engineering surveys

Engineering surveys are conducted for the dam, the reservoir and other associated works.

Generally, the topographic survey of the area is carried out and the contour plan is prepared.

The horizontal control is usually provided by triangulation survey and the vertical control by

precise leveling.

(a) Dam site: For the area in the vicinity of the dam site, a very accurate triangulation survey is

conducted. A contour plan to a scale of 1/250 or 1/500 is usually prepared. The contour interval

is usually 1 m or 2 m. The contour plan should cover an area at least up to 200m upstream and

400m downstream and for adequate width beyond the two abutments.

(b) Reservoir: For the reservoir, the scale of the contour plan is usually 1/15,000 with a contour

interval of 2 m to 3 m, depending upon the size of the reservoir. The area-elevation and

storage-elevation curves are prepared for different elevations up to an elevation 3 to 5m higher

than the anticipated maximum water level (M.W.L).

3.7.2 Geological investigations

Geological investigations of the dam and reservoir site are done for the following purposes.

i. Suitability of foundation for the dam.

ii. Water tightness of the reservoir basin

iii. Location of the quarry sites for the construction materials.

3.7.3 Hydrological investigations

The hydrological investigations are conducted for the following purposes :

60

i. To study the runoff pattern and to estimate yield.

ii. To determine the maximum discharge at the site.

Run off pattern and yield

The most important aspect of the reservoir planning is to estimate the quantity of water likely

to be available in the river from year to year and season to season. For the determination of the

required storage capacity of a reservoir, the runoff pattern of the river at the dam site is

required. If the stream gauging has been done for a number of years before the construction of

the dam, the runoff pattern will be available from the record. It is generally assumed that the

runoff pattern will be substantially the same in future also. The available record is used for

estimating the storage capacity. The inflow hydrographs of two or three consecutive bad years

when the discharge is low are frequently used for estimating the required capacity. However, if

the stream gauging records are not available, the runoff and yield have to be estimated

indirectly by the empirical (or) statistical methods. These are:

i. Runoff expressed as a percentage of rainfall.

ii. Runoff expressed as a residual of rainfall after deducting losses due to evaporation,

transpiration and ground water accretion.

iii. Run off expressed as a function of mean annual temperature and rainfall.

Maximum discharge

The spillway capacity of the dam is determined from the inflow hydrograph for the worst flood

when the discharge in the river is the maximum. Flood routing is done to estimate the

61

maximum outflow and the maximum water level reached during the worst flood. The methods

for the estimation of the maximum flood discharge are:

i. Empirical relations mostly correlated with the catchment area

ii. Statistical methods

iii. Unit hydrograph method

iv. Flood frequency studies

Usually for big reservoirs, a 1000 years flood is taken for spillway design

Site Investigations

Site investigations will have to be carried for the following reasons:-

• Location and condition of the bedrock. In Kenya the bedrock is often very close to

the surface and can easily give (spillway) construction or seepage problems.

• Location of permeable layers which might cause excessive leakage or even

undermine the dam.

• Location of foundation material under the wall and possible other structures.

Bedrock

Bedrock at shallow depths should be detected. The weathered parts under the dam’s

foundation can cause serious leakage and should therefore be removed. Heavy fractured

bedrock can cause serious losses through infiltration and another dam site should be

considered, or clay blanket applied to the bottom of the reservoir. The spillway could be badly

affected by the bedrock as simple excavation and ripping might not be possible, requiring

expensive blasting with the risk of fissuring the rock. On the other hand the presence of the

62

bedrock might enable the construction of the spillway on the bedrock thereby avoiding the

erosion problems common to earth channels. In this connection it might be more economical

to raise the dam use the bedrock as a natural spillway, in order to avoid expensive blasting.

Permeable layers

All permeable layers, like gravel and sand (old river-beds) or murram and laterite under the

dam or underlying the reservoir should be detected as extensive losses and even dam failure

can occur if their presence is not discovered. As a result a rather extensive investigation of the

dam site is required. International handbooks advise on investigation into the substrata as

deep as the dam is expected to be high.

Foundation

The test pits sunk in order to detect sand lenses and layers under the dam to be, will serve also

for testing suitability of the walls foundation. In Kenya there are generally no problems with

the foundation as the materials in site are consolidated (natural compaction) in such a way that

practically any earth wall can be build on it without any problems. Only in swampy areas

should care be exercised. Heavy structures, coming with the construction of the dam, like all-

concrete spillways or power stations etc. will require additional foundation tests.

Visual tests

With the eye a first classification of the available soils can be made, being unsuitable, heavy,

clays can be detected and classified as undesirable, as their swelling, shrinking and extensive

cracking makes them highly dangerous construction or foundation material. Soils with most

particles visible with the naked eye should be classified as sandy and generally considered

63

unsuitable for homogeneous dam construction soils with around 50% of the particles visible

with the naked eye should be considered good for homogeneous dam construction. Soils with

hardly any particles visible with the naked eye should be treated carefully and preferably

laboratory tested, as the clay fraction will probably be too large. Organic material rich soils

should be avoided.

LABORATORY TESTS

Tests are done for two purposes:

• Suitability of foundation and construction material

• During the construction the checking of the placed material, whether it meets the

required compaction specifications or not.

If there is a possibility to get testing done by experienced people in a laboratory undisturbed

samples should be taken from test pits, right away after excavation and be brought to a (field)

laboratory in sealed containers where at least the following test should be carried out:

(i) Gradation test: to see if a fairly even distribution of granularity is available

(ii) Water contents: of the soil on site in order to compute the additional water

requirements for optimum compaction.

(iii) Atterberg limits: to evaluate volume change potential and shooting strength of soils.

