Risk Assessment Of Debris Flow Along The Yakang Expressway-Yukang Section

Yakang highway which is the first highway connecting Ganzi Tibetan prefecture with the mainland will become the transportation junction in West Sichuan area.It is constructed at the transitional zone between western margin of Sichuan basin and eastern margin of Tibetan plateau.Great difficulties for highway construction and maintenance are caused by the complex terrain,lofty mountains,deep valleys and steep slope in this zone.The highway passes through the intersection of Longmen Mountain fault zone,Xianshuihe fault zone and Anninghe fault zone.Since favorable conditions for debris flow development are created by high precipitation,frequent and intense earthquake,extremely strong faulting and increased human economic activities in this intersection,highway construction as well as local people's lives and property are threatened.Therefore,deep research on the formation mechanism and development features of debris flow in the highway region,and the establishment of reliable debris flow hazard assessment system can provide disaster evaluation and early warning information.They also have great scientific significance and practical value.Through on-the-spot investigation and document browsing,the mechanism of debris flow is realized.Assessment factors contributing more largely to the debris flow hazard in searching area are determined.Assessment factor database is established in units with drainage basin as hazard assessment unit.31 debris-flow watersheds which may have impact on Yakang highway are selected based on the movement features of debris flow.The occurrence probability of debris flow in debris-flow watersheds is simulated through the establishment of BP neural network functional approximation.Global searching ability of genetic algorithm is used to optimize the initial weight value and threshold value of BP neural network,strengthen the network convergence,and improve the prediction accuracy.Debris flow hazard assessment is modeled for GA-BP neural network in research area.Finally,hazard assessment of debris flow along highway with drainage area as unit is realized,and the assessment result is strongly reliable with a certain practical significance.The main research tasks in the thesis are as follows:(1)DEM as the base map and the real situation in research area as consideration,ArcGIS10.2 platform is used to divide the research area into watersheds.The research area is divided into sub-watersheds with the threshold value of 1km2,3km2,5km2 and 10km2 respectively.After the sub-watersheds in research area are acquired,the assessment units influencing the highway are selected according to the movement features of debris flow,the disaster form of debris flow and Google Earth's overplay analysis of three-dimensional real terrain.Finally,31 assessment units are determined to study the debris flow hazard along the highway.(2)Based on the condition of debris flow formation in the research area,assessment factor system for highway debris flow is constructed,which includes relief,average gradient,fault density,cutting density,seismic intensity,mean annual precipitation,annual precipitation variation coefficient and NDVI.In this thesis,the 9 factors are used to establish assessment factor database with watershed as a unit.(3)BP neural network is applied to build assessment model of debris flow hazard.Besides,the initial weight and threshold value of BP neural network are optimized with genetic algorithm in order to improve the network performance.Compared with non-optimized BP neural network,the optimized one through training has a better performance.Therefore,it is concluded that this network model is suitable for the study of hazard assessment of debris flow along highway.(4)The divided watershed units are used to assess the debris flow hazard along the research highway.It is concluded that there is 1 extremely dangerous watershed,12 high-dangerous watersheds,6 moderate-dangerous watersheds,and 12 low-dangerous watersheds.