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Introduction
Landslide is the result of a wide variety of geoenvironmental processes which involve geological, meteorological and human factors. The main factors which influence landsliding were discussed by Varnes (1984) and Hutchinson (1995). Most important inherent factors are bedrock geology (lithology, structure, degree of weathering), geomorphology (slope gradient, aspect, and relative relief), soil (depth, structure, permeability, and porosity), land use/land cover, and hydrologic conditions. Landslides are triggered by many extrinsic causative factors such as rainfall, earthquake, blasting and drilling, cloudburst, and flashfloods.
Tehri reservoir (67 km long) was developed due to the construction of a 260.5 m high dam across the confluence of Bhagirathi and Bhilangana rivers on the highly rugged Lesser Himalayan terrain. Several studies have indicated that impoundment of the reservoir has induced negative impacts on the surrounding geoenvironmental system (AHEC 2008). A number of villages are situated all around the rim of the reservoir. Due to readjustment of slopes during drawdown conditions of the reservoir, the slopes on which villages are located have been rendered unstable in many areas in addition to loss of huge areas of farm land. Geoenvironmental factors such as slope, relative relief, hydro-geological condition, lithology, and structural discontinuity are responsible for the slope instability in the reservoir rim region. Characterization of landslide causative factors and a comprehensive landslide probability mapping are the most important planning strategies for mitigation. Identification of the landslide probable zones are based on the assumption that landslides in the future will be more likely to occur under those conditions which led to the past and present instability (Varnes 1984; Carrara et al.1995; Guzzetti et al.1999; Kanungo et al.2006). ‘Landslide susceptibility zonation’ (LSZ) term adheres to principles of landslide hazard zonation (LHZ) and landslide susceptibility mapping (LSM). For landslide study, Varnes (1984) defined zonation as division of the land surface into areas and ranking these areas according to the degree of actual or potential hazard from landslides or other mass movement on the slopes. On the other hand, Brabb (1984) introduced the term ‘landslide susceptibility’, which is the spatial probability of occurrence of landslide based on a set of geoenvironmental factors. Both terminologies have been extensively used in landslide hazard study (Anbalagan 1992; Pachauri and Pant 1992; Nagarajan et al.1998; Gupta