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A dam is a barrier across flowing water that obstructs, directs or slows down the flow, often creating a reservoir, lake or impoundments. In Australian and South African English, the word "dam" can also refer to the reservoir as well as the structure. Most dams have a section called a spillway or weir over which, or through which, water flows, either intermittently or continuously.

Dams generally serve the primary purpose of retaining water, while other structures such as levees and dikes are used to prevent water flow into specific land regions.

Etymology

The word dam existed already in Middle English, but stems probably from Middle Dutch. In the Netherlands dams were often applied to block rivers in order to regulate the water level and to prevent the sea from entering the marsh lands. Such dams were often the beginning of a city because it was easy to cross the river at such a place. For instance Amsterdam started with a dam through the river Amstel and Rotterdam started with a dam through the river Rotte.

History

Most of the first Dams were built in Mesopotamia up to 7,000 years ago. These were used to control the water level, for Mesopotamia's weather affected the Tigris and Euphrates rivers, and could be quite unpredictable. The earliest recorded dam is believed to have been on the Nile river at Kosheish, where a 15 meter high masonry structure was built about 2900 B.C. to supply water to capital of Memphis.

The oldest surviving and standing dam in the world is The Grand Anicut, also known as the Kallanai, is an ancient dam in Tamil Nadu state in southern India. The Grand Anicut is the most ancient surviving irrigation work in the Kaveri River Delta.

It is attributed to the Chola king Karikalan, and dates back to the 2nd century. It is considered the oldest water-diversion structure in the world still in use. The Kaveri River forms the boundary between the Erode and Salem districts. The Bhavani River joins the Kaveri at the town of Bhavani, where the Sangameswarar Temple, an important pilgrimage spot in southern India, was built at the confluence of the two rivers. Sweeping past the historic rock of Tiruchirapalli, it breaks into two channels at the island of Srirangam, which enclose between them the delta of Thanjavur, the garden of South India.

The northern channel is called the Kollidam (Kolidam); the other preserves the name of Cauvery, and empties into the Bay of Bengal at Poompuhar, a few hundred miles south of Chennai (Madras). On the seaward face of its delta are the seaports of Nagapattinam and Karikal. Irrigation works have been constructed in the delta for over 2,000 years.

The Kallanai is a massive dam of unhewn stone, 329 meters (1,080 feet) long and 20 meters (60 feet) wide, across the main stream of the Cauvery. The purpose of the dam was to divert the waters of the Cauvery across the fertile Delta region for irrigation via canals. The dam is still in excellent repair, and supplied a model to later engineers, including the Sir Arthur Cotton's 19th-century dam across the Kollidam, the major tributary of the Cauvery. The area irrigated by the ancient irrigation network of which the dam was the centrepiece was 69,000 acres (280 square kilometers). By the early 20th century the irrigated area had been increased to about 1,000,000 acres (4,000 square kilometers).

In ancient China, the Prime Minister of Chu (state), Sunshu Ao, is the first known hydraulic engineer of China. He served Duke Zhuang of Chu during the reign of King Ding of Zhou (606 BC-586 BC), ruler of the Eastern Zhou Dynasty. His large earthen dam flooded a valley in modern-day northern Anhui province that created an enormous irrigation reservoir (62 miles in circumference), a reservoir that is still present today.

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Types of dams

Dams can be formed by human agency, natural causes, or by the intervention of wildlife such as beavers. Man-made dams are typically classified according to their structure, intended purpose or height.

Based on structure and material used, dams are classified as timber dams, embankment dams or masonry dams, with several subtypes.

Intended purposes include providing water for irrigation or town or city water supply, improving navigation, creating a reservoir of water to supply industrial uses, generating hydroelectric power, creating recreation areas or habitat for fish and wildlife, flood control and containing effluent from industrial sites such as mines or factories. Few dams serve all of these purposes but some multi-purpose dams serve more than one.

According to height, a large dam is higher than 15 meters and a major dam is over 150 meters in height. Alternatively, a low dam is less than 30 m high; a medium-height dam is between 30 and 100 m high, and a high dam is over 100 m high.

A saddle dam is an auxiliary dam constructed to confine the reservoir created by a primary dam either to permit a higher water elevation and storage or to limit the extent of a reservoir for increased efficiency. An auxiliary dam is constructed in a low spot or saddle through which the reservoir would otherwise escape. On occasion, a reservoir is contained by a similar structure called a dike to prevent inundation of nearby land. Dikes are commonly used for reclamation of arable land from a shallow lake. This is similar to a levee, which is a wall or embankment built along a river or stream to protect adjacent land from flooding.

An overflow dam is designed to be over topped. A weir is a type of small overflow dam that can be used for flow measurement.

