Application of Synthetic Unit Hydrograph on HEC-HMS Model for Flood Forecasting

The surface runoff model of the HEC-HMS hydrological modeling system is mainly simulated through the unit line method. The simulated results usually had big errors because of the flood with different net rainfall magnitude using the same set of unit lines. In this paper, the Huan River Basin was selected as the study area for the flood forecasting using HEC-HMS model. In the process of the HEC-HMS hydrological model construction, the ArcGIS software was used to extract the watershed information according to the river DEM data. The net rainfall was calculated through the initial constant rate loss model. The surface runoff of the basin was calculated by the Snyder unit line model, and the basis was calculated by the exponential decay model. The river flow convergence was calculated by the Muskingum method. Based on the rainfall runoff data of 17 floods, three sets of Snyder unit lines were calculated according to the net rainfall intensity, and then three large, medium and small floods were employed to verify the flow process of the exit section of the basin. The model was calibrated and verified using historical observed data. The results showed that: The determination coefficients and coefficients of agreement for all the flood events were above 0.92, and the relative errors in peak discharges were all within the acceptable range, which belongs to A-Level forecast. The simulation accuracy of the model in the Huan River basin can be enhanced by synthesizing the Snyder unit line in the HEC-HMS model according to the net rain intensity.