dam(redirected from Dental dams)
Also found in: Dictionary, Thesaurus, Medical, Legal, Financial, Wikipedia.
dam,barrier, commonly across a watercourse, to hold back water, often forming a reservoir or lake; dams are also sometimes used to control or contain rockslides, mudflows, and the like in regions where these are common. Dams are made of timber, rock, earth, masonry, or concrete or of combinations of these materials. Timber is seldom used in dams because timbers are impermanent and their height is limited. Rock-fill dams consist of an embankment of loose rock with either a core impervious to water (e.g., clay) or a watertight face on the upstream side. Earth dams may be either simple embankments of earth or embankments reinforced with a core of cement or with an upstream surface made watertight. Masonry and concrete dams are either gravity dams or arch dams (either single-arch or multiple-arch). Gravity dams are dependent upon their own weight for resistance to the pressure of the water. Single-arch dams are curved upstream and are usually constructed in narrow canyons or gorges where the rocky side walls are strong enough to withstand the tremendous lateral thrust of the dam that is caused by the pressure of the water. Dams of the multiple-arch type consist of a number of single arches supported by buttresses. Dams may also be constructed with roller-compacted concrete, in which thin layers of concrete are compacted as if they were earth layers; this produces a far stronger dam, without the need for full forms.
Dams have been constructed from early times to provide a ready supply of water for irrigation and other purposes. One of the earliest large dams for this purpose was a marble structure built c.1660 in Rajputana (Rajasthan), India. A dam used only to impound water is often called a barrage; the largest such barrage is the Syncrude Tailings Dam in Canada, which impounds 540 million cubic meters of water.
Most modern dams are constructed for multiple purposes, e.g., to provide for irrigation, to aid flood control and hence improve the navigability of waterways, and especially to furnish power for hydroelectric plants. Notable dams built to provide hydroelectric power include the AswanAswan
, city (1986 pop. 190,579), capital of Aswan governorate, S Egypt, on the Nile River at the First Cataract. It is one of the driest cities in the world.
..... Click the link for more information. Dam in Egypt, the Kariba DamKariba Dam
, hydroelectric project, in Kariba Gorge of the Zambezi River, on the Zambia-Zimbabwe border, S central Africa; built 1955–59. One of the world's largest dams, it is 420 ft (128 m) high and 1,900 ft (579 m) long.
..... Click the link for more information. in Zambezi, the Daniel Johnson Dam in Canada, the Guri Dam in Venezuela; the Itaipú Dam between Brazil and Paraguay, and the Three Gorges DamThree Gorges Dam,
607 ft (185 m) high and 7,575 ft (2,309 m) long, on the Chang (Yangtze) River, central Hubei prov., China, 30 mi (48 km) W of Yichang. The largest concrete structure in the world, the dam itself was constructed from 1994 to 2006.
..... Click the link for more information. in China, which is the largest hydropower dam in the world. The Grand Coulee DamGrand Coulee Dam
, 550 ft (168 m) high and 4,173 ft (1,272 m) long, on the Columbia River, N central Wash., NW of Spokane; built 1933–42 as a key unit in the Columbia basin project of the U.S. Bureau of Reclamation.
..... Click the link for more information. , located near Spokane, Wash., is the largest hydropower dam in the United States. The 20th cent. witnessed many great dam projects in the United States (see Central Valley projectCentral Valley project,
central Calif., long-term general scheme for the utilization of the water of the Sacramento River basin in the north for the benefit of the farmlands of the San Joaquin Valley in the south, undertaken by the U.S. Bureau of Reclamation in 1935.
..... Click the link for more information. ; Missouri River basin projectMissouri River basin project,
comprehensive plan authorized in 1944 for the coordinated development of water resources of the Missouri River and its tributaries, draining an area of c.
..... Click the link for more information. ; Tennessee Valley AuthorityTennessee Valley Authority
(TVA), independent U.S. government corporate agency, created in 1933 by act of Congress; it is responsible for the integrated development of the Tennessee River basin.
