A
reservoir is an
artificial lake used to store
water. Reservoirs are often created by building a reinforced
dam, usually out of
concrete,
earth,
rock, or a mixture across a
river or
stream. Once the dam is completed, the
stream fills the reservoir. When a reservoir is predominantly man-made (rather than being an adaptation of a natural basin) it may be called a
cistern. The term reservoir is also often used to describe underground reservoirs such as an
oil or
water well.
Types
Valley dammed reservoir
The more common dam across a valley relies on naturally formed features to form the water tight elements. Generally,
engineers look for
dam sites which are narrow with a broad area upstream; the
valley sides can then act as natural walls and the broad area upstream makes a large reservoir for the height. 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 or historic sites must be moved. For example, the temples of
Abu Simbel were moved before the construction of the
Aswan Dam (which created
Lake Nasser from the
Nile in
Egypt).
At the start of construction the river must be diverted, often through a
tunnel. Then the foundation is prepared. Once that is done, building of the dam can start. This may take anywhere from a few months to a few years, depending on its size and complexity. After the dam is complete, the diversion is removed or plugged, and the river fills the area upstream of the dam.
Bank-side reservoir
Where water is taken from a river of variable quality or quantity, it is common to construct bank-side reservoirs to store water pumped or
siphoned from the river. Such reservoirs are usually built partly by excavation and partly by the construction of a complete encircling bund or
embankment. Both the floor of the reservoir and the bund must have an impermeable lining or core, often made of puddled clay. The water stored in such reservoirs may have a residence time of several months during which time normal biological processes are able to substantially reduce many contaminants and almost eliminate any turbidity. The use of bank-side reservoirs also allows a water abstraction to be closed down for extended period at times 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 for all the water taken from the
River Thames and
River Lee with many large reservoirs visible along the approach to
London Heathrow Airport.
Service reservoir
Many service reservoirs are constructed as
water towers, often as elevated structures on concrete pillars where the landscape is relatively flat. Other service reservoirs are entirely underground, especially in more hilly or mountainous country. In the
United Kingdom Thames Water has many underground reservoirs beneath
London built in the 1800s by the
Victorians, most of which are lined with thick layers of brick.
Honor Oak 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.
Operation
A raw water reservoir does not simply hold water until it is needed. It is the first part of the
water treatment process. The time the water is held for 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 within the water.
Water can be released from the reservoir, generally by
gravity, to be cleaned for
drinking water,
generate electricity, or simply maintain the downstream flow. In the event that major
rainfall occurs, water can be released via a
spillway to avoid over-topping and compromising the integrity of the dam. 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 reservoir draws down but also to allow water of a specific quality to be discharged into the downstream river as compensation water.
Levels
The terminology for reservoirs varies from country to country. In the
United States the normal maximum level of a reservoir lake is called
full pool, while the minimum level it can function at is
dead pool. The water below this point is also called the dead pool, while the water in between is called the
conservation pool. Full pool may have different levels in summer and winter, or based on the local wet and dry seasons.
Once a reservoir reaches dead pool, it is below the level at which the dam can release it downstream. At this point, the streambed beyond the dam goes nearly or completely dry, and electricity production stops. This is also often the point at which intakes for
municipal water systems begin to suck air in, and must be extended into deeper water, where
stagnant water quality is much poorer. This can be done either permanently with longer pipes, or temporarily with large
hoses floated on small
barges, such as until a severe
drought or dam
repairs are over.
Hydroelectricity
Hydroelectric dam in cross section
A
hydroelectric power station consists of large
turbines at the base of a dam. Water from the reservoir behind the dam is channeled through pipes and delivered to the turbines, which in turn, spin a
generator to produce
electricity.
Controlling watercourses
Reservoirs can be used in a number of ways to control how water flows through downstream waterways.
Irrigation
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.
Flood control
Commonly known as an "
attenuation" or "
balancing" 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. 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 behind the dam but as soon as the flow rate reduces the water behind the dam slowly releases 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 climate change. They are called "Flood Detention Reservoirs".Because these reservoirs will remain dry for long periods, there will be a question as to the stability of the clay core as it could dry out. A British company Instant Barrage Services has developed an interesting composite core fill as an alternative to clay, made from recycled materials that seems to work and has a much lower carbon footprint.
Compensation
If a standard reservoir is built on a river which is used as a source of power, a compensation reservoir may also be built to guarantee a sufficient flow of water downstream during the working hours of the water-powered industries.
Canals
Recreational-only Kupferbach reservoir near
AachenWhere 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 to cross a range of hills through
locks.
Recreation
Reservoirs often provide for
recreational uses. Most reservoirs are built for a civic purpose, but still allow
fishing,
boating, and other activities. At most reservoirs, special rules apply for the safety of the public.
Modelling reservoir management
There is a wide variety of software for modelling reservoirs, from the specialist Dam Safety Program Management Tools (DSPMT) to the relatively simple
WAFLEX, to integrated models like the
Water Evaluation And Planning system (WEAP) that place reservoir operations in the context of system-wide demands and supplies.
History
Dry climate and water scarcity in
India led to early development of water 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 province of India, constructed in the 11th century, covered 650 square meters.
In Sri Lanka large reservoirs have been created by ancient Sinhalese kings in order to save the water for irrigation. The famous Sri Lankan king Parākramabāhu I of Sri Lanka stated " do not let a drop of water seep into the ocean without benefitting mankind ". He created the reservoir named Parakrama Samudra(sea of King Parakrama), which has astonished archeologists.List of reservoirs
Largest reservoir by region
List of reservoirs by area
The following are the world's ten largest reservoirs by surface area:
List of reservoirs by volume
Other uses
A reservoir may also refer to the water for a steam iron or humidifier, or collected from a dehumidifier or air conditioner. The term is also used for perfumes and liquid air fresheners, as well as the paint bottle for airbrushes, or for any other spray or atomizer.
The wind chests of pipe organs and the wind bags of bagpipes are reservoirs that maintain air at pressure ready for use.
Inkjet printers have reservoirs of ink to supply their print heads.See also
