Storage reservoir is one having gates and valves installed at its spillway and at its sluice outlets. The retarding basin is one with uncontrolled and ungated outlets. The cost of gate installation is saved and re are no gates hence, possibility of human error and negligence in ir operation is eliminated.

A reservoir planned and constructed to serve various purposes toger is a multipurpose reservoir. It is designed to protect downstream areas from floods, to conserve water, irrigation, industrial needs, hydroelectric purposes, etc. Bhakra dam and Nagarjuna Sagar dam are important multipurpose dams.

A reservoir planned, designed and constructed for one purpose is called a single purpose reservoir whereas a reservoir planned and constructed to serve various purposes toger is a multipurpose reservoir. A small storage reservoir constructed within a city water supply system is called distribution reservoir

A flood control reservoir protects downstream areas by storing a portion of flood flows to minimize flood peaks. The entire inflow entering reservoir is discharged or gradually released to recover capacity for next flood. It is also called as a mitigation reservoir.

Storage reservoir is one having gates and valves installed at its spillway and at its sluice outlets whereas retarding basin is one with uncontrolled and ungated outlets. It provides more flexibility in operation and better control.

A small storage reservoir constructed within a city water supply system is called a distribution reservoir. It stores water during hours of no demand or less demand and supply water from ir storage during critical time of maximum demand.

Storage reservoirs are preferred on large rivers and require better control. It is provided with gated spillway and sluiceways for more flexibility of operation, better control and to increase usefulness of reservoir.

There are two basic types of flood-mitigation reservoirs i.e. storage reservoir and retarding reservoirs. Storage reservoir is one having gates and valves installed at its spillway and at its sluice outlets whereas retarding basin is one with uncontrolled and ungated outlets.

When floods occur reservoir gets filled and discharges through sluiceways and water level goes on rising until flood has subsided. The inflow becomes equal or less than outflow. The water gets completely withdrawn until stored water is completely discharged.

Since retarding reservoir is not always filled much of land below maximum reservoir level will be submerged only temporarily and occasionally. The automatic regulation of outflow depends upon availability of water. The maximum discharging capacity should be equal to maximum safe capacity of channel d/s.