Editing Dam (section)

== Types==
{{more citations needed section|date=May 2014}}
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.

=== By structure ===
Based on structure and material used, dams are classified as easily created without materials, [[arch-gravity dam]]s, [[embankment dam]]s or [[masonry dam]]s, with several subtypes.

==== Arch dams ====
{{main|Arch dam}}

[[File:Gordon Dam.jpg|thumb|upright|[[Gordon Dam]], [[Tasmania]], is an [[arch 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 normal [[Fluid 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 [[Normal (geometry)|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 [[abutment]]s (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=https://www.pbs.org/wgbh/buildingbig/dam/basics.html#arch |title=Arch Dam Forces |publisher=[[PBS]] |access-date=7 January 2007}}</ref> 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.

[[File:Barrage Daniel-Johnson2 edited.jpg|thumb|left|[[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 Dam]] on the Colorado River is a constant-angle arch dam.

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

The multiple-arch dam consists of a number of single-arch dams with concrete buttresses as the supporting abutments, as for example the [[Daniel-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|Gravity dam}}
[[File:ThreeGorgesDam-China2009.jpg|thumb|The [[Three Gorges Dam]] is a hydroelectric gravity dam, and the [[List of largest power stations|world's largest power station]] by [[Nameplate capacity|installed capacity]] (22,500&nbsp;[[Megawatt|MW]]).]]
[[File:Grand Coulee Dam spillway.jpg|thumb|The [[Grand Coulee Dam]] is 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.<ref>British Dam Society http://www.britishdams.org/about_dams/gravity.htm {{Webarchive|url=https://web.archive.org/web/20110831091428/http://www.britishdams.org/about_dams/gravity.htm |date=31 August 2011 }}</ref> 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 the [[Parallelogram law|resultant]] of 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|date=March 2019}} 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|date=March 2019}} 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.<ref>{{Cite report |url=https://www.ferc.gov/sites/default/files/2020-04/chap3.pdf |title=Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter III - Gravity Dams |date=4 March 2016 |publisher=Federal Energy Regulatory Commission |access-date=2024-11-24}}</ref>

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 of [[flood]] is 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 Dam]] is a solid gravity dam and [[Braddock Locks & Dam]] is a hollow gravity dam.{{citation needed|date=March 2019}}

==== Arch-gravity dams ====
[[File:Hoover Dam (aerial view) - 30 April 2009.jpg|thumb|The [[Hoover Dam]], shown in 2009 with the [[Mike O'Callaghan–Pat Tillman Memorial Bridge]] under construction, is an example of an arch-gravity dam.]]
{{main|Arch-gravity dam}}

A gravity dam can be combined with an arch dam into an [[arch-gravity dam]] for 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 ====
[[File:Koshi.jpg|thumb|The [[Koshi Barrage]] of [[Nepal]]]]
{{main|Barrage (dam)|l1=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-decommissioned [[Red Bluff Diversion Dam]] on the [[Sacramento River]] near [[Red Bluff, California]].

Barrages that are built at the mouths of rivers or lagoons to prevent [[tide|tidal incursions]] or use the tidal flow for [[tidal power]] are known as [[tidal barrage]]s.<ref name="urlDams and Development: An Overview" />

