Hazard Assessment Of Debris Flow In Lanzhou Based On GIS

Lanzhou has undulating topography,thick loess cover,poor geological conditions and less vegetation coverage.So Lanzhou has become a frequent occurrence area of debris flow,which seriously threatens people's lives and property safety.The study of debris flow,includes two aspects,one is the mechanism analysis of debris flow,and the other is properties of debris flow,which includes debris flow susceptibility and the attributions of velocity,flow volume,water depth and impact force.In this study,the terrain factors(including Slope,Aspect,Slope of Slope,Slope of Aspect,Curvature,Slope Length,Surface Roughness,Surface Relief,Elevation Variation Coefficient and Surface Cutting Depth)were extracted,based on the ArcGIS software and high-resolution Aster DEM.And then the spatial relationship between debris flows and terrain factors were analyzed Based on the five evaluation indexes(topography,lithology,landform,rainfall and vegetation coverage),Three methods(i.e.the grey correlation degree method,rough dependency method and fuzzy mathematics method)were used to build debris flow sensitivity map.At last,a case study of Yuantuomao ditch,we deeply analyzed the relationship among the water depth,the flow velocity,the flow volume,caculated the impulsive force of debris flow and local stone impact force,and built the evaluation index system of debris flow hazard for Yuantuomao ditch.The conclusions were as the following:(1)From the relationship between debris flow and hazard factors(including rainfall,lithology,vegetation and landform),the debris flow mainly occurred in the area of 10~30%of vegetation coverage,with 65~80mm of monthly rainfall and the lithology layer of loess,pelite,clay and sub-clay sand;(2)After evaluating the model and from the ROC curve,the confidence intervalfrome the grey correlation degree method,rough dependency method and fuzzy mathematics method were(0.596,0.721),(0.597,0.722)and(0.748,0.847),respectively.And calculated the area under the ROC curve,the accuracy and reliability from the method of fuzzy mathmatics was better than others due to the large area;(3)From the sensitivity results of the debris flows in Lanzhou,the region of about 20.55% was located in higher sensitive area and 4.97%was high sensitive area.For example,the districts in higher sensitive area were Honggu,Xigu,Chengguan,Qilihe and Yongdeng;(4)From the debris flow study of Yuantuomao ditch,after analyzing the relationship among water level,average water depth,maximum water depth,flow velocity and flow volume in Yuantuomao ditch through field surveys and models,we found that the average water depth and maximum water depth were increased in linearly with the water level,the flowvelocity was increased in logarithmically with the water level,and the flow volume was increased in exponentially with the water level.