Elements of Dam Engineering:
• Introduction, classification, comparative study of
different types of dams, selection of type of dam,
selection of site of dam, preliminary and final
investigations of dam sites, fixation of storage
capacity, reservoir losses, sedimentation in reservoirs,
3. Elements of Dam Engineering
Dam: A dam is an impervious barrier constructed
across a river or stream to store water on its a
upstream side. The side of barrier on which water is
stored is called upstream side and the other side of the
barrier is called downstream side.
4. Purpose of Dam
• Dam is generally most suitable in hilly area where deep
valleys are available which gives a deep storage of
water. The stored water on its upstream side serves
various purpose such as:
• Flood Mitigation
• Water Supply
• Fishery and wild life Preservation
• Hydro-electric Power Generation
5. Flood Control
7. Water Supply
8. Hydro Electric Power Generation
10. Classification of Dams
Classification as per function and use
• This is the most common type of dam
normally constructed to store excess flood
water which can be utilized later when
demand exceeds the flow in river. The
Storage dams may be constructed for various
purposes such as irrigation, water
supply, hydro-power generation etc.
they may be made of concrete, stone or earth
or rock fill etc.
11. Storage Dam
12. Classification of Dams
• This type of dams are mainly constructed to
control flood. This type of dam stores water
temporarily and releases it gradually at a safe
rate when the flood recedes. Detention dam
provides safeguard against possible damage
due to flood on the downstream side of it.
Sometimes a detention dam may also be used
as storage dam.
13. Detention Dams
14. Classification of Dams
• The purpose of diversion dam is necessarily
different. It is constructed to divert the river
water into canal, conduit etc. For this purpose,
mostly a weir or low level dam is constructed
across the river to raise the water level which can
be diverted as per the needs. This type of dam
may be used for water supply, irrigation or some
15. Diversion Dam
16. Classification of Dams
Classification as per hydraulic design
Overflow Dam: An overflow dam is built to allow
the overflow of surplus discharge above the top
of it. They are generally built of masonry or
concrete and they are gravity type of dam.
Usually dams are not designed as overflow for
their entire length. Only few meters of its length
is kept as overflow section.
17. Overflow Dam
18. Classification of Dams
• In this type of dam, water is not allowed to overtop the
dam. The top of the dam is fixed at a higher elevation
than the expected maximum flood level. Since water is
not allowed to overtop, it can be constructed of large
fill, masonry, concrete etc.
19. Non-Overflow Dam
20. Classification as per Structural
• It is a solid concrete or masonry dam that all
external forces are resisted by its own weight
or gravity forces.
• An arch dam is curved masonry or concrete
dam which has convex portion facing
upstream. It resists major portion of water
pressure by arch action.
• The self weight of the dam is comparatively
lesser than gravity dam.
21. Gravity Dam & Arch Dam
22. Classification as per
• It consists of sloping membrane or deck on
upstream which is supported by number of
buttress or piers. These buttress are
constructed of reinforcements concrete
and supported by struts or bracings.
23. Buttress Dam
24. Buttress Dam
25. Classification as per Structural
• They are constructed of locally available soils, gravels
and sands, which resists all external forces by it shear
strength. These types of dams are more suitable up to
moderate height. They are generally trapezoidal in
section. Earth dam, earth and rock fill dam are the
example of this type of dam.
26. Embankment Dams
27. Earthen Dams
28. Rock Fill Dam
29. Classification as per material of
• These dams are built of rigid materials such
as masonry, concrete, steel or timber. In
earlier times dams were mostly built of stone
masonry which have been replaced now-adays by concrete.
• These dams are built of non-rigid materials
such as earth, rock fill etc. Earth Dam, Rock
fill Dam etc are the examples of this type.
30. Rigid Dams & Non-Rigid Dams
31. Embankment Dams
• They are the most ancient type of dams that can
be build by naturally available materials with
minimum of processing. These dams are not
suited for sites where good foundations is not
available at a reasonable depth for concrete or
masonry dam to construct.
