Dams

Dams are structures designed to restrict the flow of a stream or river, thus forming a pond, lake , or reservoir behind the wall. Dams are used for flood control, for production of hydroelectric power, to store and distribute water for agriculture and human populations, and as recreation sites.

Classification of dams

Dams may be classified according to the general purpose for which they are designed. These include storage, diversion, and detention.

Storage dams are built to provide a reliable source of water for short or long periods of time . Small dams, for example, are often built to capture spring runoff for use by livestock in the dry summer months. Storage dams can be further classified by the specific purpose for which the water is being stored, such as municipal water supply, recreation, hydroelectric power generation, or irrigation .

Diversion dams are typically designed to raise the elevation of a water body to allow that water to be conveyed to another location for use. The most common applications of diversion dams are supplying irrigation canals and transferring water to a storage reservoir for municipal or industrial use.

Detention dams are constructed to minimize the impact of flooding and to restrict the flow rate of a particular channel. In some cases, the water trapped by a detention dam is held in place to recharge the subsurface groundwater system. Other detention dams, called debris dams, are designed to trap sediment.

Large dams frequently serve more than one of these purposes and many combine aspects of each of the three main categories. Operation of such multipurpose dams is complicated by sometimes opposing needs. In order to be most effective, storage behind a flood control dam should be maintained at the lowest level possible. After a detention event occurs, water should be released as quickly as possible, within the capacity of the downstream channel. Conversely, the efficient and economic operation of storage and diversion dams requires that water levels be maintained at the highest possible levels. Releases from these reservoirs should be limited to the intended user only, such as the power generating turbines or the municipal water user. Operators of multipurpose dams must balance the conflicting needs of the various purposes to maintain the reliability, safety, and economic integrity of the dam. Operators must use a variety of information to predict the needs of the users, the expected supply, and the likelihood of any abnormal conditions that might impact the users or the dam itself. Failure to do so can threaten even the largest of dams.

During the El Niño of 1983, climate and hydrologic forecasts failed to predict abnormally heavy spring runoff in the Rocky Mountains. Dam operators along the Colorado River maintained high water storage levels, failing to prepare for the potential of the flooding. By the time operators began to react, water was bypassing the dams via their spillways and wreaking havoc throughout the system. Ultimately, the Glen Canyon dam in Arizona was heavily impacted with flood flows eroding large volumes of rock from within the canyon walls that support the dam. Fortunately, the flooding peaked and control was regained before the dam was breached.

Dam construction

There are four main types of dams: arch, buttress, gravity, and embankment dams. The type of construction for each dam is determined by the proposed use of the structure, qualities of the intended location, quantity of water to be retained by the structure, materials available for construction, and funding limitations.

Arch dams use an upstream-facing arch to help resist the force of the water. They are typically built in narrow canyons and are usually made of concrete . Good contact between the concrete and the bedrock are required to prevent leakage and ensure stability. A dome dam is a special variant with curves on the vertical and horizontal planes, while the arch dam is only curved on the horizontal plane . In addition, dome dams are much thinner than arch dams.

A buttress dam is characterized by a set of angled supports on the downstream side that help to oppose the force of the water. This design can be employed in wide valleys where a solid bedrock foundation is not available. Because of the steel framework and associated labor needed for construction, these dams are no longer economically viable.

The gravity dam withstands the force of the water behind it with its weight. Made of cement or masonry, this type of dam normally utilizes a solid rock foundation but can be situated over unconsolidated material if provisions are made to prevent the flow of water beneath the structure. The solid, stable nature of this dam is favored by many and often incorporated into the spillway designs of embankment dams.

An embankment dam uses the locally available material (rocks , gravel, sand , clay, etc.) in construction. Just as with gravity dams, the weight of embankment dams is used to resist the force of the water. The permeability of the materials that make up these dams allows water to flow into and through the dam. An impervious membrane or clay core must be built into them to counteract the flow and protect the integrity of the structure. Because the materials are locally available and the construction of these dams is relatively simple, the cost of construction for this type of dam is much lower than the other types. Embankment dams are the most common.

Impact of dams

Dams have long been acknowledged for providing electricity without the pollution of other methods, for flood protection, and for making water available for agriculture and human needs. Within recent decades, however, the environmental impacts of dams have been debated. While dams do perform important functions, their effects can be damaging to the environment. People have begun to question whether the positive contributions of some dams are outweighed by those negative effects.

The damming of a river will have dramatic consequences on the nature of the environment both upstream and downstream of the dam. The magnitude of these effects are usually directly related to the size of the dam.

Prior to dam construction, most natural rivers have a flow rate that varies widely throughout the year in response to varying conditions. Of course once constructed, the flow rate of the river below a dam is restricted. The dam itself and the need to control water releases for the various purposes of the particular dam result in a flow rate that has a smaller range of values and peaks that occur at times related to need rather than the dictates of nature. In cases where the entire flow has been diverted for other uses, there may no longer be any flow in the original channel below the dam.

