Once it is decided to build a reservoir on a river by constructing a dam across it, it is necessary to arrive at a suitable design capacity of the reservoir. As has been discussed in section 4.5.1,

The reservoir storage generally consists of there main parts which may be broadly classified as: 

1. Inactive storage including dead storage 

2. Active or conservation storage, and 

3. Flood and surcharge storage. 

In general, these storage capacities have to be designed based on certain specified considerations, which have been discussed separately in the following Bureau of Indian Standard codes: 

• IS: 5477 Fixing the capacities of reservoirs- Methods 

• (Part 1): 1999 General requirements 

• (Part 2): 1994 Dead storage 

• (Part 3): 1969 Line storage 

• (Part 4): 1911 Flood storage 

The data and information required for fixing the various components of the design capacity of a reservoir are as follows: 

1. Precipitation, run-off and silt records available in the region; 

2. Erodibility of catchment upstream of reservoir for estimating sediment yield; 

3. Area capacity curves at the proposed location; 

4. Trap efficiency; 

5. Losses in the reservoir; 

6. Water demand from the reservoir for different uses; 

7. Committed and future upstream uses; 

8. Criteria for assessing the success of the project; 

9. Density current aspects and location of outlets; 

10. Data required for economic analysis; and 

11. Data on engineering and geological aspects. 

These aspects will be explained in detail in the following sections.