Dam is a barrier placed across a river to stop the flow of water. Dams vary in size from small earth or rock barriers to concrete structures as high as a skyscraper.
Throughout history, wherever people settled, an important first concern was to locate an adequate water supply. In many regions, streams full of water during certain seasons of the year become dry at other times, perhaps when water is most needed. At first, people built small dams of brush, earth, and rock that would store enough water for immediate needs. But floods frequently washed these dams away. As communities grew and populations increased, people learned to construct larger dams that would provide a more permanent and abundant water supply, even during droughts covering several years. Later, people learned how to harness the energy of falling water released from dams and use that energy to produce electric power for homes and industries.
The effects of dams
As a barrier across a river or stream, a dam stops the flow of water. A dam stores the water, creating a lake or reservoir behind it. The stored water is then made available for irrigation, town and city water supplies, and many other uses. The dam also raises the water surface from the level of the original riverbed to a higher level. Raising the water level permits water to be diverted by the natural flow of gravity to adjacent lands. The stored water also flows through hydraulic turbines, producing electric power that is used in homes and industries. Water released from the dam in uniform quantities assures water for fish and other wildlife in the stream below the dam. Otherwise, the stream would go dry there. Water released in larger quantities permits river navigation throughout the year. Where dams create large reservoirs, floodwaters can be held back and released gradually over longer periods without overflowing riverbanks.
Reservoirs or lakes created by dams provide recreational areas for boating and swimming. They give refuge to fish and wildlife. They help preserve farmlands by reducing soil erosion caused by flooding.
Some people are concerned about how dams can affect the environment. Rivers carry material called sediment eroded from the land over which they flow. When a river enters a reservoir, the speed of the flowing water slows, and sediment falls to the reservoir bed. Much of the sediment held back in the reservoir would otherwise be carried downstream. If too much sediment is stored, conditions downstream can be changed. For example, the riverbed can erode when it is not replenished with sediment. Also, farmland can be deprived of the fine-grained sediment called silt that would normally be deposited when the river floods. Silt carries nutrients that are important for fertilizing soil.
Types of dams
Dams are classified by the material used to construct them. Dams built of concrete, stone, or other masonry are called masonry dams. Dams built of earth or rocks are called embankment dams. Engineers generally choose to build embankment dams in areas where large amounts of earth or rocks are available.
Masonry dams.
Today, nearly all masonry dams are built of large blocks of concrete. There are three main kinds of masonry dams: gravity, arch, and buttress.
Gravity dams depend entirely on their own weight to resist the tremendous force of the oncoming water. They are the strongest and most massive dams. Engineers build a gravity dam on a solid rock foundation. The dam transfers the force of the water downward to the foundation below. Gravity dams can hold back enormous amounts of water. However, they are costly to build because they require so much concrete.
Arch dams curve toward the flow of water. They are usually built in narrow canyons. As the water pushes against the dam, the arch transfers the water’s force outward to the canyon wall. An arch dam requires much less concrete than a gravity dam of the same length.
Buttress dams depend for support on a series of vertical supports called buttresses. The buttresses run along the dam’s downstream face—that is, the side facing away from the water’s flow. The downstream face of a buttress dam usually slopes outward at about a 45-degree angle. The sloping face and the buttresses transfer the force of the water downward to the dam’s foundation. Buttress dams, like gravity dams, are usually built in wide valleys where long dams are needed.
Embankment dams
are constructed of materials dug out of the ground, including rocks, gravel, sand, silt, and clay. They are also known as fill dams. An earth-fill dam is an embankment dam in which compacted earth materials make up more than half the dam. Engineers construct earth-fill dams by hauling materials into place and compacting them layer upon layer with heavy rollers. Dam builders grade the materials by density, placing the finest, such as clay, in the center to form a waterproof core. In some cases, the builders use concrete cores. They place the coarser materials outside the core and cover them with a layer of rock called riprap. The riprap serves as an outer protection against water action, wind, rain, and ice. In addition, the builders pump thinned-out cement, called grout, into the foundation to fill cracks and make the foundation watertight.
Where rocks are available, it may prove most economical to build a rock-fill dam. Most dams of this type are constructed of coarse, heavy rock and boulders. Many of them have a covering of concrete, steel, clay, or asphalt on the upstream side. This covering makes the dam watertight. Combinations of rock and earth result in a type of dam called an earth-and-rock-fill dam.
