DAM
A dam is a structure that impounds water or restricts water flow. Dams are categorized by their structure: gravity dams are large, heavy masses made of concrete or masonry, and rely on their weight to remain immobile and resist the forces exerted by the upstream water. Embankment dams are large earthworks consisting of rocks, clay, sand, soil, or gravel. A buttress dam consists of a sloped, concrete face supported on the downstream side by numerous triangular buttresses. An arch dam is a curved concrete wall that transfers the force of the impounded waters to the valley walls.
The earliest known dam is the Jawa Dam in Jordan, built around 3000 BCE. The Hittite Empire built several dams between the 17th and 13th centuries BCE in Turkey. The Roman Empire began building significant dams until the first century CE – typically masonry gravity dams with vertical faces on both upstream and downstream sides. In medieval Europe, dams were used to power water wheels for milling and mining. In the late 18th century, the process of designing dams began to transform from an informal practice to an engineering discipline rooted in science. In the 20th century, the widespread availability of concrete and power construction machinery led to an explosion in the number of large dams built around the world. The modern era also saw the emergence of arguments against dam construction, starting as early as the 1870s with objections to the Thirlmere Dam in Britain.
Dams provide for irrigation, hydropower, water supply, flood management, recreation, inland navigation, and fish farming. Irrigation is a critical application of dams: in 2006, between 12% and 15% of the world's population relied on food that was irrigated by water that originated in reservoirs impounded by dams. Dams can generate hydropower, which is a clean and renewable source of electricity. Many dams supply water for domestic or industrial use. Dams that support flood management don't entirely eliminate all possibility of flood, but generally reduce the peak flood level (height) to a safe limit.
Dams are often one component of a larger project. Many dams include power plants that run water through a generator to produce electricity. Many dam projects include spillways, which are structures that provide a controlled release of excess water from the reservoir into the river downstream, preventing the dam from overflowing and possibly failing. Dam outlets are structures which permit the reservoir to be partially drained for the purpose of purging sediment from the floor of the reservoir, generating hydropower, or increasing the water flow downstream. When a dam is placed in a river where it would prevent the movement of boats, locks may be incorporated into the dam project.
Dams occasionally fail, sometimes causing large floods and loss of life. Many principles governing the design of safe dams have been developed based on lessons learned from dam failures. Dams can fail for many reasons: rock weakness at the abutments, erosion of the foundation under the dam, shearing where the dam meets rock, or sliding over its foundation. Dams built in Turkey, India, Ethiopia, and China without the consent of downstream nations have led to notable international disputes, amid increasing water scarcity driven by population growth and the impacts of climate change.
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