Dams introduction

INTRODUCTION

WATER RESOURCES DEVELOPMENT PROJECTS
Water-resources engineering is concerned with the utilisation of water, control of water, and
water quality management. Water is utilised for various beneficial purposes such as
irrigation, water supply, hydropower and navigation. Water is controlled and regulated for a
variety of purposes such as flood control, land drainage, sewerage and bridges so that it does
not cause damage to property, inconvenience to the public, or loss of life. Water-quality
management or pollution control is also an important phase of water resources engineering to
maintain the required quality of water for municipal and irrigation uses and to preserve the
environment and ecological balance.
The development of water resources of a region requires the conception, planning,
design, construction, and operation of various facilities to utilise and control water, and to
maintain water quality. Because each water-development project is unique, it is not possible
to give a standard solution. The special conditions of each project should be tackled through
an integrated application of the fundamental knowledge of many disciplines.
The Planning of a water-resources development project involves systematic
consideration of the original statement of purpose, evaluation of alternatives and the final
decision. It forms the basis for the decision to accept or to reject a particular project. It
involves a thorough study of various alternatives, selecting the best alternative and the
methods for the implementing the same to achieve the optimum benefits. It is the most
important aspect of the total engineering for the project. Planning is relatively easier if there
is only one objective to be achieved. In multiple objectives, the planning becomes more
difficult. The planning of the entire river-basin consisting of a number of projects is even
more difficult because whatever is done at one site, it would affect other projects elsewhere in
the basin. The planning of a water-resources development project generally consists of the
following steps: (1) Statement of objectives, (2) Collection of data (eg, location, size,
physiography, climate, history, ,population, Precipitation, evaporation, transpiration, stream
flow, sediment, water quality, rock and soil type, ground water, minerals, erosion,
topographic and other maps of the area, type of vegetation, fish and wild life, population
statistics, data of people and various institutions, various industries, means of transportation,
market, tourism, recreation, land, taxes, water rights, population control, land zoning, land
ownership, administrative pattern, types and locations of various projects, opinions of
different sections of society, land classification, crop water requirements, climatic data, types
of crops, drinking water needs, industrial water needs, quality of water, need of power,
variation of power demands, alternate energy sources, record of past floods, extent of
damage caused, storm-water drainage requirements, existing water traffic patterns,
alternate means of transport, existing recreational facilities, natural attractions, scenic
data, existing waste discharge methods, location, time and character of waste, water
pollution regulations, quality standards, type of fish and wild life, their migratory habits,
protection requirements, etc), (3) Future projections, (4) Project formulation, (5) Project
evaluation, and (6) Environmental considerations
Various Purposes
The water-resources development projects are planned to serve various purposes as follows:
(A) Main Purposes: The following are the main purposes.

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1. Irrigation: The objective of irrigation is to increase the agricultural production.
Various works required for irrigation are dams, reservoirs, wells, canals, distribution systems,
drainage facilities, farmland grading, etc.
2. Hydroelectric powers: The objective of hydroelectric power development is
generation of electric powers for economic development and improving living standards. The
works include forebay, penstocks, turbines, generators, transformers, transmission lines, etc.
3. Flood control: Flood control works are required for prevention or reduction of flood
damage, protection of areas, river regulation, recharging of water, etc. The various works and
measures include dams, storage reservoirs, levees, flood walls, channel improvements, flood
ways, flood-plain zoning, flood forecasting, etc.
4. Domestic and industrial water supply: The objective to be achieved is to provide
safe and adequate water for domestic, industrial, commercial, municipal and other uses. The
various works and measures adopted are dams, reservoirs, wells, conduits, pumping plants,
treatment plants, distribution systems, etc.
5. Navigation: Inland navigation facilities are provided for transportation of goods and
passengers. The various works and measures adopted are dams, reservoirs, canals, locks,
channel improvement, harbour improvement, etc.
(B) Secondary Purposes: The following secondary purposes are also served by various
projects.
1. Recreational: The objective is to provide recreational facilities for the health and
welfare of the people. Various works and measures include reservoirs, swimming pools,
facilities for boating and water sports, preservation of scenic beauty, etc.
2. Fish and wild life: The objectives in this case are improvement of habitat for fish and
wild life, prevention of loss of fish and wild life, enhancement of sport facilities, expansion of
commercial fishing, etc. The various works and measures are wild life refuges, fish
hatcheries, fish ladders, reservoirs, regulation of stream flows, pollution control, land
management, etc.
3. Drainage control: The objectives of drainage control are better agricultural
production, urban development, protection of public health, prevention of waterlogging and
salinity, etc. The various works and measures are ditches, tile drains, levees, pumping
stations, etc.
4. Watershed management: The various objectives of the watershed management are
conservation and improvement of soil, sediment reduction, runoff retardation, forest and
grass land improvement, etc. The various works and measures include soil-conservation
practices, forest and range management practices, debris detention dams, small reservoirs and
farm ponds.
5. Sediment control: The objectives are reduction or control of silt load in streams,
prevention of silting of reservoirs, preservation of fertility of soil, etc. The various works and
measures adopted include soil conservation, afforestation, desilting works, revetment works,
bank stabilisation, check dams, special reservoir operation, etc.
6. Salinity control: The objective is abatement or prevention of contamination of
agricultural, industrial and municipal water supplies by different salts. The various works and
measures include reservoirs for augmentation of low stream flows, barriers, ground water
recharge, coastal jetties, etc.
7. Pollution abatement: The objectives is protection or improvement of quality of water

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supplies for municipal, domestic, industrial and agriculture, aquatic life and recreation. The
various works and measures include treatment facilities, reservoirs for augmenting low flows,
sewage-collection systems, legal measures to control pollution, cleaning of polluted rivers,
etc.
8. Insect control: The objective is protection of public health, recreational values,
forests, crops, and land. The various works and measures include drainage, extermination
measures, proper design and operation of reservoirs and associated works.
9. Artificial precipitation: The objective is to induce artificial precipitation within
meteorological limits of the basin. The various works and measures include cloud-seeding
equipment, meteorological instrumentation, etc.
(C) Miscellaneous Purposes: In addition to above main and secondary purposes, water
resources development projects serve the following miscellaneous purposes:
1. Employment: The objective is to provide employment and other sources for increased
income in backward areas having a lot of unemployment and underdevelopment.
2. Acceleration of public works: The objective is to accelerate the construction of
various public works for the development of region. Sometimes, the projects are planned that
they may be executed on cost-sharing basis among various agencies, such as central
government, state government, local bodies and private organisation, to accelerate the
development.
3. Development of new water resources policies: The objective is to initiate new
policies for the development, composition, formulation and execution of the water-resources
projects.
Classification
The water-resources development projects are usually classified on the basis of the number of
purposes serves. (1) Single purpose projects: These projects are designed and operated to
serve only one basic purpose. (2) Multipurpose projects: These projects are designed and
operated to serve two or more purposes.
It may be noted that a project which is designed for single purpose but which
incidentally also serves other purposes is not a multipurpose project. Most of the major
water-resources development projects in India are multipurpose projects. These projects are
designed to serve a number of purposes to make effective use of water resources of the
country. Although water resources are quite abundant but because the population is also
increasing at a fast rate, it is desirable that the maximum use shall be made of the available
water resources by developing multipurpose projects. Moreover, multipurpose projects are
generally more economically viable because the increase in costs is often not proportional to
the increase in benefits. Additional benefits are, therefore, obtained with a small increase in
cost.
Water Requirements of Multipurpose Projects
As discussed above, a multipurpose project is designed to serve a number of purposes. The
purposes usually served are irrigation, hydropower, flood control, water supply, navigation,
recreation, fish and wild life and sanitation. The water requirements for these functions are
quite different. The successful use of stored water in a multipurpose project can be made for
various purposes after studying the various requirements. If these requirements are
compatible, the stored water is used more effectively because it would simultaneously serve
more than one purpose. Water requirements for various functions are briefly reviewed below:

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1. Irrigation: Water requirements for irrigation in India are mostly seasonal. The
maximum demand of water for irrigation is during the winter months for Rabi crops.
However, there is usually a small demand of water for Kharif crops during the summer
months just prior to the onset of the monsoon. Water requirements for irrigation are generally
higher in a year of low rainfall. But the average demand does not vary greatly from year to
year if the irrigated area remains the same. Because irrigation is a sort of insurance against
drought, it is desirable to reserve as much storage as possible for irrigation use.
2. Hydropower: Water requirements for hydropower depend upon the type of area
served. The power demand generally has a marked seasonal variation. However, most of the
hydropower plants are connected to a power grid and, therefore, there is considerable
flexibility in their operation. The water requirements for hydropower can generally be
coordinated with other uses of water. During the period when hydropower production is low,
thermal and nuclear plants can be run to full capacity. Moreover, hydropower production
does not make consumptive use of water as the water released for hydropower can serve
other purposes. Thus hydropower production is quite compatible with other uses. Water
released for irrigation and water supply may be used to produce hydropower. However, when
irrigation demands are low in the rainy season, water has to be released only for hydropower.
If the power production is limited only during the period when irrigation demand exists, the
load factors for the plant will go down and there will be a loss in overall efficiency.
3. Flood control: The basic requirement for flood control is that there should be a lot of
empty space in the reservoir so that the flood water can be stored. The flood control use is,
therefore, not compatible with other uses which require that adequate water should be stored
in the reservoir. However, the flood control requirement is seasonal as it is only during the
rainy season.
4. Water supply: Requirements for domestic water supply are more or less constant
throughout the year. However, the demand is somewhat more during the summer months.
With an increase in population, the water demand increases from year to year, and this factor
should be considered while planning a water supply project. Adequate reserve should be
maintained to avoid water shortage during the periods of drought.
5. Navigation: Requirements for inland navigation are that there should be adequate
flow in the river to maintain the required water depth. Water is released from a storage
reservoir to sustain downstream flow for navigation. There is a marked seasonal variation in
the demand. Generally, peak releases are required during the summer months when the
natural flow is low.
6. Recreation: The basic requirements for recreation is that the reservoir should remain
nearly full during the recreation season to permit boating, fishing, swimming and other water
sports. Moreover, there should not be sudden large drawdowns which may create several
problems. However, reservoirs are seldom designed to serve recreation alone. The reservoirs
designed for other purposes may have recreation as an incidental purpose.
7. Fish and wild life: For protection of fish, there should not be large and rapid
fluctuations in water level of the reservoir, particularly during the spawning period.
Moreover, the flow of water of downstream of the dam should not be completely stopped, as
it would lead to the destruction of fish and wild life. Fish ladders or other suitable
arrangements are required at the dams to permit migratory fish to travel upstream as well as
downstream. However, the construction of a reservoir causes a major change in habitat for
existing fish and wild life, and there may be a decrease or an increase in species of fish and
wild life.

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8. Sanitation: The requirement for proper sanitation is that there should be adequate
flow downstream of the dam. Sanitation requirements are compatible with other uses as these
can be easily combined with the release of water for other uses. Sometimes there is another
requirement in some areas that the reservoir should be operated such that there is less
mosquito growth. This is usually achieved by causing rapid fluctuations of water level.

RESERVOIRS
A reservoir is a large, artificial lake created by constructing a dam across a river (Figs).
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. This is especially so for a Reservoir
country like India in which about 75% of the
total precipitation occurs during the monsoon
season from June to September. Most of the
rivers carry very little or no water during non-
monsoon period, except the Himalayan rivers,
which also carry a substantial discharge in the
non-monsoon period due to melting of snow.
During the period of low flow, it is not possible Dam
to meet the water demands for various purposes
such as irrigation, water supply and hydroelectric power. To regulate the water supplies, a
reservoir is created on the river to store water during the rainy season. The stored water is
later released during the
period of low flows to
meet the demand. In the
monsoon season, the
reservoir store excess
water when the Reservoir
discharge in the river is
high. Thus besides
releasing the water
during the period of low Dam Spillway
flows, the reservoirs also
help in flood control.
Types of Reservoirs
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. Because in
most of the cases, a single purpose reservoir is not economically feasible, it is the general
practice in India to develop multipurpose reservoirs. The various purposes served by a
multipurpose reservoir include (i) irrigation (ii) municipal and industrial water supply, (iii)
flood control (iv) hydropower, (v) navigation, (vi) recreation, (vii) development of fish and
wild life, (viii) soil conservation (ix) pollution control and (x) mosquito control.

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Depending upon the purpose served, the reservoirs may be broadly classified into four types:
(1) Storage (or conservation) reservoirs, (2) Flood control reservoirs, (3) Multipurpose
reservoirs, (4) Distribution reservoirs, and (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 in
India are usually constructed for irrigation, the 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 reservoir.
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 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
flood water is discharged downstream till the outflow reaches the safe capacity of the channel
downstream. When the discharge exceeds the safe capacity, 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. The flood control reservoirs are of two
types: (i) Detention reservoirs and (ii) Retarding reservoirs.
i. Detention reservoirs: A detention reservoir stores excess water during floods and
releases it after the flood. It is similar to a storage reservoir but is provided with large gated
spillways and sluiceways to permit flexibility of operation. The discharge from a detention
reservoir to the downstream channel is regulated by gates. In the earlier stages of a flood,
the gates are left open and the water is
released subjected to the safe carrying
capacity B of the channel downstream. In
the later stages of the flood when the
discharge downstream exceeds the
maximum capacity of the downstream
channel, the gates are kept partially closed.
There is basically no difference between
the detention reservoir and a storage
reservoir except that the former has a
larger spillway capacity and sluiceway capacity to permit rapid drawdown just before or after
a flood. The reservoir is quickly emptied and thus the full reservoir capacity is made available
again for moderating a subsequent flood after a short interval. In this manner, the available
capacity is more effectively used. When the natural inflow is greater than controlled outflow
rate (B), the excess water is stored in the reservoir. The volume of stored water is indicated
by dotted area. When the discharge is less than B the stored water is released. The volume of
released water is shown by hatched area. Because of detention reservoir, the flood peak is
reduced from A to B. Thus the effect of reservoir on a flood is to reduce the peak discharge by
absorbing a volume of flood water when the flood is rising, and releasing the same later
gradually when the flood is receding. Advantages (1) The detention reservoirs provide more
flexibility of operation and better control of outflow than retarding reservoirs. Large

