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Dams

Dams

Dams are structural barriers built to obstruct or control the flow of water in rivers and streams. They are designed to serve two broad functions. The first is the storage of water to compensate for fluctuations in river discharge (flow) or in demand for water and energy. The second is the increase of hydraulic head , or the difference in height between water levels in the lake created upstream of the dam and the downstream river.

By creating additional storage and head, dams can serve one or more purposes:

  • Generating electricity;
  • Supplying water for agricultural, industrial, and household needs;
  • Controlling the impact of floodwaters; and
  • Enhancing river navigation.

They can be operated in a manner that simultaneously augments downstream water quality, enhances fish and wildlife habitat, and provides for a variety of recreational activities, such as fishing, boating, and swimming.

Classes of Dams

Four major classes of dams are based on the type of construction and materials used: embankment, gravity, arch, and buttress.

Embankment.

Embankment dams typically are constructed of compacted earth, rock, or both, making them less expensive than others that are constructed of concrete. Consequently, more than 80 percent of all large dams are of this type. Embankment dams have a triangular-shaped profile and typically are used to retain water across broad rivers.

Gravity.

Gravity dams consist of thick, vertical walls of concrete built across relatively narrow river valleys with firm bedrock. Their weight alone is great enough to resist overturning or sliding tendencies due to horizontal loads imposed by the upstream water.

Arch.

Arch dams, also constructed of concrete, are designed to transfer these loads to adjacent rock formations. As a result, arch dams are limited to narrow canyons with strong rock walls that can resist the arch thrust at the foundation and sides of the dam.

Buttress.

Buttress dams are essentially hollow gravity dams constructed of steel-reinforced concrete or timber.

Planning for Dams

Careful planning throughout the siting, design, and construction of dams is necessary for optimal utilization of rivers and for preventing catastrophic dam failure . These planning phases require input from engineers, geologists, hydrologists, ecologists, financiers, and a number of other professionals.

Designers must first evaluate alternative solutions and designs for meeting the same desired objective, whether the goal is to allocate water supply, improve flood control, or generate electricity. Each alternative requires a comprehensive cost-benefit analysis and feasibility study for evaluating its physical, economic, ecological, and social impact.

Once an alternative has been selected, a number of important considerations enter into the design and construction of the dam. These include:

  • Hydrological evaluation of climate and streamflows;
  • Geologic investigation for the foundation design;
  • Assessment of the area to be inundated by the upstream lake (also called a reservoir) and its associated environmental and ecological impacts;
  • Selection of materials and construction techniques;
  • Designation of methods for diverting stream flow during construction of the dam;
  • Evaluation of the potential for sediments to accumulate on the reservoir bottom and subsequently reduce storage capacity; and
  • Analysis of dam safety and failure concerns.

When a dam is put into operation, or commissioned, water is released from the upstream reservoir over a spillway or through gates in a manner to satisfy intended objectives. Operating rules for maximizing power generation, for example, include maintaining hydraulic head. In contrast, water levels in flood control reservoirs must be periodically reduced to allow for new storage during anticipated periods of flood hazard. Operating issues, however, can easily become complex and highly politicized and may be difficult to resolve. This is particularly true for river systems containing several reservoirs, for dams serving multiple purposes, and in cases where adverse social, ecological, and environmental impacts are significant.

Overview of Dam-Building

The first dam for which reliable records exist was built on the Nile River sometime before 4000 b.c.e. near the ancient city of Memphis. Remains of other historic dams have been located at numerous sites bordering the Mediterranean Sea and throughout the Middle East, China, and Central America. The oldest continuously operating dam still in use is the Kofini Dam, which was constructed in 1260 b.c.e. on the Lakissa River in Greece.

Today, there are approximately 850,000 dams located around the world. Of the more than 40,000 that are categorized as large dams, more than half are located in China and India. It is estimated that 24 countries currently generate more than 90 percent of their electrical power from dams, and 70 countries rely on dams for flood control.

Dams in the United States.

Large-scale construction of dams occurred in the United States during the postWorld War II years and reached its peak in the 1960s. The organizations that have been primarily responsible for dam-building are the U.S. Army Corps of Engineers, the Bureau of Reclamation (part of the U.S. Department of the Interior), and a number of public and private utility developers.

Since the nineteenth century, the U.S. Army Corps of Engineers has been engineering rivers to accommodate river traffic, control floods, produce electricity, and provide irrigation waters. Four of the largest dams constructed by the Corps include Garrison, Oahe, Fort Peck, and Fort Randall Dams.

