(Homonym: damn).
1. A barrier constructed to contain the flow or water or to keep out the sea; SYN. dike, dyke, levee.
2. Female parent of an animal especially domestic livestock.
Structure built to hold back water in order to prevent flooding, to provide water for irrigation and storage, and to provide hydroelectric power. The biggest dams are of the earth- and rock-fill type, also called embankment dams. Early dams in Britain, built before about 1800, had a core made from puddled clay (clay which has been mixed with water to make it impermeable). Such dams are generally built on broad valley sites. Deep, narrow gorges dictate a concrete dam, where the strength of reinforced concrete can withstand the water pressures involved.
A valuable development in arid regions, as in parts of Brazil, is the underground dam, where water is stored on a solid rock base, with a wall to ground level, so avoiding rapid evaporation. Many concrete dams are triangular in cross section, with their vertical face pointing upstream. Their sheer weight holds them in position, and they are called gravity dams. They are no longer favored for very large dams, however, as they are expensive and time-consuming to build. Other concrete dams are built in the shape of an arch, with the curve facing upstream: the arch dam derives its strength from the arch shape, just as an arch bridge does, and has been widely used in the 20th century. They require less construction material than other dams but are the strongest type.
Buttress dams are used when economy of construction is important or foundation conditions preclude any other type. The upstream portion of a buttress dam may comprise series of cantilevers, slabs, arches or domes supported from the back by a line of buttresses. They are usually made from reinforced and prestressed concrete.
Earth dams have a watertight core wall, formerly made of puddle clay but nowadays constructed of concrete. Their construction is very economical even for very large structures. Rock-fill dams are a variant of the earth dam in which dumped rock takes the place of compacted earth fill.
Major dams include: Rogun (Tajikistan), the world's highest at 335 m/1,099 ft; New Cornelia Tailings (US), the world's biggest in volume, 209 million cu m/7.4 billion cu ft; Owen Falls (Uganda), the world's largest reservoir capacity, 204.8 billion cu m/7.2 trillion cu ft; and Itaipu (Brazil/Paraguay), the world's most powerful, producing 12,700 megawatts of electricity. The Three Gorges Dam on the Chang Jiang was officially inaugurated December 1994.
Although dams can service huge irrigation schemes and are a reliable and cheap source of power, they cause many environmental problems such as the forcible removal of local communities, waterlogging and salinization of land in the area, and loss of habitat. For example, the world's biggest hydroelectric dam and irrigation project which is currently under construction on the Narmada river, central India, has attracted huge protests as it will displace up to a million people and submerge large areas of forest and farmland. Similarly, the Kansa dam in Zimbabwe flooded habitat used by the rhinoceros, one of the world's most endangered mammals.
There is also controversy as to the effectiveness of large dams as the reservoirs tend to fill with silt from upstream. This leads to a gradual reduction in reservoir depth and hence the volume of water held back by the dam which in turn reduces the power delivered by the hydroelectric turbines. + prikaži više