Study On Flash Flood Simulation And Early Warning In Chabagou Watershed Of Loess Plateau Based On HEC-HMS

Flash flood disasters are defined as flood events caused by the surge of water levels in small and medium-sized watersheds in mountain areas within a few hours after the start of intensive rain.Due to the steep terrain of the watersheds in the mountain area,the formation of flash floods is relatively short,and the flash floods are severely destructive,which not only caused huge economic losses,but also posed a threat to the lives of mountain residents.Therefore,flash flood disasters have always been a key issue to be solved urgently in the prevention and control of natural disasters.Due to the unique underlying surface conditions,flash floods are prone to occur in the hilly and gully areas of the Loess Plateau.For example,the “7.26” flood disaster in 2017 caused heavy losses in Zizhou County.The loss and transform mechanism is very complex,and in recent years because of the impact of climate change,the frequency of mountain flood disasters has increased.How to more accurately simulate the process of flash floods is still to be studied in depth.Therefore,this thesis aims at the simulation of mountain flood disasters and the construction of early warning indicators for mountain flood disasters in Chabagou watershed.In this study,using Arc GIS technology,the HEC-geo HMS hydrological extension module is used to model the Chabagou watershed through a DEM,the Chabagou watershed is divided into 15 sub-basins,and the relevant data such as soil type and land use classification are used to estimate the initial value of the parameters of the model.Considering the complexity of the mechanism of loss and transform in Chabagou watershed,the HEC-HMS hydrological model is applied to select different calculation methods of loss and transform.Among them,the Muskingum method and the recession method are used in the routing model and baseflow model.The loss model adopts the SCS curve number method and the deficit and constant method;the transform model adopts the SCS hydrograph method and the Snyder hydrograph method.Based on this,the SCS curve number method + SCS hydrograph method + Muskingum method + Recession method is established as scheme 1 and Deficit and Constant method + Snyder hydrograph method + Muskingum method + Recession method as Scheme 2.Two different method systems are designed to simulate the flash flood process in Chabagou watershed.By collecting data of eight flooding events from 2006 to 2015,the period from 2006 to 2012 is selected as calibration period and 2013 to 2015 as the verification period.The relevant parameters in the two schemes are optimized.The results of the study show that the calibration pass rate in scheme 1 is 100%,reaching the A-level accuracy of the Hydrological Information Forecasting Standard,and the average NSE is 0.85,reaching the B-level accuracy;in the verification period,the pass rate is 100%,reaching the A-level accuracy,the average NSE is 0.72,which achieves Blevel accuracy.The calibration pass rate in Scheme 2 is 80%,reaching the B-level accuracy,average Nash coefficient is 0.71,reaching the B-level accuracy;the pass rate in the verification period is 66.7%,reaching the C-level accuracy,an average of Nash coefficient is 0.58,reaching the C-level accuracy.In addition,the average difference in peak time,average peak relative error,average runoff depth relative error and average NSE in Scheme 1 are better than those in Scheme 2.It can be seen that the HEC-HMS model based on Scheme 1 used in the flash flood simulation of Chabagou Basin's applicability is much better.Meanwhile,the MATLAB neural network time series model is used to fit the water stage-discharge data of Caoping Hydrological Station to establish a functional relationship.Based on the early warning stage data obtained from the survey,the corresponding flash flood critical discharge is calculated by it.In addition,combined with the “Atlas of Design Storms and Floods in Small and Medium-sized Watersheds in Shaanxi Province”,the Yulin City rainstorm intensity formula are used,and the Chicago hyetograph is used to refer to the site of the "726" flood disaster-Zizhou County,Yulin City,downstream of Chabagou Cao Ping Village conducts heavy rain design research.By continuously changing the frequency of rainstorm,the rainfall is input into the model to calculate the discharge,and finally the critical rainfall value required for the simulated runoff to be close to the critical flow of flash flood is finally determined,so that the critical rainfall data is used as the early warning indicator of mountain flood in the study area.In summary,the HEC-HMS model has good applicability in the Chabagou watershed of the Loess Plateau,and can provide theoretical guidance for the hydrological simulation of mountain flood disasters in the typical watersheds of the Loess Plateau.It can also provide important reference application value with integrated prevention and control as well as the forecast and early warning of flash flood disasters in the small and medium watersheds of the Loess Plateau.