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Dam

Barrier that impounds water or underground streams

A dam is a structure that impounds water or restricts its flow. 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; and arch dams use a curved concrete wall to transfer the weight of the water to the surrounding valley walls. 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 using water from reservoirs impounded by dams. Dams generate hydropower, providing a clean and renewable source of electricity, and also supply water for household and industrial needs. An early dam was 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. In the 1st 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. The most visible part of a dam is the barrier that retains water, but most dams contain additional structures that perform important functions. When a dam blocks a navigable river, locks may be incorporated into the project to allow ships to pass through. Fish ladders are included in many dams to enable fish to migrate upstream. Spillways are often included to safely release excess reservoir water downstream and prevent catastrophic overflows. Dam outlets allow reservoirs to be partially drained to purge sediment, perform maintenance, or increase water flow downstream. A dam may be deliberately removed for various reasons: if it poses a safety hazard, if the dam no longer fulfills its original purpose, to restore fish migration routes, or to improve the health of downstream rivers by improving sediment flow. 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.