DAM
A dam is a structure that impounds or restricts the flow of water. Dams are classified into four basic types: gravity dams are massive structures made of concrete or masonry that rely on their weight to resist the force of impounded water. Embankment dams are large earthworks consisting of rocks, clay, sand, soil, or gravel. Buttress dams consist of a sloped, concrete face supported on the downstream side by a series of triangular buttresses. Arch dams use a curved concrete wall to redirect the force of the water outwards into the surrounding valley walls.
An early dam was the Jawa Dam in modern Jordan, built around 3000 BCE. The Hittite Empire built several dams in modern Turkey between the 17th and 13th centuries BCE. By the first century CE, the Roman Empire began building masonry gravity dams – typically with vertical faces on both upstream and downstream sides. In medieval Europe, dams powered water wheels for milling and mining. The process of designing dams began to transform in the late 18th century from an informal practice to an engineering discipline rooted in science. In the 20th century, the widespread availability of concrete and heavy construction machinery led to the creation of many large-scale dam projects worldwide. Organized opposition to dam projects emerged in the modern era, including protests in the 1870s against 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: about 20% of the world's arable land is irrigated by water that originated in reservoirs impounded by dams. Dams generate hydropower, providing a clean and renewable source of electricity, and also supply water for household and industrial needs. Dams that support flood management reduce downstream water levels to a safe limit.
Dams are often a component of a larger project. Many dams incorporate power plants that run water through generators to produce electricity. Spillways are often included to safely release excess reservoir water downstream and prevent catastrophic overflows. Dam outlets are structures which permit the reservoir to be partially drained to purge sediment, generate electricity, or increase water flow downstream. When a dam blocks a navigable river, locks may be incorporated into the project to allow ships to pass through.
Dams occasionally fail, resulting in flooding 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: earthquakes, weak rocks at the abutments, water leaking within or under the dam, or the dam sliding over its foundation. Beyond structural issues, dams also present political challenges caused by water scarcity, population growth, and the impacts of climate change. Dams built in countries such as Turkey, India, Ethiopia, and China have led to international disputes with downstream nations.
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