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Chapter 1: Introduction to Dam Engineering
1.1 General
A dam is a barrier constructed across a river or a natural stream to create a reservoir for
impounding water (for Irrigation, Hydro-power, Water-supply, Flood Control, Navigation,
Fishing and Recreation), or to facilitate diversion of water from the river, or to retain debris
flowing in the river along with water.
The construction of dams ranks with the earliest and most fundamental of civil engineering
activities. All great civilizations have been identified with the construction of storage reservoirs
appropriate to their needs, in the earliest instances to satisfy irrigation demands arising through
the development and expansion of organized agriculture. Operating within constraints imposed
by local circumstance, notably climate and terrain, the economic power of successive
civilizations was related to proficiency in water engineering. Prosperity, health and material
progress became increasingly linked to the ability to store and direct water.
In an international context, the proper and timely utilization of water resources remains one of
the most vital contributions made to society by the civil engineer. Dam construction represents a
major investment in basic infrastructure within all nations. The annual completion rate for dams
of all sizes continues at a very high level in many countries, e.g. China, Turkey and India, and to
a lesser degree in some more heavily industrialized nations including the United States.
Dams are individually unique structures. Irrespective of size and type they demonstrate great
complexity in their load response and in their interactive relationship with site hydrology and
geology. In recognition of this, and reflecting the relatively indeterminate nature of many major
design inputs, dam engineering is not a stylized and formal science. As practiced, it is a highly
specialist activity which draws upon many scientific disciplines and balances them with a large
element of engineering judgment; dam engineering is thus a uniquely challenging and
stimulating field endeavor.
The history of dam building dates back to antiquity, and is bound up with the earlier civilizations
of the Middle East and the Far East. Countless small dams, invariably simple embankment
structures, were constructed for irrigation purposes in, for example, China, Japan, India and Sri
Lanka. Certain of these early dams remain in existence.
The dam built at Sadd-el-Kafara, Egypt, around 2600 BC, is generally accepted as the oldest
known dam of real significance. Constructed with an earth-fill central zone flanked by rock
shoulders and with rubble masonry face protection, Sadd-el-Kafara was completed to a height of
14m. The dam breached, probably in consequence of flood overtopping, after a relatively short
period of service.
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Numerous other significant dams were constructed in the Middle East by early civilizations,
notably in modern Iraq, Iran and Saudi Arabia. The Marib embankment dam, completed in the
Yemen around 750 BC to service a major irrigation project, was an example of particular note, as
this dam was raised to a final height of 20m. The first significant masonry dam, the 10m high
Kesis Golu (North) dam in Turkey, dates from the same period.
1.2 Reservoir site selection
Before we discuss about reservoir site selection first we have to define reservoir. Reservoir is an
artificial lake in which water is impounded for domestic and industrial use, irrigation,
hydroelectric power, flood control, or other purposes. Reservoirs are formed behind a dam. The
size and water content of a reservoir are controlled by a dam. The reservoir volume is usually
defined by its live or dead storage capacity. Live storage capacity is the entire volume that can be
withdrawn from the reservoir, whereas dead storage is the volume of water remaining in the
reservoir when it is emptied to its (legislative) low-water level.
Fig. 1.1 Dam and Reservoir
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Investigations
1.
2.
3.
Engineering surveys
 Conducted for dam, reservoir and other associated work.
 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.
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.
Hydrological investigations
 The hydrological investigations are conducted for the following purposes :
i.
To study the runoff pattern and storage capacity.
ii.
To determine the maximum discharge at the site.
10 procedures for selection of site for reservoirs during dam surveying
1.
The first point you have to consider is the geological conditions of the land. The
percolation losses of the catchment should be less i.e. it should maintain maximum
runoff.
2. Suitable dam site should exist and the infiltration rate of soil should be less.
3. The reservoir should be in a place such that the quantity of leakage should be less.
4. The site should be deep as possible because less land is required, less evaporation and
less reduction in water spread area.