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reservoirs are usually detention reservoirs. (2) The discharge from various detention
reservoirs on different tributaries of a river can be adjusted according to the carrying capacity
of the d/s channel. Disadvantages (1) The detention reservoirs are more expensive than the
retarding reservoirs because of high initial cost and maintenance cost of gates and the lifting
machinery. (2) Due to the possibility of human error or negligence, a disaster can occur.
ii. Retarding Reservoirs: A retarding reservoir is provided with spillways and
sluiceways which are ungated. The maximum combined discharging capacity of all spillways
and sluiceways is limited to the safe-carrying capacity of the channel downstream. The
retarding reservoir stores a portion of the flood when the flood is rising and releases it later
when the flood is receding. However, in this case, the discharge downstream cannot be
controlled because there are no gates. There is an automatic release of water, depending upon
the level of water in the reservoir. As the flood occurs, the reservoir gets filled and at the
same time, the discharge from the spillways and sluiceways occurs. When the elevation of
water in the reservoir increases, the discharge through spillways and sluiceways also
increases. The water level in the
reservoir goes on rising until the
flood starts receding when the
inflow is reduced and it becomes
equal to or less than the outflow.
After that stage has reached, the
water level in the reservoir starts
falling and it continues till the
stored water has been completely
discharged and the water level
has reached the lowest sluiceway
level. Fig. shows the peak reduction mechanism of a retarding reservoir. The stored water in
the reservoir (dotted), which is later released, is shown hatched. A favourable location for a
retarding reservoir is just above the area or the city to be protected against floods. A retarding
reservoir is also usually located on a tributary of a river just upstream of its confluence to
protect the area downstream of it. Advantages (1) The retarding reservoirs are relatively less
expensive than detention reservoirs. (2) As the outflow is automatic, there is no possibility of
a disaster due to human error or negligence. Disadvantages (1) The retarding reservoirs do
not provide any flexibility of operation as the outflow is automatic. (2) The discharge from
retarding reservoirs on different tributaries of a river may coincide and cause heavy flood in
the river downstream.
3. Multipurpose Reservoirs: A multipurpose reservoir is designed and constructed to
serve two or more purposes. In India, 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 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

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used for municipal water supply.
5. Balancing reservoir: A balancing reservoir is a small reservoir constructed d/s of the
main reservoir for holding water released from the main reservoir.
Storage Capacity of a Reservoir
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 characteristics. 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 upto a certain level of water, engineering surveys are usually conducted. For
preliminary estimates of the capacity, the Survey of India maps can be used. These maps are
available to a scale of 1 cm = 500 m (RF = 1/50000) and 1 cm = 2500 m (R.F = 1/250000),
with contours marked on it. For accurate de-termination 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 (RF = 1/10,000 or
1/50000) 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. In addition to finding out the capacity of a reservoir, the
contour map of the reservoir can also be used to determine the land and property which
would be submerged when the reservoir is filled upto various elevations. It would enable one
to estimate the compensation to be paid to the owners of the submerged property and land.
The time schedule, according to which the areas should be evacuated, as the reservoir is
gradually filled, can also be drawn.
(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. An
elevation-storage volume is
plotted between the storage
volume as abscissa and the
elevation as ordinate.
Generally, the volume is
calculated in Mm3 or M ha-
m. 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

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h
∆V = ( A1 + A2 )
2
where h is the contour interval. Therefore the total storage volume V is
h
V = ( A1 + 2 A2 + 2 A3 + 2 A4 + ... + 2 An−1 + An )
2
where n 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
h
∆V =
3
(
A1 + A2 + A1 A2 )
3. Prismoidal formula: According to the prismoidal formula, the storage volume
between three successive contours is given by
h
∆V = ( A1 + 4 A2 + A3 )
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The total storage volume is
h
V = (( A1 + An ) + 4( A2 + A4 + A6 + ... + An−1 ) + 2( A3 + A5 + A7 + ... + An−2 ))
3
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 upto the last but
one area is determined by the prismoidal formula, and that of the last segment is determined
by the trapezoidal formula.
4. Storage Volume from cross-sectional areas: In the absence of adequate contour
maps, the storage volume can be computed from the cross-sectional areas of the river. Cross-
sectional areas (a1, a2 ... etc) are obtained from the cross-sections of the river taken upstream
of the dam upto the u/s end of the reservoir at regular interval d. The volume is determined
from the prismoidal formula. The formula is applicable for odd number of sections
d
V = ((a1 + a n ) + 4(a 2 + a 4 + a6 + ... + a n−1 ) + 2(a3 + a5 + a7 + ... + a n− 2 ))
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(c) Combined Diagram: It is the usual practice to plot both the elevation-area curve and
the elevation- storage curve on the same paper. It is important to note the abscissa markings
as the areas and volumes increase in the opposite directions.
Basic Terms and Definitions
A large number of terms are commonly used for reservoir planning. These terms are defined
below. It may be noted that various terms are sometimes used to indicate the same quantity.
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 upto 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
(NPL). Normal conservation level (NCL) It is the highest level of the reservoir at which

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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 upto FRL, the normal conservation level and the FRL become
identical.
2. 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).
3. 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.
4. 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.
5. Surcharge storage: The surcharge storage is the volume of water stored above the
full reservoir level upto 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.
6. 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.
7. 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

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reservoir when the water level in the reservoir falls. Thus the banks of the reservoir act like
mini reservoirs. The bank storage 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.
8. Valley storage: The volume of water held by the natural river channel in its valley
upto 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 upto 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.
9. 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 Mha-m/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.
10 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 organisation to the consumers that the safe yield will be
available to them. It is therefore also called the firm yield or the guaranteed yield.
11. 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.
12. Average yield: The average yield is the arithmetic average of the firm yield and the
secondary yield over a long period of time.
13. Design yield: 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 are
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.

11

DAMS
A dam is a hydraulic structure of fairly impervious material built across a river to create a
reservoir on its upstream side for impounding water for various purposes. A dam and a
reservoir are complements of each other. A distinction should be made between a weir and a
dam. A weir is also a structure
built across a river; however, its
purpose is not to store water but
to divert it. Thus there is no
reservoir on the upstream of a Reservoir
weir. If there is a small storage Buttress Dam
reservoir on its upstream, the
weir is called a storage weir.
Dams are generally constructed
in the mountainous reach of the
river where the valley is narrow
and the foundation is good.
Generally, a hydropower station
is also constructed at or near the
dam site to develop hydropower.
Sometimes, a pickup weir is constructed on the downstream of a dam quite away from it in
the boulder reach or the alluvial reach of the river to divert the water released from the dam
into canals for irrigation and other purposes. Dams are probably the most important hydraulic
structure built on the rivers. These are very huge structure. Thousands of workers and
engineers work for a number of years in the construction of a dam.
Classification of Dams
Dams can be classified according to different criteria, as given below:
(a) Based on Function Served: Depending upon the function served, the dams are of the
following types -
1. Storage dams: Storage (or conservation) dams are constructed to store water during
the rainy season when there is a large flow in the river. The stored water is utilized later
during the period when the flow in the river is reduced and is less than the demand. The water
stored in the reservoir is used for a number of purposes, such as irrigation, water supply and
hydropower. Storage dams are the most common type of dams and in general the dam means
a storage dam unless qualified otherwise.
2. Detention dams: Detention dams are constructed for flood control. A detention dam
retards the flow in the river on its downstream during floods by storing some flood water.
Thus the effect of sudden floods is reduced to some extent. The water retained in the
reservoir is later released gradually at a controlled rate according to the carrying capacity of
the channel downstream of the detention dam. Thus the area downstream of the dam is
protected against flood.
3. Diversion dams: A diversion dam is constructed for the purpose of diverting water of
the river into an off-taking canal (or a conduit). A diversion dam is usually of low height and
has a small storage reservoir on its upstream. The diversion dam is a sort of storage weir
which also diverts water and has a small storage. Sometimes, the terms weirs and diversion
dams are used synonymously.
4. Debris dams: A debris dam is constructed to retain debris such as sand, gravel, and