• Earth dams are made of locally available soils,
sands and gravel with trapezoidal in section.
They are economical and suitable for almost all
type of available foundation.
32. Embankment Dams
33. Earth Dam
• This type of dam is constructed of a single
kind of materials with stone pitching at
upstream side to safeguard against erosion.
Also have a rock toe at downstream to drain
out the seepage water through the body of the
34. Earth Dam
• The zoned type of dam consists of more than one
kind of material. I consists of a central
impervious core made of clay and outer pervious
zone made of mixtures of earth and gravel. It
also has rock toe at downstream side and stone
pitching on upstream side.
35. Earth Dam
• This type of section is used when impervious
material is available in lesser quantity at site. It
consists of thin impervious core of diaphragm
made of clay, cement concrete or bituminous
concrete which is surrounded by earth or rock
fill. They are also called sometimes as thin core
dams. The construction of this type of dam is
limited to small dams only. It has upstream
stone pitching and downstream stone blanket.
36. Earth Dam
37. Characteristics of Earth Dam
• They are normally constructed when huge quantity of
material e.g. soils, gravels are available locally.
• They are suitable for almost all type of available
• They resist all external forces acting mainly by shear
strength of soil.
• They can be built rapidly with relatively unskilled labour
because they use locally available material in large
• They are comparatively cheaper than concrete and arch
• They allow easy increase in their height if
needed, without much difficulties.
38. Characteristics of Earth Dam
• They require a separate spillway away from the
• They require heavy maintenance cost and
• They are more susceptible to be damaged by
floods than any other type of dam.
39. Rock fill Dam
• It is the type of embankment dam which uses various
sizes of materials to provide stability. It also has
impervious membrane on upstream face to provide
• Impervious membrane is usually made of concrete. Rock
fill dam is preferred when plenty of rocks are available
from nearby quarry.
• Rock fill dams require foundation which will result in a
minimum settlement. The foundation should be free
from all foreign materials like silt, clay, sand etc. The
upstream and downstream slopes of a rock fill dams
depend on the type of impervious membrane and its
location. They are cheaper then concrete dam and can be
built rapidly if proper rock is available.
40. Rock fill Dam
41. Combined Earth and Rock fill
• It is a composite embankment dam. In this
upstream consists of soil where as downstream
portion is filled with rock. Upstream has a
riprap. With cement grouted core wall to check
seepage. Riprap makes upstream slope stronger
against seepage and damage due to wave action.
42. Combined Earth and Rock fill
43. Concrete Dams
• They are Categorized as rigid dams because they
are constructed of rigid material like concrete.
They may be either straight or curved in plan.
These types of dams are normally best suited on
solid rock foundations. The construction of such
dams requires heavy mechanized plants,
concrete, aggregate, cement and sand.
44. Concrete Dams
45. Gravity Dams
• It is a solid concrete dam which resists all
external forces by its own weight. It needs a
sound rock foundation because it transmits all
the forces including self weight to the
foundation. Most of the gravity dams are
provided with an overflow portion known as
spillway within the body of the dam
46. Gravity Dams
• It is Stronger and more stable than any other
type of dam
• It can house an overflow spillway to pass
excess flood water safely.
• It can built of any height provided suitable
foundation is available to bear all the loads
coming on it.
• The failure of a gravity dam is not sudden at
all. It gives sufficient time for evacuation of
area downstream of it.
47. Gravity Dams
• Its construction is possible only on sound rock
• Initial cost is higher.
• It needs skilled labor and mechanized plants for
• It may take more time in construction, if
manufacturing and transporting equipments are
48. Gravity Dams
49. Arch Dam
• It is curved Concrete Dam. The self weight of this
dam is quite less compared to gravity dam it
transmit major portion of water load to the
• It is particularly suited in Deep Georges where
length is same compared to its height.