..... Click the link for more information. ). The Oroville DamOroville Dam,
770 ft (235 m) high and 7,600 ft (2,317 m) long, on the Feather River, N Calif., near the city of Oroville. The largest unit of the Feather River project, the dam was built (1957–68) to provide electric power, drinking water, and irrigation for central and S California.
..... Click the link for more information. , located in California, the tallest in the United States, is 770 ft (235 m) high; the Rogun Dam, in Russia, the tallest in the world, is 1,100 ft (335 m) high. A large dam in Panama forms Gatún LakeGatún Lake
, artificial lake, 163 sq mi (422 sq km), Colón Prov., Panama, formed by the impounding of the Chagres River. Gatún Dam (completed 1912), 1 1-2 mi (2.4 km) long and 115 ft (35 m) high, controls the level of the lake (c.
..... Click the link for more information. , the key to the Panama Canal system.
See A. H. Cullen, Rivers in Harness: The Story of Dams (1962); N. Smith, A History of Dams (1972); D. Jackson, Great American Bridges and Dams (1988); A. H. J. Dorsey, ed., Large Dams: Learning from the Past, Looking at the Future (1997).
a hydraulic-engineering structure that blocks off a river or other watercourse to raise the water level behind itself, to create a head at its location, and to create a reservoir.
The significance of dams for water management is diverse. The raising of the water level and the increase in depth in the upper pool contribute to navigation and timber flotation, as well as to water retention for the needs of irrigation and water supply. The creation of a head at the dam makes possible the use of the river’s discharge to generate power. The presence of a reservoir makes possible regulation of the discharge, that is, an increase in the water discharge in the river during the low periods and a reduction of maximum discharge during high water, which could otherwise cause destructive flooding. Dams and reservoirs have a substantial effect on rivers and adjacent areas. The river discharge conditions, water temperature, and duration of freeze-up of the river are altered; fish migration is impeded; the shores of the river behind the dam are inundated; and the microclimate of the littoral areas is altered. A dam is usually the main structure of a hydraulic power system.
Dam construction began at the same time as hydraulic engineering, in connection with the significant development of artificial irrigation among the farming peoples of Egypt, India, and China. The erection of dams was necessary for the construction of water-power installations and later of hydroelectric power plants. The use of water resources for energy was the basic incentive for increasing the size and improving the design of dams, as well as for the appearance of hydraulic power systems on numerous rivers.
On the territory of the USSR, water mills with dams were built as early as the time of Kievan Rus’. In the 17th to 19th centuries, the mining, metallurgical, textile, paper, and other industrial sectors in the Urals, the Altai, Karelia, and the central regions of Russia mainly used the mechanical energy of water-power installations. The dams of such installations were insignificant in size and were built of local materials.
The construction of high-capacity hydroelectric power plants with large concrete and earth dams began only under Soviet power, after the adoption of the plan of GOELRO (the State Commission for the Electrification of Russia). The first concrete spillway of the Volkhov Hydroelectric Power Plant was built in 1926, and the high concrete dam of the Dnieper Hydroelectric Power Plant was built in 1932 (its maximum height was about 55 m). The spillway of the Nizhniaia Svir’ Hydroelectric Power Plant was the first dam built on loose clay ground. Dams built in the 1950’s to 1970’s on rivers carrying a great deal of water included hydraulic-fill earth dams on the Volga near Kuibyshev and Volgograd, the concrete dams of the Bratsk Hydroelectric Power Plant on the Angara (height, 128 m) and the Krasnoiarsk Hydroelectric Power Plant on the Enisei (124 m), the 300-m rock-fill dam of the Nurek Hydroelectric Power Plant on the Vakhsh River, and the arch dam of the Saian Hydroelectric Power Plant on the Enisei (height, 242 m; length of crest, 1,070 m; under construction as of 1975). The design and construction of dams in the USSR are on a high technical level, which has enabled Soviet dam construction to occupy a leading place in the world.
