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This article is about structures for water impoundment. For other uses, seeDam (disambiguation).

TheEdersee DaminHesse,Germany

Adamis a barrier that stops or restricts the flow ofsurface wateror underground streams.Reservoirscreated by dams not only suppress floods but also provide water for activities such asirrigation,human consumption,industrial use,aquaculture,andnavigability.Hydropoweris often used in conjunction with dams to generate electricity. A dam can also be used to collect or store water which can be evenly distributed between locations. Dams generally serve the primary purpose of retaining water, while other structures such asfloodgatesorlevees(also known asdikes) are used to manage or prevent water flow into specific land regions.

The worddamcan be traced back toMiddle English,[1]and before that, fromMiddle Dutch,as seen in the names of many old cities,[2]such asAmsterdamandRotterdam.

Ancient dams were built in Mesopotamia and the Middle East for water control. The earliest known dam is theJawa DaminJordan,dating to 3,000 BC. Egyptians also built dams, such as Sadd-el-Kafara Dam for flood control. In modern-day India, Dholavira had an intricate water-management system with 16 reservoirs and dams. The Great Dam of Marib in Yemen, built between 1750 and 1700 BC, was an engineering wonder, and Eflatun Pinar, a Hittite dam and spring temple in Turkey, dates to the 15th and 13th centuries BC. TheKallanai Damin South India, built in the 2nd century AD, is one of the oldest water regulating structures still in use.

Roman engineers built dams with advanced techniques and materials, such as hydraulic mortar and Roman concrete, which allowed for larger structures. They introduced reservoir dams, arch-gravity dams, arch dams, buttress dams, and multiple arch buttress dams. In Iran, bridge dams were used for hydropower and water-raising mechanisms.

During the Middle Ages, dams were built in the Netherlands to regulate water levels and prevent sea intrusion. In the 19th century, large-scale arch dams were constructed around the British Empire, marking advances in dam engineering techniques. The era of large dams began with the construction of the Aswan Low Dam in Egypt in 1902. The Hoover Dam, a massive concrete arch-gravity dam, was built between 1931 and 1936 on the Colorado River. By 1997, there were an estimated 800,000 dams worldwide, with some 40,000 of them over 15 meters high.

History

Ancient dams

Early dam building took place inMesopotamiaand theMiddle East.Dams were used to control water levels, for Mesopotamia's weather affected theTigrisandEuphratesRivers.

The earliest known dam is theJawa DaminJordan,100 kilometres (62 mi) northeast of the capitalAmman.This gravity dam featured an originally 9-metre-high (30 ft) and 1 m-wide (3.3 ft) stone wall, supported by a 50 m-wide (160 ft) earthen rampart. The structure is dated to 3000 BC.[3][4]However, the oldest continuously operational dam isLake Homs Dam,built inSyriabetween 1319-1304 BC.[5]

TheAncient EgyptianSadd-el-Kafara Damat Wadi Al-Garawi, about 25 km (16 mi) south ofCairo,was 102 m (335 ft) long at its base and 87 m (285 ft) wide. The structure was built around 2800[6]or 2600 BC[7]as adiversion damfor flood control, but was destroyed by heavy rain during construction or shortly afterwards.[6][7]During theTwelfth Dynastyin the 19th century BC, the Pharaohs Senosert III,Amenemhat III,andAmenemhat IVdug a canal 16 km (9.9 mi) long linking theFayum Depressionto theNilein Middle Egypt. Two dams called Ha-Uar running east–west were built to retain water during the annual flood and then release it to surrounding lands. The lake calledMer-werorLake Moeriscovered 1,700 km2(660 sq mi) and is known today as Birket Qarun.[8]

By the mid-late third millennium BC, an intricate water-management system inDholavirain modern-dayIndiawas built. The system included 16 reservoirs, dams and various channels for collecting water and storing it.[9]

One of the engineering wonders of the ancient world was theGreat Dam of MaribinYemen.Initiated sometime between 1750 and 1700 BC, it was made of packed earth – triangular in cross-section, 580 m (1,900 ft) in length and originally 4 m (13 ft) high – running between two groups of rocks on either side, to which it was linked by substantial stonework. Repairs were carried out during various periods, most importantly around 750 BC, and 250 years later the dam height was increased to 7 m (23 ft). After the end of theKingdom of Saba,the dam fell under the control of theḤimyarites(c. 115 BC) who undertook further improvements, creating a structure 14 m (46 ft) high, with five spillways, two masonry-reinforced sluices, a settling pond, and a 1,000 m (3,300 ft) canal to a distribution tank. These works were not finished until 325 AD when the dam permitted the irrigation of 25,000 acres (100 km2).

Eflatun Pınaris aHittitedam and spring temple nearKonya,Turkey. It is thought to date from the Hittite empire between the 15th and 13th centuries BC.

