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DAMS: How They Work

 Dams have many different functions to perform. A dam can be either a storage, diversion or detention dam. Storage dams are built to trap water for later use, like an artificial reservoir. Diversion dams usually stop a river's natural course so that water can be sent off to a different place. Diversion dams are often used for irrigation. Detention dams are constructed to either stop or slow the amount of water in a river. This is mostly to prevent an area from flooding.

 All dams are classified as being overflow or nonoverflow. Overflow dams have to have a very strong crest, or else the fast flowing water will start to erode it. Nonoverflow dams can not take the risk of having water overtop them, so many of them have spillway structures. Spillway structures can be a tunnel that takes away excess water, or they can be sections of a dam that are meant to be overtopped.

DAMS: History

 Dams were first used to control flooding and to provide water for irrigation and drinking. The first record of a dam was from the Greek historian Herodotus. The dam was built across the Nile River in about 2900 B.C. to protect the city of Memphis from flooding. Dams were popular with the Romans, and two of their dams are still in use today, almost 2000 years later. Dam construction was mostly lost after the Romans until the 1800s.

 There are now about 50,000 major dams in the U.S. and dams are continuing to be built throughout the world. In fact, every day a new dam exceeding 50 feet is built, along with many smaller ones.

DAMS: Design, Structure, Problems

 Dams are usually classified according to the materials used to construct them and on their basic design. Earthfill dams have a center made of clay, and an outside layer of strong, compacted earth. When earthfill dams are being built, the river has to be diverted via tunnel or temporary diversion dam. Earthfill dams are nonoverflow, so they must have spillway structures to prevent serious damage.

 Concrete gravity dams hold back water by using their own weight. The side facing the water is perpendicular while the opposite side gradually slopes down. Since there is more pressure near the base of the dam, the dam is more broad there. There is not as much pressure near the top of the dam, so it is not as broad there.

 Concrete arch dams use an arch design to hold back the water. They are built inside narrow, steep canyons. The side that faces the water arches in. When water pushes on it, the dam just pushes into the canyon walls. Arch dams can be very thin because they can hold back a lot of force with their arch design.

 If water finds a way to seep through a dam, the pressure will not let it stop. The hole will get larger, and will require costly repairs. Also, when rivers are stopped, wildlife and vegetation can die off. Areas behind the dam are generally flooded, causing more harm to the environment.

PICTURES FOR:

Grand Coulee Dam, located on the Columbia River in Washington

Itaipu Dam, largest dam in the world. Located between Brazil and Paraguay

Yellowtail Dam, on the Bighorn River in eastern Montana