==== Embankment dams ====
[[File:Chatuge Dam is an earthen embankment dam in North Carolina, United States.jpg|thumb|[[Chatuge Dam]] is an earthen embankment dam in [[North Carolina]]]]
{{main|Embankment dam}}
Embankment dams are made of [[Soil compaction|compacted]] earth, 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.<ref>{{cite web|url=http://wesa.fm/post/us-army-corps-engineers-wants-you-enjoy-rivers-safely#stream/0 |title=The U.S. Army Corps of Engineers Wants You To Enjoy The Rivers, Safely |publisher=90.5 WESA |date=23 June 2017 |access-date=2018-07-18}}</ref> Fixed-crest dams are designed to maintain depth in the channel for navigation.<ref>{{cite web|url=https://www.lrp.usace.army.mil/Media/News-Releases/Article/1218267/army-corps-waterways-partners-focus-on-fixed-crest-dam-safety/ |archive-url=https://web.archive.org/web/20180718001516/https://www.lrp.usace.army.mil/Media/News-Releases/Article/1218267/army-corps-waterways-partners-focus-on-fixed-crest-dam-safety/ |url-status=dead |archive-date=18 July 2018 |title=Army Corps, waterways partners focus on fixed-crest dam safety > Pittsburgh District > News Releases |publisher=Lrp.usace.army.mil |date=2017-06-19 |access-date=2018-07-18}}</ref> 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.<ref>{{cite web|author=Bob Bauder |url=https://triblive.com/local/allegheny/13875841-74/family-of-kayaker-swept-over-dashields-dam-sues-us-army-corps-of |title=Family of kayaker swept over Dashields Dam sues U.S. Army Corps of Engineers |publisher=TribLIVE |date=2017-05-20 |access-date=2018-07-18}}</ref>

=== 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, and [[Dam removal|removal]] costs and affects the dams' potential range and magnitude of environmental disturbances.<ref>{{Cite conference |last1=Carter |first1=Edward F. |last2=Hosko |first2=Mary Ann |last3=Austin |first3=Roger |date=1997 |title=Guidelines for Retirement of Dams and Hydroelectric Facilities |conference=Waterpower '97 |url=https://cedb.asce.org/CEDBsearch/record.jsp?dockey=0106463 |publisher=ASCE |pages=1248–1256}}</ref>

====Large dams====
The [[International Commission on Large Dams]] (ICOLD) defines a "large dam" as "A dam with a height of {{convert|15|m|abbr=on}} or greater from lowest foundation to crest or a dam between {{convert|5|m|abbr=on}} metres and 15 metres impounding more than {{convert|3|e6m3|acre-ft|lk=out}}".<ref>{{cite web |title=Definition of a Large Dam |url=https://www.icold-cigb.org/GB/dams/definition_of_a_large_dam.asp |publisher=International Commission on Large Dams |access-date=23 January 2021}}</ref> "Major dams" are over {{convert|150|m|abbr=on}} 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 |access-date=1 August 2007 |archive-url=https://archive.today/20070704103642/http://www.iucn.org/themes/wani/eatlas/html/technotes.html |archive-date=4 July 2007 |url-status=dead |df=dmy }}</ref> The ''Report of the World Commission on Dams'' also includes in the "large" category, dams which are between {{convert|5|and|15|m|abbr=on}} high with a reservoir capacity of more than {{convert|3|e6m3|acre-ft|lk=out}}.<ref name="urlDams and Development: An Overview">{{cite web|url=http://www.dams.org/report/wcd_overview.htm |title=Dams and Development: An Overview |date=16 November 2000 |quote=Box 1. What is a large dam? |access-date=24 October 2010 |url-status=dead |archive-url=https://web.archive.org/web/20101028045913/http://www.dams.org/report/wcd_overview.htm |archive-date=28 October 2010 }}</ref> [[Hydropower]] dams can be classified as either "high-head" (greater than 30 m in height) or "low-head" (less than 30 m in height).<ref name="Poff Hart 2002">{{Cite journal|last1=Poff|first1=N. Leroy|last2=Hart|first2=David D.|date=2002-08-01|title=How Dams Vary and Why It Matters for the Emerging Science of Dam Removal;... |journal=BioScience|language=en|volume=52|issue=8|pages=659–668|doi=10.1641/0006-3568(2002)052[0659:HDVAWI]2.0.CO;2|issn=0006-3568|doi-access=free}}</ref>

