2. Dams
• Dam is a solid barrier constructed at a suitable
location across a river valley to store flowing water.
• Storage of water is utilized for following objectives:
• Hydropower
• Irrigation
• Water for domestic consumption
• Drought and flood control
• For navigational facilities
• Other additional utilization is to develop fisheries
4. • Heel: contact with the ground on the upstream side
• Toe: contact on the downstream side
• Abutment: Sides of the valley on which the structure of the dam rest
• Galleries: small rooms like structure left within the dam for checking
operations.
• Diversion tunnel: Tunnels are constructed for diverting water before the
construction of dam. This helps in keeping the river bed dry.
• Spillways: It is the arrangement near the top to release the excess water
of the reservoir to downstream side
• Sluice way: An opening in the dam near the ground level, which is used
to clear the silt accumulation in the reservoir side.
5. TYPES OF DAMS
• Gravity Dams:
• These dams are heavy
and massive wall-like
structures of concrete
in which the whole
weight acts vertically
downwards
Reservoir
Force
As the entire load is transmitted on the small area of foundation, such dams are constructe
where rocks are competent and stable.
6. • Bhakra Dam is the highest
Concrete Gravity dam in
Asia and Second Highest in
the world.
• Bhakra Dam is across river
Sutlej in Himachal Pradesh
• The construction of this
project was started in the
year 1948 and was
completed in 1963 .
• It is 740 ft. high above the deepest foundation as straight concrete dam being more than three
times the height of Qutab Minar.
• Length at top 518.16 m (1700 feet); Width at base 190.5 m (625 feet), and at the top is 9.14 m (30
feet)
• Bhakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world.
7.
8. Buttress Dam:
• Buttress Dam – Is a
gravity dam reinforced by
structural supports
• Buttress - a support that
transmits a force from a
roof or wall to another
supporting structure
This type of structure can be considered even if the foundation rocks are little weaker
9. • These type of dams are
concrete or masonry dams
which are curved or convex
upstream in plan
• This shape helps to transmit
the major part of the water
load to the abutments
• Arch dams are built across
narrow, deep river gorges, but
now in recent years they have
been considered even for little
wider valleys.
Arch Dams:
10.
11. Earth Dams:
• They are trapezoidal in
shape
• Earth dams are
constructed where the
foundation or the
underlying material or
rocks are weak to support
the masonry dam or
where the suitable
competent rocks are at
greater depth.
• Earthen dams are
relatively smaller in height
and broad at the base
• They are mainly built with
clay, sand and gravel,
hence they are also
known as Earth fill dam or
Rock fill dam
12. • Gravity dam: rigid monolithic structure
– Trapezoidal cross section
– Minimal differential movement tolerated
– Dispersed moderate stress on valley floor and walls
• Arch dam: high strength concrete wall
– Convex faces upstream
– Thin walled structure
– Relatively flexible
– Huge stresses imposed on valley walls and floor
• Earth dams: bank or earth or rock with
impermeable core
– Core of clay or concrete, extended below ground
– Sand or gravel drains built to cut fluid pressure
– Low stress applied to valley floor and walls
13. Geotechnical considerations
Dam site
• Topography- a place is selected ideally in narrow
gorge or small valley with enough catchment area
behind dam is calculated.
• Technically- a site should have strong,
impermeable and stable rock body.
• Constructionally- site should not be much away
from the deposits of material which required for
construction.
• Economical
• environmental
14. Geological investigation
• General geology of area-
– In detailed mapping of the area reveals the facts
like
• Topographic features
• Natural drainage patterns
• General characters and structures of rock formation like
stratification, folding and faulting
• Trend and rate of erosion in the area.
15. • Lithology-
– It is the most imp. Factor in dam construction
investigation.
– Lithology in and around area with surface and
subsurface lithology is investigated.
– These studies reveals the composition and
textures of rock at site.
– It gives the idea about the rock type as
igneous, sedimentary or metamorphic.
– It also shows that the area is made up of single
thick layer of same rock type or with multiple
layers.
16. • Structures-
– These involves detail study of and mapping of
planes of weakness like faults, fold, joints,
bedding plane etc.
– Dip and strike:
Strength of unfractured stratified rock is greater when
stresses are acting normal to the bedding planes .
Horizontal beds are stable base
Dipping upstream are stable
Dipping down stream unstable
17. • Fault:
– The faulted rocks are generally shattered and weak
along the rupture surface
– Unexpected and different rock types may occur on
either sides of fault.
– Faulted land surfaces are the favorable sites for
shocks during earthquakes.
– Small faults and shear zones are treated with some
methods
– Deep and major faults are avoided at construction
site.
– Loading in reservoir may activate previously
inactive fault.
– Dip of faults also plays a major role in selecting a
site for dams.
