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Two idiots making youtube videos that noone watches. so if u are nice(yea right) then watch our videos |
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{{otheruses1|structures for water impoundment}} |
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our youtube channel is damienownz11 |
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{{distinguish|Damn}} |
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[[Image:Karun3-dam.jpg|thumb|right|upright|[[Karun-3 dam]], [[Iran]].]] |
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[[Image:Hydroelectric dam.svg|thumb|right|Hydroelectric dam in cross section.]] |
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A '''dam''' is a barrier that [[Reservoir|impound]]s [[surface water|water]] or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as [[floodgates]], [[levee]]s, and [[Dike (construction)|dikes]] are used to manage or prevent water flow into specific land regions. |
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==History== |
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[[Image:Anaicut.JPG|thumb|right|[[Grand Anicut]] dam on river [[Kaveri]] (1st-2nd Century CE)]] |
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The word ''dam'' can be traced back to [[Middle English]],<ref>[http://www.bartleby.com/61/45/D0014500.html The American Heritage® Dictionary of the English Language, Fourth Edition]</ref> and before that, from [[Middle Dutch]], as seen in the names of many old cities.<ref>Source: Tijdschrift voor Nederlandse Taal- en Letterkunde (Magazine for Dutch Language and Literature), 1947. The first known appearance of the word ''dam'' stems from 1165. However, there is one village, Obdam, that is already mentioned in 1120. The word seems to be related to the Greek word ''taphos'', meaning ''grave'' or ''grave hill''. So the word should be understood as ''dike from dug out earth''. The names of more than 40 places (with minor changes) from the Middle Dutch era (1150 - 1500 CE) such as [[Amsterdam]] (founded as 'Amstelredam' in the late 12th Century) and [[Rotterdam]], also bear testimony to the use of the word in Middle Dutch at that time.</ref> |
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Most of early dam building took place in [[Mesopotamia]] and the [[Middle East]]. Dams were used to control the water level, for Mesopotamia's weather affected the [[Tigris]] and [[Euphrates]] rivers, and could be quite unpredictable. |
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The earliest known dam is situated in Jawa, [[Jordan]], 100 km northeast of the capital [[Amman]]. The gravity dam featured a 9 m high and 1 m wide stone wall, supported by a 50 m wide earth rampart. The structure is dated to 3000 BC.<ref>Günther Garbrecht: "Wasserspeicher (Talsperren) in der Antike", Antike Welt, 2nd special edition: Antiker Wasserbau (1986), pp.51-64 (52)</ref><ref>S.W. Helms: "Jawa Excavations 1975. Third Preliminary Report", Levant 1977</ref> The [[Ancient Egypt]]ian Sadd Al-Kafara at Wadi Al-Garawi, located about 25 kilometers south of [[Cairo]], was 102 m long at its base and 87 m wide. The structure was built around 2800<ref name="Günther Garbrecht 52f.">Günther Garbrecht: "Wasserspeicher (Talsperren) in der Antike", Antike Welt, 2nd special edition: Antiker Wasserbau (1986), pp.51-64 (52f.)</ref> or 2600 B.C.<ref name=Bazzasowr> {{cite web |url=http://www.fao.org/world/Regional/RNE/morelinks/Publications/English/HYSTORY-OF-WATER-RESOURCES.pdf |title=overview of the hystory of water resources and irrigation management in the near east region |accessdate=2007-08-01 |author=Mohamed Bazza |authorlink= |coauthors= |date=28-30 |year=2006 |month= October Food and Agriculture Organization of the United Nations pages= }}</ref> as a [[diversion dam]] for flood control, but was destroyed by heavy rain during construction or shortly afterwards.<ref name="Günther Garbrecht 52f."/><ref name=Bazzasowr /> The [[Ancient Romans|Romans]] were also great dam builders, with many examples such as the three dams at [[Subiaco]] on the river [[Anio]] in [[Italy]]. Many large dams also survive at [[Merida]] in [[Spain]]. |
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The oldest surviving and standing dam in the world is believed to be the Quatinah barrage in modern-day [[Syria]]. The dam is assumed to date back to the reign of the Egyptian pharao [[Sethi]] (1319-1304 BC), and was enlarged in the Roman period and between 1934-38. It still supplies the city of [[Homs]] with water. |
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The [[Kallanai]] is a massive dam of unhewn stone, over 300 meters long, 4.5 meters high and 20 meters (60 ft) wide, across the main stream of the Kaveri. The basic structure dates to the 2nd Century AD.<ref name=IsisWiebe>{{cite journal |last= Wiebe |first= Bijker |title= Dikes and Dams, Thick with Politics |journal= Isis |volume=98 |date= 2007 |pages= 109–123 |url= http://www.journals.uchicago.edu/ISIS/journal/issues/v98n1/980107/980107.html|accessdate=2007-07-01 |doi= 10.1086/512835 |format= {{dead link|date=June 2008}} – <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3AWiebe+intitle%3ADikes+and+Dams%2C+Thick+with+Politics&as_publication=Isis&as_ylo=&as_yhi=&btnG=Search Scholar search]</sup> }}</ref> The purpose of the dam was to divert the waters of the Cauvery across the fertile Delta region for irrigation via canals. |