{{as of|2021}}, ICOLD's World Register of Dams contains 58,700 large dam records.<ref name="UNU-2021">{{cite book |last1=Perera |first1=Duminda |display-authors=et al.|title=Ageing Water Storage Infrastructure: An Emerging Global Risk |date=2021 |publisher=United Nations University Institute for Water, Environment and Health |location=Hamilton, Canada |isbn=978-92-808-6105-1 |url=https://inweh.unu.edu/wp-content/uploads/2021/01/Ageing-Water-Storage-Infrastructure-An-Emerging-Global-Risk.pdf |access-date=23 January 2021 |format=Report Series, Issue 11}}</ref>{{RP|6}} The tallest dam in the world is the {{convert|305|m|ft|adj=mid|-high|abbr=on}} [[Jinping-I Dam]] in [[China]].<ref>{{cite web |title=The Jinping-I Double Curvature Arch Dam sets new world record |url=http://en.powerchina.cn/2016-12/28/content_27870606.htm |website=en.powerchina.cn |access-date=16 November 2018 |archive-date=17 November 2018 |archive-url=https://web.archive.org/web/20181117022403/http://en.powerchina.cn/2016-12/28/content_27870606.htm |url-status=dead }}</ref>

==== Small dams ====

[[File:Dam in Europe at Autumn as viewed from FPV drone.webm|thumb|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, [[hydropower]] production, 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.<ref>{{Cite journal |last1=Nathan |first1=R. |last2=Lowe |first2=L. |date=2012-01-01 |title=The Hydrologic Impacts of Farm Dams |url=https://www.tandfonline.com/doi/abs/10.7158/13241583.2012.11465405 |journal=Australasian Journal of Water Resources |volume=16 |issue=1 |pages=75–83 |doi=10.7158/13241583.2012.11465405 |issn=1324-1583}} <!-- as of 3 July 2023, doi is still broken --></ref> Small scale dams have the potential to generate benefits without displacing people as well,<ref>{{Cite web|title=Why small-scale hydroelectric plants benefit local communities|date=3 September 2015 |url=https://www.weforum.org/agenda/2015/09/why-small-scale-hydroelectric-plants-benefit-local-communities/|publisher=World Economic Forum|language=en|access-date=2020-05-11}}</ref> and small, decentralised hydroelectric dams can aid rural development in developing countries.<ref>{{Cite journal|last=Faruqui|first=N. I.|title=Small Hydro for Rural Development|date=1994|journal=Canadian Water Resources Journal|language=en|volume=19|issue=3|pages=227–235|doi=10.4296/cwrj1903227|issn=0701-1784|doi-access=free|bibcode=1994CaWRJ..19..227F }}</ref> In the United States alone, there are approximately 2,000,000 or more "small" dams that are not included in the [[United States Army Corps of Engineers|Army Corps of Engineers]] [[National Inventory of Dams|National Inventory of dams]].<ref>{{Cite book|last=Graf|first=WL|title=Sustaining Our Water Resources|publisher=National Academy Press|year=1993|location=Washington DC|pages=11–42|chapter=Landscapes, commodities, and ecosystems: The relationship between policy and science for American rivers}}</ref> Records of small dams are kept by state regulatory agencies and therefore information about small dams is dispersed and uneven in geographic coverage.<ref name="Poff Hart 2002" />

