Study On Hazards Assessment Of Debris Flow In The Bailong River Basin For Different Scales

Debris flow is a devastating geological disaster occurred in mountain rivers and valleys.The special terrain conditions, natural conditions and geological environment determine the inevitability of collapse, landslide and debris flow disasters. This paper selects Bailong River Basin as the study area and aims to analyze the development and distribution of debris flow in the study area in order to understand the controlling and triggering factors suited for different scales. Meanwhile, this paper aims to establish the suitable index systems combined with the information method、AHP analysis method、gray correlation degree and fuzzy evaluation method、entropy weight method and Liu Xilin’s model to evaluate the debris flow hazards, and obtained the reasonable research methods and regional assessment results. The main results of the article are as follows:(1) There are three main distribution characteristics in Bailong River Basin of southern Gansu:distributing along the river valley; distributing along the deep faults; distributing along the soft rock and the places where the human activities are concentrated. The research selected a total of nine factors:the elevation, slope, landslide density, precipitation index, lithology, cumulative distance from fault interface, seismic, land use and NDVI to establish the evaluation index system for the large scale of Bailong River basin. The results showed that the location of each level is roughly equal, the high-risk and extremely high-risk level are mainly distributed in the Bailong River, Beiyu River, the fault of Shiji dam-Linjiang-Yamenkou, the distribution is consist with the soft rock; the very mild and mild level are mainly distributed in the northwest region, higher alpine tundra areas. Meanwhile, the results showed that the zoning results of debris flow hazards by information method were more in line with the actual distribution characteristics that the smaller the density of debris flow gully, the lower the risk level, the greater the density of debris flow gully, the higher the risk level. Therefore, the information method is more accurate than the AHP analysis method in the debris flow hazards assessment.(2) Due to the precipitation factor and geological conditions in the whole region can be seen as equivalent, and therefore they will not be considered. In view of this, the paper selected five factors for the evaluation of the small area as Zhouqu County. The Principal Component Analysis(PCA) method was applied to determine the weight of each index:normalized difference vegetation index (0.219), slope (0.207), the basin area (0.205), land use (0.185), relative height difference (0.184). Then the weighted summation model was used to combine with the assignment layer of each index to obtain the conclusion that the risk of the right bank of the Bailong River Basin is high compared with the left bank. The result is more accurate than the evaluation of the Bailong River basin.(3) This paper extracted nine factors to evaluate large debris flow gully and the correlation value are as follows:relative height difference (1.000),watershed cutting density (0.926),main channel length (0.873),the ratio of sediment supply along the length (0.857),tectonic density (0.846),maximum possible amount of the running-out (0.817),population density (0.809),watershed area (0.792),maximum daily precipitation (0.775); Similarly, eight factors were extracted for the small debris flow gully evaluation and the correlation value are as follows:relative height difference (1.000),average slope (0.884),maximum hourly precipitation (0.809), main channel longitudinal slope (0.754),the ratio of sediment supply along the length(0.748),rock weathering degree (0.721),loose material reserves (0.715),average width (0.710).This paper used Gray correlation degree and fuzzy evaluation method, entropy weight method and Liu Xilin’s model for single channel hazards assessment, and the results of three methods access to a correspondance percentage of about80%. At the same time, we found that the results are consistent with the fuzzy comprehensive evaluation method via the field survey and the situation analysis for the differences of debris flow gully. Therefore, it is more reliable for single gully debris flow hazards assessment based on the fuzzy comprehensive evaluation method.