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[[Du Jiang Yan Irrigation System|Du Jiang Yan]] is the oldest surviving irrigation system in China that included a dam that directed waterflow. It was finished in 251 B.C. A large earthen dam, made by the [[Prime Minister]] of [[Chu (state)]], [[Sunshu Ao]], 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.<ref name="needham volume 4 part 3 271"> |
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Needham, Joseph (1986). ''Science and Civilization in China: Volume 4, Part 3''. Taipei: Caves Books, Ltd. |
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</ref> |
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In the [[Iran]], [[bridge]] dams was used to power a [[water wheel]] working a [[water]]-raising [[mechanism]]. The first was built in [[Dezful]], which could raise 50 [[cubit]]s of water for the [[water supply]] to all houses in the town. Also [[diversion dam]]s were known.<ref name=Hill-Engineering>[[Donald Routledge Hill]] (1996), "Engineering", p. 759, in {{Harvard reference |last1=Rashed |first1=Roshdi |last2=Morelon |first2=Régis |year=1996 |title=[[Encyclopedia of the History of Arabic Science]] |publisher=[[Routledge]] |isbn=0415124107 |pages=751-795}}</ref> [[Milling]] dams were introduced which the Muslim engineers called the ''Pul-i-Bulaiti''. The first was built at Shustar on the River [[Karun]], [[Iran]], and many of these were later built in other parts of the [[Islamic world]].<ref name=Hill-Engineering/> Water was conducted from the back of the dam through a large pipe to drive a water wheel and [[watermill]].<ref name=Lucas>Adam Lucas (2006), ''Wind, Water, Work: Ancient and Medieval Milling Technology'', p. 62. BRILL, ISBN 9004146490.</ref> |
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In the [[Netherlands]], a low-lying country, ''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 often marked the beginning of a town or city because it was easy to cross the river at such a place, and often gave rise to the respective place's names in [[Dutch language|Dutch]]. For instance the Dutch [[capital city|capital]] [[Amsterdam]] (old name Amstelredam) started with a ''dam'' through the river [[Amstel]] in the late 12th Century , and [[Rotterdam]] started with a ''dam'' through the river Rotte, a minor tributary of the [[Nieuwe Maas]]. The central square of Amsterdam, believed to be the original place of the 800 year old dam, still carries the name ''[[Dam Square]]'' or simply ''the Dam''. |
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==Types of dams== |
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Dams can be formed by human agency, natural causes, or even by the intervention of wildlife such as [[beaver]]s. Man-made dams are typically classified according to their size (height), intended purpose or structure. |
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===By size=== |
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International standards define ''large dams'' as higher than 15 meters and ''major dams'' as over 150 meters in height.<ref>{{cite web|url=http://www.iucn.org/themes/wani/eatlas/html/technotes.html|quote=A large dam is defined by the industry as one higher than 15 meters high and a major dam as higher than 150.5 meters.|work=Watersheds of the World|title=Methodology and Technical Notes|format=HTML|accessdate=2007-08-01}}</ref> |
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The tallest dam in the world is the 300 meter high [[Nurek Dam]] in [[Tajikistan]].<ref name=gbor97p108109> Guinness Book of Records 1997 Pages 108-109 ISBN 0-85112-693-6</ref> |
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===By purpose=== |
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Intended purposes include providing water for [[irrigation]] to town or city [[water supply]], improving navigation, creating a reservoir of water to supply industrial uses, generating [[hydroelectricity|hydroelectric power]], creating recreation areas or [[habitat (ecology)|habitat]] for fish and wildlife, retaining wet season flow to minimise downstream [[flood control|flood]] risk and containing [[effluent]] from industrial sites such as [[mining|mines]] or factories. Few dams serve all of these purposes but some multi-purpose dams serve more than one. |
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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 (construction)|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 [[flood]]ing. |
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An ''overflow dam'' is designed to be over topped. A [[weir]] is a type of small overflow dam that are often used within a river channel to create an impoundment lake for water abstraction purposes and which can also be used for flow measurement. |
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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. |
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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. |
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A ''[[diversionary dam]]'' is a structure designed to divert all or a portion of the flow of a [[river]] from its natural course. |
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===By structure=== |
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Based on structure and material used, dams are classified as [[timber]] dams, [[arch-gravity dam]]s, [[embankment dam]]s or [[masonry dam]]s, with several subtypes. |
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====Masonry dams==== |
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=====Arch dams===== |
