Research On Design Flood Of Small Watershed In Semi-arid Loess Gully Region Based On HEC-HMS Model

Design flood research has always played an important role in engineering planning and mountain flood disasters prevention.Reasonable design flood calculation is of great significance for balancing project construction safety and reducing project investment.Especially in the semi-arid loess gully region of China,the lack of data and the intensification of human activities have led to many uncertainties in the design flood calculation of this type of area.In addition,most of the traditional design flood calculation methods ignore the underlying surface of the basin,and the default full basin infiltration,which weakens the flood peak flow to a certain extent,and brings some hidden dangers to the project planning and construction.Therefore,the paper takes Dashagou-a small watershed in the semi-arid loess gully region of Gansu Province as the research area.Develop two sets of scenarios(without considering the underlying surface of the basin and considering the underlying surface of the basin)using the HEC-HMS hydrological model for simulation calculations.On the one hand,the applicability of the HEC-HMS hydrological model in the semi-arid loess gully region is tested,so as to solve the shortcomings of the traditional design flood calculation method;on the other hand,the response of the runoff flood peak flow to the underlying surface change of the basin is studied.The main results are as follows:(1)According to the model parameter requirements and the applicability of each module calculation method,the flow calculation module selects the initial and constant;the direct runoff calculation module selects kinematic wave;the river flood convergence calculation selects the Muskingum Cunge and the rainfall calculation module selection frequency storm.Based on the combination of these modules,the HEC-HMS hydrological model can be applied to semi-arid loess gully-free data areas.(2)The reduction factor of frequency storm is derived from the long-term meteorological data observed by the US National Weather Service,and Combined with the average point and surface rainfall of different watershed areas in the United States.There is a certain gap between the value and the actual value of the study area.Therefore,this study introduced 0.948,0.888,0.867 and 0.846 for the point rainfall of 1 hour,3 hours,6 hours and 24 hours to reduce the error.(3)By comparing and analyzing the results of the model without considering the underlying surface of the watershed and the instantaneous unit line method and the inference formula method.The results show that the relative errors of the design frequencies(P=1%,2%,5%,10%,20%)calculated by the HEC-HMS hydrological model and the instantaneous unit line method are: 1.0%,6.7 %,11.4%,15.0%,6.8%;the relative errors with the inference formula method are 16.8%,8.7%,2.8%,2.9%,and 8.1%,respectively.The relative error of HEC-HMS hydrological model and both are less than 20% and the average relative error is less than 10%,within the error tolerance.Therefore,the design flood calculation results of the HEC-HMS hydrological model are reasonable,and the model has good applicability in the semi-arid loess gully region.(4)Considering the conditions of the underlying surface of the watershed,the parameters of the area of the impervious area in different years are introduced.The results show that the different design frequency results calculated by HEC-HMS hydrological model are larger than those without considering the underlying surface,and the runoff flood peak flow increases with the increase of the watertight area of the basin.This indicates that the response of the runoff peak flow in the small watershed to the change of the underlying surface of the basin is obvious.Considering the calculation results of the underlying surface condition of the basin,the actual flood situation of the small watershed in the semi-arid loess gully region can be reproduced more realistically.The results have a strong application value for the regional flood control planning and whether the existing existing projects have risks.