A check dam is a small dam designed to reduce flow velocity and control soil erosion. Conversely, a wing dam is a structure that only partly restricts a waterway, creating a faster channel that resists the accumulation of sediment.

A dry dam is a dam designed to control flooding. It normally holds back no water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream.

Diversionary dams

A diversionary dam is a structure designed to divert all or a portion of the flow of a river from its natural course.

Timber dams

Timber dams were widely used in the early part of the industrial revolution and in frontier areas due to ease and speed of construction. Rarely built in modern times by humans due to relatively short lifespan and limited height to which they can be built, timber dams must be kept constantly wet in order to maintain their water retention properties and limit deterioration by rot, similar to a barrel. The locations where timber dams are most economical to build are those where timber is plentiful, cement is costly or difficult to transport, and either a low head diversion dam is required or longevity is not an issue. Timber dams were once numerous, especially in the North American west, but most have failed, been hidden under earth embankments or been replaced with entirely new structures. Two common variations of timber dams were the crib and the plank.

Timber crib dams were erected of heavy timbers or dressed logs in the manner of a log house and the interior filled with earth or rubble. The heavy crib structure supported the dam's face and the weight of the water.

Timber plank dams were more elegant structures that employed a variety of construction methods utilizing heavy timbers to support a water retaining arrangement of planks.

Very few timber dams are still in use. Timber, in the form of sticks, branches and withes, is the basic material used by beavers, often with the addition of mud or stones.

Embankment dams

Embankment dams are made from compacted earth, and have two main types, rock-fill and earth-fill dams. Embankment dams rely on their weight to hold back the force of water, like the gravity dams made from concrete.

Rock-fill dams

Rock-fill dams are embankments of compacted free-draining granular earth with an impervious zone. The earth utilized often contains a large percentage of large particles hence the term rock-fill. The impervious zone may be on the upstream face and made of masonry, concrete, plastic membrane, steel sheet piles, timber or other material. The impervious zone may also be within the embankment in which case it is referred to as a core. In the instances where clay is utilized as the impervious material the dam is referred to as a composite dam. To prevent internal erosion of clay into the rock fill due to seepage forces, the core is separated using a filter. Filters are specifically graded soil designed to prevent the migration of fine grain soil particles. When suitable material is at hand, transportation is minimized leading to cost savings during construction. Rock-fill dams are resistant to damage from earthquakes. However, inadequate quality control during construction can lead to poor compaction and sand in the embankment which can lead to liquefaction of the rock-fill during an earthquake. Liquefaction potential can be reduced by keeping susceptible material from being saturated, and by providing adequate compaction during construction. An example of a rock-fill dam is New Melones Dam in California.

Earth-fill dams

It has been suggested that Earthen dam be merged into this article. (Discuss) Proposed since July 2007.

Earth-fill dams, also called earthen, rolled-earth or simply earth dams, are constructed of well compacted earth. A homogeneous rolled-earth dam is entirely constructed of one type of material but may contain a drain layer to collect seep water. A zoned-earth dam has distinct parts or zones of dissimilar material, typically a locally plentiful shell with a watertight clay core. Modern zoned-earth embankments employ filter and drain zones to collect and remove seep water and preserve the integrity of the downstream shell zone. An outdated method of zoned earth dam construction utilized a hydraulic fill to produce a watertight core. Rolled-earth dams may also employ a watertight facing or core in the manner of a rock-fill dam. An interesting type of temporary earth dam occasionally used in high latitudes is the frozen-core dam, in which a coolant is circulated through pipes inside the dam to maintain a watertight region of permafrost within it. Examples of earth-fill dams include the 300 meter high Nurek Dam in Tajikistan, the tallest dam in the world , and Oroville Dam, the tallest dam in the United States.

Embankment dam with Central Asphalt Concrete Core

A third embankment dam type is built with asphalt concrete core. The majority of such dams are built with rock and or gravel as main fill material. Almost 100 dams of this design have now been built world- wide since the first dam was completed in 1962. All dams built have an excellent performance record. This type of asphalt is a viscoelastic- plastic material that can adjust to the movements and deformations imposed on the embankment as a whole and to settlements in the foundation. The flexible properties of the asphalt makes such dams especially suited in earthquake regions.

Masonry dams

Masonry dams are of either the gravity or the arch type.

Gravity dams

In a gravity dam, stability is secured by making it of such a size and shape that it will resist overturning, sliding and crushing at the toe. The dam will not overturn provided that the moment around the turning point, caused by the water pressure is smaller than the moment caused by the weight of the dam. This is the case if the resultant force of water pressure and weight falls within the base of the dam. However, in order to prevent tensile stress at the upstream face and excessive compressive stress at the downstream face, the dam cross section is usually designed so that the resultant falls within the middle at all elevations of the cross section (the core). For this type of dam, impervious foundations with high bearing strength are essential.

