Risk Assessment Of Geological Disasters In The Segment From Lameng To Zhongshan In The Lower Reaches Of Nujiang River

The study area is located in the valley area of the lower reaches of Nujiang River at the south end of Mt.Hengduan.The complex structural background,broken rock(soil),frequent and strong earthquakes,abundant rainfall,and strong human activities in the area lead to the development of landslides,collapses,debris flows and other geological disasters under the coupling effect of virious internal and external dynamics,and it has brought great trouble to the people's life and development,studying its developmental characteristics and conducting risk zoning are of great practical significance for the prevention and mitigation of disasters in the research area.By means of data collection,remote sensing interpretation and field investigation,then based on the comprehensive analysis of the geological environment conditions in the study area,the development laws and influencing factors of geological disasters were studied and the risk zoning evaluation of geological disasters was completed.On the basis of previous studies,this paper has made the following understanding and progress:(1)The geological environment conditions are complex and geological disasters are frequent in the study area.Generally,the main types of geological disasters are landslide,collapse and debris flow.The spatial and temporal distribution characteristics of these geological disasters are obvious and the development characteristics are also different.The main features are as follows:(1)most of them are distributed along the left bank of Nujiang River;(2)most of them occur in the period of strong earthquake,heavy rainfall and strong human activities;(3)landslides are mainly distributed along both sides of the tributaries,and the main type is rock landslide,the scale is mainly large and medium,the deformation and failure modes are mainly bending and fracturing;(4)collapses are mainly distributed along the sides of main roads,and the main type is rock collapse,the scale is mainly small and medium,the deformation and failure mode is mainly sliding;(5)debris flows are mainly distributed along the main stream of Nujiang River,and the main type is gully-type debris flow,the scale is mainly small and medium,all of them are rain-induced and the loose solid matter mainly comes from the erosion in slope or gully.(2)The formation of geological disasters in the study area is controlled by the coupling of many factors,among which the fault,human activity,lithology and vegetation are the main factors.In this paper,11 impact factors have been selected and assessed as evaluation indexes,consisting of the slope gradient,slope aspect,elevation,relief amplitude,structure of bank slope,lithology,fault,annual rainfall,river,vegetation and road,and on this basis,the risk assessment system is constructed.Then using the analytic hierarchy process method to get the weight value of multi factor combination of different types of disasters.The results show that the factors such as fault,road(human activity),lithology and vegetation have higher weight values,which have a greater impact on the occurrence of disasters.(3)The AHP-Information method and the weight of evidence method were used to evaluate the risk of geological disasters in the study area,and the ROC curve was used to compare the reliability of the two methods.The results show that the risk zoning of AHP-Information method and weight of evidence method can better reflect the spatial reality of existing geological disasters,but the reliability of weight of evidence method is higher,which is more suitable for the risk assessment of geological disasters in the study area.(4)The risk level of geological disasters is divided into three categories:high risk,medium risk and low risk.The results show that the overall risk level of geological disasters in the study area is relatively high.The area of high risk area is 1095 km~2,which account for 29%of the whole area,mainly distributed in Zhongshan,Mucheng,Bizhai and Lameng;the area of medium risk area is 1924 km~2,which account for 50%of the whole area,mainly distributed in Mengnuo,Jiufang,Wanxing,Taiping and Heyuan;the area of low risk area is 808 km~2,which account for 21%of the whole area,mainly distributed in Pingda,Menpeng and Xiaomengtong.