5. The rock should not have high permeability; preferred rocks area shales, slates, schists,
gneisses and crystalline rocks such granites etc.
6. The price of real estate for the reservoir, including roads, relocation, and dwelling must
be less as possible.
7. The topography of the reservoir should be such that it should maintain required capacity
without submerging excessive land and other properties.
8. The valley to the reservoir should avoid carrying a high percentage of silt to reservoir or
dam.
9. The rocks and soil should not contain any minerals or salts which are harmful to the
concrete construction of the dam.
10. You should have a proper idea about the chances of the earthquake in that place.
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Zones of Storage Reservoir
The storage capacity of a reservoir is designated by several zones which are demarcated by
certain pool levels as stated below (Fig. 1.2).



Normal Pool Level (N.P.L); It is the maximum elevation of the water surface which is to
be stored in the reservoir during the normal working period. The water above this level
will flow out over the spill way crest. This water level is also known as full reservoir
level (F.R.L.).
Maximum Pool Level (M.P.L.); It is the maximum elevation to which the water surface
is allowed to rise in the reservoir during the period of flood. This level can be controlled
by providing spill way gate.
Minimum Pool Level; It is the lowest elevation to which the water level is allowed to
drop in a reservoir. This level is fixed by providing outlets in the dam. In case of
hydroelectric power generation, the minimum head is essential. So, the water level of the
reservoir should not drop below the minimum pool level.
The following are the various storage zones:
a) Dead storage; the volume of water which is stored below the minimum pool level of a
reservoir is known as dead storage. This water cannot be utilized under normal operating
condition.
b) Useful or live storage; the volume of water stored in the reservoir between the normal
pool level and minimum pool level is known as useful storage or live storage. This water
can be used for various purposes, such as irrigation, water supply, fishery, hydroelectric
power generation, etc.
c) Surcharge storage; the volume of water stored in a reservoir between the normal pool
level and maximum pool level is known as surcharge storage. This Storage is not meant
for any work, but it is mainly for the detention of flood water so that it may not cause any
danger on the downstream side. This storage is automatically exhausted by flowing over
the spill way crest when the spill way gates are opened.
d) Valley storage; the minimum volume of water which flows through a river before the
construction of dam is known as valley storage. This water may always be available from
the river even before the formation of reservoir. So, the effective storage capacity is given
by.
Effective storage = (live storage + surcharge storage — valley storage).
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Fig.1.2 Storage zones
1.3 Classification of Dams
Various types of dams are constructed based on their usage. These dams are classified based on
construction material used, structure and design, functions, hydraulic design and size.
Classification Based on Construction Material used for a Dam
1.
Rigid Dam
A dam is said to be Rigid Dam if it is constructed using rigid construction materials such as
masonry, concrete, steel, timber etc. The basic shape of rigid dam is triangular. Rigid dams
constructed using different rigid materials are discussed below.
a. Masonry Dam
Masonry dams are built using either stone masonry or brick masonry. Cement mortar is
used to join the masonry blocks. Gravity dam, arch dam etc. are examples of masonry
dams.
b. Concrete Dam
Concrete is most commonly used material to construct a dam. Most of the major dams in
the world are built using concrete. Gravity dams, arch dam, Buttress dam etc. can be
constructed using concrete.
c. Timber Dam
Timber dams generally used for temporary purposes such as to divert the water for the
construction of main dam, to control flood water flow etc. Timber dams are suitable up to
9 meters height.
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d. Steel Dam
Steel dams are also used for temporary requirements like timber dams. Steel plates and
inclined struts are used for the construction of steel dam. This type of dam is suitable up
to 15 to 18 meters of height.
2. Non-Rigid Dam
Non-rigid dams are constructed using non rigid materials such as earth, rocks etc. Basic
shape of non-rigid dam is Trapezoidal.
 Earth-fill and rock-fill dams are non-rigid dams.
