Demanding Patterns for Various Types of Reservoirs

1. Single purpose conservation reservoir.

This reservoir stores water only to meet one particular demand.
In all such cases, the demand for water can be predicted quite accurately for different parts of the year and required storage capacity can be accurately worked out as explained earlier.
The worked out storage capacity should be sufficient for other uses also.
The storage capacity should be justified on cost-benefit considerations also.

In such reservoirs there is no specific down stream demand.
The only down stream demand, in this case, is the rate of release of water from the reservoir.
The rate of release of water should be such that it does not exceed the safe carrying capacity of the down stream channel.

Since these reservoirs have to store huge quantities of flood water temporarily, their capacities should be ample so cause sufficient lowering of the peak of the flood discharge.
The capacity of flood control reservoir is dependent upon following factors:

(i) Hydrograph of the worst flood that is likely to enter the reservoir.

(ii) Down stream permissible high flood level.

(iii) Safe carrying capacity of the down stream channel.

The hydrograph of the worst flood inflow can be determine from the hydrological investigations of the catchment area.
The peak safe rate of outflow from the reservoir can be found out from the downstream channel conditions.
After having found out these two factors, the capacity of the reservoir require to modify the inflow peak of a value equal toor less than the down stream safe peak can be determin by trial and error method or graphical method use with the help of flood routing.
Flood routing is the process by which the hydrograph of the modifi or moderat flood can be determine.
The methods of flood routing have been explain a little later.
Hence maximum capacity of such reservoirs should be equal to design inflow less safe release in the downstream channel.
However economic background should also be consider while fixing reservoir capacity.

Nowadays practically all the reservoirs are constructed as multi-purpose reservoirs.
They have to serve more than one purpose simultaneously.
A reservoir made for conserving water for irrigation purpose may be use for flood control also and if sufficient head is available it may further be use for hydroelectric power generation also.
The water release from reservoir may be running the turbine for hydro-electric power before it is led through canals for irrigation.
The reservoir may also be acting as source of a water supply scheme along with its use for irrigation, flood control and hydroelectric power generation.

Following may be combinations for a multipurpose reservoir:

(i) Irrigation + Power

(ii) Irrigation + Power + Flood control

(iii) Irrigation + Water supply

(iv) Flood control + Water supply

(v) Irrigation + Power + Flood control + Water supply.

Besides the above combinations, fisheries, recreation etc.
also develop along the banks of reservoir. Combination of various purposes for which water released from reservoir is use is considere the most effectively utilized water.
But all the uses of water from a reservoir have their own characteristics.
All the uses are not entirely compatible with each other.
But by careful and scientific planning, all the uses may be brought into a good agreement.
This planning is also known as schedule of operations.
Hence the main feature of multi-purpose reservoirs is a scientific schedule of operations, with the help of which reservoir water can be utilized in the best possible manner.