A RESERVOIR (from French réservoir – a "storehouse" ) is a storage space for fluids. These fluids may be water, hydrocarbons or gas. A reservoir usually means an enlarged NATURAL or ARTIFICIAL LAKE, STORAGE POND or IMPOUNDMENT created using a dam or lock to store water. Reservoirs can be created by controlling a stream that drains an existing body of water. They can also be constructed in river valleys using a dam. Alternately, a reservoir can be built by excavating flat ground or constructing retaining walls and levees .
Tank reservoirs store liquids or gases in storage tanks that may be elevated, at grade level, or buried. Tank reservoirs for water are also called cisterns .
Underground reservoirs are used to store liquids, principally either water or petroleum , below ground.
* 1 Types
* 1.1 Reservoirs dammed in valleys * 1.2 Bank-side reservoir * 1.3 Service reservoir
* 2 History
* 3 Uses
* 4 Operation
* 4.1 Terminology * 4.2 Modelling reservoir management
* 5 Safety
* 6 Environmental impact
* 6.1 Whole life environmental impact
* 6.2 Climate change
* 6.3 Biology
* 6.4 Human impact
* 8 See also * 9 References * 10 External links
RESERVOIRS DAMMED IN VALLEYS
A dam constructed in a valley relies on the natural topography to
provide most of the basin of the reservoir. Dams are typically located
at a narrow part of a valley downstream of a natural basin. The valley
sides act as natural walls, with the dam located at the narrowest
practical point to provide strength and the lowest cost of
construction. In many reservoir construction projects, people have to
be moved and re-housed, historical artifacts moved or rare
environments relocated. Examples are the temples of
(which were moved before the construction of the Aswan
Construction of a reservoir in a valley will usually need the river to be diverted during part of the build, often through a temporary tunnel or by-pass channel.
In hilly regions, reservoirs are often constructed by enlarging existing lakes. Sometimes in such reservoirs the new top water level exceeds the watershed height on one or more of the feeder streams such as at Llyn Clywedog in Mid Wales . In such cases additional side dams are required to contain the reservoir.
Where the topography is poorly suited to a single large reservoir, a
number of smaller reservoirs may be constructed in a chain, as in the
Where water is pumped or siphoned from a river of variable quality or
size, bank-side reservoirs may be built to store the water. Such
reservoirs are usually formed partly by excavation and partly by
building a complete encircling bund or embankment , which may exceed 6
km (4 miles) in circumference. Both the floor of the reservoir and
the bund must have an impermeable lining or core: initially these were
often made of puddled clay , but this has generally been superseded by
the modern use of rolled clay. The water stored in such reservoirs may
stay there for several months, during which time normal biological
processes may substantially reduce many contaminants and almost
eliminate any turbidity . The use of bank-side reservoirs also allows
water abstraction to be stopped for some time, when the river is
unacceptably polluted or when flow conditions are very low due to
drought. The London water supply system is one example of the use of
bank-side storage: the water is taken from the
Service reservoirs store fully treated potable water close to the point of distribution. Many service reservoirs are constructed as water towers , often as elevated structures on concrete pillars where the landscape is relatively flat. Other service reservoirs can be almost entirely underground, especially in more hilly or mountainous country. In the United Kingdom, Thames Water has many underground reservoirs, sometimes also called cisterns , built in the 1800s, most of which are lined with brick. A good example is the Honor Oak Reservoir in London, constructed between 1901 and 1909. When it was completed it was said to be the largest brick built underground reservoir in the world and it is still one of the largest in Europe. This reservoir now forms part of the southern extension of the Thames Water Ring Main . The top of the reservoir has been grassed over and is now used by the Aquarius Golf Club.
Service reservoirs perform several functions, including ensuring sufficient head of water in the water distribution system and providing water capacity to even out peak demand from consumers, enabling the treatment plant to run at optimum efficiency. Large service reservoirs can also be managed to reduce the cost of pumping, by refilling the reservoir at times of day when energy costs are low.
Circa 3 000 BC, the craters of extinct volcanoes in
Dry climate and water scarcity in India led to early development of stepwells and water resource management techniques, including the building of a reservoir at Girnar in 3000 BC. Artificial lakes dating to the 5th century BC have been found in ancient Greece. An artificial lake in present-day Madhya Pradesh state of India, constructed in the 11th century, covered 650 square metres (7,000 sq ft).
