DAM’S
Irrigation and Hydraulic Structures
UNIT-I
by
Lakshmi T
Assistant Professor
MGIT
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DAMS
• Dam, structure built across a stream, a river, or an estuary to
retain water.
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.
OR
A dam is a barrier constructed across a river or a
natural stream to create a reservoir for impounding water or
to facilitate diversion of water from the river or to retain
debris flowing in the river along with water.
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These are the purposes to construct a dam……………..
• Irrigation,
• Hydropower,
• Water-supply,
• Flood Control,
• Navigation,
• Fishing and Recreation.
Dams may be built to meet the one of the above
purposes or they may be constructed fulfilling more than
one.
As such, Dam can be classified as: Single-purpose and
Multipurpose Dam.
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Different parts & terminologies of Dams:
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• Crest: The top of the Dam. In some cases these may
be used for providing a roadway or walkway over the
dam.
• Parapet walls: Low Protective walls on either side of
the roadway or walkway on the crest.
• Heel: Portion of Dam in contact with ground or river-
bed at upstream side.
• Toe: Portion of dam in contact with ground or river-
bed at downstream side.
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• Spillway: It is the arrangement made (kind of passage)
near the top of dam for the passage of surplus/ excessive
water from the reservoir.
• Abutments: The valley slopes on either side of the dam
wall to which the left & right end of dam are fixed to.
• Gallery: Level or gently sloping tunnel like passage (small
room like space) at transverse or longitudinal within the
dam with drain on floor for seepage water.
• These are generally provided for having space for drilling
grout holes and drainage holes.
• These may also be used to accommodate the
instrumentation for studying the performance of dam.
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• Sluice way: Opening in the dam near the base, provided
to clear the silt accumulation in the reservoir.
• Free board: The space between the highest level of
water in the reservoir and the top of the dam.
• Dead Storage level: Level of permanent storage below
which the water will not be withdrawn.
• Diversion Tunnel: Tunnel constructed to divert or change
the direction of water to bypass the dam construction
site. The dam is built while the river flows through the
diversion tunnel.
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Types?
Dams may be classified in different ways on the basis of
their…………..
• Function
• Hydraulic design
• Material of construction
• Structural design
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Based on the functions of dam, it can be classified as follows:
1. Storage dams
2. Diversion dams
3. Detention dams
4. Debris dams
5. Coffer dams
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Storage dams: They are constructed to store water during the
rainy season when there is a large flow in the river.
• Many small dams impound the spring runoff for later use in
dry summers.
• Storage dams may also provide a water supply, or improved
habitat for fish and wildlife.
• They may store water for hydroelectric power generation,
irrigation or for a flood control project.
• Storage dams are the most common type of dams and in
general the dam means a storage dam unless qualified
otherwise
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Diversion dams:
A diversion dam is constructed for
the purpose of diverting water of
the river into an off-taking canal (or
a conduit).
They provide sufficient pressure for
pushing water into ditches, canals,
or other conveyance systems. Such
shorter dams are used for
irrigation, and for diversion from a
stream to a distant storage
reservoir.
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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
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Debris dams: A debris dam is constructed to retain debris such as sand, gravel,
and drift wood flowing in the river with water. The water after passing over a
debris dam is relatively clear.
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Coffer dams: It is an enclosure constructed around the construction site to
exclude water so that the construction can be done in dry.
A cofferdam 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.
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Classification according to Hydraulic design
• Overflow Dams
• Non-Overflow Dams
• Rigid Dams
• Non-Rigid Dams
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Overflow dam
The dam body is made of strong material as concrete and flow is
allowed over the dam crest (Concrete dams)
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Non-Overflow dam:
Flow is not allowed over the embankment crest for reasons of
dam safety. (earth, rock) dams.
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RIGID DAM.
It is constructed with rigid material such as stone, masonry, concrete,
steel, or timber.
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NON-RIGID DAM (EMBANKMENT DAMS)
• It is constructed with non-rigid material such as earth,
tailings, rockfill etc.
• Earthen dam – gravel, sand, silt, clay etc
• Tailings dam – waste or refuse obtained from mines
• Rockfill dam – rock material supporting a water tight
material on the u/s face
• Rockfill composite dam – Rockfill on the d/s side and earth
fill on the u/s side
• Earthen dams are provided with a stone masonry or
concrete overflow (spillway) section. Such dams are called
composite dams.
• In some cases, part of the length of the dam is constructed
as earth dam and the rest (excluding the spillway) as a
masonry dam. Such dams are called masonry cum earthen
dams.
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• BASED ON STRUCTURAL BEHAVIOUR:
• Gravity Dams: A gravity dam is a massive sized dam
fabricated from concrete or stone masonry. They are
designed to hold back large volumes of water.
• By using concrete, the weight of the dam is actually able
to resist the horizontal thrust of water pushing against it.
This is why it is called a gravity dam.
• Gravity dams are well suited for blocking rivers in wide
valleys or narrow gorge ways. Since gravity dams must
rely on their own weight to hold back water, it is
necessary that they are built on a solid foundation of
bedrock.
• Examples of Gravity dam: Grand Coulee Dam (USA), (
Nagarjuna Sagar Dam (India) and Itaipu Dam ( Between
Brazil and Paraguay).
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• Earth Dams: An earth dam is made of earth (or soil) built up by compacting
successive layers of earth, using the most impervious materials to form a core and
placing more permeable substances on the upstream and downstream sides.