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[[Image:Hoover dam.jpg|right|thumb|[[Hoover Dam]], a concrete [[arch-gravity dam]] in the [[Black Canyon of the Colorado|Black Canyon]] of the [[Colorado River]].]] |
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{{main|Arch dam}} |
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{{see also|Geotechnical engineering}} |
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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 [[Fluid pressure#Hydrostatic pressure|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 [[surface normal|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 basterds 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.<ref name=pbsarch>{{cite web|url=http://www.pbs.org/wgbh/buildingbig/dam/basics.html#arch|title=Arch Dam Forces|accessdate=2007-01-07}}</ref> |
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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. |
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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. |
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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. |
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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. |
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=====Gravity dams===== |
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[[Image:Gilboa Dam.jpg|thumb|right|The [[Schoharie Reservoir#Gilboa Dam|Gilboa Dam]] in the [[Catskill Mountains]] of [[New York State]] is an example of a "solid" gravity dam.]] |
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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 (physics)|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. |
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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. |
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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|Itaipu Dam]] is a hollow gravity dam. A gravity dam can be combined with an arch dam, an [[arch-gravity dam]], for areas with massive amounts of water flow but less material available for a purely gravity dam. |
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====Arch-gravity dams==== |
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{{main|Arch-gravity dam}} |
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====Embankment dams==== |
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[[Image:San Luis Dam and Gianelli Powerhouse.jpg|thumb|The [[San Luis Dam]] near Los Banos, California is an [[embankment dam]].]] |
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{{main|Embankment dam}} |
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Embankment dams are made from [[Soil compaction|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. |
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=====Rock-fill dams===== |
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[[Rock (geology)|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 [[earthquake]]s. However, inadequate quality control during construction can lead to poor compaction and sand in the embankment which can lead to [[Earthquake liquefaction|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]]. |
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=====Earth-fill dams===== |
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{{main|Embankment dam}} |
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Earth-fill dams, also called earthen, rolled-earth or simply earth dams, are constructed as a simple [[Embankment dam|embankment]] 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. |
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Because earthen dams can be constructed from materials found on-site or nearby, they can be very cost-effective in regions where the cost of producing or bringing in concrete would be prohibitive. |
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=====Asphalt-Concrete Core===== |
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A third type of embankment dam is built with [[asphalt concrete]] core. The majority of such dams are built with rock and/or gravel as the main fill material. Almost 100 dams of this design have now been built worldwide since the first such dam was completed in 1962. All asphalt-concrete core dams built so far have an excellent performance record. The type of asphalt used 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]] make such dams especially suited in [[earthquake]] regions. |
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====Cofferdams==== |
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[[Image:Dam Coffer.jpg|thumb|right|A cofferdam during the construction of [[canal lock|locks]] at the [[Montgomery Point Lock and Dam]].]] |
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{{main|Cofferdam}} |
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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 [[pile|piling]], cofferdams are used to allow construction on the [[Foundation (architecture)|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. |
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====Timber dams==== |
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[[Image:Dam Timber Crib.jpg|thumb|right|A timber crib dam in [[Michigan]], photographed in 1978.]] |
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[[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''. |
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''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. [[Splash dam]]s were timber crib dams used to help float [[logging|logs]] downstream in the late 19th and early 20th centuries. |
