LANDSLIDE HAZARD MAPPING IN WESTERN NEPAL: A GIS PERSPECTIVE

The tectonically active Himalayan Mountains are susceptible to landslides that create hazardous living conditions for inhabitants in the area. Each year approximately 300 people are killed in Nepal as a result of active landslides. One of the most active areas for hazardous landslides is Andhi Khola Watershed in Western Nepal. Thus, knowing the characteristics and locations of past and present landslides is fundamental to understanding the geologic situation of the region and to planning for safe human occupation. We constructed a landslide hazard map that can be used to ensure proper planning and minimization of deaths caused by poor citing of building in landslide susceptible areas. Data used to construct the landslide hazard maps were obtained from published works, topographical and geological maps, satellite images, and aerial photographs. Collection and verification of data were accomplished via fieldwork. Major landslides that occurred during the last fifty years were mapped, but only active landslides were analyzed. Map compilation was accomplished using Arc View GIS 3.2®. Thematic map layers showing slope, geology, soils, vegetation cover, and faults were created. Two maps (statistical-hazard and qualitative-hazard) showing landslide hazards were created. Three hazard zones were differentiated on the hazard maps: high, medium, low. The statistical-hazard map classified 10% of the area as high hazard, 70% medium hazard, and 20% low hazard. The qualitative-hazard map classified 23% of the area as high hazard, 22% medium hazard, and 55% low hazard. High hazard zones are primarily located in bush and grassland, rocky terrain, slopes ranging between 25° and 45°, slope aspects between 135° and 225°, and elevations ranging between 1000 and 1600 m. Eighty-eight landslides were mapped in the study area; 75 of the landslides are active, 8 are stable and 5 are dormant. Instability is influenced by joint pattern, rock weathering, shallow soils, geomorphic, geodynamic and groundwater conditions. The major triggers of landslide events are heavy rainfall and steepening of slopes through road cutting and river undercutting. The GIS database is a dynamic way to provide accurate information in a timely manner. It also allows the continual updating of the landslide map.