Dam (Encyclopedia of Science)
Dams are structures that hold back water in a stream or river, forming a lake or reservoir behind the wall. Dams are used as flood control devices and as sources of hydroelectric power and water for crops. Dams are designed to resist the force of the water against them, the force of standing waterot a running stream.
There are five main types of dams: arch, buttress, earth, gravity, and rock-fill. Arch dams are curved upstream, into the water they hold back. They are typically built in narrow canyons, where the high rocky walls of the canyon can withstand the pressure of the water as it pushes off the arch and against the walls.
A buttress dam uses the force of the water to support it. A slab of concrete is tilted at a 45-degree angle and has buttresses (supports) on the opposite side of the water. While the water pushes down on the slab, the buttresses push up against it. These counterforces keep the slab in balance. Because of the large number of steel beams needed in construction, however, these dams are no longer popular because steel and labor are too expensive.
An earth dam may be a simple embankment or mound of earth (gravel, sand, clay) holding back water. An earth dam might also have a core of cement or a watertight material lining the upstream side.
A gravity dam, made...
(The entire section is 577 words.)
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