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''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. |
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Very few timber dams are still in use. Timber, in the form of sticks, branches and withes, is the basic material used by [[beaver]]s, often with the addition of mud or stones. |
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====Steel dams==== |
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{{main|Steel dam}} |
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[[Image:088808pv.jpg|thumb|right|Red Ridge steel dam, b. 1905, Michigan.]] |
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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. |
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====Beaver dams==== |
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{{main|Beaverdam (habitat)}} |
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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== |
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===Power generation plant=== |
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[[Image:Water turbine.jpg|thumb|right|upright|[[Water turbine|Hydraulic turbine]] and [[electrical generator]].]] |
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{{main|Hydroelectricity}} |
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As of 2005, hydroelectric power, mostly from dams, supplies some 19% of the world's electricity, and over 63% of [[renewable energy]].<ref name="REN21-2006">[http://www.ren21.net/globalstatusreport/download/RE_GSR_2006_Update.pdf Renewables Global Status Report 2006 Update], ''[[REN21]]'', published 2006, accessed 2007-05-16</ref> Much of this is generated by large dams, although [[China]] uses small scale hydro generation on a wide scale and is responsible for about 50% of world use of this type of power.<ref name="REN21-2006" /> |
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Most hydroelectric power comes from the [[potential energy]] of [[dam]]med water driving a [[water turbine]] and [[electrical generator|generator]]; to boost the power generation capabilities of a dam, the water may be run through a large pipe called a [[penstock]] before the [[turbine]]. A variant on this simple model uses [[pumped storage hydroelectricity]] to produce electricity to match periods of high and low demand, by moving water between [[reservoir (water)|reservoirs]] at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. |
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===Spillways=== |
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[[Image:Llyn Brianne spillway.jpg|right|upright|thumb|Spillway on [[Llyn Brianne]] dam, [[Wales]] soon after first fill.]] |
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{{main|Spillway}} |
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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 [[floodgate]]s designed to control the flow through the spillway. Types of spillway include: 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 over topped and washed away in the event of a large flood. Fusegate elements are independent free-standing block set side by side on the spillway which work without any remote control. They allow to increase the normal pool of the dam without compromising the security of the dam because they are designed to be gradually evacuated for exceptional events. They work as fixed weir most of the time allowing overspilling for the common floods. |
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The spillway can be gradually [[erosion|eroded]] by water flow, including [[cavitation]] or [[turbulence]] of the water flowing over the spillway, leading to its failure. It was the inadequate design of the spillway which led to the 1889 over-topping of the [[South Fork Dam]] in [[Johnstown, Pennsylvania|Johnstown]], [[Pennsylvania]], resulting in the infamous [[Johnstown Flood]] (the "great flood of 1889"). |
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Erosion rates are often monitored, and the risk is ordinarily minimized, by shaping the downstream face of the spillway into a curve that minimizes turbulent flow, such as an [[ogee curve]]. |
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==Dam creation== |
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===Common purposes=== |
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{| class="wikitable" |
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! Function |
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!Example |
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|- |
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!style="text-align:center"| Power generation |
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| [[Hydroelectric power]] is a major source of electricity in the world. Many countries have rivers with adequate water flow, that can be dammed for power generation purposes. For example, the [[Itaipu]] on the [[Paraná River]] in [[South America]] generates 14 [[Watt|GW]] and supplied 93% of the energy consumed by [[Paraguay]] and 20% of that consumed by [[Brazil]] as of 2005. |
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|- |
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!style="text-align:center"| Water supply |
