AHP-Based Susceptibility Assessment on Debris Flows in Semiarid Mountainous Region: A Case of Benzilan-Changbo Segment in the Upper Jinsha River, China

The semiarid mountainous region is characterized by sparse vegetation and rich source of loose deposits, which is favorable for the formation of debris flows. Benzilan-Changbo segment in the upper reaches of Jinsha River is selected in this study. Based on field investigation and interpretation of remote sensing images, the development characteristics of regional debris flows in the semiarid area are studied. Six assessment indices are selected, including lithology, structural fault, slope gradient, relative height of the watershed, annual average rainfall and normalized vegetation index. Based on GIS, the assessment model of debris flow susceptibility for semiarid region was built using AHP (analytical hierarchy process) method, so as to obtain the grid map of debris-flow susceptibility zoning in the study area. The study area is divided into small watershed as one unit for regional statistical analysis and classification. Finally, the debris-flow susceptibility assessment map based on small watershed analysis can be obtained. The assessment results show that the very high susceptibility area and high susceptibility area of debris flow are mainly distributed in the northeast, central and southwest banks of Jinsha River, with an area of about 1040.9 km2, accounting for 35.7% of the study area. The area of low susceptibility and very low susceptibility for debris flow covers about 1341.7 km2, accounting for 46.0% of the study area, mainly distributed in the high altitudes of western and southeastern study area. Through examination, the prediction successful rate of debris flow susceptibility is 62.2%. The present actual areas of debris flow with extremely high and high susceptibility are about 651.4 km2, accounting for 51.1% of the total assessed debris-flow area, indicating the favorable comparative test results of susceptibility assessment against real debris flow occurrence.