Because water is held behind the dam and often released from some depth, the temperature of the water below the dam is usually lower than it would be prior to dam emplacement. The temperature of the water flow is often constant, not reflecting the natural seasonal variations that would have been the case in the free-flowing river. Similarly, the chemistry of the water may be altered. Water exiting the lake may be higher in dissolved salts or have lower oxygen levels than would be the case for a free-flowing river.

Impoundments increase the potential for evaporation from the river. Because the surface area of a lake is so great when compared to the river that supplies it, the loss of water to evaporation must be considered. In some desert areas, potential annual evaporation can be greater than 7 ft (2.1 m), meaning that over the course of one year, if no water flowed into or out of the system, the reservoir would drop in elevation by 7 ft (2.1 m). At Lake Mead on the Colorado River in Arizona and Nevada, evaporation losses in one year can be as great as 350 billion gal (1.3 trillion l).

The impoundment of water behind a dam causes the velocity of the water to drop. Sediment carried by the river is dropped in the still water at the head of the lake. Below the dam, the river water flows from the clear water directly behind the dam. Because the river no longer carries any sediment, the erosive potential of the river is increased. Erosion of the channel and banks of the river below the dam will ensue. Even further downstream, sediment deprivation affects shoreline processes and biological productivity of coastal regions.

This problem has occurred within the Grand Canyon below Glen Canyon Dam. After the construction of the dam was completed in 1963, erosion of the sediment along the beaches began because of the lack of incoming sediment. By the early 1990's, many beaches were in danger of disappearing. In the spring of 1996, an experimental controlled flood of the river below Glen Canyon Dam was undertaken to attempt to redistribute existing sediments along the sides of the channel. While many of the beaches were temporarily rebuilt, this redistribution of sediments was short lived. Research on this issue is continuing, however, the fundamental problem of the lack of input sediment for the river downstream of the dam remains unresolved.

The environmental changes described above create a new environment in which native species may or may not be able to survive. New species frequently invade such localities, further disrupting the system. Early photographs of rivers in the southwest desert illustrate the dramatic modern invasion of non-native plants. Entire lengths of these rivers and streams have been transformed from native desert plants to a dense riparian environment. Native species that formerly lived in this zone have been replaced as a result of the changes in river flow patterns. The most commonly cited species affected by the presence of dams is the salmon . Salmon have been isolated from their spawning streams by impassable dams. The situation has been addressed through the use of fish ladders and by the use of barges to transport the fish around the obstacles, but with only limited success.

David B. Goings

Resources

books

Dunar, Andrew J., and Dennis McBride. Building HooverDam: An Oral History of the Great Depression. New York: Twayne Pub, 1993.

Fradkin, Philip L. A River No More: The Colorado River and the West. Berkeley: University of California Press, 1996.

High Country News. Western Water Made Simple. Washington, DC: Island Press, 1987.

Keller, Edward. Environmental Geology. Upper Saddle River: Prentice-Hall, Inc., 2000.

Pearce, Fred. The Dammed: Rivers, Dams, and the ComingWorld Water Crisis. London: Bodley Head, 1992.

Reisner, Marc. Cadillac Desert: The American West and its Disappearing Water. New York: Penguin Books Ltd., 1993.

periodicals

"Water: The Power, Promise, and Turmoil of North America's Fresh Water." National Geographic Special Edition. November 1993.

other

"Colorado River Watershed." Arizona Department of Water Resources. [cited October 16, 2002.] <http://www.adwr.state.az.us/AZWaterInfo/OutsideAMAs/UpperColoradoRiver/Watersheds/coloradoriver.html>.

"Lower Colorado River Operations." United States Bureau of Reclamation. [cited October 16, 2002]. <http://www.lc.usbr.gov/lcrivops.html>.

"Controlled Flooding of the Colorado River in Grand Canyon." United States Geological Survey. February 14, 1999 [cited October 16, 2002]. <http://az.water.usgs.gov/flood.html>.

KEY TERMS

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Arch dam

—A thin concrete dam that is curved in the upstream direction.

Buttress dam

—A dam constructed of concrete and steel that is characterized by angled supports on the downstream side.

El Niño

—The phase of the Southern Oscillation characterized by increased sea water temperatures and rainfall in the eastern Pacific, with weakening trade winds and decreased rain along the western Pacific.

Embankment dam

—A simple dam constructed of earth materials.

Gravity dam

—A massive concrete or masonry dam that resists the force of the water by its own weight.

Impoundment

—The body of water, a pond or lake, that forms behind a dam.

Permeability

—The capacity of a geologic material to transmit a fluid, such as water.

Spillway

—A passage for water to flow around a dam.