The second group, the Bureau of Reclamation, was established in 1902, when Congress passed the National Reclamation Act. The Bureau was initially charged with developing irrigation and power projects in seventeen western states and has been responsible for the construction of more than six hundred dams and reservoirs, including the massive Hoover, Shasta, Glen Canyon, and Grand Coulee Dams.

The third organization responsible for dam construction encompasses various power administrations, such as the Tennessee Valley Authority, the largest public power company in the United States, as well as others operating under the Federal Power Act of 1920, which provided for the licensing of privately built dams to produce electric power. In part because of this mid-twentieth-century dam-building era, the U.S. dam population has approached 75,000. More recently, however, the rate of dam construction in the United States is exceeded by the rate of decommissioning . In many cases, maintenance costs for aging infrastructure, significant social and ecological impacts, high construction costs, and the reduced availability of suitable sites have made alternatives to dams more viable.

see also Army Corps of Engineers, U.S.; Bureau of Reclamation, U.S.; Cost-Benefit Analysis; Hoover Dam; Hydroelectric Power; Recreation; Reservoirs, Multipurpose; Supply Development; Tennessee Valley Authority.

John W. Nicklow

Bibliography

Linsley, Ray K. et al. Water Resources Engineering, 4th ed. New York: McGraw-Hill, 1992.

Mays, Larry W. Water Resources Engineering. New York: John Wiley & Sons, 2001.

Morris, Gregory L., and Jiahua Fan. Reservoir Sedimentation Handbook. New York:McGraw-Hill, 1998.

U.S. Department of Interior, Bureau of Reclamation. Design of Small Dams, 3rd ed. Denver, CO: U.S. Government Printing Office, 1987.

World Commission on Dams. Final Report: Dams and DevelopmentA New Framework for Decision-Making. Cape Town, South Africa: World Commission on Dams, 2000.

Internet Resources

The World Commission on Dams. <http://www.dams.org>.

BIG DAMS

The world's two tallest dams are located in Tajikistan in the city of Vakhsh where they tower over 335 meters, or 1,100 feet tall (Rogun) and 300 meters, or 985 feet tall (Nurek). The Three Gorges Dam in China, a concrete gravity dam scheduled for completion in 2009, will be 175 meters tall (574 feet), the equivalent of a 48-story building.

When completed, Three Gorges Dam will be the world's largest hydropower facility with a generation capacity of 18,200 megawatts. It will simultaneously supply flood storage and enhance navigation along the Yangtze River. The structure will create a reservoir more than 600 kilometers long and 1,100 meters wide, capable of storing 39.3 billion cubic meters of water.

Construction of the dam, which began in 1993, requires the inundation of 632 square kilometers of existing land and will cause the permanent relocation of over 1.2 million people.

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dam

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 Aswan Dam in Egypt, the Kariba Dam in Zambezi, the Daniel Johnson Dam in Canada, the Guri Dam in Venezuela; the Itaipú Dam between Brazil and Paraguay, and the Three Gorges Dam in China, which is the largest hydropower dam in the world. The Grand Coulee Dam, 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 project; Missouri River basin project; Tennessee Valley Authority). The Oroville Dam, 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 Lake, 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).

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Dam

Dam

Dams are structures that hold back water in a stream or river, forming a lake or reservoir behind the wall. Dams are used as flood control devices and as sources of hydroelectric power and water for crops. Dams are designed to resist the force of the water against them, the force of standing waternot a running stream.

Dam construction

There are five main types of dams: arch, buttress, earth, gravity, and rock-fill. Arch dams are curved upstream, into the water they hold back. They are typically built in narrow canyons, where the high rocky walls of the canyon can withstand the pressure of the water as it pushes off the arch and against the walls.

A buttress dam uses the force of the water to support it. A slab of concrete is tilted at a 45-degree angle and has buttresses (supports) on the opposite side of the water. While the water pushes down on the slab, the buttresses push up against it. These counterforces keep the slab in balance. Because of the large number of steel beams needed in construction, however, these dams are no longer popular because steel and labor are too expensive.

An earth dam may be a simple embankment or mound of earth (gravel, sand, clay) holding back water. An earth dam might also have a core of cement or a watertight material lining the upstream side.