(iv) Proctor Compaction: test indicates the greatest dry unit weight obtainable under

optimum water contents. This should be controlled during construction. Usually 95% of

the optimum compaction is specified as minimum construction requirement.

64

Above described tests are to ensure good placing and compaction of the soils to be used for

construction, in case laboratory equipment is available. Otherwise compaction will have to be

done by feeling and between five and ten percent – volume wise – water should be added to

ensure decent compaction. The percentage mentioned is commonly required with the soils in

Kenya. Water should be added at the borrow area, to ensure better distribution in the soil.

3.8 Dam Design

The basic design of the embankment will include but not necessarily be limited to all the

components below

Dam Axis.

The dam axis should normally be designed straight unless special topography features dictate a

bend or a curved axis. The dam axis should be located in such a way that the minimum amount

of backfill will be required for the embankment. A narrowing down of the contour intervals on

the topographical map usually indicates this.

Foundation.

It should be expected that the whole foundation area of the dam will have to be cleared of all

vegetation and the topsoil containing organic materials, as their rotting will create seepage

paths and localized settlements. Equally should all the sand (or mud) be cleared from the

riverbed. No foundation slopes of over 20% (max 30%) should be allowed. Steep slopes will

create either dangerous sliding plane, through embankment settlement against the naturally

compacted original soil (in situ) making seepage lines (planes). In case of riverbed gullies

65

unequal settlement between two embankment sections might occur, also creating seepage

planes.

Height of Wall.

Establish approximate height of the dam wall plus the required freeboard (safety). It is noted

that dams with a height under 5.00 meters should be carefully considered before further design

as the freeboard is usually around 1.00 – 1.50 meters, evaporation in arid areas is 2.00 – 2.50

meters per year leaving 1.50 meters or less out of the 5.00 meters for actual net storage or

consumption, which means relative large quantities of backfill for little effective storage. Only

special cases will economically justify the construction of very low dams.

Slopes of Wall.

Slopes of the dam are dependent on the height as well as the materials used. The following

slopes are recommended:

TABLE

1

DAM HEIGHT

(m)

DAM TYPE

SLOPES

Upstream Downstream

Below 5

Good granular distribution

1:2.5 1:2

5 – 10

Good granular distribution;

very clayey

1:2.5

1:2.5

1:2

1:2.5

10-15

Good granular distribution,

clay

1:2.5

1:3

1:2.5

1:2.5

66

Application of a clay core does not change the slopes, however, in cases that the foundation

proves to be bad (e.g. clay) flatter slopes should be selected.

Crest Width.

The crest width of the dam should be 5.0 meters minimum in case heavy machinery will be

used for the construction, as this is a reasonable minimum width for the machines to be

maneuvered comfortably. A 1.5 meters to 2.0 meters crest width can be employed in cases of

labor intensive construction methods.

Dam crest.

If the dam crest is not used for traffic, it should be covered with 200 mm gravel to avoid drying

out and shrinking.

Core Trench.

A core trench is used to cut off bad underlying layers. In case machinery will be used for the

construction the minimum width of the core trench should be about 1.5 times the excavation

width of the machine. The depth of the core trench that should have been established by the

test pits will have to be confirmed by the Resident Engineer during the excavation, the trench

should not exceed a depth of 1/3 – 1/2 of the dam’s height.

Filter.

A toe filter and drain are used to prevent piping or seepage appearing on the downstream

slope. The toe filter is made from sand. In case graded sand is available it should be employed

if not any sand without organic matters to be found in the vicinity can be used. The thickness of

the toe filter can be 1.0 meter, of width 5.0 – 10 meters depending on the height of the dam.

67

The extent of the filter should be up to an elevation 4.0 to 5.0 meters under the crest level of

the embankment. The toe drain made from rocks or riprap of variable size (25mm – 250mm

diameter) to be placed against the sand filter to prevent the water draining from the filter of

carrying too many soil particles (piping). The toe drain is kept outside the embankment for

easier construction purposes, simple lorry dumping is possible and no angled filters are

required. It also forms a barrier against cattle wandering up the embankment.

Upstream Slope Protection.

In case of a long reservoir (large fetch) when strong wave development can be expected, the

upstream slope of the embankment should be protected by rip-rap (rough stone pitching) over

the area, which will be affected by the wave action.

Draw Off.

A draw off system or compensation outlet should be included, as pumping straight from the

reservoir will prove to be awkward with the fluctuating water levels. Either mentioned system

will be crossing in or under the embankment for either gravity supply or connection to a

pumping system. As this crossing pipe is a possible seepage hazard care should be taken with

the backfill (well compacted) around the pipe concrete collars should be constructed at set

intervals around the pipe to lengthen the seepage path and render it less straight.

68

CHAPTER 4: MATERIALS AND METHODOLOGY

4.1 Identification of dam site and type

A reconnaissance survey was carried out along River Kiu to determine the best type of dam and

the best location of the dam at the sections neighboring Nzeveni and Itumbule villages. Google

earth was used to acquire the coordinates of the site and also to acquire satellite imagery of the

site.

4.2 Data acquisition

The following datasets were collected:-

1. Demographic data

This was obtained from the Makueni District Statistics Office and from personal communication

with the area sub chief. The data includes:-

-population size (census 2009)

-population density

-number of households

-average household size

-average livestock ownership numbers

-institution population

69

2. Climatic data for Machakos weather station(this was the nearest station to the area of

study)

This data was collected from the Kenya Meteorological Department. This includes: -

-mean monthly rainfall data for the period 1990-2012

-mean monthly wind speeds for the period 1999-2003

-mean monthly pan evaporation for the period 1995-2000

3. Other datasets

A survey was carried out to establish the water demand categories and numbers for Nzeveni

and Itumbule villages

Land use information was obtained from the Machakos and Makueni districts farm

management handbook.

