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Dam reservoir
A reservoir is a place or containment area where large amounts of water are stored or reserved for use at a later time. A reservoir can be a natural holding place such as a lake or pond from which a city may obtain their water supply. Most reservoirs, however, are artificially constructed to capture, collect, and store water for use when it is most needed. These man-made reservoirs are built by erecting a dam across a narrow valley in the path of a river or stream or excavating a type of catchment basin in a level tract of land.[1]

Smaller reservoirs in the form of small ponds or tanks are used on farms and in cities. In a city, water may be stored in small holding tanks or reservoirs called standpipes. These tanks are built at a slightly higher elevation than the city’s buildings, creating enough pressure in effect to force the water to the tops of the buildings for use.[2]

Water in a reservoir may be stored and used for a range of purposes. Off-stream, consumptive uses for the water may include irrigation as a drinking water supply. In-stream, non-consumptive uses for the water include hydroelectric power, flood protection and control, navigation, esthetic and recreational development, and wildlife habitat improvement programs. Reservoirs may also be constructed to improve the existing holding capacity of a lake or create new lakes on rivers, creeks, or coulees.[3]

All major river systems throughout the world today have reservoirs in their drainage basins and a number have multiple or cascades of reservoirs. In fact, it is been estimated that about 25 percent of the world's water that was previously flowing out to the ocean is now impounded in reservoirs.[4]


[edit] History

The very first use of reservoirs dates back to ancient times. Reservoirs provided protection from flooding and severe drought.

About 4,000 years ago, China, Egypt, and Mesopotamia began to build smaller reservoirs to hold water for drinking and to irrigate crops. These reservoirs were very rudimentary, constructed by blocking a river or stream with soil and sticks. The process involved damming a natural depression or pit, or forming a depression alongside a river and then excavating a channel to divert the water from the river into it. This eventually led to the fabrication of larger sized reservoirs.[5]

In time, the notion of using reservoirs as a source of power caught on. Waterwheels first were used to move the water. Today, this has been replaced with the use of hydroelectric power.[6]

[edit] Proliferation of Super Dams

The construction of man-made reservoirs proliferated in the first part of the 20th century, particularly in the U.S. In the 1930s, construction of large dams such as the Hoover Dam between Arizona and Nevada and its reservoir, Lake Mead, set a precedent in dam and reservoir construction, standing as an example of the “efficient application of engineering techniques to water management.”[7] 

According to information from the Data of World Registers of Dams, an organization that records all dam structures worldwide measuring more than 49 feet (15 m) in height, 1968 was the breakout year in dam building activity. More intensive dam construction ensued throughout the 1970s. By 1982, there were more than 35,000 dams around the world higher than 49 feet (15 m), excluding China.[8]

The trend since the 1970s has been to build larger dams and reservoirs. There are more than 28 dams measuring 656 feet (200 m) high, with another 23 planning to be built or already under construction. The number of reservoirs is 73, including another 17 on which work has already started. When these new super dams are constructed they will make the size and height of the Hoover Dam and its reservoir, Lake Mead, will pale in comparison.[9]

For the most part, reservoir construction today is largely carried out in North America and Europe. Most of the dams and reservoirs being constructed today are in developing countries such as Africa, Latin America, and Asia.[10]

Today, there are about 40,000 large reservoirs worldwide used for water supply, power generation and flood control.[11]

[edit] How it Works

Surface water flows naturally into the reservoir from different sources such as a lake, river, stream, or by rain or even ground water. Alternatively, water can be mechanically pumped with the aid of electric water pumps. A combination of the two is also possible.