• Very small portion of water pressure is transmitted
to foundation hence it can be built on moderate or
• It has less initial cost as compared to Gravity Dam.
50. Arch Dam
• It needs skilled labour, sophisticated formwork
and specialized design.
• Construction time is normally Large.
• It needs very strong abutments of solid rock to
resist arch thrusts
• It is not suitable if solid rocks are not available.
51. Arch Dam
52. Buttress Dams
• They may be considered as lightened version of
the gravity dam. A buttress dam consists of a
continuous inclined upstream face supported by
downstream buttresses at regular intervals. They
may be of deck slab type. Multiple arch
type, multiple dome type or bulk head type.
53. Buttress Dams
• It requires less materials for construction
• It can be constructed on even weak
foundation as pressure on foundation is
• The water pressure acts normal to the
inclined deck. Hence the vertical
components of water pressure stabilizes
the dam against overturning and sliding.
54. Buttress Dams
• It requires more for work than solid
• As Thickness of upstream concrete surface
is less, it is more liable to get deteriorated
• It requires constant maintenance and
• Life of dam is less as compared to other
55. Buttress Dams
56. Selection of Site for Dam
• Suitable foundations must be available at the selected
site for a particular type of dam. The foundation should
be free from seams and faults. It is however possible to
improve the foundation conditions by adopting suitable
• Dam site should have a narrow valley to reduce its length
it should store maximum volume of water. A major
portion of dam should be located on high ground as
compared to river basin. This will reduce the cost of dam
and facilitates easy drainage of dam section as well.
57. Foundation of Dam
59. Selection of Site for Dam
• Dam site should form deep reservoir with
small water surface to reduce (i) evaporation
loss and (ii) Submergence area (iii)and
control on weed growth. The quantity of
leakage through the sides and bed of selected
site should be minimum. Reservoir site with
the presence of permeable rocks reduces the
water tightness of the reservoir.
• For larger storage in reservoir, dam site
should be located at the confluence of two
61. Selection of Site for Dam
• The geological conditions of catchment
should be such that it yields maximum
runoff. It should also have minimum
percolation losses. The catchment area
should avoid or exclude water from
tributaries carrying high percentage of silt
62. Catchment Area
63. Selection of Site for Dam
• A suitable site for spillway should be
available near dam site when spillway is to
be located separately from dam. E.g. for
earth or rock fill dam. There is no special
site requirement for the spillway if it is to
be built inside the dam.
65. Selection of Site for Dam
• Huge amount of materials is required for
the construction of dam. Therefore
construction materials should easily be
available either locally or near vicinity of
the site so as to reduce the transportation
66. Selection of Site for Dam
• The dam Site should be easily approachable
so that it can economically be connected to
the important towns, cities etc. by rails or
• Healthy environmental conditions must be
available at dam site to set up colonies,
residential quarters for labours and other
67. Selection of Site for Dam
68. Factors Governing the
selection of Type of Dam
• Selection of the kind of dam is the first task. The
choice and selection of dam at a particular place in
the river may depend on the following factors.
• This is the first factor which governs the choice of
dam for a site
• (a) A low rolling plain topography gives choice of
an earth dam with a separate site of spillway
• (b) A low narrow V-shaped valley with sound rock
in the abutment suggests an arch dam.
70. Factors Governing the
selection of Type of Dam
(2) Geology and Foundation Conditions:
• Next Important factor for the choice of Dam.
• (a) Solid rock foundation with no fault or fissures, any
type of dam can be constructed.
• Rocks like granite, gneiss and schist provide good
foundation for a gravity dam.
• Poor rock or gravel foundation suggests choice of an
earthen dam, rock fill dam or low concrete gravity
• Silt and fine sand foundation pose the problem of
seepage settlement etc. Hence such foundations are
suitable only for earth dam or low concrete gravity
dam. They are not suitable for rock fill dam.