Other types of dams.
Engineers build timber dams where lumber is available and the dam is relatively small. The builders weight down the timber with rock. Planking or other watertight material forms the facing. Metal dams have watertight facings and supports of steel. Dams with movable gates are built where it is necessary to let large quantities of water, ice, or driftwood pass by the dam. A roller dam has a large horizontal roller between supporting pillars called piers. Operators can raise and lower the roller to allow ice and other materials to pass through the dam without much loss of reservoir water level.
How dams are built
Planning.
To construct a dam, the builders must first gather and study much information. Geologists and engineers must examine the site for its formation, quality of foundation, and availability of suitable construction materials. They drill beneath the surface to determine the condition, quality, and location of the rock formation under the damsite. Hydrologists (scientists who study water) make a careful analysis of the stream flow. Surveyors measure and survey the land and map the area to be covered by the reservoir that the dam creates.
Engineers must determine the amount of mud, silt, and debris that will flow into the reservoir. Over a number of years, the sediment in the reservoir can build up and reduce the space available for storing water. Engineers must design the reservoir to reduce the amount of sediment deposited, and to maximize the sediment flow downstream.
If the dam is to generate power, the plans must provide outlets that will connect to generating equipment. If the water is to be used for irrigation or municipal supply, the plans must include outlets to control its release to canals or aqueducts.
Government authorities or private developers must buy or relocate all property in the reservoir area. This occasionally requires the relocation of entire towns and sections of highways or railroads. Before construction of a dam is begun, governments may require an expert assessment that evaluates the different types of environmental changes that might result from the project. This report is sometimes called an environmental impact statement or an environmental impact assessment report. Federal, regional, and local government agencies then review the report and present it to the public. Based on their evaluation and the public reaction, these agencies may require planners to revise the project. At times, they may even order a project to be abandoned.
Construction.
After a damsite has been selected, the builders must divert the flow of water from the riverbed so they can excavate the foundation and place the concrete, earth, or rock. Dam builders often excavate half the riverbed at one time while the other half carries the flow of the river. In some cases, builders bore a tunnel through an adjacent canyon wall. The tunnel permits the entire flow of the river to pass around the damsite. To accomplish this diversion, the builders erect small temporary dams called cofferdams upstream to divert the river into the tunnel. After the builders have made the dam high enough, they close the diversion tunnel with gates and permanently plug it.
Dam safety.
In designing a dam, engineers must provide a way to bypass water when the reservoir is full, without overtopping the dam. For this purpose, they construct a passage called a spillway. Spillways act as safety valves by releasing excess waters that the reservoir cannot contain. A spillway may be a channel apart from the dam or a section of the dam over which water can flow freely. The excess water flows from the reservoir through the spillway and back to the downstream river or drainage channel. A spillway must be large enough to handle the water from a major flood.
Dams can create serious safety hazards. If a dam collapses, it can cause enormous property damage, injury, and sometimes death. A dam can collapse because of faulty construction or an earthquake. Erosion can also lead to a dam’s collapse. A dam can erode from the inside by water leaking into the embankment, the foundation, or structures attached to the dam. If the spillway is too small, water may flow over the top of the dam and cause erosion.
History
Dams have influenced civilization for thousands of years, especially cultures that depended on irrigation. The Egyptians built the earliest known dam on the Nile River about 2800 B.C. But dams probably were built much earlier. The ancient Romans built dams of cut stone throughout the Roman Empire. Some of these dams are still in use today.
The earliest dams in North America provided power for grist mills and sawmills. American colonists probably built their first dam in 1634 to operate a sawmill in South Berwick, Maine. In the 1800’s, significant advances were made in the design and construction of masonry dams, especially in Europe. Dams built in the United States during the late 1800’s and early 1900’s supplied water to dry lands in the West and opened the area to settlement. During the 1900’s, improvements in engineering techniques and building materials led to the construction of higher and longer dams than ever before. These dams brought electric service to remote areas and water to arid regions.
Three Gorges Dam, on the Yangtze River in China, is the world’s largest dam. Construction on the dam began in 1994. The dam began generating electric power at full capacity in 2010.
