WHAT IS A DAM?

� A dam is a barrier built across a stream, river or estuary to holdand control the flow of water for uses such as drinking watersupplies, irrigation, flood control and hydropower generationetc.

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BENEFITS OF DAMS� The benefits of dams are usually to the advantage of humans. They

may include:

� Irrigation

� Hydro-electric production

� Flood control

� Recreational opportunities

� Navigation

� Industrial and Domestic water supply

� Aeration of water

� For animals the benefits may include:

� Larger numbers of fish and birds in the reservoir

� Greater habitat diversity

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DISADVANTAGES OF DAMS

Impacts on Environmental and Ecosystem of the area

• Changes in temperature and flow/sediment transport in the riverdownstream from the dam

• Loss of flowing water habitat and replacement with standingwater (reservoir) habitat

• Interruption of animal movements along the course of the river

• Possible alteration of the fish community in the region of theriver

• Interruption of genetic exchange among populations inhabitingthe river course

• Reduction in the delivery of river nutrients to downstreamsection of the river because of entrapment by the reservoir

• The loss of the floodplain habitat and connectivity between theriver and bordering habitats upland

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CLASSIFICATION OF DAMS:

Dams are classified on several aspects, some of the important aspects are as follow:

1) Based on Hydraulic Design:

� Over flow dams (e.g. concrete dams)

� Non over flow dams (e.g. embankment dams)

2) Based on Structural Design:

� Gravity dams

� Arch dams

� Buttress dams

3) Based on Usage of Dam:

� Storage dams

� Diversion dams

� Detention dams

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CLASSIFICATION OF DAMS:

4) Based on Construction Material:

� Concrete / Masonary dams

� Earthfill dams

� Rockfill dams

� Earth and rockfill dams

� Concrete faced rockfill dams (CFRD)

5) Based on Capacity:

� Small dams

� Medium dams

� Large dams

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Following are the important factors considered for the selection of sitefor a dam:

SITE SELECTION OF A DAM

1) Catchment characteristics

2) Length of dam

3) Height of dam

4) Foundation conditions

5) Availability of suitable Spillway location

6) Availability of suitable construction materials

6) Storage capacity

7) Construction and maintenance cost

8) Access to the site

9) Options for diversion of river during construction

10) Compensation cost for property and land acquisition

11) Quality of water

12) Sediment transport

13) Environmental conditions

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The choice of dam is decided upon by examining foundation conditions,load strains, temperature and pressure changes, chemicalcharacteristics of ground water and possible seismic activity.

The followings important factors are considered for the selection of typeof dams:

SELECTION OF DAM TYPE

1) Topography

2) Geology and nature of foundation� Bearing capacity of the underlying soil

� Foundation settlements

� Permeability of the foundation soil

� Foundation excavation

3) Hydraulic Gradient

4) Availability of construction materials

5) Economics

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6) Spillway location

7) Safety considerations

8) Earthquake zones

9) Purpose of dam

10)Aesthetic considerations

11)Life of the Dam

SELECTION OF DAM TYPE

The optimum type of dam for a specific site is determined by estimates of cost and construction programme for all design solutions which are technically valid.

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SPILLWAYS

� Spillways: The purpose of spillway is to pass flood watersafely downstream when the reservoir is full.

� The Spillways can be� Uncontrolled (Normally)

�Controlled

� Note: Concrete dams normally incorporate an over-fall orcrest spillway, but embankment dams generally require aseparate side-channel or shaft spillway structure locatedadjacent to the dam.

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Types of Spillways

a. Overflow spillways

b. Chute spillways

c. Side-channel spillways

d. Shaft spillways

e. Siphon spillways

f. Service & Emergency spillways

SPILLWAYS

Acknowledgment: Some text and pictures are taken from the lecture notes of

Clayton J. Clark II (Department of Civil & Coastal Engineering, Gainesville,

Florida) http://www.ce.ufl.edu/~clark/28

OVERFLOW SPILLWAYS

Section of a dam that allows water to pass over its crest widely used on gravity, arch, & buttress dam

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CHUTE SPILLWAYS

Auxiliary Spillway of Tarbela Dam Service Spillway of Tarbela Dam

formed by spillways that flow over a crest into a steep-sloped open channel*chute width is often constant: -narrowed for economy

-widened to decrease discharge velocity

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SIDE CHANNEL SPILLWAYS

Spillway in which flow, after passing over the crest, is carried away ina channel running parallel to the crest

* used in narrow canyons in which there is sufficient crest lengthfor overflow or chute is available

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SHAFT SPILLWAY

Shaft spillway at Ladybower Reservoir

Water drops through a vertical shaft in a the foundation material to a horizontal conduit that conveys the water past the dam

*often used where there is not room enough for other spillways*possible clogging with debris a potential problem; screens and trashracks protect inlet

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SIPHON SPILLWAY

Siphon PrincipleTypical Siphon Spillway

Air vent used automatically maintain the water-surface elevationlarge capacity not needed, good for limited space

* At low flow: it acts like an overflow spillway* At high flow: the siphon action removes the water through the structure until reservoir drops to the elevation at the upper lip of entrance

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SERVICE AND EMERGENCY SPILLWAY

Submerged Orifice type Spillway at Mangla Dam

Service and Emergency Spillways-extra spillways provided on a project in rare case of extreme floods (emergency)-used to convey frequently occurring outflow rates (service)

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