Assessment Of Debris Flow Hazard In Zhouqu County

Zhouqu County has a great topographic relif as well as complex geological condition. As a consequence, rocks in this area are very weak and have a lot of fractures which lead to frequent occurrances of geological hazards such as debris flow and landslide. Espeically, debris flow causes significant impact on the local areas, and it has severely restricted the local economic development and infrastructure construction. Therefore, it is of great necessity to carry out an analysis of the causes and distribution of debris flow and establish an appropriate harzard evaluation system, so as to provide a help for the local disaster prevention and construction.In this paper, a detailed research on the regional geological background and the identification of influencing factors on debris flow are firstly conducted, which are then followed by an establishment of relationships between those existing debris flow points and the various impact factors. The Analysis Hierarchy Process (AHP) method is applied to build the evaluation system of debris flow considering seven impact factors, which are the existing hazard point, slope angle, relative elevation, engineering geological rock group, the distance to fault, NDVI (Normalized Difference Vegetation Index), and the average annual rainfall. With the help of ArcGIS Spatial analysis function, a25×25meter grid is used as the basic evaluation unit to achieve the assessment of the study area of debris flow hazard. The Bailongjiang River in the study area is very sensitive to debris flow, therefore the paper attempts to use the valley as evaluation unit together with three impact factors (i.e. the existing hazard point, geomorphic information entropy, average annual rainfall value) to evaluate eighty-five valley on debris flow hazard. Overall, the evaluation results have been proved as in good agreement with the reality, and the following understanding has been summarized:(1) The very special topographical conditions is the major cause of debris flow. Besides, sufficient rainfall is also providing an indispensible triggering factor. In general, debris flow in this area shows several features, including the universality, heterogeneity, multiple and periodicity etc.(2) The field investigation has found that the number of debris flow is84in total, and the average development density is0.028/km2. The lithology and geological structure of the area is significantly controlling the distribution of debris flow, and the rainfall intensity is the main triggering factor. Debris flows in Zhouqu County are mainly located in around Bailongjiang and Gongba river basin, and mainly developed in soft and hard rocks. In the areas of having similar rock and soil types, the strongger the structure and tectonic movements are, the more active the debris flows are developing.(3) Regarding to the risk assessment on the debris flow in the whole region of Zhouqu County, the evaluation result has been divided into five levels, including the very-high risk zone, danger zone, moderate-risk zone, low risk zone and very-low risk zone. The evaluation has showed that, the very-high risk areas of debris flow are located in the Bailongjiang basin and along part of the Gongba river basin, which are accounted for35.4percent of the region’s area.(4) The study area of Bailongjiang River is the most disaster-prone areas for debris flow, it is tried to use each divided valley as evaluation unit for risk assessment. Using the hydrology function of ArcGIS together with a catchment threshold value of10000, The Bailongjiang watershed is divided into85basins, which is basically in agreement with the actual situation. Since the geomorphic information entropy can reflect the intensity of tectonic activities, it is used here to represent the susceptibility of geological environment in the basin. The results shows that most of Bailongjiang river valley (52in total) are at from the middle-stage to old stage, among which33basins are at the stage of young, where the downstream debris flow activity are in higher frequency.(5) Similarly, this paper has also used the AHP method by considering the geomorphic information entropy value, the existing disaster hazard points and average annual rainfall values, to evaluate the debris flow hazard of Bailongjiang river watershed. As a result, there are24basins of very-high risk for debris flow,11of high risk,19of middle-risk,11of low risk, and20of very-low risk, when the threshold of catchment is similarly set as10000. In this paper, several typical debris flow basins are used for the verification of the evaluation results, the results show a high matching between the debris flow risk assessment based on the AHP method and the reality. Hence, it is concluded that the evaluation method in this paper can reflect the disaster development law in an objective manner, and be able to provide reliable references for the regional disater prevention work in the future.