Countries worldwide consider small hydropower plants (SHPs) important for their energy strategies, and there has been a notable increase in interest in SHPs.<ref name="Couto Olden 2018">{{Cite journal|last1=Couto|first1=Thiago BA|last2=Olden|first2=Julian D.|date=2018|title=Global proliferation of small hydropower plants – science and policy|journal=Frontiers in Ecology and the Environment|language=en|volume=16|issue=2|pages=91–100|doi=10.1002/fee.1746|issn=1540-9309|doi-access=free|bibcode=2018FrEE...16...91C }}</ref> Couto and Olden (2018)<ref name="Couto Olden 2018" /> 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 of [[Colorado]] a non-jurisdictional dam is defined as a dam creating a [[reservoir]] with 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.<ref>{{Cite web|title=DWR Dam Safety Non-Jurisdictional Dam |publisher=Colorado Information Marketplace |url=https://data.colorado.gov/Water/DWR-Dam-Safety-Non-Jurisdictional-Dam/6smc-zj6j/data |language=en|access-date=2020-05-11}}</ref> In contrast, the state of [[New Mexico]] defines 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.<ref>{{Cite journal|date=7 December 2009|title=Evaluation of Non-Jurisdictional Dams|url=https://www.ose.state.nm.us/dams/submittal/EvaluationOfNonJurisdictionalDams.pdf|journal=Office of the State Engineer, Dam Safety Bureau}}</ref> Most US dams, 2.41&nbsp;million of a total of 2.5&nbsp;million dams, are not under the jurisdiction of any public agency (i.e., they are non-jurisdictional), nor are they listed on the [[National Inventory of Dams]] (NID).<ref name="Brewitt Colwyn 2020">{{Cite journal|last1=Brewitt|first1=Peter K.|last2=Colwyn|first2=Chelsea L. M.|date=2020|title=Little dams, big problems: The legal and policy issues of nonjurisdictional dams|journal=WIREs Water|language=en|volume=7|issue=1|pages=e1393|doi=10.1002/wat2.1393|issn=2049-1948|doi-access=free|bibcode=2020WIRWa...7E1393B }}</ref>

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.<ref name="Brewitt Colwyn 2020" /> For example, according to the [[National Park Service|US 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."<ref>{{Cite journal|date=25 May 2010|title=Director's Order #40: Dam Safety & Security Program|url=https://www.nps.gov/subjects/policy/upload/DO_40_5-25-2010.pdf|journal=United States Department of the Interior, National Park Service}}</ref> Small dams can be dangerous individually (i.e., they can fail), but also collectively,<ref>{{Cite journal|last1=Fencl|first1=Jane S.|last2=Mather|first2=Martha E.|last3=Costigan|first3=Katie H.|last4=Daniels|first4=Melinda D.|date=2015-11-05|editor-last=Deng|editor-first=Z. Daniel|title=How Big of an Effect Do Small Dams Have? Using Geomorphological Footprints to Quantify Spatial Impact of Low-Head Dams and Identify Patterns of Across-Dam Variation|journal=PLOS ONE|language=en|volume=10|issue=11|pages=e0141210|doi=10.1371/journal.pone.0141210|issn=1932-6203|pmc=4634923|pmid=26540105|bibcode=2015PLoSO..1041210F|doi-access=free}}</ref> as an aggregation of small dams along a river or within a geographic area can multiply risks. Graham's 1999 study<ref>{{Cite journal|last=Graham|first=W J|date=September 1999|title=A Procedure for Estimating Loss of Life Caused by Dam Failure|url=https://www.usbr.gov/ssle/damsafety/TechDev/DSOTechDev/DSO-99-06.pdf|journal=U.S. Department of the Interior, Bureau of Reclamation}}</ref> 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<ref name="Pisaniello 2009">{{Cite journal|last=Pisaniello|first=John D.|date=2009|title=How to manage the cumulative flood safety of catchment dams|url=http://www.scielo.org.za/scielo.php?script=sci_abstract&pid=S1816-79502009000400001&lng=en&nrm=iso&tlng=en|journal=Water SA|volume=35|issue=4|pages=361–370|issn=1816-7950}}</ref>) 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.<ref name="Pisaniello 2009" /> Scholars have noted that more research is needed to better understand the environmental impact of small dams<ref name="Couto Olden 2018" /> (e.g., their potential to alter the flow, temperature, sediment<ref>{{Cite journal|last1=Ashley|first1=Jeffrey T. F.|last2=Bushaw-Newton |first2=Karen|last3=Wilhelm|first3=Matt|last4=Boettner|first4=Adam|last5=Drames|first5=Gregg|last6=Velinsky|first6=David J.|date=March 2006|title=The Effects of Small Dam Removal on the Distribution of Sedimentary Contaminants |journal=Environmental Monitoring and Assessment|language=en|volume=114|issue=1–3|pages=287–312|doi=10.1007/s10661-006-4781-3|pmid=16565804|bibcode=2006EMnAs.114..287A |s2cid=46471207|issn=0167-6369}}</ref><ref name="Poff Hart 2002" /> 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 a [[levee|dike]] to prevent inundation of nearby land. Dikes are commonly used for reclamation of arable land from a shallow lake, similar to a [[levee]], which is a wall or embankment built along a river or stream to protect adjacent land from flooding.

