Landslide hazard zonation , mapping and investigation of triggering factors in Phewa lake watershed , Nepal

landslide also referred to as mass movement, slope failures, slope instability, and terrain instability is the mass movement, usually sudden, of soil and debris down a steep slope (Cruden, 1991). They are caused by a variety of factors such as heavy rains, earthquake ground shaking or geological forces. Different types of landslides move down slope at a wide range of speeds (Varnes, 1978). Most of the terrain in the mountainous areas has been generally subjected to slope failure under the influence of a variety of causal factors and triggered by events such as earthquake or extreme rainfall (HMG/N,1999). Heavy rainfall and ground water on the hills saturate rocks and soil that decreases shear strength. These phenomena not only cause loss of life and property, they also pose severe threats to physical infrastructure, lake and disrupt social and economic development (Pradhan, 2007). The more rapidly moving landslides may pose a greater hazard to life because they can destroy dwellings or damage roads quickly and with little warning. Slower moving landslides will gradually cause increasing amounts of damage, but the expected movement can be anticipated (UNDRO, 1991). Several landslides occur every year in Himalayan region; damage caused by landslides is estimated to cost more than US $ 1 billion in economic losses and landslides cause more than 200 deaths every year (Pradhan, 2007).