71. Geology and Foundation
72. Factors Governing the
selection of Type of Dam
(3)Availability of material for Construction
• The Construction materials must be available locally o
near the dam site in order to achieve economy in dam
construction. The local availability of sand, gravel,
crushed stone suggests concrete gravity dam. However, if
coarse and fine grained soils are available locally, an
earth dam may be suitable.
(4) Spillway Size and Location
• Spillway is required for safe disposal of of flood water.
When separate site of spillway is available earthen dam
may be preferred. In case of large capacity spillway, an
overflow concrete gravity dam having overflow section in
the middle will be the best choice.
73. Factors Governing the
selection of Type of Dam
• The provision of roadway at the top of dam
requires the choice of earth dam or gravity
• Spillway are important auxiliary works of dams,
provided to dispose of surplus floodwater safely
which cannot be stored in the reservoir.
Spillways are invariably provided in all the dams
and often called safety valve for the dam. It is
necessary to provide a spillway of sufficient
capacity so as to avoid water from overtopping
the dam. Overtopping of dam may lead to failure
of dam resulting in serious damage to the
Requirements of a spillway
• A Spillway should fulfill the following
• The spillway should have sufficient capacity
• The location of spillway should provide safe
disposal of water without toe erosion.
• Spillway should be hydraulically and structure
• Usually spillway should be accomplished by an
energy dissipation work on its downstream side.
76. Components Parts of a Spillway
• The various component parts of a spillway
are as under
(i) Control Structure
• Control Structure consists of a weir which may
be sharp. It is a major component of a spillway.
It regulates and controls the surplus water from
the reservoir. It does not allow the discharge of
water below from reservoir level and allows the
discharge of water below from reservoir level
and allows only when water surface in the
reservoir rises above that level.
77. Components Parts of a
(ii) Discharge Channel
• Discharge Channel It is provided to convey
the surplus water released through control
structures to the stream bed below the
dam safely. The discharge channel may be
the downstream face of spillway itself, or
open channel excavated along the ground
surface or a closed conduit placed through
or under a dam.
78. Components Parts of a
79. Components Parts of a
• They are usually provided on the downstream
side of the spillway. High Voltage water coming
through Spillway may cause serious damage to
the toe of dam and to the adjacent structures.
This high energy of flow must be dissipated
before it flows back to river. For this, energy
dissipaters are provided
80. Components Parts of a
81. Components Parts of a
Entrance and Outlet Channels
• They are not required in case of an overflow
spillway. However entrance channels are
provided to draw water from reservoir and
convey it to control structure. Similarly Outlet
channels are provided to carry the spillway flow
in river channel below the dam. Similarly, outlet
channels are provided to carry the spillway flow
to river channels below the dam.
82. Types of Spillways
• According to the prominent features related to various
components of spillway viz. control Structures, discharge
channel etc. The spillways may be classified in the
• Free Overfall
• Ogee or overflow spillway
• Trough Spillway
• Shaft Spillway
• Side Chanel Spillway
• Tunnel Spillway
• Siphon Spillway.
83. Types of Spillways
• This is the simplest type of Spillway which
consists of a low height narrow crusted Weir
having downstream face either vertical or nearly
vertical. This type of Spillway is suitable for low
earth dam, low concrete masonry gravity dam or
low thin arch dam. It is not suitable for high
84. Free Over-fall
85. Types of Spillways
Ogee or Overflow Spillway
• It is the modified form of drop spillway suitable
for high gravity dam, arch dam and buttress
dam. The overflow water is guided smoothly over
the crest and profile of spillway. This type of
spillway is more preferable on valleys where
width of river is more to provide sufficient crest
length and river bed can be protected from
scouring at reasonable cost.
86. Ogee or Overflow Spillway
87. Types of Spillways
• Trough spillway Is provided when it is not
possible to provide an overflow spillway such as
in case of embankment dam or due to erodible
nature of stream bed in case of concrete masonry
dams. It discharges the surplus flood through a
steep sloped open channel. The crest of spillway
is kept normal to its centre line. It consists of a
discharge channel to the river.
