Updated on October 22, 2011 hassam moreContact Author 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.
An example of a large dam is the Glen Canyon Dam in Arizona. It is about 700 feet tall and is made of concrete. 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. According to their functions, dams serve three main purposes: storage, diversion, or detention. Storage damsTheyare 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.
The specific purpose of a storage dam influences its design and determines the amount of reservoir needed. Where multiple purposes are involved e.g. both power and irrigation, reservoir allocation is usually made for each separate use. The volume of storage establishes the height and width of the dam. 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.
The development of modern construction equipment has made massive earthfill dams economical. The Rogun and Nurek dams in Tajikistan, the world’s highest, are earthfill structures. Canada’s Syncrude Tailings, which will be the world’s most massive, is also an earthfill structure. 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 damage or even failure, due to water seepage. The rockfill dam uses rock instead of earth to provide stability. It has an impervious, watertight membrane, usually an upstream facing of impervious soil, concrete paving, or steel plates; or it may have a thin interior core of impervious soil. Rockfill embankment dams and zoned-embankment dams are the most common embankment dams.