 They are usually called embankment dams.
a) Earth-fill Dam
Earth-fill dams (also called an earthen dams or terrain dams) are made of ordinary soil
which is cheaply available. This type of dam is suitable where the foundation soil is very
weak and not strong enough to carry the weight of masonry dam. Since it is constructed
using soil the cost of construction is very less compared to rigid type dam.
b) Rock Fill Dam
Rock fill dams are constructed using rocks and boulders. Upstream side of dam is built
with dry rubble masonry and loose rock fill is provided on the downstream side. A
reinforced concrete slab layer is also provided on the upstream side to make it water tight.
It is more stable than earthen dams and its flexible nature helps it better against
earthquake forces.
Classification Based on Structure and Design of a Dam
Depending upon structure and design dams are classified into
1. Gravity Dam
A Gravity dam is a structure which resists the external forces by its own weight or selfweight. Gravity dams are generally constructed by using masonry or concrete. Various
external forces like water pressure, uplift pressure, wave pressure, ice pressure, earth
quake pressure etc. are resisted by its self-weight only which acts vertically downwards.
So, good foundation is required to construct gravity dam preferably rocky strata under the
dam. The shape of cross section of gravity dam is approximately triangular in shape.
Infiltration gallery can be provided within the dam to resist uplift pressure. The failure of
gravity dam may occur due to sliding, overturning or crushing at toe. Hence, higher
factor of safety is recommended for the design of gravity dam.
2. Arch Dam
An arch dam is curved in plan with its convex upstream. Various forces coming onto the
dam are resisted by its arch action. It is constructed using masonry or concrete but
requires less material compared to gravity dam. The loads coming onto the dam are
transferred to the abutments of dam. So, abutments must be stronger and generally
natural formations like hills are used as abutments. Arch dams are generally preferred for
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narrow valleys. Arch dam is economical when the length of dam is less than its height.
So, this type of dams can be build up to greater heights.
3. Buttress Dam
A buttress dam contains face slab, buttresses and base slab. Face slab is provided on the
upstream side with some inclination and this slab is supported by series of buttresses
which are nothing but supports. Base slab acts as foundation for the whole dam which
receives the load from buttresses and face slab. Buttress dam is either straight or curved
in plan. Greater the height of dam higher the number of buttresses. The space available
between the buttresses can used for several purposes like water treatment plant
installation, power plant installation etc.
4. Embankment Dam
Embankment dams come in two types: the earth-filled dam (also called an earthen dam or
terrain dam) made of compacted earth, and the rock-filled dam. A cross-section of an
embankment dam shows a shape like a bank, or hill. Most have a central section or core
composed of an impermeable material to stop water from seeping through the dam. The
core can be of clay, concrete, or asphalt concrete. This dam type is a good choice for sites
with wide valleys. They can be built on hard rock or softer soils. For a rock-fill dam,
rock-fill is blasted using explosives to break the rock. Additionally, the rock pieces may
need to be crushed into smaller grades to get the right range of size for use in an
embankment dam.
Classification Based on Functions of a Dam
Based on Functions Dams are classified into
1) Storage Dam
Storage dam is constructed to store water on the upstream side especially during rainy
seasons and is released during dry weather season or when there is higher demand of
water. The stored water can also be used to generate power, irrigation, water supply etc.
2) Diversion Dam
Diversion dam is a dam which is constructed to divert the flow of water into other
channel or canal. This type dams are generally used to fill the irrigation channels. A
diversion dam is usually called a weir or a barrage.
3) Detention Dam
The main purpose of Detention dam is to control floods. During flood period, it stores the
water and protects the downstream side from damage due to floods. After the flood
period the stored water is released at a controlled rate without affecting the downstream
side.
4) Debris Dam
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Debris dam is built to retain the debris of the river water. Sand, gravel, driftwood etc. are
debris generally flow with river water. The water on the downstream side of debris dam
is clear.
5) Cofferdam
Cofferdam is temporary structure which generally acts like diversion dam. Cofferdam
provides dry area for the construction of main dam by diverting the water flow into other
watercourse. It is constructed on upstream side or fully around the construction site.