TheKallanaiis constructed of unhewn stone, over 300 m (980 ft) long, 4.5 m (15 ft) high and 20 m (66 ft) wide, across the main stream of theKaveriRiver inTamil Nadu,South India.The basic structure dates to the 2nd century AD[10]and is considered one of the oldest water diversion or water regulating structures still in use.[11]The purpose of the dam was to divert the waters of the Kaveri across the fertile delta region for irrigation via canals.[12]

Du Jiang Yanis the oldest survivingirrigationsystem in China that included a dam that directed waterflow. It was finished in 251 BC. A large earthen dam, made bySunshu Ao,theprime ministerofChu (state),flooded a valley in modern-day northernAnhuiProvince that created an enormous irrigation reservoir (100 km (62 mi) in circumference), a reservoir that is still present today.[13]

Roman engineering

TheRoman dam at CornalvoinSpainhas been in use for almost two millennia.

Roman damconstruction was characterized by "the Romans' ability to plan and organize engineering construction on a grand scale."[14]Roman planners introduced the then-novel concept of largereservoirdams which could secure a permanentwater supplyfor urban settlements over the dry season.[15]Their pioneering use of water-proof hydraulicmortarand particularlyRoman concreteallowed for much larger dam structures than previously built,[14]such as theLake Homs Dam,possibly the largest water barrier to that date,[16]and theHarbaqa Dam,both inRoman Syria.The highest Roman dam was theSubiaco DamnearRome;its record height of 50 m (160 ft) remained unsurpassed until its accidental destruction in 1305.[17]

Roman engineers made routine use of ancient standard designs like embankment dams and masonry gravity dams.[18]Apart from that, they displayed a high degree of inventiveness, introducing most of the other basic dam designs which had been unknown until then. These includearch-gravity dams,[19]arch dams,[20]buttress dams[21]andmultiple arch buttress dams,[22]all of which were known and employed by the 2nd century AD (seeList of Roman dams). Roman workforces also were the first to build dam bridges, such as theBridge of Valerianin Iran.[23]

Remains of theBand-e Kaisardam, built by the Romans in the 3rd century AD

InIran,bridge dams such as theBand-e Kaisarwere used to providehydropowerthroughwater wheels,which often powered water-raising mechanisms. One of the first was the Roman-built dam bridge inDezful,[24]which could raise water 50cubits(c. 23 m) to supply the town. Alsodiversion damswere known.[25]Millingdams were introduced which theMuslim engineerscalled thePul-i-Bulaiti.The first was built at Shustar on the RiverKarun,Iran, and many of these were later built in other parts of theIslamic world.[25]Water was conducted from the back of the dam through a large pipe to drive a water wheel andwatermill.[26]In the 10th century,Al-Muqaddasidescribed several dams in Persia. He reported that one inAhwazwas more than 910 m (3,000 ft) long,[27]and that it had many water-wheels raising the water intoaqueductsthrough which it flowed into reservoirs of the city.[28]Another one, the Band-i-Amir Dam, provided irrigation for 300 villages.[27]

Middle Ages

14th c. Shāh Abbās arch dam

Shāh Abbās Arch (Persian: طاق شاه عباس), also known asKurit Dam,is the thinnest arch dam in the world and one of the oldest arch dams in Asia. It was constructed some 700 years ago inTabas county,SouthKhorasan Province,Iran.It stands 60 meters tall, and in crest is a one meter width. Some historians believe the dam was built by Shāh Abbās I, whereas others believe that he repaired it.

In theNetherlands,a low-lying country, dams were often built to block rivers to regulate the water level and to prevent the sea from entering the marshlands. 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 influenced Dutch place names. The present Dutch capital,Amsterdam(old nameAmstelredam), started with a dam on the riverAmstelin the late 12th century, andRotterdambegan with a dam on the riverRotte,a minor tributary of theNieuwe Maas.The central square of Amsterdam, covering the original site of the 800-year-old dam, still carries the nameDam Square.

Industrial revolution

An engraving of theRideau Canallocks atBytown

The Romans were the first to buildarch dams,where thereaction forcesfrom the abutment stabilizes the structure from the externalhydrostatic pressure,but it was only in the 19th century that the engineering skills and construction materials available were capable of building the first large-scale arch dams.

Three pioneering arch dams were built around theBritish Empirein the early 19th century. Henry Russel of theRoyal Engineersoversaw the construction of theMir Alam damin 1804 to supply water to the city ofHyderabad(it is still in use today). It had a height of 12 m (39 ft) and consisted of 21 arches of variable span.[29]

In the 1820s and 30s, Lieutenant-ColonelJohn Bysupervised the construction of theRideau CanalinCanadanear modern-dayOttawaand built a series of curved masonry dams as part of the waterway system. In particular, theJones Falls Dam,built byJohn Redpath,was completed in 1832 as the largest dam inNorth Americaand an engineering marvel. In order to keep the water in control during construction, twosluices,artificial channels for conducting water, were kept open in the dam. The first was near the base of the dam on its east side. A second sluice was put in on the west side of the dam, about 20 ft (6.1 m) above the base. To make the switch from the lower to upper sluice, the outlet of Sand Lake was blocked off.[30]

Masonry arch wall,Parramatta,New South Wales,the first engineered dam built in Australia

Hunts Creek near the city ofParramatta,Australia,was dammed in the 1850s, to cater to the demand for water from the growing population of the city. The masonryarch damwall was designed by Lieutenant Percy Simpson who was influenced by the advances in dam engineering techniques made by theRoyal EngineersinIndia.The dam cost £17,000 and was completed in 1856 as the first engineered dam built in Australia, and the second arch dam in the world built to mathematical specifications.[31]

The first such dam was opened two years earlier inFrance.It was the first French arch dam of theindustrial era,and it was built by François Zola in the municipality ofAix-en-Provenceto improve the supply of water after the1832 cholera outbreakdevastated the area. Afterroyal approvalwas granted in 1844, the dam was constructed over the following decade. Its construction was carried out on the basis of the mathematical results of scientific stress analysis.

