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Thursday, December 22, 2011

Types Of Dams

A dam is a barrier built across a water course to hold back or control the water flow. Some dams divert the flow of river water into a pipeline, canal, or channel. Others raise the level of inland waterways to make them navigable by ships and barges. Many dams harness the energy of falling water to generate electric power. Dams also hold water for drinking and crop irrigation, and provide flood control.
A beaver dam is an example of a small dam. It is made by using sticks and mud to slow down the flow of a stream or a river. This causes water to pool behind the jam of sticks and mud which results in a new pond being built.
Large dams are more complex to build and take a lot of work, power, time and money. A dam can be made of concrete, rocks, wood or earth.

The most important load that a dam must support is the water behind it. The water pushes on the dam, creating water pressure. Water pressure increases with the depth of the water.In greater depths, there is more water "piled up," which causes the pressure to be greater at the bottom than at the surface. A dam's design must enable it to withstand greater pressure at the bottom than at the top. As a result, many dams are built in a triangular shape. The wide bottom withstands the great load of the water deep below the surface, while the thinner top of the dam need not use unnecessarily costly materials.

 

Dam Structure and Design

Earthfill Dams

 

Earthfill dams typically have a water-impermeable clay core, and a water cut-off wall from their base to bedrock to prevent underground seepage. During construction, the stream or river must be diverted either through the dam-site by means of a conduit, or around it by means of a tunnel.
Earthfill dams require supplementary structures as spillways for discharging water from behind the dam. If sufficient spillway capacity is not provided, an earthfill dam may be damaged or even destroyed by the erosive water flowing over its crest. Unless special precautions are taken, such dams are also subject to serious

  

Embankment or Rockfill Dams

 

The rockfill dam uses rock instead of earth to provide stability.Rockfill embankment dams have a mound of loose rock covered with a waterproof layer on the upstream side. The waterproof layer may be made of concrete, flat stone panels, or other impervious materials. Zoned-embankment dams include an impervious core surrounded by a mound of material that water can penetrate. The supporting mound is usually made of loose rock or earth. The core might be built from concrete, steel, clay, or any impervious materials. Embankment dams hold back water by the force of gravity acting upon their mass. Embankment dams require more material because loose rock and earth are less dense than concrete.

Gravity Dams


Gravity dams hold back water only by the sheer force of their weight pushing downward. To do this, gravity dams must consist of a mass so heavy that the reservoir water cannot push the dam downstream or tip it over. They are much thicker at the base than the top. As water becomes deeper, it exerts more horizontal pressure on the dam. Gravity dams are relatively thin near the surface of the reservoir, where the water pressure is light. A thick base enables the dam to withstand more intense water pressure at the bottom of the reservoir.Most gravity dams are made from concrete, a mixture of port land cement, water, and aggregates.

Concrete Arch
 
Concrete arch dams are built in narrow, steep-walled canyons. The canyon walls take up the thrust exerted by the arch and the pressure of the water. Such dams can be extraordinarily thin.Arch dams can be less expensive to build than gravity dams.


Buttress Dam


A buttress dam consists of a face supported by several buttresses on the downstream side. Buttress dams are made of concrete reinforced with steel. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley.  

 

Classification of Dams

 Storage Dams

 

They are constructed to impound water in periods of surplus supply for use in periods of deficiency. 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.

Diversion Dams

 

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 

Detention Dams  

 

They minimize the effect of sudden floods and trap sediment 

 

Overflow Dams  

 

They carry water discharge over their crests, and are made of materials that will not be eroded by such discharges. Non-overflow dams are designed not to be overtopped, and may include earth and rock in their structure. Often the two types are combined to form a composite structure.
To prevent a dam from being overtopped, spillway structures carry off excess water. In earthfill dams, essential spillways are built as separate structures, often a shaft or tunnel adjacent to the dam. With concrete gravity dams, the downstream side of the structure acts as the spillway.

 

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