12

drift wood flowing in the river with water. The water after passing over a debris dam is
relatively clear.
5. Coffer dams: A
coffer dams is not actually a
dam. It is rather an
enclosure constructed
around the construction site
to exclude water so that the
construction can be done in
dry. A coffer dam is thus a
temporary dam constructed
for facilitating construction.
A coffer dam is usually
constructed on the upstream
of the main dam to divert
water into a diversion tunnel
(or channel) during the
construction of the dam.
When the flow in the river
during construction of the
dam is not much, the site is
usually enclosed by the
coffer dam and pumped dry.
Sometimes a coffer dam on
the downstream of the dam
is also required.
(b) Based on Hydraulic Design: On the basis of hydraulic design, dams may be classified as
1. Overflow dams: An overflow dam is designed to act as an overflow structure. The
surplus water which cannot be
retained in the reservoir is permitted
to pass over the crest of the overflow
dam which acts as a spillway. The
overflow dam is made of a material
which does not erode by the action of
overflowing water. Generally,
cement concrete is used in overflow
dams and spillways. Most of the
gravity dams have overflow sections
for some length and the rest of the
length as a non-overflow dam.
However, sometimes the entire
length of the dam of low height is
designed as an overflow dam. The
overflow dam is also called the
spillway section.
2. Non-overflow dams: A non-
overflow dam is designed such that
there is no flow over it. Because

13

there is no overflow, a non-overflow
dam can be built of any material, such
as concrete, masonry, earth, rockfill
and timber. As already mentioned, the
non-overflow dam is usually provided
in a part of the total length of the
dam. However, sometimes the non-
overflow dam is provided for the
entire length and a separate spillway
is provided in the flanks or in a saddle
away from the dam. Fig shows a non
overflow earth dam.
(c) Based on Materials of Construction: Based on the materials used in construction, the
dams are classified as follows: (1) Masonry dam, (2) Concrete dam, (3) Earth dam, (4)
Rockfill dam, (5) Timber dam, (6) Steel dam, (7) Combined concrete-cum-earth dam, and (8)
Composite dam.
(d) Based on Rigidity: On the basis of the rigidity, the dams are classified into 2 types:
1. Rigid dams: A rigid dam is quite stiff. It is constructed of stiff materials such as
concrete, masonry, steel and timber. These dams deflect and deform very little when
subjected to water pressure and other forces.
2. Non-rigid dams: A non-rigid dam is relatively less stiff compared to a rigid dam. The
dams constructed of earth and rockfill are non-rigid dams. There are relatively large
settlements and
deformations in a
non-rigid dam.
Rockfill dams are
actually neither
fully rigid nor fully
non-rigid. These
are sometimes
classified as semi-
rigid dams.
(e) Based on the
structural action:
This is the most
commonly used
classification of
dams. Based on the
structural action,
the dams are
classified as
(1) Gravity dams,
(2) Earth dams,
(3) Rockfill dams,
(4) Arch dams,
(5) Buttress dams,
(6) Steel dams, and

14

(7) Timber dams.
1. Gravity dams: A
gravity dam resists the water
pressure and other forces due
to its weight (or gravitational
forces). Thus the stability of a
gravity dam depends upon its
weight. The gravity dams are
usually made of cement
concrete. In the past, the
gravity dams were made of
stone masonry, (Nagarjuna
Sagar Dam (AP), Krishnaraja
Sagar Dam (KN), Gandhi
Masonry Gravity Dam
Sagar Dam (MP), d, Jawai
(Non Overflow)
Dam(RJ)) but now the
masonry dams are rarely
constructed, except for very small heights. The gravity dams are generally straight in plan
(i.e. axis is straight from one abutment to the other) and are called straight gravity dams.
However, sometimes they are slightly curved in plan, with convexity towards the upstream
and are called curved-gravity dams (Hoover dam). The gravity dams are approximately
triangular in cross-section, with apex at the top. The gravity dams are generally more
expensive than earth dams but are more durable. They are quite suitable for the gorges with
very steep slopes. They require strong rock foundation. However, if the foundation consists
of soil, the height of the gravity dams is usually
limited to 20 m or so. A gravity dam is called solid
gravity dam when it is a solid mass of concrete (or
masonry) with no hollow spaces inside the dam,
except for small drainage galleries or shafts. On the
other hand, a hollow gravity dam has large hollow
spaces left within the body of the dam for the
purpose of reducing the weight and for more
effective use of concrete for resisting the stresses.
Hollow gravity dams are similar to buttress dams.
Hollow gravity dams are rarely constructed these
days. Hence, the gravity dam, in general, means a Concrete Gravity Dam
solid gravity dam. Most of the gravity dams with Overflow Section
constructed in India are straight solid gravity dams.
Bhakra dam (structural height of 226 m) was the
highest concrete gravity dam of the world when built
(surpassing then existing 221 m high Hoover dam in
USA). At present, it is second highest after Grand
Dixence Dam in Switzerland (284 m high). Koyna
Dam (MR), Rihand Dam (UP), Sardar Sarovar Dam,
etc are few examples of concrete gravity dams.
Nagarjuna Sagar Dam (structural height of 125 m) is
the highest masonry dam of the world. It is also the
largest (storage capacity) dam of India.

15

Advantages
(i) Gravity dams are quite strong, stable and durable.
(ii) Gravity dams are quite suitable across moderately wide valleys and gorges having
steep slopes where earth dams, if constructed, might slip.
(iii) Gravity dams can be constructed to very great heights, provided good rock
foundations are available.
(iv) Gravity dams are well adapted for use as an overflow spillway section. Earth dams
cannot be used as an overflow section. Even in earth dams, the overflow section is usually a
gravity dam.
(v) Gravity dams are specially suited to such areas where there is very heavy downpour.
The slopes of the earth dams might be washed away in such an area.
(vi) The maintenance cost of a gravity dam is very low.
(vii) The gravity dam does not fail suddenly. There is enough warning of the imminent
failure and the valuable property and human life can be saved to some extent.
(viii) Gravity dam can be constructed during all types of climatic conditions.
(ix) The sedimentation in the reservoir on the upstream of a gravity dam can be
somewhat reduced by operation of deep-set sluices.
Disadvantages
(i) Gravity dams of great height can be constructed only on sound rock foundations.
These cannot be constructed on weak or permeable foundations on which earth dams can be
constructed. However, gravity dams upto 20 m height can be constructed even when the
foundation is weak.
(ii) The initial cost of a gravity dam is usually more than that of an earth dam. At the
sites where good earth is available for construction and funds are limited, earth dams are
better.
(iii) Gravity dams usually take a longer time in construction than earth dams, especially
when mechanised plants for batching, mixing and transporting concrete are not available.
(iv) Gravity dams require more skilled labour than that in earth dams.
(v) Subsequent raising is not possible in a gravity dam.
(2) Earth dams: An earth dam is made of earth (or soil). It resists the forces exerted
upon it mainly due to shear strength of the soil. Although the weight of the earth dam also
helps in resisting the forces, the
structural behaviour of an earth dam
is entirely different from that of a
gravity dam. The earth dams are
usually built in wide valleys having Earth Dam
flat slopes at flanks (abutments). Reservoir
The foundation requirements are
less stringent than those of gravity
dams, and hence they can be built at
the sites where the foundations are
less strong. They can be built on all
types of foundations. However, the height of the dam will depend upon the strength of the

