Meaning of DAM in English

DAM

barrier built across a stream, river, or estuary to confine and check the flow of water for such uses as human consumption, irrigation, flood control, and electric power generation. Dams are categorized according to their profiles (cross sections) and construction materials. The principal types are earth fill, rock fill, gravel fill, solid-masonry gravity, solidmasonry arch, arch gravity, structural masonry, and steel or timber. The first four types have been used from antiquity, while the others have been developed in the 19th and 20th centuries. The choices made by modern engineers regarding the materials and design that are best suited for a particular dam depend upon complicated analyses of foundation conditions, load strains, temperature and pressure changes, the chemical characteristics of local groundwater, and the probability of seismic activity. For centuries dam design was based upon previous experience. Remains of ancient earth-fill dams still exist in India and Sri Lanka. The earliest recorded dam is believed to be a masonry structure 15 m (49 feet) high that was built across the Nile River in Egypt around 2900 BC. A rockfill dam built around 1300 BC in what is modern Syria is the oldest dam still in use. Dam builders in ancient Rome did not show the engineering genius that was employed in other projects, but Byzantine engineers utilized the Roman masonry arch to build a prototypical arch-gravity dam in AD 550. There was little dam building in Europe for several centuries; it began in earnest only when the Industrial Revolution gave rise to the need for water power in the 18th and 19th centuries. The profession of civil engineering emerged in the 1850s, mainly through the influence of the Scottish engineer, William J.M. Rankine. During the next 100 years theoretical knowledge that had been accumulating since the discoveries of Isaac Newton, G.W. Leibniz, Robert Hooke, and others was finally brought to bear on dam design. Modern dams generally fall into two categories: earth fill (embankment) and masonry (concrete). Earth-fill dams are usually used to retain water across broad rivers; the reason is partly that huge amounts of material are needed and earth and rock are usually more available and less expensive, but another reason is that earth fill and rock fill tend to adapt to deformation caused by movements in the dam foundation. The profile of an earth-fill dam, such as the Aswan High Dam across the Nile in Egypt, looks like a broad-based triangle. Although these dams are generally known more for their length than their height, the Nurek Dam in Tajikistan is 300 m (984 feet) high. The bulk of the construction material is a highly compactable soil that stays in place because of its enormous weight. The danger to such a structure comes from the water that gradually permeates the entire embankment and emerges on the downstream slope; the dam can eventually be washed away. Engineers build strata of different permeabilities into the embankment either to stop this process or to allow seeping water to drain harmlessly through the dam. These designs usually include a core of impermeable clay and transition zones of more permeable materials. There are several different design possibilities for a masonry dam. The gravity dam utilizes the downward force of the weight of the construction materials to resist the horizontal force of the water. The base, where the force of the water is greatest, is made of concrete; its width is roughly three-fourths the height of the dam. Concrete-buttress dams have less material in the wall itself through the use of support buttresses around the outside base. Extra rigidity is possible if the buttresses can be linked together, as in the multiple-arch dam, but such a structure is possible only where no movement is anticipated in the buttress foundations. The Daniel Johnson Dam in Quebec has 14 buttresses across its crest (length) of 1,314 m (4,311 feet). The arch dam is built in a convex arch facing the reservoir. It has the unique advantage of reinforcement from the water pressure itself, which keeps the masonry joints closed tight. The strength of this design was tested in Italy in 1963, when the Vaiont Dam remained basically undamaged after a huge quantity of soil and rock slid into the reservoir, causing water to surge over the top of the dam. Once water has been retained by dams it can be utilized. Outlets called sluices, or gates, allow sufficient water through the dam for irrigation, water supply, or power generation; they can also control the level of water in the riverbed below the dam for ecological reasons. Some dams have special gates in the form of stepped pools, locks, or fish ladders for the upstream and downstream passage of migratory fish. Sluices are also used to drain the silt that accumulates behind a dam. In addition to causing additional pressure on the structure, accumulated silt can eventually fill a reservoir. The most important auxiliary structure of a dam is a spillway, the lack of which greatly increases the probability of structural failure. The spillway automatically discharges any water in excess of the capacity of the dam due to heavy rain or landslide. Spillway water is usually diverted along the side of the dam or made to shoot out from the dam in what is called a ski-jump spillway. In this way the dam's foundation is not eroded. structure built across a stream, river, or estuary to retain water. Its purposes are to meet demands for water for human consumption, irrigation, or industry; to reduce peak discharge of floodwater; to increase available water stored for generating hydroelectric power; or to increase the depth of water in a river so as to improve navigation. An incidental purpose can be to provide a lake for recreation. Auxiliary works at a dam may include spillways, gates, or valves to control the discharge of surplus water downstream from the dam; an intake structure conducting water to a power station or to canals, tunnels, or pipelines for more distant use; provision for evacuating silt carried into the reservoir; and means for permitting ships or fish to pass the dam. A dam therefore is the central structure in a multipurpose scheme aiming at the conservation of water resources. The multipurpose dam holds special importance in less developed countries, where a small nation may reap enormous benefits in agriculture and industry from a single dam. Dams fall into several distinct classes, by profile and by building material. The decision as to which type of dam to build depends largely on the foundation conditions in the valley and the construction materials available. Broadly, the choice of materials now lies between concrete, soils, and rock fill. Although in the past a number of dams were built of jointed masonry, this practice is now largely obsolete. The monolithic form of concrete dams permits greater variations in profile, according to the extent to which water pressure is resisted by the deadweight of the structure, is transferred laterally to buttresses, or is carried by horizontal arching across the valley to abutments formed by the sides of the valley. Additional reading Norman Smith, A History of Dams (1971), contains an outstanding detailed record of ancient dams. Nicholas J. Schnitter, A History of Dams (1994), chronicles the construction and use of dams from ancient civilization to the present. Two histories of individual dams are Anne D. Rassweiler, The Generation of Power: The History of Dneprostroi (1988), providing the political background to the dam's development; and Andrew J. Dunar and Dennis McBride, Building Hoover Dam (1993), an oral history of those involved in the project. Alfred R. Golz (ed.), Handbook of Dam Engineering (1977), is a manual of design and construction. Earlier works are J. Guthrie Brown (ed.), Hydro-Electric Engineering Practice, 3 vol., 2nd ed. (196470), a comprehensive textbook dealing principally with British and European practice; James L. Sherard et al., Earth and Earth-Rock Dams (1963), on the design and construction of foundations and embankments; and Calvin Victor Davis and Kenneth E. Sorenson (eds.), Handbook of Applied Hydraulics, 3rd ed. (1969, reprinted 1984), a classic work on the basic principles of hydraulic engineering and the design of hydraulic structure. J. Laginha Serafim and R.W. Clough (eds.), Arch Dams (1990), collects papers from an international workshop on this type of dam. J. Guthrie Brown The Editors of the Encyclopdia Britannica

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