It is defined as the hydrograph of direct runoff resulting from unit depth (1cm) of rainfall excess occurring uniformly over the the basin and at a uniform rate for a specified duration (D hour).
Assumption of unit hydrograph
(i) The effective rainfall is uniformly distributed within its duration
(ii)The effective rainfall uniformly distributed throughout the whole area of catchment.
(iii) The characteristics of the drainage basin are assumed to be constant.
(iv) Ordinate of unit hydrograph do not contain base flow.
(v)The shape of the hydrograph is same for system of same duration
(vi)Hydrograph of storm of longer duration may be constructed by superimposing unit hydrograph of shorter duration .
Limitation Of Unit Hydrograph
(i) The assumption of uniform intensity of rainfall throughout its duration and overall area of catchment is not possible
(ii) Unit hydrograph gives good results for catchment area less than 5000km^2.
(iii) It is valid only for catchment having unchanged characteristics with respect to time.
(iv) This method is not applied where snowfall form a significant parts of ppt
(v) The hydrograph of longer duration constructed by superposition principle are not very correct .
Application Of Unit Hydrograph
To construct storm hdrograph of design storm which are required in the design of hydraulic structure.
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