Depending upon the purposes served, the reservoirs may be classified into following categories.
Storage or conservation reservoirs
Flood control reservoirs
Distribution reservoirs
Multipurpose reservoirs.
1. Storage or Conservation Reservoirs.
These reservoirs are primarily used to maintain minimum supplies of water for irrigation, hydroelectric generation, domestic and industrial water supply schemes, etc.
during lean months of discharge in the rivers.
We know that discharge in the rivers remains changing day to day, and season to season.
During high floods the excess water in the river goes waste while in dry months it may not be sufficient to meet the minimum needs.
The storage reservoir is constructed to store the excess water of floods and released gradually as and when required.
2. Flood Control Reservoirs.
This reservoir is also called flood mitigation reservoir.
The main purpose of this reservoir is to temporarily store the flood water and release slowly at a safe rate after the floods, so that it may not cause any damage on the down stream side.
So this reservoir may be said a flood prevention reservoir also.
This reservoir requires provision of large spillways
and sluice-ways so that excess stored flood water is rapidly releas downstream,
but only at the safe rate. Figure 9.1 shows hydrographs at certain point on the river.
ABC is the natural hydrograph at the dam site having a maximum flood discharge
After construction of the dam the hydrograph gets modified because of the development of a reservoir at the back of the dam.
AB’DC is the modified hydrograph showing maximum flood discharge as Q2.
Thus, because of reservoir the flood discharge has reduce from Q1 to Q2.
Shaded are ABDB’A represents the storage to be provid in the reservoir.
The area DEC represents the excess volume being released from the reservoir after the floods have receded.
Flood control reservoirs may be further classified into two categories.
(i) Storage reservoir of Detention basins.
(ii) Retarding basins or retarding reservoirs.
(i) Storage reservoirs or Detention basins.
The reservoirs whose spillways and sluice outlets are fitted with gates and valves, are known as storage reservoirs or detention basins. Gated spillways and gated sluiceways provide more flexibility in operation.
They help in exercising better control on the reservoir and thus reservoir water can be used more wisely and usefully.
This reservoir is costly as it involves cost of gates and valves.
(ii) Retarding basins or retarding reservoirs.
In these reservoirs there are no gates at spillways and sluice outlets.
In this case sluice and spillway’s joint maximum discharging capacity is at the most equal to the maximum safe carrying capacity of the channel down stream.
As floods occur, the reservoir first of all gets filled up, upto normal level.
At this time sluice outlets are discharging out water from the reservoir.
As level in the reservoir increases further, discharge through sluice ways also increases.
At certain level of reservoir, water also starts escaping through ungated spillways.
As the level of water further rises the discharge over spill ways also increases.
At some particular level a balance will be struck between the inflow and outflow of the reservoir.
At this time level of water in the reservoir will become stable.
This condition happens only when flood inflow in the reservoir is equal to the outflow from the reservoir.
Now when floods occurring recede, inflow in the reservoir will also decrease but the outflow is at the same maximum rate.
Hence flood water which had accumulated in the reservoir will now be slowly flowing out of the reservoir.
This reservoir has the following advantages over detention basin.
(a) Gates are not required to be provided at sluice ways and spillway crests.
(b) Since there are no gates at the spillways, chances of human error in opening the gates during floods cannot take place.
(c) Since water from the reservoir is driven out in few days after floods, the land during maximum floods remains submerged only temporarily.
This submerged land can be used for growing very good crops.
But no habitation on this land should be allowed.
Retarding basins are preferred on small rivers.
3. Distribution Reservoir.
It is a small capacity reservoir which is mainly constructed to meet the water supply requirements of a particular city.
It is made of masonry or cement concrete and may be covered from the top.
This reservoir is filled by treated water at some constant rate.
Since demand of water remains fluctuating during the day, water may have to be drawn from this reservoir at times at rate much more than the inflow rate.
Hence these reservoirs allow pumping units and treatment units to work at predetermined constant rates.
During no demand or very little demand the water coming from treatment units and pump units goes on storing in the reservoir.
During peak demand this stored water from the reservoir is used to make up the required supplies.
4. Multipurpose Reservoirs.
The reservoir planned and designed keeping only one purpose in view is known as single-purpose reservoir.
Reservoir planned and designed keeping more than one purpose in view is called Multi purpose reservoir.
For example a reservoir designed to protect the downstream areas from floods and also to conserve water for irrigation, water-supply, hydroelectric purpose etc.
shall be called a Multi-purpose reservoir.
Bhakra Dam, Nagarjuna Saggar Dam are the examples of important multi-purpose projects of India.
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