A gravity dam, made of cement or masonry, withstands the force of the water behind it with its weight. To accomplish this, a gravity dam's base must have a width that is at least two-thirds the total height of the dam. The dam wall is typically given a slight curve, which adds extra strength and watertightness.

Rock-fill dams are embankments of loose rocks covering a watertight core, such as clay. The upstream side of a rock-fill dam might also be constructed with a watertight material.

Impact of dams

While dams can help save lives, irrigate farmland, and provide hydroelectric power, they can also damage farmland and the environment.

Building a dam changes the ecology of the surrounding area, flooding the habitats of plants and animals. Currently, before a dam is built a full-scale environmental impact study is made to determine if any endangered species would be threatened by a dam's construction.

In some areas of the world, however, progress far outweighs the need to protect endangered species or the lives of many people. This seems to be the case, with the giant Three Gorges Dam being built on the Yangtze River in central China. Expected to be completed sometime in 2003, the dam will create the world's largest hydroelectric project and a huge new lake. It will stretch nearly 1 mile (1.6 kilometers) across and tower 575 feet (175 meters) above the world's third-longest river. Its reservoir would stretch more than 350 miles (563 kilometers) upstream. By the time the newly created reservoir reaches its maximum height in 2008, it is estimated that 1.1 million people will have been relocated (some sources say as many as 1.9 million people). Many international organizations have criticized the project, saying it threatens the environment and dislocates many people who are merely being resettled in already overcrowded areas.

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dam

dam1 • abbr. decameter(s). dam2 / dam/ • n. a barrier constructed to hold back water and raise its level. ∎  a barrier of branches in a stream, constructed by a beaver to provide a deep pool and a lodge. ∎  any barrier resembling a dam. • v. (dammed , dam·ming ) [tr.] build a dam across (a river or lake). ∎  hold back or obstruct (something): the closed lock gates dammed up the canal. dam3 • n. the female parent of an animal, esp. a domestic mammal.

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dam

dam Barrier built to confine water (or check its flow) for irrigation, flood control or electricity generation. The first dams were probably constructed by the Egyptians 4500 years ago. Gravity dams are anchored by their own weight. Single-arch dams are convex to the water they retain, supported at each end by river banks. Multiple-arch and buttress dams are supported by buttresses rooted in the bedrock. The cheapest commercial source of electricity comes from hydroelectric projects made possible by dams, such as the Aswan High Dam, Egypt.

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dam

dam 1 barrier checking the downward flow of water. XII (in mulnedam ‘mill-dam’). — (M)LG., (M)Du. dam, f. a base repr. also by OE. fordemman (ME. demme), OFris. demmen, Goth. faurdamnjan dam up, close up; of doubtful orig.
Hence dam vb. XVI.

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dam

dam.
1. Bank or barrier of earth, masonry, etc., built across a stream to obstruct its flow and raise its level, to form a reservoir, or to make water available to turn a mill-wheel, etc.

2. Causeway over swampy ground.

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dam

dam 2 †dame, lady XIII; female parent XIV. var. of DAME.

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Dams

DAMS

DAMS. SeeHydroelectric Power .

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dam

damam, Amsterdam, Assam, Bram, cam, cham, cheongsam, clam, cram, dam, damn, drachm, dram, exam, femme, flam, gam, glam, gram, ham, jam, jamb, lam, lamb, mam, mesdames, Omar Khayyám, Pam, pram, pro-am, ram, Sam, scam, scram, sham, Siam, slam, Spam, swam, tam, tram, Vietnam, wham, yam •in memoriam • ad nauseam •iamb, Priam •grandam • Edam • goddam •quondam • Potsdam • cofferdam •Rotterdam • Oxfam • Birmingham •Abraham • logjam • CAD-CAM •minicam • Nicam •Eelam, Elam •flimflam • oriflamme • Suriname •ad personam • diazepam • tangram •ashram • telegram • milligram •epigram • centigram • dithyramb •program, programme •cardiogram • radiogram • echogram •mammogram •aerogramme (US aerogram) •microgram • dirham •electrocardiogram • ideogram •heliogram • diaphragm • diagram •parallelogram • kilogram • hologram •encephalogram • anagram •monogram • sonogram • kissogram •pentagram • cryptogram • photogram •tam-tam • wigwam • whim-wham

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dam

dam symbol for decametre(s)

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DAMS

DAMS (USA) defense against missiles system(s)
• Deputy Assistant Military Secretary

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