16

foundation material. The section of an earth dam can be homogeneous when the height of the
dam is not great. Generally, the earth dams are of zoned sections, with an impervious zone
(called core) in the middle and relatively pervious zones (called shells or shoulders) enclosing
the impervious zone on both sides. If the earth dam is built on a pervious foundation, a
concrete cutoff wall or a steel sheet pile line is also provided in the continuation of the core
section. Moreover, a drainage filter or a rock toe is provided on the downstream to carry
away the water that seeps through the dam and its foundation. Earth dams are usually cheaper
than the gravity dams if
suitable earth in abundant
quantity is easily
available near the site.
Nowadays majority of
dams constructed are of
this type. The highest
dams of the world are
earth dams (Rongunsky
dam Rusia, 325 m and
Nurek dam, Rusia, 317 m)
as well as the largest
capacity dams (New
Cornelia dam, USA and
Tarbela dam, Pakistan).
The highest dam of India
is Tehri dam (261 m),
which is earth and rockfill
type. Hirakund dam
(Orissa) is the longest
earthen dam of the world
(25.3 km).
Advantages
(i) Earth dams are usually cheaper than gravity dams if suitable earth for construction is
available near the site.
(ii) Earth dams can be constructed on almost all types of foundations, provided suitable
measures of foundation treatment and seepage control are taken.
(iii) Earth dams can be constructed in a relatively short period.
(iv) The skilled labour is not required in construction of an earth dam. Earth dams can be
raised subsequently.
(vi) Earth dams are aesthetically more pleasing than gravity dams.
(vii) Earth dams are more earthquake-resistant than gravity dams.
Disadvantages
(i) Earth dams are not suitable for narrow gorges with steep slopes.
(ii) An earth dam cannot be designed as an overflow section. A spillway has to be
located away from the dam.
(iii) Earth dams cannot be constructed in regions with heavy downpour, as the slopes
might be washed away.

17

(iv) The maintenance cost of an earth dam is quite high. It requires constant supervision.
(v) Sluices cannot be provided in a high earth dam to remove slit.
(vi) An earth dam fails suddenly without any sign of imminent failure. A sudden failure
causes havoc and untold miseries.
(3) Rockfill dams: A rockfill dam is built of rock fragments and boulders of large size. An
impervious membrane is placed on the
rockfill on the upstream side to reduce
the seepage through the dam. The
membrane is usually made of cement
concrete or asphaltic concrete. In early
rockfill dams, steel and timber
membrane were also used, but now
they are obsolete. A dry rubble cushion
is placed between the rockfill and the
membrane for the distribution of water
load and for providing a support to the
membrane. Sometimes, the rockfill
dams have an impervious earth core in
the middle to check the seepage instead
of an impervious upstream membrane. The earth core is placed against a dumped rockfill. It
is necessary to provide adequate filters between the earth core and the rockfill on the
upstream and downstream sides of the core so that the soil particles are not carried by water
and piping does not occur. The side slopes of rockfill are usually kept equal to the angle of
repose of rock, which is usually taken as 1.4:1 (or 1.3:1). Rockfill dams require foundation
stronger than those for earth
dams. However, the foundation
requirements are usually less
stringent than those for gravity
dams. Rockfill dams are quite
economical when a large
quantity of rock is easily
available near the site. Thiem
dam (PJ), Ramganga dam (UP),
etc. are rockfill dams in India,
while Mica dam (242 m,
Canada), and Chicoasen dam
(240 m, Maxico) are highest
rockfill dams. Rockfill dams
have almost the same
advantages and disadvantages
over gravity dams as
discussed for earth dams.
However, they have the
following particular
advantages and disadvantages
over earth dams.
(a) Advantages
(i) Rockfill dams are

18

quite inexpensive if rock fragments are easily available.
(ii) Rockfill dams can be constructed quite rapidly.
(iii) Rockfill dams can better withstand the shocks due to earthquake than earth
dams.
(iv) Rockfill dams can be constructed even in adverse climates.
(b) Disadvantages
(i) Rockfill dams require more strong foundations than earth dams.
(ii) Rockfill dams require heavy machines for transporting, dumping and
compacting rocks.
(4) Arch Dams: An
arch dam is curved in
plan, with its convexity
towards the upstream
side. An arch dam
transfers the water
pressure and other forces
mainly to the abutments
by arch action. An arch
dam is quite suitable for
narrow canyons with
strong flanks which are
capable of resisting the
thrust produced by the
arch action. The section
of an arch dam is
approximately triangular
like a gravity dam but
the section is
comparatively thinner.
The arch dam may have
a single curvature or
double curvature in the
vertical plane. Generally,
the arch dams of double
curvature are more
economical and are used
in practice. The arch
dams are not common in
India. Only one arch dam has been
constructed in India at Iddukki in Kerala
so far. Its height is 169 m. Outside of
India, there are many arch dams over
200 m (Juguri dam (272 m), Russia,
Vaiont dam (262 m), Italy, Manvoisin
dam (237 m) Switzerland). The arch
dam requires good quality concrete for
resisting the stresses. The quantity of

19

concrete required in an arch dam is less than that for a gravity dam, but it is not necessarily
less expensive because of high cost of concrete and form work. The arch dams are subjected
to large stresses because of changes in temperature shrinkage of concrete and yielding of
abutments.
Advantages
(i) An arch dam requires less concrete as compared to a gravity dam as the
section is thinner.
(ii) Arch dams are more suited to narrow, V-shaped valley, having very steep
slopes.
(iii) Uplift pressure is not an important factor in the design of an arch dam
because the arch dam has less width and the reduction in weight due to uplift does
not affect the stability.
(iv) An arch dam can be constructed on a relatively less strong foundation
because a small part of load is transferred to base, whereas in a gravity dam full load
is transferred to base.
Disadvantages
(i) An arch dam requires good rock in the flanks (abutments) to resist the thrust.
If the abutments yield, extra stresses develop which may cause failure.
(ii) The arch dam requires sophisticated formwork, more skilled labour and
richer concrete.
(iii) The arch dam cannot
be constructed in very cold
climates because spalling of
concrete occurs due to
alternate freezing and
thawing.
(iv) The arch dams are
more prone to sabotage.
(v) The speed of
construction is relatively
slow.
(5) Buttress Dams:
Buttress dams are of three types: Buttress Dam (d/s side)
(i) Deck type, (ii) Multiple arch-
type, and (iii) Massive-head
type. A deck type buttress dam
consists of a sloping deck
supported by buttresses.
Buttresses are triangular
concrete walls which transmit
the water pressure from the deck
slab to the foundation. Buttresses Multiple Arch Buttress
are compression members. The Dam (u/s side)
deck is usually a reinforced
concrete slab supported between