DIRECT WATER SUPPLY
Gibson Reservoir , Montana
Many dammed river reservoirs and most bank-side reservoirs are used to provide the raw water feed to a water treatment plant which delivers drinking water through water mains. The reservoir does not merely hold water until it is needed: it can also be the first part of the water treatment process. The time the water is held before it is released is known as the retention time. This is a design feature that allows particles and silts to settle out, as well as time for natural biological treatment using algae , bacteria and zooplankton that naturally live in the water. However natural limnological processes in temperate climate lakes produce temperature stratification in the water, which tends to partition some elements such as manganese and phosphorus into deep, cold anoxic water during the summer months. In the autumn and winter the lake becomes fully mixed again. During drought conditions, it is sometimes necessary to draw down the cold bottom water, and the elevated levels of manganese in particular can cause problems in water treatment plants.
Hydroelectric dam in cross section.
In 2005 about 25% of the world's 33,105 large dams (over 15 metres in height) were used for hydroelectricity. However of 80,000 dams of all sizes in the U.S., only 3% produce electricity. A reservoir generating hydroelectricity includes turbines connected to the retained water body by large-diameter pipes. These generating sets may be at the base of the dam or some distance away. In a flat river valley a reservoir needs to be deep enough to create a head of water at the turbines; and if there are periods of drought the reservoir needs to hold enough water to average out the river's flow throughout the year(s). Run-of-the-river hydro in a steep valley with constant flow needs no reservoir.
Some reservoirs generating hydroelectricity use pumped recharge: a high-level reservoir is filled with water using high-performance electric pumps at times when electricity demand is low, and then uses this stored water to generate electricity by releasing the stored water into a low-level reservoir when electricity demand is high. Such systems are called pump-storage schemes.
Reservoirs can be used in a number of ways to control how water flows
through downstream waterways: DOWNSTREAM WATER SUPPLY – water may
be released from an upland reservoir so that it can be abstracted for
drinking water lower down the system, sometimes hundred of miles
further downstream. IRRIGATION – water in an irrigation reservoir
may be released into networks of canals for use in farmlands or
secondary water systems.
Irrigation may also be supported by
reservoirs which maintain river flows, allowing water to be abstracted
for irrigation lower down the river. FLOOD CONTROL – also known
as an "attenuation" or "balancing" reservoirs, flood control
reservoirs collect water at times of very high rainfall, then release
it slowly during the following weeks or months. Some of these
reservoirs are constructed across the river line, with the onward flow
controlled by an orifice plate . When river flow exceeds the capacity
of the orifice plate, water builds up behind the dam; but as soon as
the flow rate reduces, the water behind the dam is slowly released
until the reservoir is empty again. In some cases such reservoirs only
function a few times in a decade, and the land behind the reservoir
may be developed as community or recreational land. A new generation
of balancing dams are being developed to combat the possible
consequences of climate change . They are called "Flood Detention
Reservoirs". Because these reservoirs will remain dry for long
periods, there may be a risk of the clay core drying out, reducing its
structural stability. Recent developments include the use of composite
core fill made from recycled materials as an alternative to clay.
CANALS – Where a natural watercourse's water is not available to be
diverted into a canal , a reservoir may be built to guarantee the
water level in the canal: for example, where a canal climbs through
locks to cross a range of hills. Recreational-only Kupferbach
Reservoirs can be used to balance the flow in highly managed systems,
taking in water during high flows and releasing it again during low
flows. In order for this to work without pumping requires careful
control of water levels using spillways. When a major storm
approaches, the dam operators calculate the volume of water that the
storm will add to the reservoir. If forecast storm water will overfill
the reservoir, water is slowly let out of the reservoir prior to, and
during, the storm. If done with sufficient lead time, the major storm
will not fill the reservoir and areas downstream will not experience
damaging flows. Accurate weather forecasts are essential so that dam
operators can correctly plan drawdowns prior to a high rainfall event.
Many reservoirs often allow some recreational uses, such as fishing
and boating .
Water falling as rain upstream of the reservoir, together with any
groundwater emerging as springs, is stored in the reservoir. Any
excess water can be spilled via a specifically designed spillway.
