The functions of reservoirs are to provide water for one or more of the following purposes. Reservoirs that provide water for a combination reservoirs.
Human consumption and/or industrial use:
•Irrigation: usually to supplement insufficient rainfall.
•Hydropower: to generate power and energy whenever water is available or to provide reliable supplies of power and energy at all times when needed to meet demand.
•Pumped storage hydropower schemes: in which the water flows from an upper to a lower reservoir, generating power and energy at times of high demand through turbines, which may be reversible, and the water is pumped back to the upper reservoir when surplus energy is available. The cycle is usually daily or twice daily to meet peak demands. Inflow to such a reservoir is not essential, provided it is required to replace water losses through leakage and evaporation or to generate additional electricity. In such facilities, the power stations, conduits and either or both of the reservoirs could be constructed underground if it was found to do so.
•Flood control: storage capacity is required to be maintained to absorb foreseeable flood inflows to the reservoirs, so far as they would cause excess of acceptable discharge spillway opening. Storage allows future use of the flood water retained.
•Amenity use: this may include provision for boating, water sports, fishing, sight seeing.
Formally, the Bureau of Indian Standards code to river valley projects -Reservoirs" defines the following types of reservoirs:
•Auxiliary or Compensatory Reservoir: A reservoir which supplements and absorbed the spill of a main reservoir.
•Balancing Reservoirs: A reservoir downstream of the main reservoir for holding water let down from the main reservoir in excess of that required for irrigation, power generation or other purposes.
•Conservation Reservoir: A reservoir impounding water for useful purposes, such as irrigation, power generation, recreation, domestic, industrial and municipal supply etc.
•Detention Reservoir: A reservoir where in water is stored for a relatively brief period of time, past of it being retained until the stream can safely carry the ordinary flow plus the released water. Such reservoirs usually have outlets without control gates and are used for flood regulation. These reservoirs are also called as the Flood Control Reservoir or
•Distribution Reservoir: A reservoir connected with distribution system a water supply project, used primarily to care for fluctuations in demand which occur over short periods and as local storage in case of emergency such as a break in a main supply line failure of a pumping plant.
•Impounding or Storage Reservoir: A reservoir with gate-controlled outlets wherein surface water may be retained for a considerable period of time and released for use at a time when the normal flow of the stream is in sufficient to satisfy requirements.
ü A reservoir constructed and equipped to provide storage and release of water for two or more purposes such as irrigation, flood control, power generation, navigation, pollution abatement, domestic and industrial water supply, fish culture, recreation, etc.
ü It may be observed that some of these objectives may be incompatible in combination.
ü For example, water may has to be released for irrigation to suit crop growing seasons, while water releases for hydropower are required to suit the time of public and industrial demands.
ü The latter will be affected not only by variations in economic conditions but also by variations over a day and night cycle.
ü Compatibility between irrigation demand and flood control strategy in operating a reservoir is even more difficult for a reservoir which intends to serve both, like the Hirakud Dam reservoir on the river Mahanadi.
ü Flood wave moderation requires that the reservoir be emptied as much as possible so that it may absorb any incoming flood peak.
ü However, this decision means reducing the water stored for irrigation. Usually, such a reservoir would be gradually emptied just before the arrival of monsoon rains, anticipating a certain flood and hoping that the reservoir would be filled to
the brim at the end of the flood season.
ü However, this anticipation may not hold good for all years and the reservoir does not get filled up to the optimal height. On the other hand, if the reservoir is not depleted sufficiently well, and actually a flood of high magnitude arrives, then the situation may lead to the flood inundations on the downstream.