20

the buttresses, which are usually equally spaced. In a multiple-arch
type buttress dam the deck slab is replaced by horizontal arches
supported by buttresses. The arches are usually of small span and
made of concrete. In a massive-head type buttress dam, there is no
deck slab. Instead of the deck, the upstream edges of the buttresses
are flared to form massive heads which span the distance between
the buttresses. The buttress dams require less concrete than gravity dams. But they are not
necessarily cheaper than the gravity dams because of extra cost of form work, reinforcement
and more skilled labour. The foundation requirements of a buttress dam are usually less
stringent than those in a gravity dam.
Advantages
(i) Buttress dams require less concrete than gravity dams.
(ii) The uplift pressure is generally not a major factor in the design of buttress
dams.
(iii) Buttress dams can be constructed on relatively weaker foundations.
(iv) Power house and water treatment plants, etc. can be housed between
buttresses.
(v) The ice pressure is relatively less important because ice tends to slide over
the inclined deck.
(vi) The vertical component of the water pressure on deck prevents the dam
against overturning and sliding failures.
(vii) Buttress dams can be designed to accommodate moderate movements of
foundations without serious damages.
(viii) Heat dissipation is better in buttress dams. Therefore, the speed of

21

construction is more.
(ix) The back of the deck and
the foundation between buttresses
are accessible for inspection.
(x) Buttress dams can be
easily raised subsequently by
extending buttresses and deck
slabs.
Disadvantages
(i) Buttress dams require
costlier formwork, reinforcement
and more skilled labour.
Consequently, the overall cost of
construction may be more than that of a gravity dam.
(ii) Buttress dams are more susceptible to damage and sabotage.
(iii) Buttress dams cannot be constructed in very cold climates because of
spalling of concrete.
(iv) Because the upstream deck slab is thin, its deterioration may have very
serious effect on the stability.
(6) Steel Dams: A steel dam consists of a steel framework, with a steel skin plate on its
upstream face. Steel dams are generally of two types: (i) Direct-strutted steel dams, and (ii)
Cantilever type steel dams. In a direct strutted steel dam, the water pressure is transmitted
directly to the foundation
through inclined struts. In a
cantilever type steel dam,
there is a bent supporting the
upper part of the deck, which
is formed into a cantilever
truss. This arrangement
introduces a tensile force in
the deck girder which can be taken care of by anchoring it into the foundation at the upstream
toe. Hovey suggested that tension at the upstream toe may be reduced by flattening the slopes
of the lower struts in the bent. However, it would require heavier sections for struts. Another
alternative to reduce tension is to frame together the entire bent rigidly so that the moment
due to the weight of the water on the lower part of the deck is utilised to offset the moment
induced in the cantilever. This arrangement would, however, require bracing and this will
increase the cost. No steel dam has been constructed in India. These are quite costly and are
subjected to corrosion. These dams are almost obsolete. Steel dams are sometimes used as
temporary coffer dams during the construction of the permanent dams. Steel coffer dams are
supplemented with timber or earthfill on the inner side to make them water tight. The area
between the coffer dams is dewatered so that the construction may be done in dry for the
permanent dam.
Advantages
(i) Steel dams can be constructed with great speeds with the modern methods of
fabrication.

22

(ii) The steel dams are usually statically determinate. Hence the section can be
designed with more confidence and more economically.
(iii) Steel dams are usually cheaper than gravity dams.
(iv) If properly fabricated, there is very little leakage through a steel dam.
(v) Steel dam can easily withstand stresses due to unequal settlements.
Disadvantages
(i) Steel dam cannot be designed as overflow dams because they are unable to
withstand stresses due to vibrations and shocks of spilling water on thin steel
section.
(ii) Steel dams require strong and deep anchorages in foundation.
(iii) Steel dams require careful regular maintenance.
(iv) The life of steel dam is short.
(v) There is concentration of bearing stresses at the points of contact of the
members.
(7) Timber Dams: A timber dam consists of a framework made of timber with a facing
of timber planks. The framework is comprised of struts and beams. It transfers the water
pressure on the upstream planks to the foundation. The timber dams are mainly of three
types: (i) A-frame type, (ii) Rock-filled crib type, and (iii) Beaver type. The A-frame type
timber dam is built of timbers and
planks making the shape of
English letter A. The stability
depends upon the weight of the
water on the deck and upon
anchorage of sills. The sill is fixed
to a ledge rock by wedge bolts or
anchor bolts for proper anchorage.
The struts are fixed to the sill and
are held in position by cross-
bracing and batten blocks. The wales are then fixed on the struts and the whole structure is
thoroughly drift-pinned to form a bent. These bents are usually spaced 2 to 4 m apart,
depending upon the height of the timber dam and the size of the timber members used.
Transverse members, called studs, are then placed across the bents, and a suitable lagging is
nailed to them to form the upstream deck to retain water. In rock-filled crib rock type of
timber dam, cribs of timber members are drift-bolted together. The timber members are
generally of round or square section and are placed at 2.5 m centres in both directions. The
bottom members of the cribs are generally pinned to the rock foundation. The space between
various members is filled with rock
fragments or boulders to give stability. A
top plank is then placed on the cribs. A
beaver type timber dam consists of
timber members of round section
forming a bent. The butts of all the
timber members point downstream.
Spacer logs are placed between the butts
and driftpinned to the other logs. The tips
of the timber members pointing upstream

23

are also driftpinned together. The bottom members are fixed to the foundation with anchor
bolts. A plank is placed on the bent to form a deck. Sometimes, a mat made of brushwood or
the branches of the trees is used as a deck. The upstream slopes are not steeper than 2: 1. This
type of timber dam is used for low heights. Timber dams are generally used as temporary
dams. These are short lived, but if well-designed, constructed and maintained, they may last
even 30 - 40 years. The timber dams are used at places where timber is available in plenty
and the height of the dam is low. However, because of shortage of timber, these dams are
becoming obsolete.
Advantages
(i) The initial cost of timber dams is low when timber is available in plenty.
(ii) Timber dams can be constructed on any type of foundation.
(iii) Timber dams are suitable when construction is for a short period.
(v) The design and construction of timber dams is easy.
(v) Timber dams can be constructed at a great speed.
Disadvantages
(i) The life of timber dams is short.
(ii) The leakage through a timber dam is quite high.
(iii) The maintenance cost of a timber dam is high.
(iv) Timber dams cannot be constructed for great height.
Selection of Site for a Dam
A dam is a huge structure requiring a lot of funds. Extreme care shall be taken while
selecting the site of a dam. A wrong decision may lead to excessive cost and
difficulties in construction and maintenance. The following factors shall be
considered when selecting the site of a dam.
1. Topography: As far as possible, the dam should be located where the river
has a narrow gorge which opens out upstream to create a large reservoir. In that
case, the length of the dam would be small and the capacity of the reservoir on its
upstream would be large. In case there is a confluence of two rivers in the selected
reach, the dam should be located downstream of the confluence to take advantage of
the flow of both rivers. The dam should be preferably located where the river bed is
high, to reduce the height and cost of the dam.
2. Suitable Foundation: Suitable foundation should exist at the site for the
particular type of dam. If suitable foundation is not available but it can be improved
by adopting various measures, the site may be considered for selection. However, in
that case, the cost of such measures should not be excessive. For gravity dams of
great height, sound rock is essential. However, earth dams can be constructed on
almost any type of foundation provided suitable measures are adopted.
3. Good Site for reservoir: As the dam is constructed to store water in the
reservoir, so the site should have the following characteristics to make a good site for a
reservoir: (i) Large storage capacity: The topography of the site should be such that the
reservoir has a large capacity to store water. (ii) Shape of reservoir basin: The reservoir
basin on the upstream of the dam should preferably be cup-shaped, with a flat
bottom but steep slopes. (iii) Watertightness of the reservoir: The geological conditions of