• A facing of crushed stone prevents erosion by wind or rain, and an ample spillway,
usually of concrete, protects against catastrophic washout should the water
overtop the dam
• Earth dam 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 behavior of an earth dam is entirely different from that of a gravity dam.
• They can be built on all types of foundations. However, the height of the dam will
depend upon the strength of the foundation material.
• Examples of earthfill dam: Rongunsky dam (Russia) and New Cornelia Dam(USA) .
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• 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.
• Hoover Dam, USA . 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.
• Examples of Arch dam: Hoover Dam (USA) and Idukki
Dam (India)
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• Buttress Dams: Buttress dams are of three types : (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 are compression members. Buttresses are typically
spaced across the dam site every 6 to 30 metre, depending upon
the size and design of the dam. Buttress dams are sometimes called
hollow dams because the buttresses do not form a solid wall
stretching across a river valley.
• The deck is usually a reinforced concrete slab supported between
the buttresses, which are usually equally spaced.
• Buttress Dam In a multiple-arch type buttress dam the deck slab is
replaced by horizontal arches supported by buttresses.
• Examples of Buttress Dam: Bartlett dam (USA) and The Daniel-
Johnson Dam (Canada)
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BASED ON TYPE
OF MATERIALS:
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.
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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.
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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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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 dams are
suitable up to 15 to 18 meters
of height.
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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.
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Factors Governing Selection of Type of Dam
Whenever it is decided to construct a dam, the first question
that one face is which type of dam will be most suitable and most
economical? Following are the factors affecting selection of dam
site by dam type.
• Topography
• Geology and Foundation Conditions
• Availability of materials
• Spillway size and location
• Earthquake zone
• Height of the Dam
• Other factors such as cost of construction and maintenance, life of
dam, aesthetics etc.
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Topography
• A narrow U-shaped valley, i.e. a narrow stream
flowing between high rocky walls, would
suggest a concrete overflow dam.
• A low plain country, would suggest an earth fill
dam with separate spillways.
• A narrow V-shaped valley indicates the choice
of an Arch dam
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• Geology and Foundation Conditions : The foundation strata should be
strong enough to carry weight of dam. Hence the dam site must be
surveyed by geologists so as to detect the thickness of foundation strata,
presence of faults, fissured material etc.
The various types of foundations generally encountered are :-
• Solid-rock foundation :Similar like granite, gnesis etc. Almost every type of
dam can be built as it is strong enough to withstand the self weight of the
dam and external forces acting on the dam.
• Gravel foundation :This type of foundation is suitable for earth and rock fill
dams. For this foundation, low concrete gravity dams upto height of 15
meter may also be suggested.
• Silt and fine sand foundation : This type of foundation suggests the
adoption of earth or very low gravity dam, upto height of 8 meter.
• Clay foundation : This foundation may be accepted for earthen dam after
special treatment.
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• Availability of Materials
Availability of materials is another important factor in selecting the
type of dam. In order to achieve economy in dam construction, the
materials required must be available locally or at short distances
from the construction site.
• Spillway Size and Location
spillway disposes the surplus river discharge. The capacity of the
spillway will depend on the magnitude of the floods to be by-
passed. The spillway is therefore much more important on rivers
and streams with large flood potential.
• Earthquake Zone
If dam is situated in an earthquake zone, its design must include
earthquake forces. The type of structure best suited to resist
earthquake shocks without danger are earthen dams and concrete
gravity dams.
• Height of Dam
Earthen dams are usually not provided for heights more than 30 m
or so. For greater heights, gravity dams are generally preferred.
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Selection of Dam Site
The selection of Dam site for constructing a dam should be governed by the
following factors.
• Small river channel width with steep side gorge: short dam crest length, leads to
large storage for small dam length
• A wide and gently sloping valley upstream of the dam site (for storage dams) and
narrow and steeply sloping valley for hydropower dams.
• River channel and valley has very flat slopes upstream of dam site (leads to large
storage for small dam heights).
• Where a deep reservoir possible, it resultantly requires less area and lesser land
costs, less surface evaporation
• Enough water flow/yield should be available to meet the demand of the area
• High sediment load tributaries are excluded
• Geology should be favorable for foundation (foundation can be designed at any
site, but it increases costs), competent hard rock is most suitable.
• Abutments are water tight, and reservoir rim allows minimum percolation and
seepage losses.
• Small river sediment rate (longer dam life) Depend on river morphology and
catchment characteristics. Gomal Zam has 10 times sediment load than Kurram
Tangi dam, thus large dead storage space is adopted.
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• Land use of reservoir area should be minimal – Less land area means
lesser compensations to be given to the land owners and lower land cost.
• Reservoir area not very sensitive to environment (wild life parks,
endangered species, historical places, monuments etc).
• No seismic and tectonic activities or active faults in and near the site.
• Socio-political stability (no unstable gestures) (Gomal-Zam, Mirani, Kurram
Tangi dams), Diamer-Basha vs Kalabagh dams.
• Reservoir and dam area should be thinly populated
• Site must have adequate stream flow record
• Site is easily accessible; approach road is present or can be developed
easily.
• Construction material available nearby easily
• Site near load center (demand area) for water+ power
• No mineral resources in reservoir area (present or future)
• Site allows a deep reservoir & small surface area (less land costs and small
evaporation losses).
• Existing infrastructure, e.g. highway, least affected. E.g. KKH and Bhasha-
Diamer dam.
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