==== Weir ====
{{main|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. It can also be used for flow measurement or retardation.

==== Check dam ====
{{main|Check dam}}
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.

==== Dry dam ====
{{main|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|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 trap [[groundwater]] and 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 the [[Fukuzato Dam]] in [[Okinawa Prefecture|Okinawa]], Japan. They are most common in [[northeastern Africa]] and the arid areas of [[Brazil]] while also being used in the [[southwestern United States]], Mexico, India, Germany, Italy, Greece, France and Japan.<ref>{{cite web |last=Yilmaz |first=Metin |title=Control of Groundwater by Underground Dams |url=http://etd.lib.metu.edu.tr/upload/1259621/index.pdf |publisher=The Middle East Technical University |access-date=7 May 2012 |date=November 2003}}</ref>

There are two types of underground dams: "sub-surface" and a "sand-storage". A sub-surface dam is built across an [[aquifer]] or 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 or [[wadi]]. 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, prevent [[evaporation]]. The stored water can be extracted with a well, through the dam body, or by means of a drain pipe.<ref>{{cite journal |last=Onder |first=H |author2=M. Yilmaz |title=Underground Dams—A Tool of Sustainable Development and Management of Ground Resources |journal=European Water |date=November–December 2005 |pages=35–45 |url=http://www.ewra.net/ew/pdf/EW_2005_11-12_05.pdf |access-date=7 May 2012}}</ref>

==== Tailings dam ====
{{main|Tailings dam}}
[[File:Bituminous geomembrane installation on a mine tailings storage facility.jpg|thumb|[[Bituminous Geomembranes (BGMs)|Bituminous geomembrane]]  installation on a mine tailings storage facility.]]

A tailings dam is typically an earth-fill embankment dam used to store [[tailings]], which are produced during [[mining]] operations after separating the valuable fraction from the uneconomic fraction of an [[ore]]. 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.<ref>{{cite book |last=Blight |first=Geoffrey E. |chapter=Construction of Tailings Dams |title=Case studies on tailings management |year=1998 |publisher=International Council on Metals and the Environment |location=Paris |isbn=978-1-895720-29-7 |pages=9–10 |chapter-url=https://books.google.com/books?id=qG9Bux3RYWMC&q=tailings+dam&pg=PA9 |access-date=10 August 2011}}</ref>

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 upon [[topography]], geology, climate, the type of tailings, and cost. An upstream tailings dam consists of [[trapezoid]]al embankments 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 tailings [[slurry]] in 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.<ref>{{cite web|title=Properties of Tailings Dams |url=http://www.mining.ubc.ca/faculty/meech/MINE290/Tailings%20Dam%20Construction%20Methods.pdf |publisher=NBK Institute of Mining Engineering |access-date=10 August 2011 |url-status=dead |archive-url=https://web.archive.org/web/20111001213037/http://www.mining.ubc.ca/faculty/meech/MINE290/Tailings%20Dam%20Construction%20Methods.pdf |archive-date=1 October 2011 }}http://mining.ubc.ca/files/2013/03/Dirk-van-Zyl.pdf</ref><ref name="taildam">{{cite book |editor-first=Raj K. |editor-last=Singhal |title=Environmental issues and management of waste in energy and mineral production: Proceedings of the Sixth International Conference on Environmental Issues and Management of Waste in Energy and Mineral Production: SWEMP 2000; Calgary, Alberta, Canada, May 30&nbsp;– June 2, 2000 |year=2000 |publisher=Balkema |location=[[Rotterdam]] [u.a.] |isbn=978-90-5809-085-0 |pages=257–260 |url=https://books.google.com/books?id=PqiYy538JFUC&q=tailings+dam&pg=PA257 |access-date=2015-11-09}}</ref>

