Reservoirs are meant to absorb a part of flood water and the excess is discharged through a spillway. The difference between Maximum Water Level and Full supply level in a reservoir is called surcharge storage or flood storage. A schematic diagram showing different storage zones of a reservoir are shown below:
A) → Free board B) → Surcharge storage C) → Active Storage capacity D) → Inactive storage capacity E) → Live storage capacity F) → Buffer storage NWL → Maximum water level or also called highest flood level (HFL) FSL → Full supply level MDDL → Minimum Draw down level DSL → Dead storage level (No outlet available to drain the water in reservoir by gravity)