Editing Dam (section)

==== 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>