The 75-miles dam nearWarwick,Australia, was possibly the world's first concrete arch dam. Designed byHenry Charles Stanleyin 1880 with an overflow spillway and a special water outlet, it was eventually heightened to 10 m (33 ft).

In the latter half of the nineteenth century, significant advances in the scientific theory of masonry dam design were made. This transformed dam design from an art based on empirical methodology to a profession based on a rigorously applied scientific theoretical framework. This new emphasis was centered around the engineering faculties of universities in France and in the United Kingdom.William John Macquorn Rankineat theUniversity of Glasgowpioneered the theoretical understanding of dam structures in his 1857 paperOn the Stability of Loose Earth.Rankine theoryprovided a good understanding of the principles behind dam design.[32]In France, J. Augustin Tortene de Sazilly explained the mechanics of vertically faced masonry gravity dams, and Zola's dam was the first to be built on the basis of these principles.[33]

Modern era

TheHoover DambyAnsel Adams,1942

The era of large dams was initiated with the construction of theAswan Low Damin Egypt in 1902, a gravitymasonrybuttress damon theNile River.Following their 1882invasion and occupation of Egypt,the British began construction in 1898. The project was designed by SirWilliam Willcocksand involved several eminent engineers of the time, including SirBenjamin Bakerand SirJohn Aird,whose firm,John Aird & Co.,was the main contractor.[34][35]Capital and financing were furnished byErnest Cassel.[36]When initially constructed between 1899 and 1902, nothing of its scale had ever before been attempted;[37]on completion, it was the largest masonry dam in the world.[38]

TheHoover Damis a massive concretearch-gravity dam,constructed in theBlack Canyonof theColorado River,on the border between the US states ofArizonaandNevadabetween 1931 and 1936 during theGreat Depression.In 1928, Congress authorized the project to build a dam that would control floods, provide irrigation water and producehydroelectric power.The winning bid to build the dam was submitted by a consortium calledSix Companies, Inc.Such a large concrete structure had never been built before, and some of the techniques were unproven. The torrid summer weather and the lack of facilities near the site also presented difficulties. Nevertheless, Six Companies turned over the dam to the federal government on 1 March 1936, more than two years ahead of schedule.[39]

By 1997, there were an estimated 800,000 dams worldwide, some 40,000 of them over 15 m (49 ft) high.[40]In 2014, scholars from theUniversity of Oxfordpublished a study of the cost of large dams – based on the largest existing dataset – documenting significant cost overruns for a majority of dams and questioning whether benefits typically offset costs for such dams.[41]

Types

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

By structure

Based on structure and material used, dams are classified as easily created without materials,arch-gravity dams,embankment damsormasonry dams,with several subtypes.

Arch dams

Main article:Arch dam
Gordon Dam,Tasmania,is anarch dam.

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 normalhydrostatic pressurebetween verticalcantileverandarch actionwill depend upon thestiffnessof the dam in a vertical and horizontal direction. When the upstream face is sloped the distribution is more complicated. Thenormalcomponent 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 theabutments(eitherbuttressorcanyonside wall) are more important. The most desirable place for an arch dam is a narrow canyon with steep side walls composed of sound rock.[42]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.

Daniel-Johnson Dam,Quebec,is a multiple-arch buttress dam.

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 constant and the variation in distance between the abutments at various levels is taken care of by varying the radii. Constant-radius dams are much less common than constant-angle dams.Parker Damon the Colorado River is a constant-angle arch dam.

A similar type is the double-curvature or thin-shell dam.Wildhorse DamnearMountain 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, as for example theDaniel-Johnson Dam,Québec, Canada. The multiple-arch dam does not require as many buttresses as the hollow gravity type but requires a good rock foundation because the buttress loads are heavy.

Gravity dams

Main article:Gravity dam
TheGrand Coulee Damis an example of a solid gravity dam.

In a gravity dam, the force that holds the dam in place against the push from the water is Earth's gravity pulling down on the mass of the dam.[43]The water presses laterally (downstream) on the dam, tending to overturn the dam by rotating about its toe (a point at the bottom downstream side of the dam). The dam's weight counteracts that force, tending to rotate the dam the other way about its toe. The designer ensures that the dam is heavy enough that the dam's weight wins that contest. In engineering terms, that is true whenever theresultantof the forces of gravity acting on the dam and water pressure on the dam acts in a line that passes upstream of the toe of the dam.[citation needed]The designer tries to shape the dam so if one were to consider the part of the dam above any particular height to be a whole dam itself, that dam also would be held in place by gravity, i.e., there is no tension in the upstream face of the dam holding the top of the dam down. The designer does this because it is usually more practical to make a dam of material essentially just piled up than to make the material stick together against vertical tension.[citation needed]The shape that prevents tension in the upstream face also eliminates a balancing compression stress in the downstream face, providing additional economy.