Among notable dams built in other countries are the 87-m multiple-arch Bartlett Dam (USA, 1939), the 112-m rock-fill Paradela Dam (Portugal, 1958), the 122-m earth-fill Serre-Pon-çon Dam (France, 1960), the 131-m rock-fill Miboro Dam (Japan, 1961), and the 284-m Grande Dixence concrete gravity dam (Switzerland, 1961).
The type and design of a dam are determined by its size and purpose, as well as by the natural conditions and the type of main building material. Dams are divided by purpose into nonover-flow and overflow types (the latter is designed only to raise the level of the upper pool). In terms of head, dams are divided into low-head, with a head of up to 10 m; medium-head, with a head of 10-40 m; and high-head, with a head of more than 40 m.
A distinction is made among three types of dams, depending on the role performed in the hydraulic power system: (1) the nonoverflow type, which serves only to block the flow of water; (2) the overflow type, which is designed to release excess water and is equipped with surface spillway openings, which may be open or have gates, or with base water outlets; (3) the station type, which has water intakes (with the corresponding equipment) and water lines driving the turbines of a hydroelectric power plant. In terms of the basic material from which dams are constructed, there are earthen, rock-fill, concrete, and wooden dams.
Earth dams are erected completely or partially from impervious soil. Such soil, which is laid on the upstream slope of the dam, forms a cutoff wall; if the soil is placed inside the body of the dam, a core is created. The presence of a cutoff wall or core makes possible the erection of the remaining portion of the dam from permeable soil or from stone (an earth core rock-fill dam). Drainage is provided along the toe of an earth dam to lead off water that seeps through the body and base of the dam. The upstream slope of the dam is protected from wave action by concrete slabs or riprap. In erecting an earthen embankment dam, the soil is mined in a quarry by excavators, transported to the construction site in dump trucks, placed in the body of the dam, leveled out by bulldozers, and packed layer by layer with rollers. The erection of a hydraulic-fill dam includes excavation of the ground with dredges or water jets and the transport of the slurry through pipe and its distribution on the surface of the dam being erected. The water then flows away, and the settling earth stabilizes itself. To prepare the base and erect an earth dam, the foundation pit in the channel of the river is protected by cofferdams, and the river is diverted through previously laid temporary conduits, which are closed after the dam has been erected.
In a rock-fill dam, the cutoff wall or core is made from reinforced concrete, asphalt, wood, metal, or polymer materials. The requirement for low water permeability also extends to the base of the dam. If the ground of the base is permeable to a great depth, it is covered in front of the dam by an upstream apron made, for example, of clay, which forms a single unit with the cutoff wall. A core dam is supplemented by the construction of a steel sheet-pile wall or an antiseepage curtain at the base. The stone in rock-fill and earth core rock-fill dams is laid in very thick layers.
Concrete dams are usually classified according to design features, depending on the shear strain conditions. Three main types of dams are thus differentiated: gravity, arch, and buttress dams. The basic material for modern concrete dams, mainly of the gravity type, is hydraulic-engineering concrete. One of the most important questions in erecting concrete dams is the reduction in water seepage in the base. For this purpose, antiseepage curtains are built at the base of high concrete dams, close to the heel. The base is drained over the remaining area to reduce the water pressure on the toe of the dam; this increases the stability of the structure. To avoid the formation of cracks as a consequence of temperature fluctuations, gravity and buttress dams are divided lengthwise into short sections, and the joints between sections are filled with “waterstops.” To prevent the appearance of cracks as a result of shrinkage of the concrete during hardening, and also to reduce temperature stresses, concrete for dams is poured in individual blocks of limited size. The components of the concrete mix and the concrete placed in the blocks are also cooled by circulation of a cooling agent from a refrigeration unit through a system of pipes laid in the body of the dam. A concrete dam in a riverbed is usually built in two stages under the protection of cofferdams, which block off the pit. During erection of the first stage of the dam, the river flows through a free portion of the bed; in the second stage it flows through openings (passages) left in the dam, which are closed at the end of construction work. If the riverbed is narrow, the concrete dam is built in one stage, with temporary diversion of the river into shore conduits.
