A huge amount of water stored behind the dam is called a reservoir. Water stored by the reservoir during different months of the year is different. This storage is divided into different zones called storage zones of a reservoir. Before understanding the storage zones of a reservoir firstly, we have to understand the different levels of water in the reservoir.
Minimum Pool Level
It is the lowest level of water which is kept for normal operation of the reservoir. Water below this level can’t be used. This level is fixed by the elevation of the lowest outlet or if the dam is a multipurpose one, the minimum required to head for the generation of power governs the Minimum pool level.
Normal Pool Level
Also called maximum conservation level. It is the level of water during the normal operation of the dam. At this level, the crest of the spillway is provided.
Maximum pool level.
It is also known as, full reservoir level. During the rainy season or flood conditions, water may get rise above the normal pool level although a spillway is provided at normal pool level. This maximum level to which water can be stored in a reservoir is termed as the full reservoir level.
Storage zones of the reservoir
Dead Storage of a Reservoir
Water stored below the minimum pool level is called dead storage. It is not of much use in the normal operation of the reservoir but it provides the minimum required head for the power generation.
Useful Storage of a Reservoir
Total amount of water stored between the minimum pool level and normal pool level is called useful storage. This water is useful for irrigation purposes and power generation purposes.
Surcharge Storage of a Reservoir
Volume of water stored between normal pool level and maximum pool level is called surcharge storage. During floods, water level rises upto the maximum pool level. This water flows down through the spillways provided in the dam.
Bank Storage of a Reservoir
Reservoirs are quite huge in size and they are confined by the earthen banks. Usually, a combination of concrete and earthen dams are constructed. Concrete dams are built at the location of spillways. These earthen dams store water due to the pores in soil and when the reservoir gets depleted, water from the earthen pores comes out. This water is called bank storage. This storage is in addition to the total volume of water stored by the reservoir in the forms of dead, useful, and surcharge storage. Hence, it increases the capacity of the reservoir.
Valley Storage of a Reservoir
It is the storage which is available in the stream channel even before the construction of the dam. In other words the volume to water stored by the river or stream in its valley before the construction of reservoir is called valley storage.
Working of Reservoir
The water level of the reservoir will normally fluctuate between the minimum pool level and the normal pool level. Water level cannot go below minimum pool level, this ensures the minimum available head for the turbines in case of power generation. This fluctuation normally happens in dead storage zone and from this storage irrigation demands are fulfilled.
During the rainy season water level increases. As soon as the water goes above the normal pool level, it is discharged through spillways at a controlled rate. Water level can reach up to the Maximum pool level during flood conditions. When flood subsides, excess water will continue to be discharged and eventually water level reaches the normal pool level.