The earliest dams were used to water farms. The early Egyptians made dams of dirt that were made to raise the level of the water, so that it could reach their farms. The pilgrims even made dams when they first came to North America. Their dams were used to turn their wheels to grind the corn and to saw the timber. An artificial lake that is made by a dam is called a reservoir. Water can pass over the dam also. The part of the dam over which the water spills is called the spillway. It can also get through by chutes, tunnels, or shafts. The pipes for conducting the water to the power turbines of hydroelectric dams are called penstocks. All the water that flows through a dam is controlled by control gates. Dams are mainly built for water supply, irrigation, electric power, flood control, and also improvement of navigation. A lot of dams have more than one purpose.
Dams that deal with irrigation, store the water, and supply the crops with it when the going gets tough are valuable to agriculture. Some dams can direct the water into pipelines and other irrigation instruments. There are also water supply dams. These dams supply the large cities with water. Flood control dams keep the river from overflowing. They release the water at the bottom of the dam and this way they can control the water below. There are also navigation dams. These are built so a ship will have the right depth of water. The passage of a river may be blocked by a waterfall or rapids. By putting in a dam, you can "drown" the problem. Some dams are built to generate power. Hydroelectric dams do just that. They direct the water into turbines. These are curved wheels that have blades as spokes. These are the main things that a dam can do. But there are other things it can do as well. They can provide recreation. People like to go swimming, they like to fish, and they like to watch the animals that are attracted to the water. Dams are very important to birds and other animals. Dams also slow down streams, so that the soil is not carried away. Dams can also create problems. The reservoirs may cover up historic places and also scenic locales. It may also cause a problem for fish, when they migrate up stream. There are other structures that are like dams too. These are called cofferdams. These dams are only temporary to hold back the water. Dikes keep the sea off land that is below sea level. There are also levees. These are artificial riverbanks that prevent the water from flooding. A very small dam that is used over a millpond or a fishpond is called a weir.
There are many types of dams and they are classified into many different groups. They are classified into groups by design, method, and materials. The usual types of masonry or concrete, rock, earth, timber, steel, and moveable dams. Concrete dams are either arched, solid gravity, or hollow gravity. Solid gravity dams are made of solid concrete. They withstand pressure. In cross section, they are triangle-shaped. They are built like this because water pressure becomes greater as the depth of the water is increased. Hollow gravity dams are made with less concrete than solid gravity dams. They rely more on their structure than their weight to resist the force of the water. There are three types of hollow gravity dams. The multiple arch dam has a number of arches supported on buttresses. A flat slab dam is a straight wall resting on buttresses. The multiple dome dam has domes instead of arches. It is like the multiple arch dam. Arched dams consist of a horizontal curve using the principle of the arch for strength against water pressure. They are curved upstream. The force of the water is then put to the canyon walls. Earth dams are made by putting clay, gravel, and sand over a river. Often a core, or a wall built inside the dam is built to prevent leakage. A wall of rocks is often laid upstream. Rock dams are made by placing rocks across a river. Over this, concrete, timber, or steel are placed for extra support.
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