24

the reservoir site should be such that the reservoir basin is watertight. The reservoir sites
having pervious rocks are not suitable. The reservoir basins having shales, slates, schists,
gneiss, granite, etc. are generally suitable. (iv) Good hydrological conditions: The
hydrological conditions of the river at the reservoir site should be such that adequate runoff is
available for storage. The catchment area of the river should give high yield. There should
not be heavy losses in the catchment due to evaporation, transpiration and percolation. (v)
Deep reservoir: The site should be such that a deep reservoir is formed after the construction
of the dam. A deep reservoir is preferred to a shallow reservoir because in the former the
evaporation losses are small, the cost of land acquisition is low and the weed growth is less.
(vi) Small submerged area: The site should be such that the submerged area is a minimum. It
should not submerge costly land and property. It should not affect the ecology of the region.
Monuments of historical and architectural importance should not be submerged. (vii) Low silt
inflow: The dam site should be such that the reservoir would not silt up quickly. The
life of the reservoir depends upon the rate of silting. The site should be selected such
that it avoids or excludes the water from those tributaries which carry a high percentage of
silt, i.e. if any tributary carries relatively large quantity of sediments, the dam should
be constructed upstream of the confluence of that tributary with the river. (viii) No
objectionable minerals: The soil and rock mass at the reservoir site should not contain any
objectionable soluble minerals which may contaminate the water. The stored water should be
suitable for the purpose for which the water is required.
4. Spillway site: A good site for a spillway should exist at or near the darn site.
The valley should be sufficiently wide to locate the spillway if it is an integral part
of the dam. If the spillway is to be located separately, the best site of spillway is that
in which there is a saddle near the dam site which is separated from it by a hillock.
In that case, the main dam can be located in the gorge and the spillway can be
constructed in the saddle.
5. Availability of materials: The dam requires a large quantity of material for
its construction. Suitable type of material in sufficient quantity should be available
at or near the dam site to reduce the cost.
6. Accessibility: The site should be easily accessible. It should be preferably
well-connected by a road or a railway line. This would facilitate transportation of
labour, materials and machinery.
7. Healthy surroundings: The surroundings of the site should be healthy and
free from mosquitos so that the labourers can comfortably live in colonies
constructed near the dam site.
8. Low cost of real estate: The cost of real estate for the reservoir site, dam, dwellings,
roads, railways, etc. should be low.
9. Minimum overall cost: The site should be such that it entails the minimum
overall cost of the project, including subsequent maintenance. Generally, two or
three probable sites are selected and rough estimates are made. The site which
entails the minimum overall cost can be tentatively selected.
10. Other considerations: For the development of a particular backward area,
the dam may be constructed in that region. Sometimes political considerations and
public opinion may affect the site of a dam.
Selection of Type of Dam
Selection of the most suitable type of dam for a particular site requires a lot of

25

judgment and experience. It is only in exceptional cases that the most suitable type
is obvious. Preliminary designs and estimates are usually required for several types
of dams before making the final selection on economic basis. The salient features of
different types of dams discussed in the preceding sections should be kept in mind
while selecting the type of dam. Various factors which govern the selection of type
of dam are discussed below:
1. Topography and valley shape. The choice or the type of dam for a
particular site depends to a large extent on the topography and the valley shape. The
following are the general guidelines. (a) If the valley is narrow, V-shaped and has
sound rock in bed and abutments, an arch dam is generally the most suitable type.
(b) If the valley is moderately wide, V-shaped and has sound rock in bed, a gravity
dam or a buttress dam may be quite suitable. (c) For a low rolling plain country,
with a fairly wide valley and alluvial soil or boulders in the bed. an earth dam or a
rockfill dam may be quite suitable.
2. Geology and foundation conditions. A dam is a very huge structure. All the
loads acting on the dam, including its own weight, are ultimately transferred to the
foundations. While selecting the type of dam for a particular site, geologic character
and thickness of rock, inclination of the bedding planes, existing faults and fissures,
permeability of strata, etc. affect the selection. Most suitable type of dam for
different types of foundation is usually determined as follows: (a) Rock foundation:
Any type of dam can be constructed on good rock foundation. Such foundations
have high bearing capacity and resistance to erosion and percolation and are ideal
for all types of dams. The most suitable type of dam for a particular site will depend
upon other factors such as availability of construction material and spillway site, etc.
If seams and fractures exist in the rock, these should be sealed by consolidation
grouting. If the rock is disintegrated at surface, it should be removed and further
disintegration should be stopped by taking suitable measures. (b) Gravel and coarse
sand foundation: The bearing capacity of foundation consisting of gravel and coarse
sand is low. For such foundations, earth dams and rockfill dams are usually selected.
However, gravity dams of low height can also be selected. There is considerable
seepage through these foundations. Cutoff walls or sheet piles are generally
provided in the foundation to reduce seepage and to prevent piping. (c) Fine sand
and silt foundations: These foundations are generally suitable only for earth dams
and low concrete dams. In such foundations, there are a number of problems such as
excessive settlement, piping, seepage erosion at the d/s toe and liquefaction failures.
Suitable measures are adopted before the construction of dams. (d) Clay foundation:
In general, clay foundations are not suitable for the construction of a dam. These
foundations have very low bearing capacity. Moreover, the settlements are quite
large, especially if the clay is nominally-consolidated or under-consolidated. In
over-consolidated clays, the settlements are relatively small. In some special cases,
low earth dams can be constructed on such foundations after properly treating and
consolidating the foundations. The maximum height of dam that can be permitted at
a site would depend upon the shearing strength and consolidation characteristics of
clay. Undisturbed samples are usually taken to determine the properties of clay. (e)
Non-uniform foundations: Non-uniform foundations may consists of soils of
different types of a combination of soils and rocks. Such foundations are not good
for the construction of dams and should be avoided as far as possible. If
unavoidable, special designs are adopted and appropriate foundation treatments are
required.