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|Many urban areas of the world are supplied with water abstracted from rivers pent up behind low dams or weirs. Examples include [[London]] - with water from the [[River Thames]] and [[Chester]] with water taken from the [[River Dee, Wales|River Dee]]. Other major sources include deep upland reservoirs contained by high dams across deep valleys such as the [[Claerwen]] series of dams and reservoirs. |
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|- |
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!style="text-align:center"| Stabilize water flow / irrigation |
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| Dams are often used to control and stabilize water ''flow'', often for [[agriculture|agricultural]] purposes and [[irrigation]].<ref>{{cite web|title=The Impact of Agricultural Development on Aquatic Systems and its Effect on the Epidemiology of Schistosomes in Rhodesia|quote=Recently, agricultural development has concentrated on soil and water conservation and resulted in the construction of a multitude of dams of various capacities which tend to stabilize water flow in rivers and provide a significant amount of permanent and stable bodies of water.|url=http://www.iucn.org/themes/ceesp/Publications/SL/CT/Chapter%208A%20-%20The%20Careless%20Technology.pdf|work=IUCN|language=English|format=PDF}}</ref> Others such as the [[Berg Strait]] dam can help to stabilize or restore the water ''levels'' of inland lakes and seas, in this case the [[Aral Sea]].<ref>{{cite web|url=http://www.fao.org/ag/agl/aglw/aquastat/countries/kazakhstan/index.stm|title=Kazakhstan|work=Land and Water Development Division|date=1998|accessdate=2007-08-01|quote=construction of a dam (Berg Strait) to stabilize and increase the level of the northern part of the Aral Sea.|format=HTML}}</ref> |
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|- |
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!style="text-align:center"| Flood prevention |
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| Dams such as the [[Blackwater dam]] of [[Webster, New Hampshire|Webster]], [[New Hampshire]] and the [[Delta Works]] are created with flood control in mind.<ref>{{cite web|url=http://www.nae.usace.army.mil/recreati/bwd/bwdfc.htm|title=Blackwater Dam|work=US Army Corps of Engineers|format=HTML|quote=The principal objective of the dam and reservoir is to protect downstream communities}}</ref> |
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|- |
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!style="text-align:center"| Land reclamation |
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| Dams (often called [[Dike (construction)|dykes]] or [[levee]]s in this context) are used to prevent ingress of water to an area that would otherwise be submerged, allowing its [[Land reclamation|reclamation]] for human use. |
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|- |
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!style="text-align:center"| Water diversion |
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| See: [[diversion dam]]. |
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|} |
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===Siting (location)=== |
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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. |
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Significant other [[engineering]] and [[engineering geology]] considerations when building a dam include: |
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* [[Permeability (fluid)|permeability]] of the surrounding rock or soil |
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* [[earthquake]] faults |
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* [[landslides]] and [[slope stability]] |
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* peak flood flows |
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* reservoir silting |
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* [[Environmental impacts of dams|environmental impacts]] on river fisheries, forests and wildlife (see also [[fish ladder]]) |
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* impacts on human habitations |
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* compensation for land being flooded as well as population resettlement |
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* removal of toxic materials and buildings from the proposed reservoir area |
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===Impact assessment=== |
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Impact is assessed in several ways: the benefits to human society arising from the dam (agriculture, water, damage prevention and power), harm or benefits to nature and wildlife (especially fish and [[endangered species|rare species]]), impact on the geology of an area - whether the change to water flow and levels will increase or decrease stability, and the disruption to human lives (relocation, loss of [[archeological]] or cultural matters underwater). |
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====Environmental impact==== |
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[[Image:Dam-pollution.JPG|thumb|250px|Wood and garbage accumulated because of a dam]] |
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{{main|Environmental impacts of dams}} |
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Dams affect 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; in addition often it is water from the colder (lower) layer which is released downstream, and this may have a different [[dissolved oxygen]] content than before. Organisms depending upon a regular cycle of temperatures may be unable to adapt; the balance of other [[fauna]] (especially [[plant life]] and [[microscopic fauna]]) may be affected by the change of oxygen content. |
