Editing Dam (section)

== Construction elements ==
=== Power generation plant ===
{{main|Hydroelectricity}}

[[File:Hydroelectric dam.svg|thumb|left|Hydroelectric dam in cross section]]
[[File:Water turbine - edit1.svg|thumb|upright|[[Water turbine|Hydraulic turbine]] and [[electric generator]]]]

{{As of|2005}}, hydroelectric power, mostly from dams, supplies some 19% of the world's electricity, and over 63% of [[renewable energy]].<ref name="REN21-2006">[http://www.ren21.net/globalstatusreport/download/RE_GSR_2006_Update.pdf Renewables Global Status Report 2006 Update]  {{webarchive|url=https://web.archive.org/web/20110718181410/http://www.ren21.net/globalstatusreport/download/RE_GSR_2006_Update.pdf |date=18 July 2011 }}  {{cite web|url=http://www.ren21.net/Portals/0/documents/activities/gsr/RE_GSR_2006_Update.pdf |title=RENEWABLES GLOBAL STATUS REPORT 2006 Update |access-date=2015-11-09 |url-status=dead |archive-url=https://web.archive.org/web/20160514070611/http://www.ren21.net/Portals/0/documents/activities/gsr/RE_GSR_2006_Update.pdf |archive-date=14 May 2016 }}, ''[[REN21]]'', published 2006, accessed 16 May 2007</ref> Much of this is generated by large dams, although [[China]] uses small-scale hydro generation on a wide scale and is responsible for about 50% of world use of this type of power.<ref name="REN21-2006" />

Most hydroelectric power comes from the [[potential energy]] of dammed water driving a [[water turbine]] and [[electric generator|generator]]; to boost the power generation capabilities of a dam, the water may be run through a large pipe called a [[penstock]] before the [[turbine]]. A variant on this simple model uses [[pumped-storage hydroelectricity]] to produce electricity to match periods of high and low demand, by moving water between [[reservoir]]s at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. (For example, see [[Dinorwig Power Station]].)

=== Spillways ===
{{main|Spillway}}
[[File:Llyn Brianne spillway.jpg|upright|thumb|Spillway on [[Llyn Brianne]] dam, [[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 have [[floodgate]]s designed 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 plug]] spillway" 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 gradually [[erosion|eroded]] by water flow, including [[cavitation]] or [[turbulence]] of the water flowing over the spillway, leading to its failure. It was the inadequate design of the spillway and installation of fish screens that led to the 1889 over-topping of the [[South Fork Dam]] in [[Johnstown, Pennsylvania]], resulting in the [[Johnstown Flood]] (the "great flood of 1889").<ref>{{cite web|title=The Club and the Dam|url=http://www.jaha.org/attractions/johnstown-flood-museum/flood-history/the-club-and-the-dam/|website=Johnstown Flood Museum|publisher=Johnstown Area Heritage Association|access-date=15 January 2018}}</ref>

Erosion rates are often monitored, and the risk is ordinarily minimized, by shaping the downstream face of the spillway into a curve that minimizes turbulent flow, such as an [[ogee]] curve.