70

4.3 Data analysis approach

4.3.1 Determination of the available reservoir volume

Survey data (coordinates and height) for the area was acquired using Google Maps Find

Altitude(http://www.daftlogic.com/sandbox-google-maps-find-altitude.htm), this data helped

develop DEM for the area using Arc Map. This DEM was used to create contours for the area.

Areas between the contours were obtained by digitizing. The trapezoidal formula was then

used to establish the reservoir inter-contour storage volumes.

Trapezoidal formula ∆( = A, �*+ + *,�

Total volume was obtained by:

( = ∆(+ + ∆(, + ∆(-… .… = ∑∆(

Where h =contour interval

The cumulated storage volumes were then computed as a function of dam height and

tabulated in Table 1.

4.3.2 Determination of design rainfall

The design rainfall was determined from 23 years records of rainfall data for Machakos weather

station obtained from the Kenya Meteorological Station. To calculate design rainfall, the mean

annual rainfall depths were ranked and fitted in a Cumulative frequency distribution function of

the Frechet type (Fisher-Tippett 2) with the `CumFreq’ cumulative frequency program with a

90% confidence interval. The confidence limits of the 90% reliability rainfall were then

71

established from `CumFreq’s output. The lower confidence limit of the values was taken to be

the design rainfall as it would result into a design that would rarely fail.

4.3.3 Catchment runoff computation

The catchment was delineated from Google Maps Find

altitude(http://www.daftlogic.com/sandbox-google-maps-find-altitude.htm) and using Arc MAP

the DEM for the area was created and by digitizing, the area for the catchment was obtained.

The perimeter was also obtained by digitizing.

The catchment soil characteristics (textural class &infiltration ranges) obtained from Shisekanu

(2006) were used to establish the hydrologic soil group (HSG) of the soils in the catchment. The

determined HSG was used with the land cover classification values to calculate a weighted

runoff coefficient for the catchment.

The annual runoff volume from the catchment to the proposed dam site was then calculated

using Nelsons Method as a product of the catchment area and the design rainfall depth and the

runoff factor.

C���ℎ������%%�D ����� = 100 × * × $ × &

Where * = ����ℎ������� $ = �;���<������� %��� & = ���%%%�����

4.3.4 Water demand computation

From the demographic data obtained from the Sub-chief’s Office (Itumbule sub-location), the

human population growth is exponential based on a 3% growth rate experienced between 1999

72

and 2009. In order to compute the total water demand for Nzeveni, the following reasonable

assumptions were made:-

i) The future increase in commercial activity and schools is directly related to the

growth of human population (M.O.W.I ,2005).

ii) As a safety factor, the livestock population growth is proportional to that of the

population and each household has an average of 3 indigenous cows,6 goats and

sheep.

iii) One household has 6 people on average

iv) The school going population is 30%of the total population(primary and/or secondary

school) (M.O.W.I, 2005)

Population forecasting was carried out for the initial, future and ultimate years using the

exponential forecasting method with a 3% growth rate. Water demand computation was then

done for the various demand categories .(i.e. Human, livestock, institutional and commercial)

using the water consumption rates given by M.O.W.I(2005) for the demand categories.

Other forms of water and reservoir storage losses were also determined as follows:

i) Annual evaporation water loss from the reservoir was computed from an average of

the annual pan evaporation data of the period 1999-2003

ii) 40% of the annual water demand in the ultimate year was taken as the seepage loss

iii) 60% of the total water demand was taken as the sedimentation loss

73

4.3.5 Required reservoir storage volume

The required reservoir storage volume was determined using analytical mass curve method.

The capacity of the reservoir is determined from the net inflow and demand. The storage is

required when the demand exceeds the net inflow. The total storage required is equal to the

sum of the storage required during the various periods

Analytical method procedure

1. Collect the stream flow data at the reservoir site during the critical dry period.

Generally, the monthly inflow rates are required. However, for very large reservoirs, the

annual inflow rates may be used.

2. Ascertain the discharge to be released downstream to satisfy water rights.

3. Determine the direct precipitation volume falling on the reservoir during the month.

4. Estimate the evaporation losses which would occur from the reservoir. The pan

evaporation data are normally used for the estimation of evaporation losses during the

month.

5. Ascertain the demand during various months.

6. Determine the adjusted inflow during different months as follows:

*�:����� %���= ������ %��� + ���� � ��� � − �;������ �− ���������� ��ℎ��<�

7. Compute the storage capacity for each months.

=����<���>� ��� = *�:�����?%��� − @����

74

8. Determine the total storage capacity of the reservoir by adding the storages required

found in Step 7.

The river inflows were tabulated using the formula below:

?%��� = C�*

Where C = ���%%���%% � ��

� = �� %�����E���%��"� ����ℎ

* = ����ℎ�������

4.3.6 Design of the dam

The dam specifications were acquired using all the parameters acquired from the previous

steps. Drawings were made using Google Sketch Up and AutoCAD

1. Limiting height of the low gravity dam

While ignoring uplift, (i.e. taking uplift coefficient c =0)

F = %GH�I + 1�

The maximum allowable compressive strength for concrete in a gravity dam should be less than

the specified compressive strength of concrete divided by 3.0 for usual load combinations.