For this type of dam, it is essential to have an impervious foundation with high bearing strength. Permeable foundations have a greater likelihood of generating uplift pressures under the dam. Uplift pressures are hydrostatic pressures caused by the water pressure of the reservoir pushing up against the bottom of the dam. If large enough uplift pressures are generated there is a risk of destabilizing the concrete gravity dam.[44][citation needed]

On a suitable site, a gravity dam can prove to be a better alternative to other types of dams. When built on a solid foundation, the gravity dam probably represents the best-developed example of dam building. Since the fear offloodis a strong motivator in many regions, gravity dams are built in some instances where an arch dam would have been more economical.

Gravity dams are classified as "solid" or "hollow" and are generally made of either concrete or masonry. The solid form is the more widely used of the two, though the hollow dam is frequently more economical to construct.Grand Coulee Damis a solid gravity dam andBraddock Locks & Damis a hollow gravity dam.[citation needed]

Arch-gravity dams

TheHoover Damis an example of an arch-gravity dam.
Main article:Arch-gravity dam

A gravity dam can be combined with an arch dam into anarch-gravity damfor areas with massive amounts of water flow but less material available for a pure gravity dam. The inward compression of the dam by the water reduces the lateral (horizontal) force acting on the dam. Thus, the gravitational force required by the dam is lessened, i.e., the dam does not need to be so massive. This enables thinner dams and saves resources.

Barrages

TheKoshi BarrageofNepal
Main article:Barrage dams

A barrage dam is a special kind of dam that consists of a line of large gates that can be opened or closed to control the amount of water passing the dam. The gates are set between flanking piers which are responsible for supporting the water load, and are often used to control and stabilize water flow for irrigation systems. An example of this type of dam is the now-decommissionedRed Bluff Diversion Damon theSacramento RivernearRed Bluff, California.

Barrages that are built at the mouths of rivers or lagoons to preventtidal incursionsor use the tidal flow fortidal powerare known astidal barrages.[45]

Embankment dams

Chatuge Damis an earthen embankment dam inNorth Carolina
Main article:Embankment dam

Embankment dams are made ofcompactedearth, and are of two main types: rock-fill and earth-fill. Like concrete gravity dams, embankment dams rely on their weight to hold back the force of water.

Fixed-crest dams

See also:Low head dam

A fixed-crest dam is a concrete barrier across a river.[46]Fixed-crest dams are designed to maintain depth in the channel for navigation.[47]They pose risks to boaters who may travel over them, as they are hard to spot from the water and create induced currents that are difficult to escape.[48]

By size

There is variability, both worldwide and within individual countries, such as in the United States, in how dams of different sizes are categorized. Dam size influences construction, repair, andremovalcosts and affects the dams' potential range and magnitude of environmental disturbances.[49]

Large dams

TheInternational Commission on Large Dams(ICOLD) defines a "large dam" as "A dam with a height of 15 m (49 ft) or greater from lowest foundation to crest or a dam between 5 m (16 ft) metres and 15 metres impounding more than 3 million cubic metres (2,400acre⋅ft) ".[50]"Major dams" are over 150 m (490 ft) in height.[51]TheReport of the World Commission on Damsalso includes in the "large" category, dams which are between 5 and 15 m (16 and 49 ft) high with a reservoir capacity of more than 3 million cubic metres (2,400acre⋅ft).[45]Hydropowerdams can be classified as either "high-head" (greater than 30 m in height) or "low-head" (less than 30 m in height).[52]

As of 2021,ICOLD's World Register of Dams contains 58,700 large dam records.[53]: 6 The tallest dam in the world is the 305 m-high (1,001 ft)Jinping-I DaminChina.[54]

Small dams

Dam in Europe at Autumn as viewed from FPV drone.

As with large dams, small dams have multiple uses, such as, but not limited to,hydropowerproduction, flood protection, and water storage. Small dams can be particularly useful on farms to capture runoff for later use, for example, during the dry season.[55]Small scale dams have the potential to generate benefits without displacing people as well,[56]and small, decentralised hydroelectric dams can aid rural development in developing countries.[57]In the United States alone, there are approximately 2,000,000 or more "small" dams that are not included in theArmy Corps of EngineersNational Inventory of dams.[58]Records of small dams are kept by state regulatory agencies and therefore information about small dams is dispersed and uneven in geographic coverage.[52]

Countries worldwide consider small hydropower plants (SHPs) important for their energy strategies, and there has been a notable increase in interest in SHPs.[59]Couto and Olden (2018)[59]conducted a global study and found 82,891 small hydropower plants (SHPs) operating or under construction. Technical definitions of SHPs, such as their maximum generation capacity, dam height, reservoir area, etc., vary by country.