Stored water may be piped by gravity for use as drinking water , to
generate hydro-electricity or to maintain river flows to support
downstream uses. Occasionally reservoirs can be managed to retain
water during high rainfall events to prevent or reduce downstream
flooding. Some reservoirs support several uses, and the operating
rules may be complex.
Most modern reservoirs have a specially designed draw-off tower that can discharge water from the reservoir at different levels, both to access water as the water level falls, and to allow water of a specific quality to be discharged into the downstream river as "compensation water": the operators of many upland or in-river reservoirs have obligations to release water into the downstream river to maintain river quality, support fisheries, to maintain downstream industrial and recreational uses or for a range of other purposes. Such releases are known as compensation water.
Water level marker in a reservoir
The terminology for reservoirs varies from country to country. In most of the world, reservoir areas are expressed in square kilometres; in the United States acres are commonly used. For volume either cubic metres or cubic kilometres are widely used, with acre-feet used in the US.
The capacity, volume or storage of a reservoir is usually divided into distinguishable areas. Dead or inactive storage refers to water in a reservoir that cannot be drained by gravity through a dam's outlet works , spillway or power plant intake and can only be pumped out. Dead storage allows sediments to settle, which improves water quality and also creates an area for fish during low levels. Active or live storage is the portion of the reservoir that can be used for flood control, power production, navigation and downstream releases. In addition, a reservoir's "flood control capacity" is the amount of water it can regulate during flooding. The "surcharge capacity" is the capacity of the reservoir above the spillway crest that cannot be regulated.
In the United States the water below the normal maximum level of a reservoir is called the "conservation pool".
In the United Kingdom, "top water level" describes the reservoir full state, whilst "fully drawn down" describes the minimum retained volume.
MODELLING RESERVOIR MANAGEMENT
There is a wide variety of software for modelling reservoirs, from
In many countries large reservoirs are closely regulated to try to prevent or minimise failures of containment.
While much of the effort is directed at the dam and its associated structures as the weakest part of the overall structure, the aim of such controls is to prevent an uncontrolled release of water from the reservoir. Reservoir failures can generate huge increases in flow down a river valley, with the potential to wash away towns and villages and cause considerable loss of life such as the devastation following the failure of containment at Llyn Eigiau which killed 17 people. (see also List of dam failures )
A notable case of reservoirs being used as an instrument of war
involved the British
Royal Air Force
WHOLE LIFE ENVIRONMENTAL IMPACT
All reservoirs will have a monetary cost/benefit assessment made before construction to see if the project is worth proceeding with. However, such analysis can often omit the environmental impacts of dams and the reservoirs that they contain. Some impacts such as the greenhouse gas production associated with concrete manufacture are relatively easy to estimate. Other impact on the natural environment and social and cultural effects can be more difficult to assess and to weigh in the balance but identification and quantification of these issues are now commonly required in major construction projects in the developed world
Reservoir Greenhouse Gas Emissions
Naturally occurring lakes receive organic sediments which decay in an anaerobic environment releasing methane and carbon dioxide . The methane released is approximately 8 times more potent as a greenhouse gas than carbon dioxide.
As a man-made reservoir fills, existing plants are submerged and during the years it takes for this matter to decay, will give off considerably more greenhouse gases than lakes do. A reservoir in a narrow valley or canyon may cover relatively little vegetation, while one situated on a plain may flood a great deal of vegetation. The site may be cleared of vegetation first or simply flooded. Tropical flooding can produce far more greenhouse gases than in temperate regions.
The following table indicates reservoir emissions in milligrams per square meter per day for different bodies of water.
LOCATION CARBON DIOXIDE METHANE
Lakes 700 9
Temperate reservoirs 1500 20
Tropical reservoirs 3000 100
Hydroelectricity And Climate Change
Depending upon the area flooded versus power produced, a reservoir built for hydro-electricity generation can either reduce or increase the net production of greenhouse gases when compared to other sources of power.
A study for the National Institute for Research in the Amazon found that hydroelectric reservoirs release a large pulse of carbon dioxide from decay of trees left standing in the reservoirs, especially during the first decade after flooding. This elevates the global warming impact of the dams to levels much higher than would occur by generating the same power from fossil fuels. According to the World Commission on Dams report (Dams And Development), when the reservoir is relatively large and no prior clearing of forest in the flooded area was undertaken, greenhouse gas emissions from the reservoir could be higher than those of a conventional oil-fired thermal generation plant. For instance, In 1990, the impoundment behind the Balbina Dam in Brazil (inaugurated in 1987) had over 20 times the impact on global warming than would generating the same power from fossil fuels, due to the large area flooded per unit of electricity generated.
