Correct Answer - Option 2 : Useful storage + surcharge storage - valley storage
i) Bank Storage: When the reservoir is filled, a certain amount of water seeps into the permeable reservoir bank. This water comes out when the reservoir gets depleted. This volume of water is known as bank storage.
ii) Useful storage: The volume of water stored in the reservoir between normal and minimum pool level is called useful storage.
iii) Surcharge Storage: The volume of water stored between the maximum pool level and normal pool level is called surcharge storage.
iv) Valley Storage: Before the construction of the dam, a variable amount of water is stored in the stream channel is called valley storage.
After construction of the dam, storage increases and there is a net increase in the storage is equal to the storage capacity of a reservoir minus natural valley storage.
Effective storage for flood mitigation = Useful Storage + Surcharge Storage – Valleys Storage