Non-jurisdictional dams

A dam is non-jurisdictional when its size (usually "small" ) excludes it from being subject to certain legal regulations. The technical criteria for categorising a dam as "jurisdictional" or "non-jurisdictional" varies by location. In the United States, each state defines what constitutes a non-jurisdictional dam. In the state ofColoradoa non-jurisdictional dam is defined as a dam creating areservoirwith a capacity of 100 acre-feet or less and a surface area of 20 acres or less and with a height measured as defined in Rules 4.2.5.1. and 4.2.19 of 10 feet or less.[60]In contrast, the state ofNew Mexicodefines a jurisdictional dam as 25 feet or greater in height and storing more than 15 acre-feet or a dam that stores 50 acre-feet or greater and is six feet or more in height (section 72-5-32 NMSA), suggesting that dams that do not meet these requirements are non-jurisdictional.[61]Most US dams, 2.41 million of a total of 2.5 million dams, are not under the jurisdiction of any public agency (i.e., they are non-jurisdictional), nor are they listed on theNational Inventory of Dams(NID).[62]

Small dams incur risks similar to large dams. However, the absence of regulation (unlike more regulated large dams) and of an inventory of small dams (i.e., those that are non-jurisdictional) can lead to significant risks for both humans and ecosystems.[62]For example, according to theUS National Park Service(NPS), "Non-jurisdictional—means a structure which does not meet the minimum criteria, as listed in the Federal Guidelines for Dam Safety, to be included in dam safety programs. The non-jurisdictional structure does not receive a hazard classification and is not considered for any further requirements or activities under the NPS dam safety program."[63]Small dams can be dangerous individually (i.e., they can fail), but also collectively,[64]as an aggregation of small dams along a river or within a geographic area can multiply risks. Graham's 1999 study[65]of US dam failures resulting in fatalities from 1960 to 1998 concluded that the failure of dams between 6.1 and 15 m high (typical height range of smaller dams[66]) caused 86% of the deaths, and the failure of dams less than 6.1 m high caused 2% of the deaths. Non-jurisdictional dams may pose hazards because their design, construction, maintenance, and surveillance is unregulated.[66]Scholars have noted that more research is needed to better understand the environmental impact of small dams[59](e.g., their potential to alter the flow, temperature, sediment[67][52]and plant and animal diversity of a river).

By use

Saddle dam

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 adiketo prevent inundation of nearby land. Dikes are commonly used for reclamation of arable land from a shallow lake, similar to alevee,which is a wall or embankment built along a river or stream to protect adjacent land from flooding.

Weir

Main article:Weir

A weir (sometimes called an "overflow dam" ) is a small dam that is often used in a river channel to create an impoundment lake for water abstraction purposes and which can also be used for flow measurement or retardation.

Check dam

Main article:Check dam

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

Dry dam

Main article:Dry dam

A dry dam, also known as a flood retarding structure, is 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 dam

Main article:Diversionary dam

A diversionary dam is designed to divert all or a portion of the flow of a river from its natural course. The water may be redirected into a canal or tunnel for irrigation and/or hydroelectric power production.

Underground dam

Underground dams are used to trapgroundwaterand store all or most of it below the surface for extended use in a localized area. In some cases, they are also built to prevent saltwater from intruding into a freshwater aquifer. Underground dams are typically constructed in areas where water resources are minimal and need to be efficiently stored, such as in deserts and on islands like theFukuzato DaminOkinawa,Japan. They are most common innortheastern Africaand the arid areas ofBrazilwhile also being used in thesouthwestern United States,Mexico, India, Germany, Italy, Greece, France and Japan.[68]

There are two types of underground dams: "sub-surface" and a "sand-storage". A sub-surface dam is built across anaquiferor drainage route from an impervious layer (such as solid bedrock) up to just below the surface. They can be constructed of a variety of materials to include bricks, stones, concrete, steel or PVC. Once built, the water stored behind the dam raises the water table and is then extracted with wells. A sand-storage dam is a weir built in stages across a stream orwadi.It must be strong, as floods will wash over its crest. Over time, sand accumulates in layers behind the dam, which helps store water and, most importantly, preventevaporation.The stored water can be extracted with a well, through the dam body, or by means of a drain pipe.[69]

Tailings dam

Main article:Tailings dam
Bituminous geomembraneinstallation on a mine tailings storage facility.

A tailings dam is typically an earth-fill embankment dam used to storetailings,which are produced duringminingoperations after separating the valuable fraction from the uneconomic fraction of anore.Conventional water retention dams can serve this purpose, but due to cost, a tailings dam is more viable. Unlike water retention dams, a tailings dam is raised in succession throughout the life of the particular mine. Typically, a base or starter dam is constructed, and as it fills with a mixture of tailings and water, it is raised. Material used to raise the dam can include the tailings (depending on their size) along with soil.[70]

