Optimization of runoff Coefficient and Concentration Time in Estimating Flood Discharge Values by SCS Method (Case Study: Catchment Basin of Kohanrood River)

Estimation of floods in a basin with various return periods is one of the effective management strategies for reducing flood damage. One of the methods for estimating flood discharge is to make synthetic unit hydrograph using the physical characteristics of the basin. The more accurate inputs of the model, the more validated results. Hence, in basins in which Instantaneous peak discharge is recorded, by comparing observed flood discharge values and modeled values by synthetic unit hydrograph, the physiographic parameters of an area can be optimized to the extent possible and accurate information can be obtained. In this study, by using the recorded data at the hydrometric station, the runoff coefficient and the lag time were calibrated by loss method and routing method of SCS unit hydrograph in the HEC-HMS software and then Flood has been modeled with different return periods, so that, the results in all the return periods have had a Nash-Sutcliff coefficient higher than 0.9 and RMSE lower than 0.5 which indicates appropriate optimization of the model. Also, the runoff coefficient and concentration time were optimized based on model calibration, which could be effective in water resource planning and soil characteristics of a watershed such as permeability.