Development Characteristics And Risk Assessment Of Debris Flow In Fanjia Gully,Hanyuan County,Ya’an City

Fanjia gully is located in Hanyuan County,Ya’an City,Sichuan Province,with steep terrain and ample water sources.Between 1998 and 2020,six debris flows disasters of various scales broke out,causing significant economic losses,and in the future,after short-term heavy rainfall or long-term continuous rainfall,large-scale debris flows are likely to be triggered again,posing a serious threat to the industrial park at the mouth of the gully.In order to scientifically evaluate the risk of debris flow in Fanjia gully and assess the effectiveness of the in-ditch barrage project,this paper analyzes the geological conditions of Fanjia gully by means of site investigation and remote sensing interpretation,discusses the characteristics of debris flow development and dynamics,uses FLO-2D to simulate the process of debris flow movement and accumulation with different rainfall frequencies before and after the implementation of the control project,compares the prevention and control effect of the in-ditch barrage and classifies the high risk zone of Fanjia gully debris flow.The high risk zone was classified,and the risk of the Fanjia gully debris flow was evaluated by using the hierarchical analysis method,and the risk of the Fanjia gully debris flow was analyzed comprehensively.The main findings of this paper are as follows.(1)Fanjia gully has a watershed area of 9.15km~2,the main ditch is 4.56km long,the average longitudinal specific drop is 204‰,a branch ditch is developed,the static storage capacity of the material source is about 432.43×10~4m~3,the dynamic storage capacity is about27.91×10~4m~3,the average annual rainfall reaches 730.8mm,and it is a storm gully type debris flow.(2)Based on the relevant information,the kinetic characteristics parameters of the Fanjia gully debris flow were calculated using empirical formulae,and the parameters of the Fanjia gully debris flow such as volume,flow velocity,peak flow rate,total amount of primary overflow and solids flushing out were obtained.(3)Under the condition of obtaining the relevant simulation parameters,FLO-2D was used to simulate the debris flow accumulation process in Fanjia gully before and after the implementation of the treatment project for 20 years,50 years and 100 years of rainfall frequency,and the flow velocity and accumulation thickness of the debris flow were obtained,indicating that the debris flow would rush to the industrial park at the mouth of the gully for50 years and 100 years of rainfall frequency before the implementation of the treatment project.After the implementation of the treatment project,check dams can effectively intercept and control the debris flow in Fanjia gully under different rainfall frequencies.(4)Hierarchical analysis was used to rate the danger of the Fanjia gully debris flow,and it was concluded that the Fanjia gully debris flow is highly dangerous under the 50-year and100-year rainfall frequency,and the high danger zone of the Fanjia gully debris flow was divided based on the FLO-2D simulation results,and the changes of the range of the high danger zone of the Fanjia gully debris flow before and after the treatment project were obtained.