There are three raised tailings dam designs, the "upstream", "downstream", and "centerline", named according to the movement of the crest during raising. The specific design used is dependent upontopography,geology, climate, the type of tailings, and cost. An upstream tailings dam consists oftrapezoidalembankments being constructed on top but toe to crest of another, moving the crest further upstream. This creates a relatively flat downstream side and a jagged upstream side which is supported by tailingsslurryin the impoundment. The downstream design refers to the successive raising of the embankment that positions the fill and crest further downstream. A centerlined dam has sequential embankment dams constructed directly on top of another while fill is placed on the downstream side for support and slurry supports the upstream side.[71][72]

Because tailings dams often store toxic chemicals from the mining process, modern designs incorporate an imperviousgeomembraneliner to prevent seepage.[73]Water/slurry levels in the tailings pond must be managed for stability and environmental purposes as well.[72]

By material

Steel dams

Main article:Steel dam
Redridge Steel Dam,built 1905,Michigan

Asteel damis a type of dam briefly experimented with around the start of the 20th century which uses steel plating (at an angle) and load-bearing beams as the structure. Intended as permanent structures, steel dams were an (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.

Timber dams

A timber crib dam in Michigan, 1978

Timberdams 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 because of their relatively short lifespan and the 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,cementis 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 theNorth AmericanWest, 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 alog houseand the interior filled with earth or rubble. The heavy crib structure supported the dam's face and the weight of the water.Splash damswere timber crib dams used to help floatlogsdownstream in the late 19th and early 20th centuries.

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

Other types

Cofferdams

Main article:Cofferdam
A cofferdam during the construction oflocksat the Montgomery Point Lock and Dam

Acofferdamis a barrier, usually temporary, constructed to exclude water from an area that is normally submerged. Made commonly of wood,concrete,orsteelsheetpiling,cofferdams are used to allow construction on thefoundationof permanent dams, bridges, and similar structures. When the project is completed, the cofferdam will usually be demolished or removed unless the area requires continuous maintenance. (See alsocausewayandretaining wall.)

Common uses for cofferdams include the construction and repair of offshore 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 and allowing a dry work environment below the surface.

Natural dams

Dams can also be created by natural geological forces.Lava damsare formed when lava flows, oftenbasaltic,intercept the path of a stream or lake outlet, resulting in the creation of a natural impoundment. An example would be the eruptions of theUinkaret volcanic fieldabout 1.8 million–10,000 years ago, which created lava dams on theColorado Riverin northernArizonain theUnited States.The largest such lake grew to about 800 km (500 mi) in length before the failure of its dam.Glacial activitycan also form natural dams, such as the damming of theClark ForkinMontanaby theCordilleran Ice Sheet,which formed the 7,780 km2(3,000 sq mi)Glacial Lake Missoulanear the end of the last Ice Age.Morainedeposits left behind by glaciers can also dam rivers to form lakes, such as atFlathead Lake,also in Montana (seeMoraine-dammed lake).

Natural disasters such as earthquakes and landslides frequently createlandslide damsin mountainous regions with unstable local geology. Historical examples include theUsoi DaminTajikistan,which blocks theMurghab Riverto createSarez Lake.At 560 m (1,840 ft) high, it is the tallest dam in the world, including both natural and man-made dams. A more recent example would be the creation ofAttabad Lakeby a landslide onPakistan'sHunza River.

Natural dams often pose significant hazards to human settlements and infrastructure. The resulting lakes often flood inhabited areas, while a catastrophic failure of the dam could cause even greater damage, such as the failure of westernWyoming'sGros Ventre landslidein 1927, which wiped out the town ofKellyresulting in the deaths of six people.

Beaver dams
Main article:Beaver dam

Beaverscreate 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.

Construction elements

Power generation plant

Main article:Hydroelectricity
Hydroelectric dam in cross section
Hydraulic turbineandelectric generator

As of 2005,hydroelectric power, mostly from dams, supplies some 19% of the world's electricity, and over 63% ofrenewable energy.[74]Much of this is generated by large dams, althoughChinauses small-scale hydro generation on a wide scale and is responsible for about 50% of world use of this type of power.[74]

Most hydroelectric power comes from thepotential energyof dammed water driving awater turbineandgenerator;to boost the power generation capabilities of a dam, the water may be run through a large pipe called apenstockbefore theturbine.A variant on this simple model usespumped-storage hydroelectricityto produce electricity to match periods of high and low demand, by moving water betweenreservoirsat 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. (For example, seeDinorwig Power Station.)

Spillways

Main article:Spillway
Spillway onLlyn Briannedam,Wales,soon after first fill

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 havefloodgatesdesigned to control the flow through the spillway. There are several types of 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 plugspillway "is a low embankment designed to be overtopped and washed away in the event of a large flood. The elements of a fuse plug are independent free-standing blocks, set side by side which work without any remote control. They allow increasing 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 weirs at times by allowing overflow in common floods.

A spillway can be graduallyerodedby water flow, includingcavitationorturbulenceof the water flowing over the spillway, leading to its failure. It was the inadequate design of the spillway and installation of fish screens that led to the 1889 over-topping of theSouth Fork DaminJohnstown, Pennsylvania,resulting in theJohnstown Flood(the "great flood of 1889" ).[75]

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 anogeecurve.