88. Trough Spillway
89. Types of Spillways
Side Channel Spillway
• It is the Spillway in which, the flow after
passing over a weir or ogee crest, is carried
away by a side channel. It is best suited
for non rigid dam like earthen dam.
90. Side Channel Spillway
91. Types of Spillways
Siphon Spillway: When available space is
limited and surplus discharge is not large
siphon spillway is often preferred. It is
based on siphon action in the shape of an
inverted pipe. Usually siphon spillway is
provided in concrete gravity dam through
92. Siphon Spillway
93. Energy Dissipation Below
• Water flowing over a spillway has high potential
energy as it glides along spillway. This large kinetic
energy rises to high velocity of flow which may
cause large scale scour/erosion at the downstream
toe, if proper arrangements are not made to
dissipate this high energy. The arrangements
provided to dissipate this energy are known as
• In general the dissipation can be achieved in two
• By developing a Hydraulic jump
• By directing the jet of water using a deflector
94. Energy Dissipation Below
A. Roller Basin
B. Deflector Bucket
C. Flip Bucket
D. Non-radial spillway and sluice buckets
E. Schoklitsch Dissipater
95. Spillway Crust Gates
• Gates provided over the crest of a
controlled spillway are termed as spillway
crest gates. An additional storage can be
obtained by providing gates on the
spillway crest. However, during floods,
these gates are lifted to make full use of
spillway capacity. Great care is needed to
be taken while operating crest gates of
earth dams so as to avoid the overtopping
of earth dam.
96. Spillway Crust Gates
• The following are some of the common types of
(i) Dropping shutters or flash boards
• They are some sort of temporary gates used only
for smaller spillway of minor importance. They
consists of wooden panels usually 1.0 to 1.25 m
high. They are hinge at bottom and supported by
struts to resist water Pressure. These wooden
panels can be raised or lowered from an
overhead cableway or a bridge.
97. Spillway Crust Gates
Radial Gates or Tainter Gates
• Radial gate has a curve water supporting face
which is made of steel. Its shape is just as
sector of a circle properly braced and hinged
at the pivot.
• The gate is thus made to rotate above a
horizontal axis. The load of the gate, water
etc. is carried on bearing which are mounted
on piers. An operating plateform is provided
to lift the the gate by means of ropes and
98. Radial Gates or Tainter Gates
99. Drum Gates
• This gate is developed by United States Bureau of
Reclamation (USBR) this gates is suitable for longer span
of the order of 40 or 50 m. The drum gate consists of
circular sector in cross section formed by skin plates
attached to internal bearing. The entire section may be
raised or lowered such that upper surface coincides with
the crest of spillway. The buoyant force due to head
water pressure underneath the drum assists in its lifting.
The drum Gate needs large recess and hence is not
suitable for smaller spillway. Some other types of
spillway gates are also used namely Vertical lift gates,
rolling Gates, Tilting Gate or Flap Gate, Bear Trap gate.
100. Drum Gates
101. Reservoir Sedimentation
• Every river carries certain amount of sediment
load. The sediment particles try to settle down
to the river bottom due to the gravitational
force, but may be kept in suspension due to the
upward current in the turbulent flow which
may overcome the gravity force.
102. Reservoir Sedimentation
• Due to these reasons, the river carries fine
sediment in the suspension as suspended loads,
and larger solids along the river bed as bed
load When the silt laden water reaches a
reservoir in the vicinity of a dam, the velocity
and the turbulence are considerable reduced.
The bigger suspended particles and most of the
bed load, therefore, gets deposited in the head
reaches of the reservoir.