Supposing concrete class 25 is to be used:

%JK = 25���, = 25 × 107�/�,

75

GH = � ��� <ℎ��%������9810#�/�,�

I = ��� �"�%������� = 2.4�/�-

1. Freeboard (FB)

OP = 1.33ℎH��5%F

ℎH = 0.0322√;. O + 0.763 − 0.271OS.,8%���O < 32#��

OP�ℎ������E������ℎ�0.9

2. Structural height (Ht)

FU = F + OP

1. Top width (Tw)

!H = 0.14FU��0.55FS.8

*����1.74� ≅ 2�

2. Base width (b)

Considering the stress criterion and a unit uplift coefficient (i.e. c = 1)

E = F3I − �

76

CHAPTER 5: DATA ANALYSIS, RESULTS AND DISCUSSIONS

5.1 Determination of dam type and location

An area with coordinates latitude 1°55'57.39"S and longitude 37°13'44.81"E was found to be

appropriate for these reasons: - the area had a narrow gorge opening upstream (from contour

map), it had a strong rock foundation which favored the construction of a gravity dam as

opposed to an earth dam, the place was also easily accessible by road which eased the

construction of the dam. A gravity dam was chosen because it is the most durable type of dam

and it requires little maintenance.

5.2 Determination of available reservoir storage volume

A survey (data given in APPENDIX 4) was carried out as illustrated in the methodology and area

was acquired as shown below. Volume was tabulated using the trapezoidal formula

77

TABLE 2: Elevation –area- volume values

Elevation(m) Area(m2) H(m) Volume(m3) Cumulative

volume(m3)

1357 2277 1 7470 7470.0

1358 12663 1 13893 21363.0

1359 15123 1 17915 39278.0

1360 20707 1 20811.5 60089.5

1361 20916 1 25240 85329.5

1362 29564 1 30344 115673.5

1363 31124 1 31742 147415.5

1364 32360 1 33700 181115.5

1365 35040 1 35521.5 196637.0

1366 36003 1 37789 200426.0

1367 39575 1 34030.5 203456.5

1368 28486 1 25778.5 314235.0

1369 23071 1 19499 333734.0

1370 15927 1 7963.5 341697.5

5.3 Rainfall data analysis

Rainfall data for Machakos weather station for the years 1990-2012 are given in Appendix 1.The

graph(FIGURE 5 ) below shows the trend of the rainfall depth for the years.

0

200

400

600

800

1000

1200

1400

1985 1990 1995 2000 2005 2010 2015

Ra

infa

ll (

mm

)

Time (Years)

RAINFALL TREND BETWEEN 1990-2012

78

The table below shows long term mean monthly rainfall as obtained from CLIMWAT data.

TABLE 3: long term mean monthly rainfall

MONTH JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC TOTAL

Rainfall

depth(mm)

66 45 103 187 69 11 7 7 10 55 185 111 856

Effective

rainfall

59 41.8 86 131 61.4 10.8 6.9 6.9 9.8 50.2 130.2 91.3 685.4

5.3.1 Computation of design rainfall

To calculate the design rainfall, the mean annual rainfall depths were ranked and fitted in a

Cumulative frequency distribution function of the Frechet type (Fisher-Tippett 2) in the

`CumFreq’ cumulative frequency program with a 90% confidence interval. The program output

is shown in APPENDIX 5.

C������ ;�%��>���"�C. O� = 1 − WX�������%��>���" = 1 − O

For 90% reliability rainfall, F=0.9 thus C.F = 1-F = 1-0.9 = 0.1

For C.F=0.1, CumFreq gave the following values;

TABLE 4: The 90% confidence intervals of the reliability rainfall

X- value

501.4

The lower confidence limit of the 90% reliability rainfall

A plot of cumulative frequency Vs the rainfall values

FIGURE 6 -A plot of calculated against observed return period

79

The 90% confidence intervals of the reliability rainfall

90% confidence intervals of the reliability rainfall

Lower Upper

491.0 572.0

The lower confidence limit of the 90% reliability rainfall above was taken as the design rainfall.

A plot of cumulative frequency Vs the rainfall values

A plot of calculated against observed return period

90% confidence intervals of the reliability rainfall

above was taken as the design rainfall.

80

81

5.4 Catchment characteristics

TABLE5: catchment characteristics

ATTRIBUTE VALUE SOURCE

Area(m2) 9.83 Arc GIS

Perimeter(m) 7056 Arc GIS

Major land cover classes(%)

Cultivated land

Grassland

Forested area

54

37

9

Rostom et.al, 2005

Soil characteristics

Texture (Depth 0-60cm below ground)

Infiltration range (mm/hr)

Hydrologic soil group

Sandy-clay-loam

15.55-22

C

Shisekanu, 2006

Slope 4% ArcGIS

TABLE 6: Runoff coefficient determination

COVER TYPE WEIGHT CONDITION

Cultivated land 0.54 Good(terraced)

Grassland 0.37 Poor (ground cover<50%)

Forested area 0.09 Fair (Woods are grazed but not burnt,

and some forest litter covers the soil)

�� <ℎ������%%���%% � �� = $. Y+ ×Z+ + $.Y, ×Z, + $.Y- ×Z-

Thus

�� <ℎ������%%���%% � �� = 0.16 × 0.24 + 0.42 × 0.37 + 0.25 × 0.09 = 0.533

$��%%���%% � �� = 0.533

The catchment area is 9.83km2. The annual runoff volume is a product of the catchment area

and the annual runoff depth.

82

*������%%;�������-� = 9.83 × 107 × 0.491 × 0.533 = 2.58[�-

This shows that there is enough runoff to fill the available reservoir capacity and also justifies

the design of the dam with a spillway

5.5 Population data & analysis

From the census data obtained from the Sub-chief’s Office (Itumbule sub-location), human

population growth is exponential and is based on a 3% growth rate experienced between1999-

2009. In order to compute the total water demand for Itumbule and Nzeveni villages, the

assumptions stated in the methodology were adapted.