26

3. Availability of construction materials. The construction of a dam requires a
huge quantity of construction material. While selecting the type of dam, the
availability of the required construction materials should be considered. If a
particular material is available in abundance at or near the dam site, the maximum
use of that material should be made to reduce the cost. The materials which are not
available near the site should be either avoided or the minimum use shall be made of
such materials. For example, if suitable aggregates such as crushed stone, gravel and
sand are available, a gravity dam may be suitable. On the other hand, if suitable soil
is available in large quantity, an earth dam may be cheaper.
4. Overall cost. The overall cost is perhaps the most important factor which
affects the selection of the suitable type of dam for a particular site. The initial cost
of the dam depends upon the availability of material, the quantity of material
required, labour and the construction methods. The cost of subsequent maintenance
depends upon the durability of the materials used and the type of construction. The
dam with a minimum overall cost is usually the best. All the factors affecting the
overall cost should be considered. For example, the quantity of concrete required for
buttress dams and arch dams is much less than that in a gravity dam, but when the
cost of form work and reinforcement is also considered, a gravity dam may have
lower overall cost. Similarly, the initial cost of an earth dam may be less than that of
a gravity dam, but when the maintenance cost is also considered, a gravity dam may
be cheaper in the long run,
5. Spillway size and location. A spillway is an overflow structure provided at
or near the dam site to discharge excess flood water to downstream. The size of the
spillway mainly depends on the maximum discharge. The selection of the most
suitable type of dam for a particular site is sometimes governed by the size and
location of spillway. In a gravity dam, the spillway section and non--overflow
section can easily be provided side by side. Therefore, if a large spillway is required
and there is no separate location to keep the spillway away from the dam, a gravity
dam will be more suitable than an earth dam. In special cases, overflow spillways
are also provided along with earth dams and rockfill dams, but a long divide wall
between the main dam and the spillway is required to keep the flow away from the
dam.
6. Earthquake hazards: If the dam site is located in a seismic zone, the most
suitable type of the dam is one which can resist the earthquake shock without much
damage. Earth dams and rockfill dams are generally more suitable for such sites,
provided suitable modifications are made in the design. However, by adopting
suitable measures and considering various forces and factors affecting the seismic
design, other types of dams can also be provided.
7. Climatic conditions: Climatic conditions should also be considered while
selecting the type of dam. In extremely cold climates, buttress and arch dams should
be avoided. These dams have thin concrete sections and are easily damaged due to
spalling of concrete which occurs due to alternate freezing and thawing. Similarly, if
there are frequent rains and the climate is extremely wet, it will be difficult to
control water content of the soil and compaction in an earth dam. Therefore, earth
dams should be avoided.
8. Diversion problems: During the construction of the dam, the river water has
to be diverted so that construction can be done in dry. If the river water cannot be
diverted through a suitable tunnel (or channel) located in one of the flanks

27

(abutments), it has to be passed over the partly constructed dam when the
construction is done in the other part. In such a case, an earth dam cannot be
provided, and the choice will be more in favour of a gravity dam or any other type
of concrete dams.
9. Environmental considerations: The dam and its appurtenant works should
be aesthetically acceptable and they should not have any adverse effect on ecology
and environment. Generally, earth dams are more suitable than concrete dams for
aesthetical consideration. They merge easily with the natural environment in the
valley. Sometimes, a particular reach of the river has good scenic beauty for
recreational facility which is likely to be spoiled by construction of a high dam and
creation of a large reservoir. In such a case, the dam site maybe shifted.
Alternatively, a low diversion dam may be constructed to divert the water to an off-
stream reservoir developed at a suitable site away from the dam to preserve the site
for recreation.
10. Roadway: If a wide, straight roadway is to be provided over the top of dam,
an earth dam or a gravity dam is more suitable than an arch dam or a buttress dam.
11. Length and height of dam: If the length of the dam is great and the height
is low, an earth dam is generally better than a gravity dam. On the other hand, if the
length is small and the height is great, a gravity dam is better.
12. Life of dam: If the expected life of the project is long, a concrete dam is
usually preferred. Earth and rockfill dams have moderate life, whereas timber dams
have short life.
13. Miscellaneous considerations: Sometimes the selection of the type of dam
is made due to other miscellaneous considerations. For example, if earth-moving
machines are cheaply available at the site, an earth dam may be preferred. Similarly,
if mixing plants, batching plants, etc. are already available near the site, a gravity
dam or a buttress dam may be selected. Likewise, if cheap labour is locally
available, a masonry dam may be preferred to a concrete dam.
Investigations of Dam and Reservoir Site
The following investigations are usually conducted locate the most suitable site for a dam
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 levelling. 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 upto 200 m upstream and 400 m downstream and for adequate
width beyond the two abutments. 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 upto an
elevation 3 to 5 m higher than the anticipated maximum water level (MWL).
2. Geological Investigations: Geological Investigations of the dam and reservoir site are
done for (a) Suitability of foundation for the dam, (b) Watertightness of the reservoir basin,
and (c) Location of the quarry sites for the construction materials. Subsurface explorations
are carried out to determine the depth of overburden to be removed for laying the foundation
of the dam, the type of rock, the nature and extent of the fault zones, if any, present in the

28

rock. The information obtained from the geological investigations is used for devising a
suitable programme of foundation treatment and grouting if necessary. Geological
investigations are conducted to detect the presence of faults, fissures, and cavernous rock
formations which have cavities and are porous. If such formations exist in small areas, they
may be treated and made watertight. However if they are wide spread, the site may have to be
abandoned. Geological investigations are also conducted for location of suitable quarries for
stones and borrow areas for soils. The quality and the quantity of the available construction
materials are also ascertained.
3. Hydrological Investigations: Hydrological Investigations are conducted (a) to study
the runoff pattern and to estimate yield and (b) to determine the maximum discharge at the
site. 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 seasons to season. For the
determination of the storage capacity of a reservoir, the runoff pattern of the river at the dam
site is required. 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 maximum outflow and the maximum water level reached during the worst flood.
The methods for the fixation of reservoir capacity, for the estimation of the maximum flood
discharge, and for flood routing are already learnt in Hydrology course.
4. Sub-surface exploration: Sub-surface exploration programme usually
includes one or more of the following methods. (i) Geophysical method, (ii)
Sounding and penetration methods, (iii) Open excavations, (iv) Exploratory boring,
and (v) Rock drilling. Foundations of a dam are usually of the following two types:
(a) Rock foundation, and (b) Alluvial foundation. High dams are usually constructed
on rock foundations. Moreover, the spillways and outlets are normally built on
sound rock. Thorough subsurface explorations of rocks are carried out to determine
the depth of overburden, location of fault zones, extent of jointing, existence of
solution cavities, presence of soluble materials, disintegration of rock, etc. The
presence and nature of clay or any other material in the seams of the jointed or
fractured zones of the rock should be specially investigated. Such materials create
problems and may even lead to failure when the reservoir is filled and very high
hydrostatic pressure develops. The depth of exploratory investigations should be
taken upto the bed rock. If the bed rock is at great depth, there should be at least one
boring upto the bed rock. Investigations must be done through all soft, unstable and
permeable strata of the overburden. For large dams, the foundations should be
thoroughly investigated to great depths. Alluvial foundations consist of sand, gravel,
silt and clay. Generally, earth dams and low gravity dams are constructed on such
foundations. For all other types of dams, rock foundations are required. Geophysical
methods and sub-surface soundings are used to determine the depth of bed rock. The
properties of soils such as permeability, density, consolidations characteristics and
shear strength are determined.

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Dams introduction

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Dams introduction