Dams
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 carried by floods and debris flows.
Large dams frequently serve more than one of these purposes and many combine aspects of each of the three main categories. Operation of such multi-purpose 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 Ninö of 1983, climate and hydro-logic 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 affected 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. 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 impact of dams has been debated. While dams do perform important functions, their effects can be damaging to the environment.
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. Once a dam is 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 large compared to that of 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 1990s, 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
KEY TERMS
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.
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.
Three Gorges
Dam The world’s largest dam is the Three Gorges Dam near Sandouping, China. The 610 ft high concrete structure extends approximately 1.3 miles across the Yangtze River valley and will create a reservoir that will extend about 350 miles upstream. Although the dam itself was completed in 2006, its hydroelectric facility was not scheduled for completion until 2009 at an estimated total cost of $30 billion. It is intended to produce hydroelectric power, provide flood control, and allow large ships to navigate up the river.
The section of the Yangtze River affected by the Three Gorges dam has experienced more than 200 catastrophic floods during the past 2, 000 years. Some 4, 000 people were killed and more than 1 million left homeless after a 1998 flood that caused an estimated $24 billion in economic losses. A 1954 flood is reported to have killed about 30, 000 people. Strong and continuous growth of China’s industrial and consumer economies also increased the demand for electrical power, much of which has historically been supplied by coal burning power plants with minimal pollution control. With a design capacity of more than 18, 000 megawatts of electricity, the dam is planned to produce the same amount of power as 15 nuclear power plants.
Despite the benefits of flood control, electricity, and improved navigation, the Three Gorges Dam has been criticized for its potential environmental impact. Filling of the reservoir behind the dam will flood 13 cities, 140 towns, and 1, 300 villages upstream from the dam and displace about 1.5 million residents. Numerous mines and factories will also be flooded without removal of potentially hazardous waste, which will likely enter the water supply as reservoir level increases. The reservoir may also concentrate a large proportion of the 700 million tons of sediment and 265 billion tons of raw sewage that enter the river each year. Finally, more than a thousand known archeological sites will be lost beneath the reservoir. Although international environmental groups protested the planning and construction of the dam, the Chinese government maintained that the benefits of dam construction will outweigh any environmental costs. Concerns about construction quality and the long term safety of the dam were also raised.
Resources
BOOKS
Billington, D.P. and D.C. Jackson. Big Dams of the New Deal Era: A Confluence of Engineering and Politics. Norman, Okla.: University of Oklahoma Press, 2006.
Brooks, K.B. and Cronon, W. Public Power, Private Dams: The Hells Canyon High Dam Controversy. Seattle: University of Washington Press, 2006.
Chetham, D. Before the Deluge: The Vanishing World of the Yangtze’s Three Gorges. (Reprint ed.) New York: Palgrave Macmillan, 2004.
David B. Goings