103. Causes of Sedimentation in
• In the catchment area the soil may get eroded
and rocks may get disintegrated due to various
reasons. The disintegrated rocks and loose soil
form the sediment which is carried by the river
and gets deposited on the reservoir bed near
the base of the dam. The sediment mainly
consists of sand and silt. The process of
deposition of sand and silt in the reservoir is
designated as reservoir sedimentation.
104. Causes of Sedimentation in
105. The following are the causes of
Characteristics of soil in Catchment area: If the catchment area
is composed of loose soil, then it may get easily eroded and get
carried away by the river. On the other hand, if the soil of the
catchment area is hard and rocky, the river would not be able
to carry sediment.
Topography of the catchment Area: If the catchment area
consists of steep slope, then it will develop high velocity of
flow which will cause more erosion of the surface soil thereby
making the river carry a lot of sediment.
Intensity of Rainfall in Catchment Area: If the intensity of
rainfall in catchment area is high, then it will increase the rate
of run-off and the river will carry more sediment.
106. The following are the causes of
Cultivation in Catchment Area: The intensive
cultivation in the catchment area will make the
soil loose and rain water will carry a lot of
sediment to the river.
Vegetation cover in Catchment Area: If the
catchment area is covered with vegetation cover
like grass, plants, forest area, etc. then the erosion
of the soil will be controlled and the river will not
carry sediment. If there is no such vegetal apron
or cover, the soil may be easily eroded and the
river will carry much sediment.
107. Effect of Sedimentation
• When the sediment laden water of the river
approaches the zone of reservoir, the velocity
of flow reduces gradually and thus the heavier
particles are settled down at the head of the
reservoir, i.e. starting zone of reservoir. This
zone is termed as delta.
108. Effect of Sedimentation
• Most of the sediments get deposited at this zone. The
fine sediments remain in suspension for a
considerable time and are carried towards the dam.
These sediments are deposited at the foot of the dam.
• The very fine particles are carried with water as
turbid flow and ultimately discharged through the
outlets. The floating debris is collected above delta.
The clear water circulates the middle portion of the
reservoir. Thus, the life of reservoir depends on the
trend of the sedimentation.
109. Effect of Sedimentation
110. Control of Sedimentation
• In order to increase the useful life of a
reservoir, it is necessary to control the
phenomenon of sedimentation. The following
steps should be taken to control the deposition
• Selection of a Reservoir Site The reservoir site
should be selected in such a way that so that
load of sediment in the flowing water from
tributaries is considerably low. If it is found that
a tributary carries heavy sediment, the reservoir
site should be shifted towards the upstream to
avoid the tributary.
111. Control of Sedimentation
• Design of Reservoir At the beginning the
reservoir should not be constructed to its full
capacity. Initially a reservoir of less capacity is
constructed. When the reservoir gets silted,
gradually the capacity should be increased stage
by stage, by increasing the height of the dam.
• Sufficient outlets should be provided in the dam
at different elevations so that the water carrying
heavy sediment may be discharged to the
downstream by opening the outlets.
112. Selection of a Reservoir Site
113. Control of Sedimentation
• Removal of Sediments The sediment already deposited
in the reservoir may be removed through the scouring
sluices provided at the bottom of the dam by loosening
the sediments by mechanical agitator.
• Afforestation New plantations should be planned in the
catchment to extend the forest area which serves as a
vegetal apron to reduce the soil erosion.
• Control of Deforestation The cutting of Trees, i.e.
deforestation in the catchment should be restricted by
the government so that the vegetal cover is not
destroyed The deforestation may cause soil erosion and
this may cause soil erosion and this may impart
sedimentation load to river water.
114. Removal of Sediments
115. Control of Sedimentation
• Control of Cultivation: The intensive cultivation
in the catchment makes the soil loose which may
then get easily carried by the rainwater to the
river. So the cultivation should be done in a
planned manner and necessary measures should
be taken to avoid the loose soil getting carried
away by the rain.
• Control of Grazing The grazing of cattle in the
catchment should be restricted and they should
not be allowed in the area where the soil can get
easily eroded by their feet.
