UM-48 : Unit Hydrograph Applications for Flood estimation, Software package No.1

Abstract

Unit hydrograph analysis technique is one of the most commonly used techniques for design flood estimation and real time flood forecasting. By definition, unit hydrograph is the direct surface runoff hydrograph resulted at the catchment outlet due to unit (1 mm/1 cm/1 inch) rainfall excess falling uniformly over the catchment in time as well as in space for the specified duration. The unit hydrograph is basically a multiplier which converts the excess rainfall to direct surface runoff. Thus it can be said that the unit hydrograph only deals with the direct surface runoff and excess rainfall. Therefore, the baseflow must be separated from the streamflow hydrograph and losses must be accounted from the average rainfall hyetograph in order to get the direct surface runoff hydrograph and excess rainfall hyetograph respectively.

The unit hydrograph can be derived by analysing the excess rainfall and direct surface runoff of various storms for the gauged catchments. Simple conventional method is used to derive the unit hydrograph from single period storms. However, Collin's method based on trial and error procedure is preferred for use in the unit hydrograph derivation from the multiperiod storms. Some times Collin's method does not converge and provides an estimate for unit hydrograph with unrealistic shape and negative ordinates. In such a situation one has to make subjective judgements for preserving the shape of the unit hydrograph with required unit volume. The use of conceptual models for unit hydrograph derivation overcomes these deficiencies associated with the Collin's method. Among many others conceptual models, Nash and Clark models are most simple and popular conceptual models for unit hydrograph derivation. Integer Nash Model is a simplified form of the conventional Nash Model.

Whenever adequate records of rainfall and runoff are available the unit hydrograph may be derived by analysing those records. However, for many small catchments the streamflow data are either limited or not at all available (i.e. the catchments are ungauged). Therefore the unit hydrograph for such catchment can only be derived using their physiographic characteristics. The procedure used for this purpose involves the development of suitable regional relationship between the unit hydrograph parameters and physiographic characteristics for the gauged catchments which are considered to be similar in hydrological and meteorological characteristics. Snyder method of unit hydrograph derivation is widely applied for the derivation of unit hydrograph for ungauged catchments. CWC derived the