Study On Design Flood Calculating Method And Application Of Small Watershed In Hilly Area Under Variable Environment

Over the past century,the global atmosphere and land surface habitats have undergone significant changes due to the combined effects of climate change,natural changes and human activities.Human activities also lead to great changes in the surface underlying surface,affecting the process of runoff generation and confluence,and then leading to sudden changes in river runoff in the basin.The changing environment makes the global flood and drought disasters occur frequently,which is much higher than ever before,and directly threatens the economic development and living environment of human society.Water conservancy projects,soil and water conservation and other engineering measures,as well as non-engineering measures of rainfall?or discharge?early warning,have been built in succession throughout the country to solve this problem.The rationality of design flood calculation methods and results directly affects the scale and layout of the project,as well as the accuracy of early warning indicators,thereby affecting the accuracy of real-time early warning.In order to discuss the design flood under changing environment,this paper takes the Peihe small watershed in Henan Province as the research object.Firstly,the hydrometeorological data and land use situation of the study area in the flood season in recent 30 years are analyzed,and the correction method is put forward in the stage of runoff generation and confluence.A HEC-HMS hydrological model suitable for studying climate and underlying surface changes was established,and the sensitivity of its parameters was analyzed.The main research contents and achievements are as follows:?1?To analyze the variation of rainfall and runoff and land use in Peihe small watershed during flood season.The result of trend analysis shows that both the precipitation and the maximum flood peak in flood season are decreasing,but the short duration rainfall and the flood peak magnitude in flood season are increasing.Most of the short duration heavy rainfall and flood peak flow occur after 2000.Through the linear regression equation of one variable,Mann-Kendall rank correlation test and Pearson III probability distribution function method,it is concluded that the study area changed abruptly around 2000.It is basically consistent with the conclusion that the cultivated land has decreased,the residential land area has increased greatly and the impervious area has increased since 2000.?2?The traditional design flood correction method for small watershed is proposed.Based on the hydrological data of flood season in the study area,API model,i.e.the pre-rainfall index model,is used to modify the relationship between rainfall and runoff in the runoff generation stage of the basin.The results show that the runoff generated after 2000 tends to increase compared with that before 2000,and is more prone to flood disasters in Hilly areas.In the confluence stage,DEM data is used to extract geomorphic parameters and the geomorphic instantaneous unit line is used instead of the generalized process line to calculate them.This method not only improves the accuracy of flood forecasting,but also reflects the geomorphic characteristics of the basin well.It is suitable for small watershed in hilly areas with lack of data.?3?The HEC-HMS hydrological model suitable for the study area was established.SCS curve method is used in runoff generation stage,SCS unit line method is used in confluence stage,basic flow is calculated by exponential regression method,flood routing is calculated by Muskingen method,and parameters are calibrated by measured rainfall and flood data in flood season with changing years as boundary points.?4?The sensitivity of key parameters of HEC-HMS hydrological model is analyzed.The sensitivity analysis of the key parameters such as CN value in runoff generation module,T_lag value in confluence module and K value in river flood routing module not only improves the accuracy of the model,but also provides a useful reference for the selection of parameters in areas lacking data.?5?The applicability of the two methods to design flood in hilly watershed is discussed.In this paper,API model combined with geomorphic unit hydrograph model and HEC-HMS distributed hydrological model are used to calculate the design flood of hilly watershed under changing environment.The former method requires less data.By extracting geomorphic characteristic parameters,it is more suitable for the areas lacking data,and the results are more accurate.The latter method considers the changes of climate and underlying surface conditions,and is more suitable for runoff yield and concentration analysis under changing environment.However,it requires high data requirements and needs to be calibrated and validated by measured data.The accuracy of runoff simulation is higher,and it is recommended to be used in areas with or without data.For areas without data,the rate of parameters in runoff yield and concentration and river flood routing can be referred to.The results of sensitivity analysis can be used to analyze the rationality of the simulation results with reference to the results of the former method.