116. Control of Sedimentation
• Construction of Check Dams: If the
tributaries of a river are found to carry heavy
sediment load, then check dams of low height
are constructed across the tributaries at
different stages to arrest the sediments just in
the catchment area. Here, the check dams
serve the purpose of detention reservoir where
the heavy sediments get settled down.
117. Construction of Check Dams
118. Control of Sedimentation
• Construction of Contour Bunds: In hilly area contour
bunds are constructed on the slope of the catchment at
different elevations so that the heavy sediments are
arrested at the base of the bunds and comparatively less
turbid water passes over the bunds.
• Stabilization of Slips: Most of the tributaries of a river
obtain the sediments from the land slides which are
caused due to heavy rainfall in the hilly area. It is
generally composed of loose soil or permeable rock
formation. These slips should be stabilized by suitable
methods so that the debris may not be carried by flowing
119. Construction of Contour Bunds
120. Density Currents
• In a Reservoir, the coarser sediment settle
down along the bottom of the reservoir, as the
muddy flow approaches the reservoir; while
the finer sediment usually remains in
suspension, and moves in a separate layer than
the clear reservoir water.
121. Density Currents
• The layer of water, containing the fine
sediment, moves below the upper clearer
reservoir water, as a density current, since its
density is slightly more than the density
difference, the water of the density current
does not mix easily with the reservoir water,
and maintains its identity for a considerable
122. Density Currents
123. Density Currents
124. Density Currents
• The density current can thus be removed
through the dam sluiceway, if they are located
properly and at the levels of the density
current. A lot of sediment load can, thus be
passed out of the reservoir, if it is possible to
locate the dam outlets and sluiceways in such a
fashion, as to vent out the density currents.
125. Reservoir Losses
• Huge Quantity of water is generally lost from
an impounding reservoir due to evaporation,
absorption, and percolation. Depending upon
which, the following losses may occur from
such a reservoir.
• Evaporation Losses
• Absorption Losses
• Percolation Losses or Reservoir leakage
126. Reservoir Losses
127. Reservoir Losses
• The evaporation losses from a reservoir depends upon
several factors as: water surface area, water
velocity, temperature, atmospheric pressure and quality
of water. The evaporation loss from the reservoir under
the given atmospheric condition can easily estimated by
measuring the standard pan evaporation and
multiplying the same by the pan coefficient.
• The evaporation losses becomes very significant in a
hot and humid country like India; and realistic
estimation of these losses is quite important. These
losses in fact vary from place to place and from season
to season, and hence monthly values of these losses are
128. Reservoir Losses
• In order to control such large scale of wastage
of water several methods have been devised by
engineers and scientists. All these methods are
based upon the efforts made to reduce the
evaporation rate from the surface of water
bodies by physical or chemical means, since
the basic meteorological factors affecting
evaporation cannot be controlled under normal
conditions. The following methods are
generally used for evaporation control.
129. Reservoir Losses
130. Reservoir Losses
Covering of the water surface
Reduction of the exposed water surface
Use of the underground storage rather than the
use of surface storage.
• Integrated operations of the reservoir.
• Use of Chemicals for retarding the evaporation
rate from the reservoir surface.
131. Reservoir Losses
• These losses do not play any significant role in
planning, since their amount, though
sometimes large in the beginning, falls
considerably as the pores get saturated. They
certainly depend upon the type of soil forming
132. Reservoir Losses
Percolation Losses or Reservoir Leakage
• For most of the reservoirs, the banks are
permeable but the permeability is so low that the
leakage is of no importance. But in certain
particular cases, when the walls of the reservoir
are made of badly fractured rocks or having
continuous seams of porous strata, serious leakage
may occur. Sometimes, pressure grouting may
have to be used to seal the fractured rocks. The
cost of grouting has to be accounted in the
economic studies of the project, if the leakage is
133. Storage Zones of a Reservoir
Normal Pool Level or Maximum Conservation Level
• It is the maximum elevation to which the reservoir
surface will rise during normal operating conditions. It
is equivalent to the elevation of the spillway crest or the
top of the spillway gates for most of the cases.