When situated on a suitable site, a gravity dam inspires more confidence in the layman than any other type; it has mass that lends an atmosphere of permanence, stability, and safety. When built on a carefully studied foundation with stresses calculated from completely evaluated loads, the gravity dam probably represents the best developed example of the art of dam building. This is significant because the fear of flood is a strong motivator in many regions, and has resulted in gravity dams being built in some instances where an arch dam would have been more economical.

Gravity dams are classified as "solid" or "hollow." The solid form is the more widely used of the two, though the hollow dam is frequently more economical to construct. Gravity dams can also be classified as "overflow" (spillway) and "non-overflow." Grand Coulee Dam is a solid gravity dam and Itaipu Dam is a hollow gravity dam.

With a height of 285 meters, the tallest gravity dam in the world is the Grande Dixence Dam in Switzerland.

Arch dams

In the arch dam, stability is obtained by a combination of arch and gravity action. If the upstream face is vertical the entire weight of the dam must be carried to the foundation by gravity, while the distribution of the normal hydrostatic pressure between vertical cantilever and arch action will depend upon the stiffness of the dam in a vertical and horizontal direction. When the upstream face is sloped the distribution is more complicated. The normal component of the weight of the arch ring may be taken by the arch action, while the normal hydrostatic pressure will be distributed as described above. For this type of dam, firm reliable supports at the abutments (either buttress or canyon side wall) are more important. The most desirable place for an arch dam is a narrow canyon with steep side walls composed of sound rock. The safety of an arch dam is dependent on the strength of the side wall abutments, hence not only should the arch be well seated on the side walls but also the character of the rock should be carefully inspected.

The highest arch dam in the world is Inguri Dam in Georgia. It is 272 metres high and it was completed in 1980.

Two types of single-arch dams are in use, namely the constant-angle and the constant-radius dam. The constant-radius type employs the same face radius at all elevations of the dam, which means that as the channel grows narrower towards the bottom of the dam the central angle subtended by the face of the dam becomes smaller. Jones Falls Dam, in Canada, is a constant radius dam. In a constant-angle dam, also known as a variable radius dam, this subtended angle is kept a constant and the variation in distance between the abutments at various levels are taken care of by varying the radii. Constant-radius dams are much less common than constant-angle dams. Parker Dam is a constant-angle arch dam.

A similar type is the double-curvature or thin-shell dam. Wildhorse Dam near Mountain City, Nevada in the United States is an example of the type. This method of construction minimizes the amount of concrete necessary for construction but transmits large loads to the foundation and abutments. The appearance is similar to a single-arch dam but with a distinct vertical curvature to it as well lending it the vague appearance of a concave lens as viewed from downstream.

The multiple-arch dam consists of a number of single-arch dams with concrete buttresses as the supporting abutments. The multiple-arch dam does not require as many buttresses as the hollow gravity type, but requires good rock foundation because the buttress loads are heavy. See Geotechnical engineering.

Steel dams

A steel dam is a type of dam briefly experimented with in around the turn of the 19th-20th century which uses steel plating (at an angle) and load bearing beams as the structure. Intended as permanent structures, steel dams were an (arguably failed) experiment to determine if a construction technique could be devised that was cheaper than masonry, concrete or earthworks, but sturdier than timber crib dams. Only two examples remain in the US.

Cofferdams

A cofferdam is a (usually temporary) barrier constructed to exclude water from an area that is normally submerged. Made commonly of wood, concrete or steel sheet piling, cofferdams are used to allow construction on the foundation of permanent dams, bridges, and similar structures. When the project is completed, the cofferdam may be demolished or removed. See also causeway and retaining wall. Common uses for cofferdams include construction and repair of off shore oil platforms. In such cases the cofferdam is fabricated from sheet steel and welded into place under water. Air is pumped into the space, displacing the water allowing a dry work environment below the surface. Upon completion the cofferdam is usually deconstructed unless the area requires continuous maintenance.

Beaver dams

Beavers create dams primarily out of mud and sticks to flood a particular habitable area. By flooding a parcel of land, beavers can navigate below or near the surface and remain relatively well hidden or protected from predators. The flooded region also allows beavers access to food, especially during the winter.

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Construction elements

Spillways

A spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream side. Many spillways have floodgates designed to control the flow through the spillway.

A service spillway or primary spillway passes normal flow. An auxiliary spillway releases flow in excess of the capacity of the service spillway. An emergency spillway is designed for extreme conditions, such as a serious malfunction of the service spillway. A fuse plug spillway is a low embankment designed to be overtopped and washed away in the event of a large flood.

Any cavitation or turbulence of the water flowing over the spillway slowly erodes the dam's wetted surfaces. To minimize that erosion (especially with maximum water elevation at the crest), the downstream face of the spillway is ordinarily made an ogee curve.

