Mapping Landslide Hazards in Western Nepal: Comparing Qualitative and Quantitative Approaches

Each year dozens of people are killed in Nepal as a result of landslides. To promote safe human occupation of this dynamic landscape, a priori understanding of the geologic circumstances of landslides, and knowledge of the characteristics and locations of landslides are essential. Furthermore, a method of constructing maps of natural hazards is necessary that is simple, efficient, and, above all, accurate. This paper assesses two different methods to identify landslides in western Nepal and demonstrates the capabilities of each technique. Eighty-eight landslides were mapped in the Andhi Khola River watershed in western Nepal. Instability in this area is influenced by joint patterns, rock weathering, shallow soils, and geomorphic, geodynamic, and groundwater conditions. The major triggers of landslides are excessive rainfalls, road cuts, and undercutting of slope toes by rivers. Two approaches, a qualitative approach and a statistical or quantitative approach, have been developed to identify landslide hazards. The qualitative map of hazards classified 23 percent of the area as high hazard, 22 percent as medium hazard, and 55 percent as low hazard. The statistical map of hazards classified 10 percent of the area as high hazard, 70 percent as medium hazard, and 20 percent as low hazard. The prediction accuracy of the statistical hazard map is better than that of the qualitative hazard map.