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Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks; for example, the daily cyclic flow variation caused by the [[Glen Canyon Dam]] was a contributor to [[sand bar]] [[erosion]]. |
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Older dams often lack a [[fish ladder]], which keeps many fish from moving up stream to their natural breeding grounds, causing failure of breeding cycles or blocking of migration paths.<ref>[http://bataviansforahealthyriver.org/dam_fact.htm Dam Fact Sheet<!-- Bot generated title -->]</ref> Even the presence of a fish ladder does not always prevent a reduction in fish reaching the [[spawn (biology)|spawning]] grounds upstream. In some areas, young fish ("smolt") are transported downstream by [[barge]] during parts of the year. Turbine and power-plant designs that have a lower impact upon aquatic life are an active area of research. |
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A large dam can cause the loss of entire [[ecosphere]]s, including [[endangered species|endangered]] and [[List of cryptids|undiscovered]] species in the area, and the replacement of the original environment by a new inland lake. |
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Depending upon the circumstances, a dam can either reduce or increase the net production of [[greenhouse gas]]es. An '''increase''' can occur if the reservoir created by the dam itself acts as a source of substantial amounts of potent [[greenhouse gas]]es ([[methane]] and [[carbon dioxide]]) due to plant material in flooded areas decaying in an [[Hypoxia (environmental)|anaerobic]] environment. According to the [[World Commission on Dams]] report (Dams And Development), when the reservoir is relatively large and no prior clearing of forest in the flooded area was undertaken, greenhouse gas emissions from the reservoir could be higher than those of a conventional oil-fired thermal generation plant.<ref>[http://www.newscientist.com/article.ns?id=dn7046 Hydroelectric power's dirty secret revealed - earth - 24 February 2005 - New Scientist<!-- Bot generated title -->]</ref> A '''decrease''' can occur if the dam is used in place of traditional power generation, since electricity produced from hydroelectric generation does not give rise to any [[flue gas emissions from fossil fuel combustion]] (including [[sulfur dioxide]], [[nitric oxide]], [[carbon monoxide]], dust, and [[mercury (element)|mercury]] from [[coal]]). |
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Large lakes formed behind dams have been indicated as contributing to earthquakes, due to changes in loading and/or the height of the water table. |
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====Human social impact==== |
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The impact on human society is also significant. For example, the [[Three Gorges Dam]] on the [[Yangtze River]] in [[China]], is more than five times the size of the [[Hoover Dam]] ([[United States|U.S.]]) will create a reservoir 600 km long, to be used for hydro-power generation. Its construction required the loss of over a million people's homes and their mass relocation, the loss of many valuable archaeological and cultural sites, as well as significant ecological change.<ref name=Bbc060520>{{cite news |
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| publisher=china-embassy |
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| url=http://www.china-embassy.org/eng/zt/sxgc/t36502.htm |
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| title=Three Gorges dam wall completed |
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| date=[[20 May]] [[2006]] |
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| accessdate=2006-05-21 |
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}}</ref> It is estimated that to date, 40-80 million people worldwide have been physically displaced from their homes as a result of dam construction.<ref>[http://internationalrivers.org/en/way-forward/world-commission-dams/world-commission-dams-framework-brief-introduction World Commission on Dams Report]</ref> |
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====Economics==== |
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Construction of a hydroelectric plant requires a long lead-time for site studies, hydrological studies, and environmental impact assessment, and are large scale projects by comparison to traditional power generation based upon [[fossil fuel]]s. The number of sites that can be economically developed for hydroelectric production is limited; new sites tend to be far from population centers and usually require extensive [[power transmission]] lines. Hydroelectric generation can be vulnerable to major changes in the [[climate]], including variation of rainfall, ground and surface [[water level]]s, and glacial melt, causing additional expenditure for the extra capacity to ensure sufficient power is available in low water years. |
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Once completed, if it is well designed and maintained, a hydroelectric power source is usually comparatively cheap and reliable. It has no fuel and low escape risk, and as an [[alternative energy]] source it is cheaper than both nuclear and wind power.{{Fact|date=August 2007}} It is more easily regulated to store water as needed and generate high power levels on demand compared to [[wind power]], although dams have life expectancies while [[renewable energy|renewable energies]] do not. |
