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)