Evaluation Of Debris Flow Susceptibility Considering Slope Instability Conditions

Debris flow is a two-phase fluid composed of solid material source and water.It has the characteristics of suddenness and strong destructive ability.Debris flow is a great threat to residents and buildings in mountainous areas.The essence of debris flow susceptibility assessment is to identify the high-risk zone of debris flow in the study area and provide data support for scientific prevention,monitoring and early warning of debris flow disasters.The assessment of debris flow susceptibility is beneficial to reduce the harm caused by debris flow disasters.At present,the evaluation of debris flow susceptibility mainly consider the disaster pregnant environmental conditions,and selects the environmental factors for debris flow formation to establish a susceptibility evaluation index system.Due to the lake of analysis of the formation mechanism of debris flow,the selection criteria of debris flow susceptibility evaluation index factors are not unified.Based on field investigation,root-soil zoning and direct shear test,this paper analyzes the stability of shallow slope in Anning River Basin under different conditions by using the limit equilibrium method,establishes the evaluation index system of debris flow susceptibility under different scales considering slope instability conditions,and evaluates the susceptibility of debris flow in Anning River Basin by using statistical analysis model.Based on the formation process of debris flow,the evaluation model of debris flow susceptibility in small watershed is constructed,and the evaluation of debris flow susceptibility in typical small watershed is completed.The main conclusions are as follows:(1)Based on the results of field investigation and indoor geotechnical experiment,the perpendicular climate-vegetation zoning and root-soil zoning in Anning River Basin were identified.The field investigation found that the climate and vegetation in Anning River Basin have perpendicular zoning phenomenon.With the increase of altitude,the perpendicular band spectrum of the climate in Anning River Basin is the low mountain subtropical zone,the middle mountain warm temperate zone,the sub alpine mid temperate zone and the alpine cold temperate zone,and the vegetation types appear grain crops,broad-leaved forest,coniferous forest,shrub and meadow in turn.According to this,the study area is divided into 12 perpendicular climate-vegetation areas.According to the differences of root depth and root type,the plant root types in the study area are divided into shallow root fibrous root type,medium deep root horizontal type,deep root horizontal type and deep root vertical type.Based on the sieving test results,the soil types in the study area are divided into medium sand and gravel sand.9 types of root-soil zones in Anning River Basin were obtained by superimposing the zoning results of plant root types and soil types.(2)The shear strength of various types of root-soil composites under different conditions was measured through direct shear tests.With reference to the calculation formula and classification standard of slope stability,the potential instability area of slope in Anning River Basin was determined.Generally,the root-soil complex in medium sand area has greater cohesion and smaller internal friction angle than that in gravel sand area.In the natural state,the slope in Anning River Basin is basically stable,but when the slope reaches the saturation state,the safety factor of the slope will decrease and potential instability areas will appear.The calculated potential instability area of gravel sand area in Anning River Basin is 621.838 km~2 more than that of medium sand area,and coniferous forest and broad-leaved forest are the main distribution areas of potential instability area.(3)The evaluation index system of debris flow susceptibility at the regional scale considering slope instability conditions is constructed.The evaluation index system of debris flow susceptibility with only disaster pregnant environmental factors and considering slope instability conditions is constructed respectively,and the certainty factor model is used to evaluate the susceptibility of debris flow.The frequency ratio analysis of the evaluation results shows that both index systems can effectively evaluate the susceptibility of debris flow in Anning River Basin.The AUC value of the debris flow susceptibility assessment results considering slope instability conditions is 0.767,which is 1.99%higher than that of the debris flow susceptibility assessment with only disaster pregnant environmental factors.(4)Based on the analysis of the source and formation process of debris flow,a debris flow susceptibility evaluation model considering slope instability conditions at a small watershed scale was constructed exploratively.According to the influencing factors of the two stages of source instability and material migration,the slope stability,topography and vegetation coverage are selected as the evaluation factors to construct the debris flow susceptibility evaluation model considering the slope instability conditions in Hot Water River Basin.