Q.- Explain the Unit hydrograph Method for estimation of peak flood discharge in a river.

Ans.-

Unit hydrograph Method

A hydrograph is a representation of hydraulic characteristic of river discharge . A point on the hydrograph shows the available discharge at that time . The area under the hydrograph came between any two intervals of time say t₁ to t₂ shows the volume of water that has passed the measuring point in time t₁ to t₂.

A concept of unit hydrograph is based on the fact that for a given point in a given stream of a basin the storm of different intensity but of the same unit duration produces hydrograph of surface run off of equal length of time and of the similar shape. The ordinates are proportional to the magnitude of rain fall with the assumption that the rain fall is distributed uniformly over the entire area.

The unit hydrograph is thus defined as the hydrograph which is produced by a rainfall of given duration uniformly distributed over the catchment and which produces a direct run off volume of a unit depth of 1 cm over the entire catchment area.

Hydrograph method consists of constructing a unit hydrograph or distribution graph for the catchment and then by use of given rainfall data, the hydro graph runoff is calculated.