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==Dam failure== |
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[[Image:Teton Dam failure.jpg|thumb|right|The reservoir emptying through the failed [[Teton Dam]].]] |
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[[Image:International special sign for works and installations containing dangerous forces.svg|thumb|right|International special sign for works and installations containing dangerous forces]] |
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Dam failures are generally catastrophic if the structure is breached or significantly damaged. Routine [[Deformation Monitoring|deformation 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 [[grout]]ing - pressure pumping [[portland cement]] [[slurry]] into weak fractured rock. |
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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. |
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The main causes of dam failure include spillway design error ([[South Fork Dam]]), geological instability caused by changes to water levels during filling or poor surveying ([[Vajont Dam]], [[Malpasset]]), poor maintenance, especially of outlet pipes ([[Lawn Lake Dam]], [[Val di Stava Dam collapse]]), extreme rainfall ([[Shakidor Dam]]), and human, computer or design error ([[Buffalo Creek Flood]], [[Dale Dike Reservoir]], [[Taum Sauk pumped storage plant]]). |
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A notable case of deliberate dam failure (prior to the above ruling) was the [[United Kingdom|British]] [[Royal Air Force]] [[Dambusters]] raid on [[Germany]] in [[World War II]] (codenamed ''"[[Operation Chastise]]"''), in which three German dams were selected to be breached in order to have an impact on German infrastructure and manufacturing and power capabilities deriving from the [[Ruhr]] and [[Eder]] rivers. This raid later became the basis for several films. |
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{{See|List of dam failures}} |
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{{clear}} |
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==Notes== |
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{{reflist|2}} |
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==See also== |
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{{commons|Dam}} |
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{{wiktionarypar|dam}} |
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* [[Beaver]] a dam-building rodent |
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* [[Canal lock]] |
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* [[Operation Chastise|Dam Busters]] |
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* [[Delta Works]] |
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* [[List of reservoirs and dams]] |
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* [[List of world's tallest dams]] |
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* [[Splash dam]] |
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* [[Zuiderzee works]] |
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==External links== |
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* [http://www.projo.com/video/?nvid=298705 Providence Journal video of the Blackstone River] |
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*[http://www.icold-cigb.net/ International Commission on Large Dams (ICOLD)] |
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*[http://en.structurae.de/structures/stype/index.cfm?id=3 Structurae: Dams and Retaining Structures] |
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*[http://infoplease.com/ipa/A0001334.html The World's Largest Dams] |
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*[http://www.planete-tp.com/en/rubrique.php3?id_rubrique=12 Dams on Planete-TP] |
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*[http://www.usbr.gov/pmts/hydraulics_lab/pubs/manuals/SmallDams.pdf "Design of Small Dams", US Bureau of Reclamation, 65MB pdf] |
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*[http://www.canadiangeographic.ca/magazine/ND05/indepth/environment.asp "Dam science"] [[Canadian Geographic]] |
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*[http://www.internationalrivers.org/ International Rivers] |
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*[http://www.dam-research.org/ Dam Research] |
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*[http://www.asphaltcoredams.com/ asphaltcoredams.com] |
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*[http://content.lib.washington.edu/cgi-bin/queryresults.exe?CISOOP=adv&CISORESTMP=%2Fsite-templates%2Fsearch_results-sub.html&CISOVIEWTMP=%2Fsite-templates%2Fitem_viewer.html&CISOMODE=thumb&CISOGRID=thumbnail%2CA%2C1%3Btitle%2CA%2C1%3Bsubjec%2CA%2C0%3Bdescri%2C200%2C0%3B0%2CA%2C0%3B10&CISOBIB=title%2CA%2C1%2CN%3Bsubjec%2CA%2C0%2CN%3Bdescri%2CK%2C0%2CN%3B0%2CA%2C0%2CN%3B0%2CA%2C0%2CN%3B10&CISOTHUMB=3%2C5&CISOTITLE=10&CISOPARM=%2Ffishimages%3Asubjec%3Adams&x=29&y=3/ University of Washington Freshwater and Marine Image Bank Collection] |
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*[http://www.ppl.nl/index.php?option=com_wrapper&view=wrapper&Itemid=82 Bibliography on Water Resources and International Law] Peace Palace Library |
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{{Global warming}} |
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Revision as of 14:26, 17 November 2008
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