Risk Assessment Of Debris Flow In Mountainous Towns ——Case Study Of Debris Flows In Daofu County Town

Daofu county is located in the northeastern part of Ganzi Prefecture,Sichuan Province.It belongs to the climate of the Qinghai-Tibet Plateau.The river network in the area is dense,the topography is complex,and the neotectonic movement is strong.Daofu county town is located in the middle of the county,and the Xianshuihe fault zone passes through the urban area.There are 10 debris flow ditches in and around Daofu county town,which have brought serious safety hazards to the county and surrounding villages.This paper takes Daofu county town and its surrounding area as the research area.On the basis of comprehensively collecting the natural environment background and existing debris flow disaster data in the study area,through field investigation,remote sensing interpretation,numerical simulation and theoretical analysis,the disaster-pregnancy background and disaster-forming conditions of the development of debris flow disaster in the study area were identified,analyzed the development characteristics of debris flow;on this basis,based on the dynamic process of debris flow,the risk assessment and division of debris flow in Daofu county town and its surrounding areas were carried out;according to the evaluation results,the corresponding debris flow prevention countermeasures were put forward.The main results achieved are as follows:1.There are 10 debris flow gully developed in the study area.The drainage area is distributed in the range of 1.00?44.08 km~2,the length of the main gully is in the range of 1.75?13.44km,the longitudinal slope of the gully bed is in the range of 135.85‰?363.25‰,and the slope of the mountain slope is in the range of 0?63°,with an average slope of 24°.The upstream of each watershed is dominated by slopes and gully provenances,and the middle and lower reaches are developed by landslide provenances,which are relatively close to the gully mouth.All watersheds have the conditions for debris flow to break out.Once encountering heavy rainfall or concentrated rainfall,debris flow can easily be induced.The types of debris flows are all rainstorm-gully type,and the fluid properties are mainly transitional,followed by sparseness;the activity is mainly high frequency,the topography of the gully accumulation area is flat,and the human activities are intense.2.Flo-2d was used to simulate the movement process of debris flow under three different rainfall frequencies(P=5%,2%,1%)under natural conditions,and data such as debris flow velocity,accumulation depth,flushing range and flushing amount were obtained..The results show that with the decrease of rainfall frequency,the maximum velocity of debris flow,the maximum accumulation depth,the flush out area and the accumulation volume gradually increase,but the average mud depth in the accumulation area does not necessarily increase.When the rainfall frequency is P=5%,all debris flows will rush out of the ditch except the Luorwa ditch and the Seismic Station ditch.The main river;when the rainfall frequency P=1%,the debris flow in Gourpu and Xiayagou can wash to the main river of Xianshui River.3.According to the numerical simulation results,combined with the architectural characteristics of the study area,two indicators,namely the accumulation depth,the product of the accumulation depth and the flow velocity,are selected to construct a dynamic process-based debris flow risk assessment model.The evaluation results show that when the rainfall frequency P=5%,the areas of high,medium and low risk areas are 7.78×10~4 m~2,12.95×10~4m~2 and 14.43×10~4 m~2;when the rainfall frequency P=2%,the areas of high,medium and low risk areas are The area of danger zone is 20.21×10~4 m~2,50.77×10~4 m~2,33.43×10~4 m~2;when the rainfall frequency P=1%,the area of high,medium and low danger zone is 60.43×10~4 m~2,89.47×10~4 m~2,63.17×10~4 m~2.Under the conditions of rainfall frequency P=5%,2%,and 1%,the debris flow accumulation areas are dominated by low-and medium-risk areas,accounting for 77.87%,80.64%,and 71.64%.The high-risk areas are mainly distributed in the debris flow paths in the accumulation area.On both sides,accounting for about 22.13%,19.36%and28.36%.4.On the basis of the risk assessment of debris flow,combined with the development and hazard characteristics of debris flow,the status of prevention and control,etc.,the prevention and control countermeasures of debris flow mainly based on interception and drainage are put forward.