Classification Based on Hydraulic Design of Dam
Based on hydraulic design dams are classified into two types as follows
1) Overflow dam - it is constructed with a crest to permit overflow of surplus water that
cannot be retained in the reservoir.
2) Non-overflow dam - it is constructed such that water is not allowed to overflow over its
crest.
Classification Based on Gross Storage of Dam
Dams are classified into three types based on gross storage capacity of dam and they are
1) Small Dam
A dam is called as small dam if its gross storage capacity is in between 0.5 to 10 MCM
(million cubic meters). Hydraulic head of small dam is generally about 7.5 to 12 meters.
2) Medium Dam
If gross storage capacity of a dam is in between 10 to 60 MCM then it is said to be
medium storage dam. Its hydraulic head is 12 to 30 meters.
3) Large Dam
A dam is said to be large dam if its gross storage capacity is above 60 MCM. Hydraulic
head of a large dam is greater than 30 meters.
1.4 Factors Affecting Selection of Type of Dam
During the early stages of planning and design, the selection of the site and the type of dam
should be carefully considered. It is only in exceptional cases that an experienced engineer can
say that only one type of dam is suitable for a given dam site. Except in cases where the selection
of type is obvious, it will be found that preliminary designs will be required for several types of
dams before it can be shown which is the best solution from the standpoint of direct costs and all
other factors. Some of the physical factors which affect the choice of the type of dam are
discussed below.
1.4.1 Topography
Topography dictates the first choice of the type of dam and the most important factor in this
respect is the shape of the valley.
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ii.
iii.
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A narrow V-shaped valley with sound rock in abutment has an arch dam as the first
choice. However, for economic arch dam it is preferable to have the top width of the
valley less than about four times its height. It is also suitable for rock-fill dam.
A narrow or moderately wide U-shaped valley with sound rock foundation is best suited
for gravity or buttress dam.
Wide valley with foundation of soil material to a considerable depth (deep over burden)
favor Earth-fill embankment dam.
1.4.2 Geology and Foundation Conditions
The foundations have to carry the weight of the dam. The dam site must be thoroughly surveyed
by geologists, so as to detect the thickness of the foundation strata, presence of faults, fissured
materials, and their permeability, slop and slip etc…
The common types of foundations encountered are:
i.
ii.
iii.
iv.
v.
Solid Rock Foundation
Because of high bearing capacity and resistance to erosion and percolation, any type of
dam can be built on such foundations. However, the choice of the type of dam will be
governed by economy of materials or overall cost. The removal of disintegrated rock
together with the sealing of seams and fractures by grouting will frequently be necessary.
Gravel Foundations (and coarse sand)
If well compacted such foundations are suitable for earth-fill, rock-fill and low concrete
gravity dams (<15m). As these foundations are frequently subject to water percolation at
high rates, special precautions must be taken to provide effective water cut offs or seals.
Silt and Fine Sand Foundations
These foundations suggest the adoption of earth dams or very low gravity dams (up to 8m
high), but they are not suitable for rock-fill dams. The main problems are settlement, the
prevention of piping, excessive percolation losses, and protection of the foundation at the
d/s toe from erosion.
Clay Foundations
Clay foundations can be used for the support of Earth-fill dams but require special
treatment. Since there may be considerable settlement of the dam if the clay is
unconsolidated and the moisture content is high, clay foundations ordinarily are not
suitable for the construction of concrete gravity dams, and should not be used for rock-fill
dams. Tests of the foundation material in its natural state are usually required to
determine the consolidation characteristics of the material and its ability to support the
supper imposed load.
Non Uniform Foundations
At certain places, a uniform foundation of the types described above may not be
available. In such a case, a non-uniform foundation of rock and soft material may have to
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be used if the dam is to be built. Such unsatisfactory conditions have to be dealt with by
special designs or appropriate foundation treatment.
1.4.3 Materials for Dam Construction
Elimination or reduction of transportation expense for construction materials, particularly those
which are used in great quantity, will effect a considerable reduction in the total cost the project.
