Reservoir
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Most often, a reservoir refers to an artificial lake, used to store water for various uses. Reservoirs are created first by building a sturdy dam, usually out of cement, earth, rock, or a mixture of all three. Once the dam is completed, a stream is allowed to flow behind it and eventually fill it to capacity. Reservoirs exist in a wide variety of shapes and sizes, and can be either natural or man-made.
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[edit] Construction of reservoirs and dams
Strictly speaking, the reservoir is the storage area created by the construction of the dam, but in general parlance the two terms are interchangeable.
There are two basic types of reservoir—the commonly seen dam across a valley or the less common fully bunded dam.
A fully bunded dam has a continuous man made embankment around its entire perimeter, most commonly using a central clay core as the waterproof element. The core is held in place by earth or rock piled either side of it in suitable volumes to resist the outward forces exerted by the water. The clay is joined directly to the natural underlying material, which itself is usually clay in order for the dam to be water tight. The reservoir is filled by mechanical pumps that draw water from an adjacent water course such as a river.
If a water tight roof is added then this dam can then be used for storing treated water before it goes to the tap and is known as a "Service" reservoir.
The more common dam across a valley relies on naturally formed features to form the water tight elements. Generally, surveyors have to find river valleys which are deep and narrow; the valley sides can then act as natural walls. The best place along the valley for building a dam has to be determined according to where the dam can best be tied into the valley walls and floor to form a water tight seal. If necessary, humans have to be re-housed and/or historic sites have to be moved, e.g. the temples of Abu Simbel before the construction of the Aswan Dam, creating Lake Nasser in the Nile River, Egypt.
First, the river must be redirected slightly so as to not disturb construction of the dam. Once that is done, building of the dam may commence. This may take anywhere from a few months to a few years, depending on its size and complexity. After the dam is complete, the river is allowed to begin filling the large area behind the dam.
Some reservoirs are entirely underground. In the United Kingdom Thames Water has many underground reservoirs beneath London built in the 1800's by the Victorians, most of which are lined with thick layers of brick. Honor Oak Service reservoir, which was completed in 1909, is the largest of this type in Europe. The roof is supported using large brick pillars and arches and the outside surface is used as a golf course.
[edit] Dam and reservoir operation
[edit] Standard reservoir
A raw water reservoir doesn't simply hold water until it is needed. It is the first part of the treatment process. The time the water is held for before it is released is known as the retention time, and is a design feature that allows larger particles and silts to settle out as well as time for the biological treatment of algae and bacteria by plankton-like creatures that naturally live within the water.
Water is then released from the reservoir, generally by gravity, to be cleaned (for drinking water). In the event that major rain occurs, water can be released, decreasing the reservoir's water level.
[edit] Hydroelectric reservoir
A hydroelectric power station consists of large turbines that rely on a gravity flow of water through pipes from the dam to turn a turbine to generate electricity. The water can then be either released to the surrounding water course or pumped back into the reservoir and reused. Generally, hydroelectric dams are built specifically for electricity generation and are not used for drinking or irrigation water.
[edit] Irrigation reservoir
Water in an irrigation reservoir is released into networks of canals mainly for use in farmlands or secondary water systems. Water in an irrigation reservoir is generally not used for drinking water, but in some cases is. As with all reservoirs, water can be released if the reservoir is too full.
[edit] Flood control reservoir
Commonly known as an "Attenuation" reservoir, these are used to prevent flooding to lower lying lands, flood control reservoirs collect water at times of unseasonally high rainfall, then release it slowly over the course of the following weeks or months. See transvasement.
[edit] Recreational reservoir
Very rarely is a reservoir built solely for a recreational purpose. Most reservoirs are built to a civic purpose, but still allow fishing, boating, and other activities. At most reservoirs, special rules apply for the safety of the public.
[edit] Largest reservoirs
Lake Volta in western Africa covers around 8,500 km². Lokka is Northern Europe's largest artificial lake, 417 km² in size. The IJsselmeer is Western Europe's largest artificial lake, 1,250 km² in size.
[edit] Surface area
The following are the world's ten largest reservoirs by surface area:
- Lake Volta (8,482 km²; Ghana)
- Kuybyshev Reservoir (6,450 km²; Russia)
- Smallwood Reservoir (5,698 km²; Canada)
- Lake Kariba (5,580 km²; Zimbabwe, Zambia)
- Bukhtarma Reservoir (5,490 km²; Kazakhstan)
- Bratsk Reservoir (5,426 km²; Russia)
- Lake Nasser (5,248 km²; Egypt, Sudan)
- Rybinsk Reservoir (4,580 km²; Russia)
- Caniapiscau Reservoir (4,318 km²; Canada)
- Lake Guri (4,250 km²; Venezuela)