Creation

Common purposes

Function Example
Power generation Hydroelectric poweris 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, theItaipu Damon theParaná RiverinSouth Americagenerates 14GWand supplied 93% of the energy consumed byParaguayand 20% of that consumed byBrazilas of 2005.
Water supply Many urban areas of the world are supplied with water taken from rivers pent up behind low dams or weirs. Examples includeLondon,with water from theRiver Thames,andChester,with water taken from theRiver Dee.Other major sources include deep upland reservoirs contained by high dams across deep valleys, such as theClaerwenseries of dams and reservoirs.
Stabilize water flow / irrigation Dams are often used to control and stabilize water flow, often foragriculturalpurposes andirrigation.[76]Others such as the Berg Strait Dam can help to stabilize or restore the water levels of inland lakes and seas, in this case, theAral Sea.[77]
Flood prevention TheKeenleyside Damon theColumbia River,Canada can store 8.76km3(2.10cu mi) of floodwater, and theDelta Worksprotects the Netherlands fromcoastal flooding.[78]
Land reclamation Dams (often calleddykesorleveesin this context) are used to prevent ingress of water to an area that would otherwise be submerged, allowing itsreclamationfor human use.
Water diversion A typically small dam used to divert water for irrigation, power generation, or other uses, with usually no other function. Occasionally, they are used to divert water to another drainage or reservoir to increase flow there and improve water use in that particular area. See:diversion dam.
Navigation Dams create deep reservoirs and can also vary the flow of water downstream. This can in return affect upstream and downstreamnavigationby altering the river's depth. Deeper water increases or creates freedom of movement for water vessels. Large dams can serve this purpose, but most often weirs andlocksare used.

Some of these purposes are conflicting, and the dam operator needs to make dynamic tradeoffs. For example, power generation and water supply would keep the reservoir high, whereas flood prevention would keep it low. Many dams in areas where precipitation fluctuates in an annual cycle will also see the reservoir fluctuate annually in an attempt to balance these different purposes. Dam management becomes a complex exercise amongst competing stakeholders.[79]

Location

The discharge ofTakato Dam

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 amasonrydam flanked by earth embankments. The current use of the land to be flooded should be dispensable.

Significant otherengineeringandengineering geologyconsiderations when building a dam include:

Impact assessment

Impact is assessed in several ways: the benefits to human society arising from the dam (agriculture, water, damage prevention and power), harm or benefit to nature and wildlife, 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 ofarcheologicalor cultural matters underwater).

Environmental impact

Main article:Environmental impacts of reservoirs
Wood and garbage accumulation due to a dam

Reservoirs held behind dams affect many ecological aspects of a river. Rivers topography and dynamics depend on a wide range of flows, whilst rivers below dams often experience long periods of very stable flow conditions or sawtooth flow patterns caused by releases followed by no releases. Water releases from a reservoir including that exiting a turbine usually contain very little suspended sediment, and this, in turn, can lead to scouring of river beds and loss of riverbanks; for example, the daily cyclic flow variation caused by theGlen Canyon Damwas a contributor tosand barerosion.

Older dams often lack afish ladder,which keeps many fish from moving upstream to their natural breeding grounds, causing failure of breeding cycles or blocking of migration paths.[80]Even fish ladders do not prevent a reduction in fish reaching thespawninggrounds upstream.[81]In some areas, young fish ( "smolt" ) are transported downstream bybargeduring parts of the year. Turbine and power-plant designs that have a lower impact upon aquatic life are an active area of research.

At the same time, however, some particular dams may contribute to the establishment of better conditions for some kinds of fish and other aquatic organisms. Studies have demonstrated the key role played by tributaries in the downstream direction from the main river impoundment, which influenced local environmental conditions and beta diversity patterns of each biological group.[82]Both replacement and richness differences contributed to high values of total beta diversity for fish (average = 0.77) and phytoplankton (average = 0.79), but their relative importance was more associated with the replacement component for both biological groups (average = 0.45 and 0.52, respectively).[82]A study conducted by de Almeida, R. A., Steiner, M.T.A and others found that, while some species declined in population by more than 30% after the building of the dam, others increased their population by 28%.[83]Such changes may be explained by the fact that the fish obtained "different feeding habits, with almost all species being found in more than one group.[83]

A large dam can cause the loss of entireecospheres,includingendangeredand undiscovered species in the area, and the replacement of the original environment by a new inland lake. As a result, the construction of dams have been opposed in various countries with some, such as Tasmania's Franklin Dam project, being cancelled following environmentalist campaigns.[84]

Large reservoirs formed behind dams have been indicated in the contribution ofseismic activity,due to changes in water load and/or the height of the water table. However, this is a mistaken assumption, because the relatively marginal stress attributed to the water load is orders of magnitude lesser than the force of an earthquake. The increased stress from the water load is insufficient to fracture the Earth's crust, and thus does not increase the severity of an earthquake.[85]

Dams are also found to influenceglobal warming.[86]The changing water levels in reservoirs are asource for greenhouse gaseslikemethane.[87]While dams and the water behind them cover only a small portion of earth's surface, they harbour biological activity that can produce large quantities of greenhouse gases.[88]