The most economical type of dam will often be the one for which materials are to be found in
sufficient quantity within a reasonable distance from the site. The availability of suitable sand
and gravel for concrete at a reasonable cost locally and perhaps even on property which is to be
acquired for the project is a factor favorable to the use of a concrete structure. On the other hand,
if suitable soils for an earth-fill dam can be found in nearby borrow pits, an earth-fill dam may
prove to be the most economical. Advantage should be taken of every local resource to reduce
the cost of the project without sacrificing the efficiency and quality of the final structure.
1.4.4 Spillway Size and Location
The spillway is a vital appurtenance of dam. Frequently its size, type and natural restrictions in
its location will be the controlling factors in the choice of the type of dam. Spillway requirements
are dictated primarily by the runoff and stream flow characteristics, independent of site
conditions or type of dam. The selection of a specific spillway types will be influenced by the
magnitudes of the floods to be bypassed. Thus, it can be seen that, on streams with large flood
potential, the spillway will become the dominant structure and the selection of the type of dam
could become a secondary consideration. The cost of constructing a large spillway is frequently a
considerable portion of the total cost of the development. In such cases, combining the spillway
and dam in to one structure may be desirable, indicating the adoption of a concrete overflow
dam. In certain instances, where excavated material from separate spillway channel may be
utilized in dam embankment, an earth-fill dam may prove to be advantageous. Small spillway
requirements often favor the selection of earth-fill or rock-fill dams, even in narrow dam sites.
1.4.5 Earthquake
If the dam lies in area that is subject to earthquake shocks, the design must include provisions for
the added loading and increased stresses. Although by including the provisions for the added
loading due to earthquake in the design of any type of dam may be adopted in these areas. Earthfill and concrete gravity dams are the best suited types in this respect.
1.4.6 Height of Dam
Earthen dams are usually not provided for heights more than 30m or so. Hence, for greater
heights, gravity dams are generally preferred.
1.5 Investigation of Dam Site
Dam site investigation requires careful planning and a considerable investment of time and
resources. Wherever possible, in situ and field test techniques should be employed to supplement
laboratory testing progarmmes. Proper and meticulous interpretation of geological and
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geotechnical data demands the closest cooperation between the engineering geologist, the
geotechnical specialist and the dam engineer. Extensive investigations are conducted to confirm
that, the site can be developed on the desired scale and at acceptable cost. The nature of soil and
rock formations present, critical to foundation integrity must be proved by subsurface
exploration. Foundation competence of the dam site must be assessed in terms of stability, loadcarrying capacity, compressibility (soils) or deformability (rocks), and effective mass
permeability. The investigative techniques to be adopted will depend upon the geomorphology
and geology of the specific site. All are assed in relation to the type and size of dam proposed.
In the case of a difficult site, the site evaluation programme can be protracted and expensive.
Expenditures may be of the order of 1% up to, exceptionally, 2.5 or 3% of the anticipated cost of
the dam. The scope of individual aspects of an investigation reflects circumstances unique to the
site.
In parallel with these investigations, extensive and detailed surveys are required to establish the
location and extent of potential sources of construction materials in reasonable proximity to the
site. Overall site viability is additionally subject to economic considerations, notably site
preparation and construction material costs. It may also be influenced by seismicity, access
development cost or other local constraints, including environmental considerations.
Generally site investigation may be broadly classified under three categories
i.
ii.
iii.
Reconnaissance
Preliminary investigation
Final investigation
Reconnaissance: Involves visiting all available sites which have a possibility of being utilized
and gathering information which will be useful for planning the detailed surveys and
investigations. The information to be collected may include geological data without any kind of
subsurface exploration, approximate estimate of stream flow data, storage capacity and head
available, etc….
Preliminary Investigation: Sufficiently precise data is collected at several sites selected during
reconnaissance to determine the most economical and suitable site among these. Preliminary
investigation usually requires the following items.
a.
b.
c.
d.
e.
