Numerical Simulation And Risk Assessment Of Debris Flow Movement Process In Mengdong Yao Township, Yunnan

A large-scale debris flow triggered by single-point heacy rainfall was occured on September 2,2018,in Mengdong Yao Township,Yunnan Province.Debris flow disaster was never occured in this area.The awareness of disaster prevention and mitigation of local government and residents were weak and most of residential houses were densely built at the mouth of the ditches.When the debris flow rushed to the township,it caused huge casualties and economic losses.From the perspective of development trend,there are a large number of landslides in the formation area,which provide abundant loose solid matter for the debris flow.The basin is in the development stage of the debris flow.When the extreme climate is encountered again,there is the possibility of triggering a large-scale debris flow again.Thus it is necessary to carry out the risk assessment of debris flow in Mengdong Yao Township,understand the entire movement process from initiation to accumulation and obtain the disaster range and various movement parameters of the debris flow.All of these will provide a strong basis for post-disaster reconstruction site selection and prevention and control measures.Combining literature review,field investigation,and data sorting,the debris flow in Mengdong Yao Township,Malipo County,Yunnan Province is taken as the research object to obtain the development background and conditions of debris flow.Then a debris flow risk assessment model based on GA-AHP and cloud matter element analysis method is established.Massflow software is used to carry out numerical simulation and risk zoning.The specific research results are as follows:(1)The development conditions of the debris flow in the study area was studied to obtain the topographical conditions of the study area.Mengdong Township is surrounded by mountains on three sides,and the exit is to the east.Because the overall slope of surrounding mountains is steep,the water catchment conditions are very superior.Abundant rainfall,relatively short surface runoff path and short confluence time provide sufficient water source and hydrodynamic conditions.The provenance conditions are abundant,mainly including landslide,slope erosion,gully erosion,and gully bed accumulation.The debris flow is in the development stage,and it is very likely that debris flow disaster will erupt again.(2)According to literatures,8 commonly used risk factors and 2 vulnerability factors are selected.The genetic algorithm is used to improve the analytic hierarchy process to obtain the index weight.Then the cloud matter element analysis is used to obtain the correlation degree of each index,which avoids the randomness and ambiguity of the evaluation factors and makes the evaluation results more objective.The comprehensive correlation degree of each debris flow gully is obtained,and a debris flow risk level assessment model based on GA-AHP and cloud matter element analysis method is established.(3)The debris flow risk level assessment model is devieded into 5 levels.It is concluded that Shuichongxiang gully is a high-risk debris flow ditch,and Fangxiangpeng gully is also a high-risk debris flow ditch.Both ditches are highly dangerous and require reasonable prevention and control measures.The debris flow risk level obtained by this model is consistent with the actual situation,which proves that the model is suitable for risk assessment of debris flow in mountainous areas.(4)A numerical simulation model of the debris flow movement processis established.The standard DEM file of the study area is obtained and a suitable Volllmy base friction model is selected.Then the debris flow flow curve and other parameters are caculated.Perform inversion analysis of the debris flow in Mengdong Yao Township is carried out,and the accuracy of the inversion reaches 74.86%.Therefore,the selected debris flow dynamics model and parameters can be used for the prediction and analysis of debris flow in the same study area.Then the movement process of three kinds of return periods,including once in 20 years,once in 50 years,and once in 100 years,are simulated.The simulation results show that the debris flow under these three conditions will bury most of the towns and downstream farmland.The high-risk area is3774km2,accounting for 39.5% of the total area.(5)The maximum depth and the maximum momentum of the debris flow are coupled as the intensity index to obtain the intensity zone.The intensity zone and the outbreak frequency are combined to carry out the risk zone of the debris flow.Then economic vulnerability is used as the vulnerability assessment index to assign values to the different types of disaster-affected bodies in the study area and divide the vulnerability zones.Then the risk zone and vulnerability zone are coupled to obtain the debris flow risk zone.This research can realize the determination of the movement parameters of the debris flow,the reproduction of the movement process,and the quantitative assessment of disaster risk.