Definition of Reservoir

when a barrier is constructed across a river in the form of dam, water gets stored on upstream side of the barrier, forming a pool of water called dam reservoir or impounding reservoirs or a storage reservoirs or a river reservoirs.

Reservoirs are mainly categorized into 2 types :

• Impounding ( into which a river flows naturally ).
• Service or Balancing.

Impounding Reservoirs

• Storage or Conservation Reservoir

It can retain excess supplies during period of peak flows and can release them gradually during low flows when required. It supplies water for useful purposes such as irrigation, power generation, domestic, industrial and municipal supply.

• Detention Reservoir

In this type the water is stored for relative short period of time, until the stream can safely carry the ordinary flow plus the released water. Such reservoir usually have outlets without control gates.

• Impounding or Storage Reservoir

A reservoir with gate-controlled outlets wherein surface water retained for a considerable time and released for use at the time when normal flow of the stream is insufficient to satisfy the requirements. It is also called as the ‘Retarding Reservoir’

• Multipurpose Reservoir

It is constructed and equipped to provide storage and release of water for two or more purposes such as irrigation, flood control, power generation, etc. This reservoir would be gradually emptied just before the arrival of monsoon rains hoping that it would be filled to the brim at the end of the flood.

Service Reservoirs

• Distribution Reservoir

It is connected with distribution system (water supply project), used primarily to care for the fluctuations in demand which occur over short period and as local storage in case of emergency such as break in a main supply line.

• Balancing 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.

• Auxillury Reservoir

It is the reservoir which supplements and absorbs the spill of the main reservoir.

Storage Zones and Control Levels of Reservoir

Full Reservoir Level ( FRL )

It includes both active and inactive storage and also flood storage, if provided for. This is the highest reservoir level that can be maintained without spillway discharge or through sluice ways.

Minimum Draw-down Level ( MDDL )

It is the level below which the water will not be drawn down so as to maintain a minimum head required in power projects.

Dead Storage Level ( DSL )

Below this level, there are no outlets to drain the water in the reservoir by gravity. Below this level silt will get accumulated during the design lifespan.

Maximum Water Level ( MWL) 

The level that is ever likely to be attained during the passage of the design flood. Sometimes called as high flood level ( HFL) or high reservoir level ( HRL )

Live Storage

This is the volume of water actually available at any time between the dead storage level and full supply level. The minimum operating level must be sufficiently above the lowest discharge outlet to avoid vortex formation and air entrainment.

Dead Storage

It is the total storage below the inverted level of the lowest discharge outlet. It is not useful and cannot be used for any purpose under ordinary operating condition.

Outlet Surcharge of Flood Storage

This is required storage between FRL and maximum water level to contain the peaks of flood that might occur when there is insufficient storage capacity for them below FRL.

Buffer Storage

This is the storage just above the dead storage level upto minimum draw-down level. Release from this zones are made in dry situation to cater for essential requirements only. Dead storage and buffer storage together is called Inactive Storage.