The low-head concrete spillway, which is widely used in hydraulic-engineering construction, is erected on a nonrock foundation and is designed to pass large discharges of water. Its basis is the spillway openings formed by a concrete flat apron and buttresses and closed by hydraulic gates. Behind the spillways, massive reinforcement of the channel is provided in the form of an apron, which is sometimes deepened to form a stilling basin, and lighter reinforcement called the downstream apron. Drainage is built under the apron. The spillway is joined to the banks or to earth dams by massive piers. A low-head concrete spillway usually is built with reinforcement, which often extends throughout the structure. To economize on materials, the apron and buttresses of such a dam are sometimes made with a light honeycomb design, in which the spaces are filled with dirt.
Low-head wooden dams of a piling and cribwork design are frequently built in forested regions (usually with spillways).
A special type of water-retaining structure is the sectional navigation dam. To erect such a dam, buttresses of steel beams are set up on a flat apron during the summer low-water period. Bridges are laid along the buttresses, and gates of simple design rest against the buttresses. The dam raises the level of the headwater, and ships and rafts pass through a lock. During the high-water period the gates and bridges are disassembled, and the girders of the buttresses are laid on the apron, opening a channel for ships and rafts across the dam.
A general trend in modern dam construction is the increase in dam height. The heights that have been achieved can be surpassed; however, the construction of two sequentially placed dams of lesser height is often economically more expedient than the use of one tall dam. The improvement in the types of dams from earth materials is being carried out with a simultaneous reduction in cost and acceleration of construction by increasing the capacity of construction and transportation equipment. Lower costs of concrete dams are being achieved through a reduction in the volume of the dams and by replacing gravity dams with buttress dams and by the wider use of arch dams. This trend has been accompanied by an improvement in and specialization of the properties of cement and concrete. The combination of the spillway and power plant building into a single structure is very efficient, since it reduces the concrete part of the upstream surface of the hydraulic power system, which is the most expensive. This problem is solved both by locating the hydraulic power units in the cavity of a tall dam and by building the water intakes in the underwater mass of a low-head power plant.
REFERENCESGrishin, M. M. Gidrotekhnicheskie sooruzheniia. Moscow, 1968.
Nichiporovich, A. A. Plotiny iz mestnykh materialov. Moscow, 1973.
Moiseev, S. N. Kamenno-zemlianye i kamenno-nabrosnye plotiny. Moscow, 1970.
Grishin, M. M., and N. P. Rozanov. Betonnye plotiny. Moscow, 1975.
Proizvodstvo gidrotekhnicheskikh rabot. Moscow, 1970.
A. L. MOZHEVITINOV
What does it mean when you dream about a dam?
A dam may signify repressed emotional energy. The dreamer may feel like crying, but instead is holding back the tears.
A barrier or structure across a stream, river, or waterway for the purpose of confining and controlling the flow of water. Dams vary in size from small earth embankments for farm use to high, massive concrete structures for water supply, hydropower, irrigation, navigation, recreation, sedimentation control, and flood control. As such, dams are cornerstones in the water resources development of river basins. Dams are now built to serve several purposes and are therefore known as multipurpose. The construction of a large dam requires the relocation of existing highways, railroads, and utilities from the river valley to elevations above the reservoir. The two principal types of dams are embankment and concrete. Appurtenant structures of dams include spillways, outlet works, and control facilities; they may also include structures related to hydropower and other project purposes. See Electric power generation, Water supply engineering
Dams are built for specific purposes. In ancient times, they were built only for water supply or irrigation. Early in the development of the United States, rivers were a primary means of transportation, and therefore navigation dams with locks were constructed on the major rivers. Dams have become more complex to meet large power demands and other needs of modern countries.