The flow of surface waters, however, can vary greatly from time to time and inflow and outflow may alternate depending on weather conditions. Reservoirs impound water and are therefore essentially constructed to hold water in high flow periods, warding off flooding. They then gradually release the water again during periods of lower flow. Simply put, a reservoir regulates water production and is therefore sometimes called a regulation reservoir.[12] A spillway is used to divert excess water during above normal periods into nearby streams and rivers to keep the reservoir at the desired level.[13]

To effectively monitor water levels, the level of the reservoir is classified as being in one of three zones. A zone for useful storage is called working storage (also known as multiple purpose capacity or operating storage). In this zone, it is normal for water level to rise and fall throughout year. The surcharge zone permits a reservoir to be used for flood control. If this is a primary purpose of the reservoir, part of working storage can also be devoted to this use. In this zone, the elevation or level of the water contained in the reservoir is at the point where it is just being to flow over the upper surface of the spillway. If a maximum flood event were to occur, the water should be able to safely pass over the spillway without damaging the dam. Dead storage is reserved for sedimentation and occurs below the normal release elevation of the water.[14] 

In fact, excessive deposition of sediments is one of the biggest problems plaguing the life span of a reservoir and reducing storage capacity. Other problems affecting reservoirs include pollution, eutrophication (the aging of water with excessive growth of algae), shoreline protection, and dam leakage.[15]

[edit] Features

[edit] Shape

Though a reservoir and a lake may have commonalities, the two are also very different. A lake usually has a round shape whereas a reservoir has a dendritic appearance with arms of the reservoir radiating from the main body, as the deepest part of the reservoir will be near the dam. The dendritic shape of a reservoir is a result of water backing up into the feeding streams as the reservoir fills up.[16]

A reservoir with high banks, created by blocking river flow with a dam, will tend to be longer and narrow in shape. Off-river storage reservoirs can take on almost any shape.[17]

[edit] Size

Reservoirs can range in size from small single purpose impoundments of a few acre-feet to huge and complex multi-purpose impoundments of millions of acre-feet.[18]

[edit] Function

Single-purpose reservoirs are typically designed to serve only one function, such as irrigation, water supply, or power generation. The prevailing trend today in construction, however, is towards multi-purpose reservoirs with more than one principal function.[19]

[edit] Types

A valley reservoir is built by constructing a barrier or dam perpendicular to a flowing river.

An off-river storage reservoir, also called an embankment reservoir or bounded reservoir is an enclosure constructed parallel to a river, subsequently supplying it with water via gravity or by pumping directly from the river.

Single reservoirs are standard, but there are also reservoir systems. A cascade reservoir consists of a series of reservoirs strung out along the path of a single river. Interbasin transfer schemes  are designed to move water through a series of reservoirs, tunnels, or canals from one drainage basin to another.[20]

[edit] References

  1. World Book Encyclopedia, Vol. 16, 2005 Edition, pg. 258
  2. World Book Encyclopedia, Vol. 16, 2005 Edition, pg. 258
  3. Reservoir. The Canadian Encyclopedia, 2008-09-30.
  4. Man-made Lakes (Reservoirs). United Nations Environment Programme, 2008-09-30.
  5. Man-made Lakes (Reservoirs). United Nations Environment Programme, 2008-09-30.
  6. Man-made Lakes (Reservoirs). United Nations Environment Programme, 2008-09-30.
  7. Man-made Lakes (Reservoirs). United Nations Environment Programme, 2008-09-30.
  8. Water Engineering and Landscape, Denis Cosgrove and Geoff Petts, Belhaven Press, pg. 189
  9. Water Engineering and Landscape, Denis Cosgrove and Geoff Petts, Belhaven Press, pg. 189
  10. Reservoir Development in the Future. United Nations Environment Programme, 2008-09-30.
  11. World Water Storage in Man-Made Reservoirs. Foundation for Water Research, April, 2005. (accessed: 2008-09-30)
  12. How does it work?, 2008-09-30.
  13. Reservoir. The Canadian Encyclopedia, 2008-09-30.
  14. Encyclopedia of Science and Technology, Volume 15, McGraw Hill, pg. 425-427
  15. Encyclopedia of Science and Technology, Volume 15, McGraw Hill, pg. 425-427
  16. The Dam Truth: About Reservoirs. The Water Line, 2008-09-30.
  17. How are Lakes and Reservoirs the Same and How are They Different? United Nations Environment Programme, 2008-09-30.
  18. How are Lakes and Reservoirs the Same and How are They Different? United Nations Environment Programme, 2008-09-30.
  19. Reservoir. Britannica, 2008-09-30.
  20. Man-made Lakes (Reservoirs). United Nations Environment Programme, 2008-09-30.