Because tailings dams often store toxic chemicals from the mining process, modern designs incorporate an impervious [[geomembrane]] liner to prevent seepage.<ref>{{cite report |last1=McLeod|first1=Harvey  |last2=Bjelkevik|first2=Annika |date=2021 |title=TAILINGS DAM DESIGN Technology Update |publisher= ICOLD Committee on Tailings Dams|url= https://www.icoldchile.cl/boletines/181.pdf|doi= |page= 91}}</ref> Water/slurry levels in the tailings pond must be managed for stability and environmental purposes as well.<ref name="taildam" />

=== By material ===
==== Steel dams ====
{{main|Steel dam}}

[[File:RedridgeSteelDam02.jpg|thumb|[[Redridge Steel Dam]], built 1905, [[Michigan]]]]

A [[steel dam]] is 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 ====
[[File:Dam Timber Crib.jpg|thumb|left|A timber crib dam in Michigan, 1978]]

[[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 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, [[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 America]]n West, but most have failed, been hidden under earth embankments, or been replaced with entirely new structures. Two common variations of timber dams were the "crib" and the "plank".

Timber crib dams were erected of heavy timbers or dressed logs in the manner of a [[log house]] and the interior filled with earth or rubble. The heavy crib structure supported the dam's face and the weight of the water. [[Splash dam]]s were timber crib dams used to help float [[logging|logs]] downstream 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|Cofferdam}}
[[File:Dam Coffer.jpg|thumb|A cofferdam during the construction of [[canal lock|locks]] at the Montgomery Point Lock and Dam]]

A [[cofferdam]] is a barrier, usually temporary, constructed to exclude water from an area that is normally submerged. Made commonly of wood, [[concrete]], or [[steel]] sheet [[Deep foundation|piling]], cofferdams are used to allow construction on the [[Foundation (engineering)|foundation]] of 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 also [[causeway]] and [[retaining 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 dam]]s are formed when lava flows, often [[basalt]]ic, intercept the path of a stream or lake outlet, resulting in the creation of a natural impoundment. An example would be the eruptions of the [[Uinkaret volcanic field]] about 1.8&nbsp;million–10,000 years ago, which created lava dams on the [[Colorado River]] in northern [[Arizona]] in the [[United States]]. The largest such lake grew to about {{convert|800|km|mi|abbr=on}} in length before the failure of its dam. [[Glacier|Glacial activity]] can also form natural dams, such as the damming of the [[Clark Fork River|Clark Fork]] in [[Montana]] by the [[Cordilleran Ice Sheet]], which formed the {{convert|7780|km2|mi2|abbr=on}} [[Glacial Lake Missoula]] near the end of the last Ice Age. [[Moraine]] deposits left behind by glaciers can also dam rivers to form lakes, such as at [[Flathead Lake]], also in Montana (see [[Moraine-dammed lake]]).

Natural disasters such as earthquakes and landslides frequently create [[landslide dam]]s in mountainous regions with unstable local geology. Historical examples include the [[Usoi Dam]] in [[Tajikistan]], which blocks the [[Murghab River]] to create [[Sarez Lake]]. At {{convert|560|m|ft|abbr=on}} high, it is the tallest dam in the world, including both natural and man-made dams. A more recent example would be the creation of [[Attabad Lake]] by a landslide on [[Pakistan]]'s [[Hunza 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 western [[Wyoming]]'s [[Gros Ventre landslide]] in 1927, which wiped out the town of [[Kelly, Wyoming|Kelly]] resulting in the deaths of six people.

===== Beaver dams =====
{{main|Beaver dam}}
[[Beaver]]s 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.