A concrete dam is a structure designed and built for the purpose of holding back water, that is usually placed across the path of a river. Dams are built for a variety of reasons, including flood control, power generation, and water management. A concrete dam is the strongest type of dam built in modern times and may take several forms. Concrete itself is a building material made from water, cement, sand and gravel, or aggregate.
In modern times, nearly every dam is made either partially or entirely from concrete. Concrete is an excellent material for constructing dams because it is very strong when under compression — pressed down or pushed together. Many designs for concrete dams take advantage of this property to produce extremely large dams, capable of holding back many cubic miles (1 cu. mile = 4.2 cu. kms) of water.
Dams constructed from concrete have three basic designs. An arch dam is a curved, relatively thin curtain of concrete, with the concave side of the curve facing down stream. This type of dam is made of solid concrete that is reinforced with steel. It relies on the pressure of the water behind it to add strength as this pressure pushes the sides of the dam into the walls on either side. Arch dams are particularly well suited for areas where a river flows at the bottom of steep canyons or gorges with solid rock walls.
A gravity dam is a type of dam that relies on its own mass to keep it in place and to hold back water. Gravity dams are often massive structures, sometimes tens of feet (1 foot = .3 meters) thick. The dam is made of concrete, but the main portion of its interior is fill rather than solid concrete. A gravity dam is usually not curved, and a cross section will resemble a right triangle, with the right angle at the bottom on the side facing the water and the side facing away from the water sloping downward so that the dam is thicker at the bottom. Gravity dams are better for areas where there is no firm bedrock or canyon walls for anchorage.
The third main type of concrete dam is the arch-gravity dam, which combines the features of both the arch dam and the gravity dam. Arch-gravity dams are curved dams that use the principle of the arch to bolster their strength but are much thicker than a typical arch dam and have a core of fill. They are designed so that their massive weight, combined with the increased strength of an arch over a straight line structure, will keep the dam in place and hold back the water.
Some concrete dams may have several small arches or buttresses or a pair of large arches or other variations, but most, if not all of these, are variations on one of the three basic designs. A relatively type of concrete, for building concrete dams, is called roller compacted concrete and uses heavy rollers to press the concrete during construction. Many newer dams are being constructed using this technique, but the designs still of the same basic types. A typical concrete dam has spillways for releasing water when needed, and many are designed to make use of water flow to generate electricity by means of hydroelectric power plants. Flowing water that turns massive turbines produces as much as 20% of the world's electricity.