Minimum Pool Level
• The lowest water surface elevation, which has to be
kept under normal operating conditions in a reservoir, is
called the minimum pool level. This level may be fixed
by the elevation of the lowest outlet in the dam or may
be guided by the minimum head required for efficient
functioning of turbines.
134. Storage Zones of a Reservoir
135. Storage Zones of a Reservoir
Useful and Dead Storage
• The volume of water stored in a reservoir between
the minimum pool and normal pool is called the
useful storage. Water stored in the reservoir below
the minimum pool level is known as the Dead
Storage, and if it is not of much use in the
operation of the reservoirs. The useful storage
may be subdivided into conservation storage and
flood mitigation storage in a multipurpose
136. Storage Zones of a Reservoir
137. Storage Zones of a Reservoir
Maximum Pool Level or Full Reservoir Level
• During high floods, water is discharged over
the spillway, but will cause the water level to
rise in the reservoir above the normal pool
level. The maximum level to which the water
rises during the worst design flood is known as
the maximum pool level.
138. Storage Zones of a Reservoir
• The volume of water stored between the
normal pool level and the maximum pool level
is called surcharge storage. Surcharge storage
is an uncontrolled storage, in the sense that it
exists only till the flood is in progress and
cannot be retained for later use.
139. Storage Zones of a Reservoir
• When the reservoir is filled up, certain amount of water
seeps into the permeable reservoir banks. This water comes
out as soon as the reservoir gets depleted. This volume of
water is known as bank storage.
• Even before a dam is constructed, certain variable amount
of water is stored in the stream channel, called Valley
• After the reservoir is formed, the storage increases, and the
actual net increase in the storage is equal to the storage
capacity of the reservoir minus the natural valley storage.
The valley storage thus reduces the effective storage
capacity of a reservoir.
140. Determining Reservoir Capacity
• Long range runoff from a catchment is known as the
yield of the catchment, Generally a period of one year
is considered for determining yield value. The total
yearly runoff, expressed as the volume of water
entering/ passing the outlet point of the catchment, is
thus known as the catchment yield, and is expressed in
Mm3 or M.ha.m
• The annual yield of the catchment up to the site of a
reservoir, located at the given point along a river, will
thus indicate the quantum of water that will annually
enter the reservoir, and will thus help in designing the
capacity of the reservoir.
141. Determining Reservoir Capacity
• After deciding the dependable yield for the
proposed reservoir tank, the reservoir capacity is
decided as follows
• The water demand is computed by estimating the
crop water requirement and any other water
demand required to meet the water supply needs,
or the downstream commitments of water release,
if any. Reservoir losses @ about 15 % of the
water demand is then added to obtain the live or
the net storage required to meet the given
142. Determining Reservoir Capacity
• Dead storage is now added to this live storage to
obtain the gross storage required to meet the
• The reservoir capacity, however, cannot exceed
the catchment yield and hence the reservoir
capacity is fixed at a value which is lesser of the
value of the assessed gross storage required to
meet the demand and ii) the assessed dependable
yield for the reservoir site. The full tank level or
full reservoir level (FRL) is finally computed
from the elevation capacity curve.
143. Determining Reservoir Capacity
• The dead storage level or dead storage required in
the above computation is usually fixed at higher
of the following values
• Dead storage= rate of silting x Life of the
• Dead Storage= 10 % of gross storage or net
• Dead Storage level being equal to the full
supply level of the off taking canal at the tank
• Modi P.N. (2011), “Irrigation water resources and water
power engineering”, Standard Book House
• Garg S.K. (2010), “Irrigation Engineering and Hydraulic
Structures”, Khanna Publishers
• Internet Websites