A two-year study of carbon dioxide and methane releases in Canada concluded that while the hydroelectric reservoirs there do emit greenhouse gases, it is on a much smaller scale than thermal power plants of similar capacity. Hydropower typically emits 35 to 70 times less greenhouse gases per TWh of electricity than thermal power plants.
A decrease in air pollution occurs when a dam is used in place of thermal power generation, since electricity produced from hydroelectric generation does not give rise to any flue gas emissions from fossil fuel combustion (including sulfur dioxide , nitric oxide and carbon monoxide from coal ).
Dams can produce a block for migrating fish, trapping them in one area, producing food and a habitat for various water-birds. They can also flood various ecosystems on land and may cause extinctions.
Dams can severely reduce the amount of water reaching countries
downstream of them, causing water stress between the countries, e.g.
Farms and villages, e.g. Ashopton can be flooded by the creation of reservoirs, ruining many livelihoods. For this very reason, worldwide 80 million people (figure is as of 2009, from the Edexcel GCSE Geography textbook) have had to be forcibly relocated due to dam construction.
The limnology of reservoirs has many similarities to that of lakes of equivalent size. There are however significant differences. Many reservoirs experience considerable variations in level producing significant areas that are intermittently underwater or dried out. This greatly limits the productivity or the water margins and limits the number of species able to survive in these conditions.
Upland reservoirs tend to have a much shorter residence time than natural lakes and this can lead to more rapid cycling of nutrients through the water body so that they are more quickly lost to the system. This may be seen as a mismatch between water chemistry and water biology with a tendency for the biological component to be more oligotrophic than the chemistry would suggest.
Conversely, lowland reservoirs drawing water from nutrient rich rivers, may show exaggerated eutrophic characteristics because the residence time in the reservoir is much greater than in the river and the biological systems have a much greater opportunity to utilise the available nutrients.
Deep reservoirs with multiple level draw off towers can discharge deep cold water into the downstream river greatly reducing the size of any hypolimnion . This in turn can reduce the concentrations of phosphorus released during any annual mixing event and may therefore reduce productivity .
The dams in front of reservoirs act as knickpoints -the energy of the water falling from them reduces and deposition is a result below the dams.
The filling (impounding) of reservoirs has often been attributed to
reservoir-triggered seismicity (RTS) as seismic events have occurred
near large dams or within their reservoirs in the past. These events
may have been triggered by the filling or operation of the reservoir
and are on a small scale when compared to the amount of reservoirs
worldwide. Of over 100 recorded events, early examples include the 60
m (197 ft) tall Marathon
Reservoirs may change the local micro-climate increasing humidity and reducing extremes of temperature, especially in dry areas. Such effects are claimed also by some South Australian wineries as increasing the quality of the wine production.
LIST OF RESERVOIRS
List of reservoirs
In 2005 there were 33,105 large dams (≥15 m height) listed by the International Commission on Large Dams (ICOLD).
LIST OF RESERVOIRS BY AREA
Lake Volta from space (April 1993). Main article: List of reservoirs by surface area
THE WORLD\'S TEN LARGEST RESERVOIRS BY SURFACE AREA
RANK NAME COUNTRY SURFACE AREA
KM2 SQ MI
2 Smallwood Reservoir Canada 6,527 2,520
3 Kuybyshev Reservoir Russia 6,450 2,490
6 Bratsk Reservoir Russia 5,426 2,095
8 Rybinsk Reservoir Russia 4,580 1,770
9 Caniapiscau Reservoir Canada 4,318 1,667
LIST OF RESERVOIRS BY VOLUME
THE WORLD\'S TEN LARGEST RESERVOIRS BY VOLUME
RANK NAME COUNTRY VOLUME
KM3 CU MI
2 Bratsk Reservoir Russia 169 41
5 Manicouagan Reservoir Canada 142 34
7 Williston Lake Canada 74 18
8 Krasnoyarsk Reservoir Russia 73 18
9 Zeya Reservoir Russia 68 16
* Water portal
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