Editing Dam (section)

==== 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}}