It was the inadequate design of the spillway that caused the overtopping of a dam that caused the infamous Johnstown Flood.

Power generation

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Other considerations

One of the best places for building a dam is a narrow part of a deep river valley; the valley sides can then act as natural walls. The primary function of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The sites are usually those where the gap becomes a minimum for the required storage capacity. The most economical arrangement is often a composite structure such as a masonry dam flanked by earth embankments. The current use of the land to be flooded should be dispensable.

Significant other engineering and engineering geology considerations when building a dam include:

• permeability of the surrounding rock or soil
• earthquake faults
• landslides and slope stability
• peak flood flows
• reservoir silting
• environmental impacts on river fisheries, forests and wildlife (see also fish ladder)
• impacts on human habitations
• compensation for land being flooded as well as population resettlement
• removal of toxic materials and buildings from the proposed reservoir area

During an armed conflict, a dam is to be considered as an "installation containing dangerous forces" due to the massive impact of a possible destruction on the civilian population and the environment. As such, it is protected by the rules of International Humanitarian Law (IHL) and shall not be made the object of attack if that may cause severe losses among the civilian population. To facilitate the identification, a protective sign consisting of three bright orange circles placed on the same axis is defined by the rules of IHL.

Dam failures are generally catastrophic if the structure is breached or significantly damaged. Routine monitoring of seepage from drains in, and around, larger dams is necessary to anticipate any problems and permit remedial action to be taken before structural failure occurs. Most dams incorporate mechanisms to permit the reservoir to be lowered or even drained in the event of such problems. Another solution can be rock grouting - pressure pumping portland cement slurry into weak fractured rock.

Dams also effect many ecological aspects of a river. Rivers depend on the constant disturbance of a certain tolerance. Dams slow the river and this disturbance may damage or destroy this pattern of ecology. Temperature is also another problem that dams create. Rivers tend to have fairly homogeneous temperatures. Reservoirs have layered temperatures, warm on the top and cold on the bottom. when the dam releases water only the cold water comes out. Now the river is way colder than it is supposed to be. Many organisms depend on a regular cycle of temperatures and when this is altered organism survival rates are also altered.

Many older dams build do not have a fish ladder, which keeps many fish from moving up stream to their natural breeding grounds. This will not let the fish breed and soon many of that species in that area will die, leaving them with a very little to none population. Dams block the migration of a sucker, a sport fish such as the Smallmouth Bass and the channel catfish.

Dams also can take over a whole city. The three gorges dam in china is making hundreds of thousands move from their homes. The water will rise and flood the cities that were build thousands of years ago. This is where many families history is and ancestry. The dam will destroy miles of land and animals. A special crocodile needs the constant flow for its survival. This dam could destroy the exsistance of this species. Many types of plants and trees will be destroyed by the high risen waters created by the dam.

Examples of failed dams

• Vega de Tera (Ribadelago, Spain) - 1959
• Baldwin Hills Reservoir - 1963
• Banqiao and Shimantan Dams - 1975
• Big Bay Dam, Mississippi - 2004
• Buffalo Creek Flood - 1972
• Camará Dam - 2004
• Dale Dike Reservoir - 1864
• South Fork Dam - 1889
• Kelly Barnes Dam - 1977
• Lawn Lake Dam - 1982
• Malpasset, Côte d'Azur, France - 1959
• Opuha Dam - 1997
• Shakidor Dam - 2005
• St. Francis Dam, Los Angeles, California - 1928
• Taum Sauk reservoir - 2005
• Teton Dam - 1976
• Tous Dam, Valencia, Spain - 1982
• Vajont Dam - 1963
• Val di Stava Dam Collapse - 1985

See also

• Delta Works
• List of reservoirs and dams
• Canal lock
• Beaver a dam-building rodent
• de:Talsperren-Katastrophen (Failed Dams; in German language)
• Ilisu Dam Campaign campaign against a planned dam in Turkey
• Megaprojects
• Dam Busters

External links

• International Commission on Large Dams (ICOLD)
• Asphalt Core for Embankment Dams
• Structurae: Dams and Retaining Structures
• "Design of Small Dams", US Bureau of Reclamation, 65MB pdf
• "Dam science" Canadian Geographic
• International Rivers Network (IRN)

Notes

1. Guinness Book of Records 1997 Pages 108-109 ISBN 0-85112-693-6

• Articles to be merged from July 2007
• Articles lacking sources from January 2007
• Articles needing cleanup from February 2007
• Cleanup tagged articles without a reason field from February 2007
• Misplaced Pages pages needing cleanup from February 2007
• Dams
• Civil engineering
• Geotechnical engineering
• Structural engineering
• Dams in California