Characteristics of landslide in Koshi River Basin, Central Himalaya

Koshi River basin, which lies in the Central Himalayas with an area of 71,500 km(2), is an important trans-boundary river basin shared by China, Nepal and India. Yet, landslide-prone areas are all located in China and Nepal, imposing alarming risks of widespread damages to property and loss of human life in both countries. Against this backdrop, this research, by utilizing remote sensing images and topographic maps, has identified a total number of 6877 landslides for the past 23 years and further examined their distribution, characteristics and causes. Analysis shows that the two-step topography in the Himalayan region has a considerable effect on the distribution of landslides in this area. Dense distribution of landslides falls into two regions: the Lesser Himalaya (mostly small and medium size landslides in east-west direction) and the Transition Belt (mostly large and medium size landslides along the river in north-south direction). Landslides decrease against the elevation while the southern slopes of the Himalayas have more landslides than its northern side. Change analysis was carried out by comparing landslide distribution data of 1992, 2010 and 2015 in the Koshi River basin. The rainfall-induced landslides, usually small and shallow and occurring more frequently in regions with an elevation lower than 1000m, are common in the south and south-east slopes due to heavy precipitation in the region, and are more prone to the slope gradient of 20 degrees similar to 30 degrees. Most of them are distributed in Proterozoic stratum (Pt-3 epsilon, Pt-3 and Pt2-3) and Quaternary stratum. While for earthquake-induced landslides, they are more prone to higher elevations (2000 similar to 3000 m) and steeper slopes (40 degrees similar to 50 degrees).