TABLE 7: Itumbule and Nzeveni Villages Population Data

VILLAGE NO. OF

HOUSEHOLDS

MALE FEMALE TOTAL

Itumbule 83 223 275 498

Nzeveni 64 170 214 384

Totals 147 882

Based on exponential population forecasting, the formula below was used:

\ = \]. ^_` Where:

P= population at a year n after t years

Po=initial population

r=population growth rate (decimal)

83

According to MOWI manual 2005, population forecasting for the purpose of water demand

projection is done for the initial year (0-5 years from the date of the commencement of the

preliminary design), future year (10 years from the initial year), and the ultimate year (20 years

from the initial year).

Based on a 3% population growth rate and exponential growth, the population at the above

years is tabulated below:

TABLE 8: Human population projections

YEAR PEOPLE

2009 882

2014 1025

2018 1156

2028 1560

2038 2106

For the purpose of estimating the water demand for livestock the following conversion factors

apply (MOWI 2005)

TABLE 9: Conversion factors

ANIMALS EQUIVALENT TO ______ LIVESTOCK

UNITS(L.U)

1 grade cow 1

3 indigenous cows 1

15 sheep or goats 1

5 donkeys 1

2 camels 1

84

TABLE 10: Livestock ownership per household

ANIMALS NUMBER PER HH(average) EQUIVALENT LU

Cows 3 1

Goats and sheep 6 0.4

The population for animals in the design years based on the assumption that livestock

population growth is proportional to that of human population, is given in the table below:

TABLE 11: Livestock Population Projections

ANIMALS TOTAL

(2009)

LU TOTAL

(2014)

LU TOTAL

(2018)

LU TOTAL

(2028)

LU TOTAL

(2038)

LU

Cows 441 147 513 171 578 193 780 260 1053 351

Goats

&Sheep

882 59 1025 69 1156 78 1560 104 2106 141

Totals 1323 206 1538 240 1734 271 2340 364 3159 492

The table below relates the growth in commercial activities with that of the human population

in accordance to the assumption adopted from MOWI 2005.

TABLE 12: Commercial Units Forecasting

COMMERCIAL NUMBER

(2009)

NUMBER

(2014)

NUMBER

(2018)

NUMBER

(2028)

NUMBER

(2038)

Hotels(low

class)

6 7 8 11 15

Bars 8 12 15 19 25

Shops (small) 26 30 35 46 63

85

There are two primary schools in the area of study (Nzeveni primary school and Itumbule

primary school) with human population of 403 and 526 pupils respectively. The table below

show s the school population projection in the design years from the assumption that the

school going population increases with increase in population. (MOWI 2005)

TABLE 13: Institutional population forecasting

YEAR TOTAL SCHOOL POPULATION

2009 929

2014 1217

2018 1643

2028 2218

2038 2994

5.6 Water demand and water losses computation

5.6.1 Water demand

The quantity of water demanded is obtained from the formula:

a��� �"@������ = ������ ������� × ������� ���E����;��

The various consumption rates have been given by MOWI 2005 as shown in APPENDIX 3. These

values of demand are used in the above equation together with the projected population to

generate the quantity of water demand as shown below:

1. Human water demand

YEAR PEOPLE RATE(L/capita/day) TOTAL(L/day)

2009 882 50 44100

2014 1025 50 51250

2018 1156 50 57800

2028 1560 50 78000

2038 2106 50 105300

86

2. Livestock water demand

YEAR LU RATE(L/LU/day) TOTAL(L/day)

2009 206 50 10300

2014 240 50 12000

2018 271 50 13550

2028 364 50 18200

2038 492 50 24600

3. Commercial water demand

Hotels water demand

YEAR NUMBER RATE(L/day) TOTAL(L/day)

2009 6 100 600

2014 7 100 700

2018 8 100 800

2028 11 100 1100

2038 15 100 1500

Shops water demand

YEAR NUMBER RATE(L/day) TOTAL(L/day)

2009 8 100 800

2014 12 100 1200

2018 15 100 1500

2028 19 100 1900

2038 25 100 2500

Bars water demand

YEAR NUMBER RATE(L/day) TOTAL(L/day)

2009 8 500 4000

2014 12 500 6000

2018 15 500 7500

2028 19 500 9500

2038 25 500 12500

4. Institutional water demand

Schools

YEAR PEOPLE

2009 929

2014 1217

2018 1643

2028 2218

2038 2994

The total water demand for the two villages was obtained as a summation of all the categories

of demand and is as shown below:

TOTAL WATER DEMAND

YEAR TOTAL (L/day)

2009 83025

2014 101575

2018 122225

2028 164150

2038 221250

Institutional

34%

Livestock

11%

Commercial

WATER DEMAND CATEGORIES

87

Institutional water demand

PEOPLE RATE(L/capita/day) TOTAL(L/day)

25 23225

25 30425

25 41075

25 55450

25 74850

The total water demand for the two villages was obtained as a summation of all the categories

of demand and is as shown below:

DEMAND

TOTAL (L/day) L/year M3/year

30304125 30304.125

37074875 37074.875

44612125 44612.125

5991475 59914.75

80756250 80756.25

Human

48%

Commercial

7%

WATER DEMAND CATEGORIES

TOTAL(L/day)

23225

30425

41075

55450

74850

The total water demand for the two villages was obtained as a summation of all the categories

/year

30304.125

37074.875

44612.125

59914.75

80756.25

FIGURE 7

88

5.6.2 Water losses

1. Seepage water loss

Seepage was taken to be 40% of the annual water demand in the ultimate year. This is

calculated below:

40100 × 80756.25 = 32302.5�-/"���

2. Evaporation from the reservoir

The amount of water lost through evaporation from the reservoir is obtained from pan

evaporation data given below and using the formula below:

$����;� ��;������ � b ��"���c = 0.7 × ���;������ ����/"����

$����;� ��;������ � d �-"���e

= $����;� ��;������ ���/"���� × $����;� �������,� Where 0.7 is the pan coefficient

TABLE14: pan evaporation

YEAR Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ANNUAL

TOTALS

1999 6.3 7 5.9 4.6 4.5 3.8 3.4 3.7 5.8 5.7 5 4.2 59.9

2000 5.5 7 7.1 5.4 4.7 3.7 3.3 4.5 5.7 6.5 4.6 4.8 62.8

2001 4.4 5.8 5.5 4.4 4.4 3.6 3.2 4.6 5.7 6 4.4 4.1 56.1

2002 5.1 6.4 5.3 5.2 3.8 3.3 3.8 3.1 6 6.3 5.6 4 57.9

2003 5.2 6.4 7.2 5.4 3.5 3.5 3.1 3.6 5.2 6.1 4.9 5.8 59.9

5 years total 296.6

5 years average 59.32

From the data above, a value of 59.32mm/year was used as the annual pan evaporation over

the design period. The corresponding reservoir evaporations were computed as shown below:

59.321000 ×

3. Sedimentation loss

60% of the total water demand was taken as the sedimentation loss. This was calculated as

shown below:

60100

TOTAL DEMAND

The total demand is therefore:

19363.43�-/"���+48453.75�=180875.93m

3/year

seepage

32%

89

× 0.7 × 466319 = 19363.43�-/"���

60% of the total water demand was taken as the sedimentation loss. This was calculated as

100 × 80756.25 = 48453.75�-/"���

�-/"���+32302.5�-/year+80756.25m3/year

Evaporation

19%

sedimentation

49%

seepage

WATER LOSSES

60% of the total water demand was taken as the sedimentation loss. This was calculated as

90

This value however is multiplied by a safety factor to cater for unaccounted for water demand

and losses. For this design, a safety factor of 1.5 was used, resulting to a total water demand of:

180875.93 × 1.5 = 271313.895�-/"���

5.7 Required reservoir capacity

This was done using analytical method. The design rainfall which was found to be 491.2mm

was used as a guide to annual precipitation data which could be used to design the dam.

The average precipitation corresponding to the driest year on record was used as the design

data as shown below:

TABLE 15: Analytical method calculations

Months Precipitation

(mm)

Inflow

(m3/s) ×10-3

Demand(m3/s) ×10

-3

Deficit(m3/s) ×10

-3

Cumulative

deficit

January 107.0 10.03 8.61 ---- 0

February 0.0 0 8.61 8.61 8.61

March 0.0 0 8.61 8.61 17.22

April 1.8 0.008 8.61 8.61 25.83

May 15.6 0.067 8.61 8.543 34.373

June 56.2 0.241 8.61 8.59 42.963

July 60.3 0.258 8.61 8.58 51.543

August 1.8 0.008 8.61 8.602 60.145

September 2.3 0.01 8.61 8.60 68.745

October 41.0 0.176 8.61 8.59 77.345

November 189.8 10.93 8.61 ----- 77.345

December 108.8 10.423 8.61 ----- 77.345

77.345

Thus the required reservoir capacity is equal to:

91

$�>� ��������;� ������ �" = 77.345 × 30.4 × 24 × 60 × 60 × 10- = 203151.28�-

This volume corresponds to the contour at 1367 which provides a cumulative volume of

fghijk. jlh

This therefore means that the required dam has a hydraulic height of mhjn − mhkn =mgl

5.8 Dam design

5.8.1 Limiting height of the low gravity dam

% = %JK3.0 = 25 × 1073.0 = 8.333 × 107

Therefore:

F = 8.333 × 1079810�2.4 + 1� = 250�

Now, 10m < 250�, �ℎ���ℎ �� ��E���� <��������<��; �"���

5.8.2 Dam dimensions

1. Hydraulic height (H)

F"����� �ℎ� <ℎ� = 1367 − 1357 = 10�

2. Freeboard (FB)

OP = 1.33ℎH��5%F

ℎH = 0.0322√;. O + 0.763 − 0.271OS.,8%���O < 32#��

92

OP�ℎ������E������ℎ�0.9

5% × 10 = 0.5��ℎ �ℎ � < 0.9, �ℎ���%���OP = 0.9 ≅ 1�

3. Structural height (Ht)

FU = F + OP = 10 + 1 = 11�

4. Top width (Tw)

!H = 0.14FU��0.55FS.8

0.14 × 11 = 1.54��ℎ ��0.55 × 10S.8 = 1.74�

FU = F + OP = 10 + 1 = 11�

*����1.74� ≅ 2�

5. Base width (b)

Considering the stress criterion and a unit uplift coefficient (i.e. c = 1)

E = F3I − � =

10√2.4 − 1 = 8.5�

5.8.3 Computation of stresses

i. Assumptions considered:

ii. Pressures considered are dam weight, water uplift, wave and silt.

iii. Tail water is nil.

