Reservoir

 A place or containment area where water is stored. Where large volumes of water are to be stored, reservoirs usually are created by the construction of a dam across a flowing stream. When water occurs naturally in streams, it is sometimes not available when needed. Reservoirs solve this problem by capturing water and making it available at later times. See also: Dam

 

 

Fig. 1  Zones of storage in a reservoir. (After R. K. Linsley et al., Water-Resources Engineering, McGraw-Hill, 1992)

 

 

 

 

In addition to large reservoirs, many small reservoirs are in service. These include varieties of farm ponds, regulating lakes, and small industrial or recreational facilities. In some regions, small ponds are called tanks. Small reservoirs can have important cumulative effects in rural regions

Purposes

Reservoirs can be developed for single or multiple purposes, such as to supply water for people and cities, to provide irrigation water, to lift water levels to make navigation possible on streams, and to generate electricity.

When used to supply water for a city, a reservoir can provide water during all parts of the year, and can even make it possible to populate dry regions. If cities have underground water available, they may rely on it; otherwise, they must have stored surface water from a reservoir, or they risk running out of drinking water.

A reservoir can supply water for irrigation of crops at precisely the time required for the maximum plant growth. Many reservoirs in dry regions capture water during wet periods and release it to farm fields at times of plant growth and stress.

Reservoirs may provide for energy generation (hydropower) through run-of-river hydroelectric plants which generate power when the water is available; or if the storage is adequate, they may provide water for energy generation at any time it is needed. Another way that reservoirs aid in energy management is through pumped storage, where electric energy is stored by pumping water to a reservoir, and releasing the water later to flow downward through a generating system. Water from reservoirs is also used to cool thermal or nuclear electricity-generating plants. See also: Pumped storage

Reservoirs also make navigation possible by maintaining river depths at required levels, usually by means of locks and dams. See also: River engineering

Another purpose of reservoirs is to control floods by providing empty spaces for flood waters to fill, thereby diminishing the rate of flow and water depth downstream of the reservoir. Such use of reservoirs has saved many lives and goods from flood damage, but reservoirs cannot control all flood problems.

Reservoirs also provide for environmental uses of water by providing water to sustain fisheries and meet other fish and wildlife needs, or to improve water quality by providing dilution water when it is needed in downstream sections of rivers. Reservoirs may also have esthetic and recreational value, providing boating, swimming, fishing, rafting, hiking, viewing, photography, and general enjoyment of nature.

 

Characteristics and configuration

 Generally, a reservoir is divided into zones of water that are reserved for different uses (Fig. 1). The zone for useful storage (also called working storage, multiple-purpose capacity, or operating storage) provides space for water storage applications. In this zone it is normal for the water level to rise and fall during the year. Sometimes people who rely on reservoirs only for recreational use complain about this, because they do not understand the necessity of using the full storage zone.

The surcharge storage zone enables the reservoir to be used for flood control. If flood control is an important purpose, part of the useful storage could also be devoted to that use. Surcharge storage occurs above the spillway crest, which is the elevation where water just begins to flow over the spillway's upper surface. The dam is designed so that the design flood, a maximum flood event, safely passes over the spillway without placing the dam at risk. The dead storage, normally reserved to be filled with sediment, occurs below the normal release elevation for water.

 

Planning

 Planning for reservoir size takes into account the statistical variation of future inflows and water demands.

The reservoir sizing process carries the risk that the capacity will be too small to meet the purposes or too large for the reservoir to fill. Therefore, the planner compiles as much historical data as possible and makes studies of how the planned reservoir would have performed if it had been in place during the historical period. These are sometimes referred to as “what if” studies. Reservoirs are complex, however, because the water supplied by the stream system is not fully predictable and is subject to variations from high to low flow periods. Thus, the water reservoir is made large enough to compensate for the risk factors.

After the location of a reservoir is determined, many aspects of the construction process must be settled through the design process, an effort that involves engineers, geologists, hydrologists, and other professionals. A major consideration is the necessity to make the dam as safe as possible to avoid placing people downstream at risk due to dam failure. While there have been few dam failures, when they do occur they may cause unacceptable levels of damage.

After a dam is built, it must be operated correctly. The key person is the operator who makes decisions about when to release or store water. In the past, reservoir operating decisions were made by rule curves, which provided the operator with simple guidelines about how much water to release and what lake levels to maintain. As the science of forecasting and the use of computers became more complex, however, reservoir operation became more sophisticated. It is not uncommon to have a reservoir control center where operators use computers to monitor weather forecasts furnished from satellite data and to simulate future demands for water in order to make decisions about water releases. They may also be bound by legal requirements to release water for downstream users, including fish and wildlife. As the demands for water increase in developed areas, conflicts over use of the water arise, especially in water-short areas. This requires the reservoir owner or management agency to develop conflict-resolution procedures.

Reservoirs require continuing attention from owners and managers. Problems may include excessive deposition of sediments on the lake bottom which reduce the capacity to store water (Fig. 2), pollution of lake waters, eutrophication (aging of waters with excessive growth of algae), shoreline protection, and associated issues such as dam leakage or settlement. Sedimentation, a naturally occurring phenomenon, is a particularly difficult problem that is impossible to fully control, and it may ruin a reservoir's capacity to store water. See also: Eutrophication; Water pollution

 

 

Fig. 2  Sediment accumulation in a typical reservoir. (After R. K. Linsley et al., Water-Resources Engineering, McGraw-Hill, 1992)

 

 

 

 

 

Controversies

The construction or alteration of reservoirs is often resisted by some people because of perceived negative side effects. One side effect is reservoir evaporation, which can consume waters that could otherwise be used in the river environment for fish and wildlife or for flushing salts through the stream system. As a result of evaporation, the microclimate around the reservoir might even be altered. Seepage caused by the reservoir impoundment can change the local patterns of underground water. In the valley floor, settlement can occur because of the weight of the water in storage. Ecosystems can be changed because the schedule of water release and the quality of the water can be altered. Also, when reservoirs are built, large numbers of residents may have to be resettled. See also: Water supply engineering

Neil S. Grigg

 

Bibliography

 N. S. Grigg, Water Resources Management, 1996

R. K. Linsley et al., Water-Resources Engineering, 1992

D. R. Maidment (ed.), Handbook of Hydrology, 1993

alifazeli = egeology.blogfa.com