Less precise site survey with the resulting topographic site map
Some investigation of the overburden
Few borings, say from 6 to 50, according to the size of the dam
Preliminary geologic investigation and corresponding report
Investigations of construction materials, e.g. earth, gravel, concrete aggregate etc…
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f. Determination of public utilities such as road, telephone lines etc… that may be affected
by the construction of the dam
g. Hydrologic studies
h. Determination of sediment load of the stream
i. Checking of high water marks for their use in determining spillway capacity
requirements.
Final Investigation: One of the several possible dam sites investigated in preliminary
investigation is elected for final, precise investigation. Final investigation involves the following
items.
a. Sufficiently precise site survey and preparation of topographic maps to serve all purposes
of design and construction of the dam
b. Accomplishment of necessary borings, test pits subsurface explorations, geologic studies
and tests on the materials in foundation and in the proposed borrow lands.
c. Determination of the type of dam to be constructed
d. Planning for the foundation treatment on the basis of subsurface investigation
e. Determination of the extent of land which would be submerged in the reservoir and the
arrangements for rehabilitation of the residents of that area.
f. Obtaining sufficient information for accurate estimate of cost
g. Determination of the final location of the dam, construction equipment, labor and other
staff members, probable source of construction materials and all other information
needed to the construction Engineer.
It may, however, be mentioned that there is no distinct line of demarcation between the
preliminary and the final investigations of dam sites.
1.6 Loads on Dams
List of all possible forces acting on a dam are as follows;
I.
II.
III.
Self-weight of the dam: It is the weight of the dam due to the action of the gravity and
always acts downward direction through CG of the body. The self-weight of the dam
depends upon the specific weight of the construction materials.
Hydrostatic pressure: It is the pressure exerted by the fluid of the dam body and always
acts perpendicular to the contact surface. The resultant pressure acts through the center of
pressure.
Uplift pressure: It is pressure exerted by the fluid on the dam surface from the underneath
and always acts towards the upward direction. The water enters in to the dam body
through the pores, cracks, fissures and causes the uplift pressure. The uplift force will
reduce the effective weight of the dam and depends the contact area where it acts. The
magnitude of the uplift pressure will depend upon the types of the foundation, drainage
provision, provision of the cutoff walls, grouting etc.
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IV.
Earthquake pressure: It is the pressure exerted on the dam body due to acceleration wave
of the earthquake. The wave acceleration direction may be vertical, horizontal as well as
in inclined direction too.
Wind pressure: It is the pressure exerted by the wind on the exposed surface of the dam
surface. The wind pressure depends upon the intensity of the wind velocity.
Wave pressure: It is the pressure exerted by the action of the wave on the dam surface.
The wave force depends upon the height of wave. The height of wave in the reservoir
depends upon the velocity of the wind and the fetching height (distance from the dam up
to the end of backwater curve).
Silt pressure: It is the pressure exerted by the silt accumulated behind the dam and could
be evaluated through the Rankine’s active earth pressure theory.
Ice pressure: It is the pressure exerted by the ice on the dam surface. It depends upon the
thickness, temperature fluctuation and restrain of the resisting force.
V.
VI.
VII.
VIII.
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1.7 Dam Free Board
Freeboard is most commonly thought of as the distance from a reservoir water surface to the top
(design crest elevation) of the dam. Freeboard is provided to prevent overtopping of the dam
crest by waves. Freeboard also provides an additional measure of safety of the dam to account
for the uncertainties in accurately estimating reservoir water surfaces and wave heights, and
uncertainties in reservoir operation and structure performance. Free board can be defined in
different terms such as:
1. Normal freeboard:
Normal freeboard is defined as the difference in elevation between the crest of the dam and the
normal reservoir water level as fixed by design requirements.
2. Minimum freeboard:
Minimum freeboard is defined as the difference in elevation between the crest of the dam and the
maximum reservoir water surface that would result should the inflow design flood occur and the
outlet works and spillway function as planned.
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