Considering 1m of the dam and w=1t/m2,

93

Designation Force

(t)

Lever arm

(m)

Moment about toe

(t.m)

Dam weight = volume

x density of concrete

W1 = 2x11x 1x2.4

W2 = +, x6.5x7.2x1x2.4

=52.8

=56.16

6.5+2/2 = 6.5

6.5x2/3 =4.33

+343.2

+243.17

W=W1+W2 =108.96

/[1 =+586.4

Uplift

U1= ½*8.5*11

=46.75 2/3x8.5=5.67

-265.1

Water pressure Pw

Pw = wh2/2 =

+,x1x10

2

=50

10/3 = 3.33

-166.5

/[����� = 586.4 − �265.1 + 166.5� = +154.8

( = Z − p = 108.96 − 46.75 = 62.21

X̅ = ∑[( = 154.8

62.21 = 2.488

� = E2 − X̅ = 8.5

2 − 2.488 = 1.762

�Urs = ;E b1 +

6�E c =

62.218.5 b1 + 6 × 1.762

8.5 c = 16.42�/�,

�Urs = ;E b1 −

6�E c = 62.21

8.5 b1 − 6 × 1.7628.5 c = 1.784�

�,

t = �1 sec, ∅

But sec, ∅=1+��∅ =1+(8.5/10)2=1.723

∴ t = 16.42 × 1.723 = 28.28

z{rH1|U}s~� = �1 tan∅ = 16.42 × 8.510 = 13.957�/�,

zK�|U}s~� = −z{rH1|U}s~� = −13.957�/�,

94

5.8.4 Dam stability analysis

1. Overturning

Righting moments, R.M = 586.4�. �

Stabilizing /overturning moments, O.M = 265.1+66.5 =331.5

O. = = /$.[Y.[

F.S = 8�7.6--+.8 =1.79

1.5<1.79<2.5 thus the dam is safe against overturning.

2. Sliding

Factor of safety against sliding (F.S.S) should be greater than 1 and is found by Eq 3.29;

F.S.S = �∑��2��

∑�

∑( − p = 62.21t ,�= 0.7

∑F = Pw (ignoring minor wave pressures)

∑F =28.233

F.S.S =S.��7,.,+,�.-- = 1.835 > 1.

Thus the dam is safe against sliding.

3. Crushing/compression

Pn toe = 16.42t/m2 = 1.874x10

4N/m

2, fcu= 25N/mm

2 = 25x10

6N/m

2.

95

Maximum allowable compressive strength should be less than the specified compressive

strength of concrete divided by 3 for usual load combinations. Thus f = 25x106/3 =

8.33x106N/m

2

Pn toe<f thus the dam is safe

4. Tension

For no tension the eccentricity e should be less than �7 or the resultant should always lie within

the middle third.

e = 3.4 and �7 =

8S.�7 = 8.45

e <�7 hence the dam is safe.

96

CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

The goal of this project was to design a multipurpose dam for water supply in Nzeveni. This goal

was however achieved by identifying a suitable site for the design of the dam using contours

generated by ArcGIS, Estimating the water demand categories in the study area using the

exponential formula and by the help of the design manual for water supply Kenya October 2005

(Kenya – Belgium study and consultancy fund), establishing the amount of water that could be

stored in the reservoir using contour area and the trapezoidal formula, and finally establishing

the dam specifications using equations specified in chapter 5.

The dam volume was found to be 203456.5m3 and the height was found to be 11m, and thus it

can be concluded that this is a small dam.

6.2 Recommendations

Before this design is actually implemented is recommended that the following should be done:

-The design could be completed by designing a water intake box and abstraction pipe and also

an overfall spillway should be designed for discharging off excess flood water.

-A suitable water treatment plant should be designed for improving the water quality before it

is used

-A supply system should also be designed to supply water to the homesteads, institutions, and

market centers.

-A bill of quantities for the entire project should be established.

97

REFERENCES

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agriculture in arid lands. Jacob Blaustein Institute for Desert Research, Ben Gurion

University of the Negev.

2. BILLY, B. (1981). Water harvesting for dryland and floodwater farming in the Navajo

Indian reservation. In: Dutt, G.R. et al (eds), Rainfall collection for agriculture in arid and

semi-arid regions: 3-8, Commonwealth Agricultural Bureaux, UK,

3. BOERS, T.M. and BEN-ASHER, J. (1982). Review of rainwater harvesting. In: Agricultural

Water Management 5: 145-158.

4. BRUINS, H.J. EVANARI, M. and NESSLER, U. (1986). Rainwater harvesting agriculture for

food production in arid zones: the challenge of the African famine. In: Applied

Geography 6 (I): 13-33.

5. Conant, J. (2009). Water for life: Community water security. California: Hesperian

foundation

6. CRITCHLEY, W.R.S., (1986). Runoff harvesting for crop production: experience in Kitui

District; 1984-1986. Paper presented to the Third National Soil and Water Conservation

Workshop, Nairobi, Kenya.

7. CRITCHLEY, W.R.S., (1987). Some lessons from water harvesting in Sub-Saharan Africa.

Report from a workshop held in Baringo, Kenya, 13-17 October 1986. World Bank,

Eastern and Southern Africa Projects Dept., Washington DC.

8. Design manual for water supply Kenya October 2005 (Kenya – Belgium study and

consultancy fund)

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9. Emiroglu, E.M. (2008). Influences on selection of the type of dam. International journal

of science and technology, 3(2), 173-189

10. FAO Irrigation and Drainage Paper ,Manual on small earth dams, a guide to sitting,

design and construction

11. FRASIER, G.W. and MYERS, L.E. (1983). Handbook of water harvesting. Agricultural

handbook No. 600. USDA, Washington DC.

12. ftp://ftp.fao.org/fi/CDrom/FAO_Training/FAO_Training/General/x6705e/x6705e02.htm

13. Garbrecht, j., and L.W.Martz. 1996. comment on “Digital Elevation Model Grid Size,

Landscape Representation, and Hydrologic Simulations” by Weihua Zhang and David

R.Montgomery.

14. GILBERTSON, D.D. (1986). Runoff (floodwater) farming and rural water supply in arid

lands. In: Applied Geography 6 (1): 5-11.

15. HILLMAN, F. (1980). Water harvesting in Turkana District, Kenya. Pastoral Network

Paper, Agricultural Admin. Unit/ODI No. 10d, Overseas Development Institute, London.