18. • Folds:
– Most notable effect of fold on rocks are
shattering and jointing along axial planes
stressing of limbs
– Dams aligned along axial regions would be
resting on most unsound rocks.
– In syncline bends dams placed on the
upstream limbs would cause leakage from
beneath the dam.
19. • Joints:
– There is no site which is free from jointing.
– Here nature of jointing, depth of joints are
considered.
– According to the nature are of joints the
appropriate engineering technique is used to
feel up the joints.
22. Depending upon the purpose served, reservoirs may be
classified as under:
1. Storage or conservation reservoirs,
- formed by constructing a dam across a river.
- to store water in the rainy season and to release it later when flow is low.
- primarily used for supply Of water for irrigation, development of hydroelectric power and
domestic and industrial water supply
2.Flood control reservoirs,
- is constructed for the purpose of flood control.
- to protect the area lying on its D/S side from the damages due to floods.
- it holds some of the flood waters of a river during the rising flood and releases them gradually
at a safe rate when the flood recedes.
- It is of two types:
a) Retarding reservoirs:
It is the one which is provided with outlets and spillway not controlled by gates or valves.
spillway is provided with the dam at such a level and capacity that the flood discharge
through it is safe for the D/S areas.
It stores a portion of the flood when the flood is rising and releases it later when the flood is
receding .it means the high flood discharge is retarded and it takes long time for the flood
water to flow completely towards the downstream area.
The flow stops when the water level falls below the crest of the spillways.
23. b) Detention Reservoir:
A detention reservoir is the one which is provided with outlets and spillway controlled by
gates or valves.
- stores excess water during flood and releases it after the flood .
- spillways with adjustable gates are provided with the dam so that the flood water may
be detained for sometimes and then released according to the situation of the D/S area
by operating the gates of the spillways.
3. Distribution reservoirs:
It is a small storage reservoir used for water supply in a city. Water is continuously pumped
in to the reservoir at a constant rate and is supplied to the consumers.
- It is rarely used for the supply of water for irrigation.
It is not formed by constructing a dam across a river it is constructed of masonry work or
concrete work in the form of a rectangular or circular tank at suitable places near the
town.
24. Investigation for reservoir planning:
1. Engineering surveys:
The area of the dam site and reservoir basin should be surveyed throughly
to prepare topographical map and contour map. From the contoured plan the
storage capacity and the water spread area of the reservoir at the various
elevations are determined.
The reservoir capacity or the volume of storage, corresponding to a given
water level in the reservoir may be determined either by trapezoidal formula or
by prismoidal formula.
2. Geological surveys:
geological investigations of the dam and reservoir site are required to
determine the following items:
I. Suitability of foundation for the dam,
II. Water tightness of the reservoir basin,
III. Location of the quarry sites for the construction materials
25. 3. Hydrological investigations:
study of runoff pattern of the river at the proposed dam site to
determine the storage capacity of the reservoir corresponding to a given
demand.
Determination of the hydrograph of the worst flood to determine the
spillway capacity and design.
26. • SELECTION OF SITE FOR A RESERVOIR:
• The topography,
• At the site river valley should be narrow so the length of the dam to be
constructed is less,
• Basin should be watertight and free from fissures, cracks, so that there is
no loss of water due to percolation.
• The site should be such that as far as possible minimum land and property
is submerged in the reservoir,
• It should be such that it avoids water from the tributaries which carry
usually high content of sediment,
• The site must be such that adequate reservoir capacity is available for the
desired purpose,
• The site should be such that a deep reservoir may be formed so that the
land costs per unit of capacity are low, evaporation loss is less and there is
less likelihood of weed growth,
• The soil and mass at the reservoir site should not contain any
objectionable soluble minerals and salts which may get dissolved in water
and deteriorate water quality.
27. • The quality of water stored in the reservoir must be satisfactory available
for its intended use,
• The site should be easily accessible by road or railway,
• The construction materials for the dam should be available in the vicinity
of the site.
• The site should be such that the costs of associated works such as roads,
rails, housing colonies for workers and staff, etc. should be low.
28. • ZONES OF STORAGE:
• The storage capacity of a reservoir is
designated by several zones by certain
water surfaces or pool levels in the
reservoir as indicated below:
• Normal pool level:
It is the maximum elevation of the
water surface which is to be stored in the
reservoir during ordinary operating
conditions. This water level is also known as
full reservoir level (F.R.L).
• Minimum pool level:
It is the lowest elevation to which the
water is drawn from the reservoir under
normal conditions. This level is fixed by
providing outlets in the dam.
• Maximum pool level:
It is the maximum elevation to which
the water surface will rise in the reservoir
during the peak flood. I t is also known as
maximum water level (M.W.L).