In addition to the standard impounded reservoir and the appurtenant structures of a dam (spillway, outlet works, and control facility), a dam with hydropower requires a powerhouse, penstocks, generators, and switchyard. The inflow of water into the reservoir must be monitored continuously, and the outflow must be controlled to obtain maximum benefits. Under normal operating conditions, the reservoir is controlled by the outlet works, consisting of a large tunnel or conduit at stream level with control gates. Under flood conditions, the reservoir is maintained by both the spillway and outlet works. See Reservoir
All the features of a dam are monitored and operated from a control room. The room contains the necessary monitors, controls, computers, emergency equipment, and communications systems to allow project personnel to operate the dam safely under all conditions. Standby generators and backup communications equipment are necessary to operate the gates and other reservoir controls in case of power failure. Weather conditions, inflow, reservoir level, discharge, and downstream river levels are also monitored. In addition, the control room monitors instrumentation located in the dam and appurtenant features that measures their structural behavior and physical condition.
All dams are designed and constructed to meet specific requirements. First, a dam should be built from locally available materials when possible. Second, the dam must remain stable under all conditions, during construction, and ultimately in operation, both at the normal reservoir operating level and under all flood and drought conditions. Third, the dam and foundation must be sufficiently watertight to control seepage and maintain the desired reservoir level. Finally, it must have sufficient spillway and outlet works capacity as well as freeboard to prevent floodwater from overtopping it.
Dams are classified by the type of material from which they are constructed. In early times, the materials were earth, large stones, and timber, but as technology developed, other materials and construction procedures were used. Most modern dams fall into two categories: embankment and concrete. Embankment dams are earth or rock-fill; other gravity dams and arch and buttress dams are concrete. See Arch, Concrete
The type of dam for a particular site is selected on the basis of technical and economic data and environmental considerations. In the early stages of design, several sites and types are considered. Drill holes and test pits at each site provide soil and rock samples for testing physical properties. In some cases, field pumping tests are performed to evaluate seepage potential. Preliminary designs and cost estimates are prepared and reviewed by hydrologic, hydraulic, geotechnical, and structural engineers, as well as geologists. Environmental quality of the water, ecological systems, and cultural data are also considered in the site-selection process.
Factors that affect the type are topography, geology, foundation conditions, hydrology, earthquakes, and availability of construction materials. The foundation of the dam should be as sound and free of faults as possible. Narrow valleys with shallow sound rock favor concrete dams. Wide valleys with varying rock depths and conditions favor embankment dams. Earth dams are the most common type.
The designers of a dam must consider the stream flow around or through the damsite during construction. Stream flow records provide the information for use in determining the largest flood to divert during the selected construction period. One common practice for diversion involves constructing the permanent outlet works, which may be a conduit or a tunnel in the abutment, along with portions of the dam adjacent to the abutments, in the first construction period. The stream is diverted into the outlet works by a cofferdam high enough to prevent overtopping during construction. A downstream cofferdam is also required to keep the damsite dry. See Cofferdam
Personnel responsible for operation and maintenance of the dam are familiar with the operating instructions and maintenance schedule. A schedule is established for collection and reporting of data for climatic conditions, rainfall, snow cover, stream flows, and water quality of the reservoir, as well as the downstream reaches. All these data are evaluated for use in reservoir regulation. Another schedule is established for the collection of instrumentation data used to determine the structural behavior and physical condition of the dam. These data are evaluated frequently. Routine maintenance and inspection of the dam and appurtenant structures are ongoing processes. The scheduled maintenance is important to preserve the integrity of the mechanical equipment.
The frequency with which instrumentation data are obtained is an extremely important issue and depends on operating conditions. Timely collection and evaluation of data are critical for periods when the loading changes, such as during floods and after earthquakes. Advances in applications of remote sensing to instrumentation have made real-time data collection possible. This is a significant improvement for making dam safety evaluations.
Throughout history there have been instances of dam failure and discharge of stored water, sometimes causing considerable loss of life and great damage to property. Failures have generally involved dams that were designed and constructed to engineering standards acceptable at the time. Most failures have occurred with new dams, within the first five years of operation.