16. HOGG, R. (1988). Water harvesting and agricultural production in semi-arid Kenya. In:

Development and Change 19: 69-87

17. House, S., Reed, B., & Shaw, R (1997) .Water Source Selection. Retrieved Nov 12, 2011,

from WELL: http:/www.Iboro.ac.uk/well/

18. http://greencleanguide.com/2012/09/07/water-losses-through-seepage/

19. http://www.fao.org/docrep/u3160e/u3160e05.htm#TopOfPage

20. http://www-personal.umich.edu/~steiss/page54.html

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21. IIT, Kharagpur. (2010). Hydraulics Structures for Flow Diversion and Storage .Lecture

Series: Module 4. India: Unpublished

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Measurements. Chelsea, Michigan: Lewis Publishers, Inc.

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25. New Jersey Storm water Best Management Practices Manual, February 2004, C H A P T E

R 5

26. Pierre J., Degoutte,G., &Lautrin,D (n.d) Choice of site and type of dam

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Table of Contents

DECLARATION…………………………………………………………………………………………………………………………………………..i

DEDICATION…………………………………………………………………………………………………………………………………………….ii

ACKNOWLEDGEMENT…………………………………………………………………………………………………………………………….iii

ABSTRACT……………………………………………………………………………………………………………………………………………….iv

LIST OF TABLES…………………………………………………………………………………………………………………………………………v

LIST OF FIGURES……………………………………………………………………………………………………………………………………..vi

LIST OF ACRONYMS………………………………………………………………………………………………………………………………..vii

CHAPTER 1: INTRODUCTION ......................................................................................................................... 1

1.1 Statement of the problem and problem analysis ......................................................................... 2

1.2 Site analysis and inventory ........................................................................................................... 5

1.3 Objectives...................................................................................................................................... 6

1.3.1 Overall objective ................................................................................................................... 6

1.3.2 Specific objectives ........................................................................................................................ 7

CHAPTER 2: LITERATURE REVIEW ................................................................................................................. 8

2.1 Sources of water ........................................................................................................................... 8

2.1.1 Factors to be considered in selecting a suitable water source ............................................. 8

2.2 Need for dams ............................................................................................................................. 10

2.2.1 The purposes of dams ......................................................................................................... 11

2.3 Reservoirs .................................................................................................................................... 14

2.4 Multipurpose Reservoirs ............................................................................................................. 17

2.5 Location of site selection of reservoir ......................................................................................... 18

2.6 Reservoir Levels .......................................................................................................................... 19

2.7 Dams ........................................................................................................................................... 24

2.7.1 Types of dams ..................................................................................................................... 24

2.7.2 Choice of site and type of dam ........................................................................................... 25

CHAPTER 3: THEORETICAL FRAMEWORK ................................................................................................... 29

3.1 Determination of design population and demand ..................................................................... 29

3.1.1 Population Estimates and Projections ................................................................................ 29

3.1.2 Population projection ......................................................................................................... 36

3.2 Design Period .............................................................................................................................. 38

3.3 Rainfall analysis ........................................................................................................................... 39

3.3.1 Rainfall characteristics ........................................................................................................ 39

3.3.2 Variability of annual rainfall ................................................................................................ 40

3.3.3 Design rainfall ..................................................................................................................... 40

3.3.4 Rainfall-runoff relationship ................................................................................................. 43

3.3.5 Runoff coefficients .............................................................................................................. 47

3.4 Catchment Area Delineation Using GIS technique ..................................................................... 52

3.5 Determination of available reservoir capacity ............................................................................ 53

3.6 Determination of the Required Capacity .................................................................................... 56

3.7 Investigations for Reservoir ........................................................................................................ 58

3.7.1 Engineering surveys ............................................................................................................ 59

3.7.2 Geological investigations .................................................................................................... 59

3.7.3 Hydrological investigations ................................................................................................. 59

3.8 Dam Design ................................................................................................................................. 64

CHAPTER 4: MATERIALS AND METHODOLOGY........................................................................................... 68

4.1 Identification of dam site and type ............................................................................................. 68

4.2 Data acquisition .......................................................................................................................... 68

4.3 Data analysis approach ............................................................................................................... 70

4.3.1 Determination of the available reservoir volume ............................................................... 70

4.3.2 Determination of design rainfall ......................................................................................... 70

4.3.3 Catchment runoff computation .......................................................................................... 71

4.3.4 Water demand computation .............................................................................................. 71

4.3.5 Required reservoir storage volume .................................................................................... 73

4.3.6 Design of the dam ............................................................................................................... 74

CHAPTER 5: DATA ANALYSIS, RESULTS AND DISCUSSIONS ........................................................................ 76

5.1 Determination of dam type and location ................................................................................... 76

5.2 Determination of available reservoir storage volume ................................................................ 76

5.3 Rainfall data analysis ................................................................................................................... 77

5.3.1 Computation of design rainfall ........................................................................................... 78

5.4 Catchment characteristics .......................................................................................................... 81

5.5 Population data & analysis.......................................................................................................... 82

5.6 Water demand and water losses computation .......................................................................... 85

5.6.1 Water demand .................................................................................................................... 85

5.6.2 Water losses ........................................................................................................................ 88

5.7 Required reservoir capacity ........................................................................................................ 90

5.8 Dam design.................................................................................................................................. 91

5.8.1 Limiting height of the low gravity dam ............................................................................... 91

5.8.2 Dam dimensions .................................................................................................................. 91

5.8.3 Computation of stresses ..................................................................................................... 92

5.8.4 Dam stability analysis .......................................................................................................... 94

CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS ............................................................................. 96

6.1 Conclusions ................................................................................................................................. 96

6.2 Recommendations ...................................................................................................................... 96

REFERENCES ................................................................................................................................................ 97