Storage space is provided in the reservoir for storing flood water temporarily in order to reduce peak discharge of a specified return period flood and to minimize flooding of downstream areas for all floods IS: 5477 ( Part 1) : 1999 equal to or lower than the return period flood considered. In the case of reservoirs envisaging flood moderation as a purpose and having separate flood control storage, the flood storage is provided above the top of conservation pool. 

Surcharge Storage

Surcharge storage is the storage between the full reservoir level (FRL) and the maximum water level (MWL) of a reservoir which may be attained with capacity exceeding the reservoir at FRL to start with. The spillway capacity has to be adequate  to pass the inflow design flood making moderation possible with surcharge storage. 

The methods that are generally used for estimate of the Design Flood for computing the Flood Storage are broadly classified as under: 

1. Application of a suitable factor of safety to maximum observed flood or maximum historical flood. 

2. Empirical flood formulae. 

3. Envelope curves. 

4. Frequency analysis. 

5. Rating method of derivation of design flood from storm studies and application of the Unit Hydrograph principle. 

Application of a Suitable Factor of Safety to Maximum Observed Flood or Maximum Historical Flood 

The design flood is obtained by applying a safety factor which depends upon the judgement of the designer to the observed or estimated maximum historical flood at the project site or nearby site on the same stream. This method is limited by the highly subjective selection of a safety factor and the length of available stream flow record which may give an inadequate sample of flood magnitudes likely to occur over a long period of time. 

Empirical Flood Formulae
The empirical formulae commonly used in the country are the Dicken‟s formula, Ryve‟s formula and Inglis‟ formula in which the peak flow is given as a function of the catchment area and a coefficient. The values of the coefficient vary within rather wide, limits and have to be selected on the basis of judgement. They have limited regional application, should be used with caution and only, when a more  accurate method cannot be applied for lack of data. 

Envelope Curves
In the envelope curve method maximum flood is obtained from the envelope curve of all the observed maximum floods for a number of catchments in a homogeneous meteorological region plotted against drainage area. This method, although useful for generalizing the limits of floods actually experienced in the region under consideration, cannot be relied upon for estimating maximum probable floods for the determination of spillway capacity except as an aid to judgement. 

Frequency Analysis
The frequency method involves the statistical analysis of observed data of a fairly long (at least 25 years) period. It is a purely statistical approach and when applied to derive design floods for long recurrence intervals, several times larger than the data, has many limitations. Hence this method has to be used with caution.