Editing Dam (section)

=== 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>