Human social impact

Dams' impact on human society is significant.Nick Cullatherargues inHungry World: America's Cold War Battle Against Poverty in Asiathat dam construction requiresthe stateto displace people in the name of thecommon good,and that it often leads to abuses of the masses by planners. He citesMorarji Desai,Interior Minister of India, in 1960 speaking to villagers upset about thePong Dam,who threatened to "release the waters" and drown the villagers if they did not cooperate.[89]

TheThree Gorges Damon theYangtze RiverinChinais more than five times the size of theHoover Dam(U.S.). It creates a reservoir 600 km (370 mi) long to be used forflood controland hydropower 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, and significant ecological change.[90]During the2010 China floods,the dam held back a what would have been adisastrous floodand the huge reservoir rose by 4 m (13 ft) overnight.[91]

In 2008, it was estimated that 40–80 million people worldwide have been displaced from their homes as a result of dam construction.[92]

Economics

Construction of ahydroelectric plantrequires a long lead time for site studies,hydrologicalstudies, andenvironmental impact assessments,and are large-scale projects in comparison to carbon-based power generation. 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 extensivepower transmissionlines. Hydroelectric generation can be vulnerable to major changes in theclimate,including variations inrainfall,ground and surfacewater levels,and glacial melt, causing additional expenditure for the extra capacity to ensure sufficient power is available in low-water years.

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 anclean energysource it is cheaper than both nuclear and wind power.[93]It is more easily regulated to store water as needed and generate high power levels on demand compared towind power.

Reservoir and dam improvements

Despite some positive effects, the construction of dams severely affects river ecosystems leading to degraded riverine ecosystems as part of the hydrological alteration.[94]One of the main ways to reduce the negative impacts of reservoirs and dams is to implement the newest nature-based reservoir optimization model for resolving the conflict in human water demand and riverine ecosystem protection.[94]

Dam removal

Main article:Dam removal

Waterandsedimentflows can be re-established by removing dams from a river. Dam removal is considered appropriate when the dam is old andmaintenance costsexceed the expense of its removal.[95]Some effects of dam removal includeerosionof sediment in thereservoir,increasedsediment supplydownstream, increased river width andbraiding,re-establishment of natural water temperatures andrecolonisationofhabitatsthat were previously unavailable due to dams.[95]

The world's largestdam removaloccurred on theElwha riverin theU.S. state of Washington(seeRestoration of the Elwha River). Two dams, theElwhaandGlynes Canyon dams,were removed between 2011 and 2014 that together stored approximately 30Mtof sediment.[95][96]As a result, the delivery ofsedimentandwoodto the downstreamriveranddeltawerere-established.Approximately 65% of the sediment stored in thereservoirseroded, of which ~10% was deposited in theriverbed.The remaining ~90% was transported to thecoast.In total, renewed sediment delivery caused approximately 60 ha ofdeltagrowth, and also resulted in increasedriver braiding.[96]

Failure

Main article:Dam failure
Teton Damfailure

Dam failures are generally catastrophic if the structure is breached or significantly damaged. Routinedeformation monitoringand monitoring of seepage from drains in and around larger dams is useful 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 rockgrouting–pressure pumpingPortland cementslurryinto weak fractured rock.

International special sign for works and installations containing dangerous forces

During an armed conflict, a dam is to be considered as an "installation containing dangerous forces" due to the massive impact of possible destruction on the civilian population and the environment. As such, it is protected by the rules ofinternational 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, aprotective signconsisting of three bright orange circles placed on the same axis is defined by the rules of IHL.

South Fork Damfailure and resultingfloodthat destroyedJohnstownin Pennsylvania in 1889

The main causes of dam failure include inadequate spillway capacity, piping through the embankment, foundation or abutments, spillway design error (South Fork Dam), geological instability caused by changes to water levels during filling or poor surveying (Vajont,Malpasset,Testalinden Creekdams), poor maintenance, especially of outlet pipes (Lawn Lake Dam,Val di Stava Dam collapse), extreme rainfall (Shakidor Dam),earthquakes,and human, computer or design error (Buffalo Creek Flood,Dale Dike Reservoir,Taum Sauk pumped storage plant).

A notable case of deliberate dam failure (prior to the above ruling) was theRoyal Air Force'Dambusters'raid onGermanyinWorld War II(codenamed "Operation Chastise"), in which three German dams were selected to be breached in order to damage German infrastructure and manufacturing and power capabilities deriving from theRuhrandEderrivers. This raid later became the basis for several films.

Since 2007, the DutchIJkdijkfoundation is developing, with anopen innovationmodel and early warning system for levee/dike failures. As a part of the development effort, full-scale dikes are destroyed in the IJkdijk fieldlab. The destruction process is monitored by sensor networks from an international group of companies and scientific institutions.

See also

Wikimedia Commons has media related toDam.

Notes

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Sources

Further reading

  • Khagram, Sanjeev.Dams and Development: Transnational Struggles for Water and Power.Ithaca: Cornell University Press 2004.
  • McCully, Patrick.Silenced Rivers: The Ecology and Politics of Large Dams.London: Zed. 2001.

External links

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