S-CURVE SYNTHESIS USING CATCHMENT CHARACTERISTICS

Abstract

The unit hydrograph technique is one of the best known methods to estimate design floods for small catchments. In order to apply this technique for ungauged catchments , several attempts have been made in past to develop synthetic unit hydrographs using catchment characteristics. The concept of S-curve is found to be of much interest in unit hydrograph theory as the unit hydrograph of any desired duration can be obtained with its help. A method is presented in this study to synthesise S-curve from measurable physiographic characteristics of the catchment. A three parametric equation has been proposed to describe the S-curve shape and an expression is derived for instantaneous unit hydrograph which is similar in form to the expression given by Nash. Relationships have been given to predict parameters of the proposed :S-curve from catchment characteristics on a regional basis. A computer program has been developed to compute the ordinates of a synthetic unit hydrograph of desired duration by giving the regional catchment characteristics as input. The values of the peak discharge and the lag time , for the synthetic unit hydrograph so obtained are found to compare reasonably well with the actual values and the values estimated by using the CWC (Central Water Commission) approach.