Terrain Analysis and Landslide Hazard Evaluation from Garhwal Himalaya: Contribution from conventional and remote sensing tools

Located in the western part of the Garhwal Himalaya, India, the upper Bhagirathi region of District Uttarkashi is subject to numerous tectonic events, viz., earthquakes, landslides, and subsidence. Extreme rainfall in monsoon and the area's shifting land use pattern combined with tectonic instability cause frequent landslides especially along the highway stretches. In this study a field survey for rock slope kinematic analysis was conducted along National Highway No. 108 connecting Dharasu to Gangotri where eleven heavily jointed rock slopes were examined and the most susceptible slopes were identified based on the joints, their spacing, aperture, roughness, filling type, and weathering state were recorded for Rock Quality Designation (RQD), Geological Strength Index (GSI), Rock Mass Rating (RMR), and Slope Mass Rating (SMR). Rock slopes at a few locations have shown a high propensity towards planar and wedge failure where the slope face and the joints are dipping in the same direction with a high dip amount for the slope i.e. up to 70° whereas in other slopes the joint set intersections have indicated wedge failure probability. Further temporal landslide inventories, from 2012 to 2018 and 2018 to 2022 were used in the spatial analytic tools to create a database for the major causative elements (topographic roughness index, slope units, slope angles, fault and lineament density, slope curvature, topographic wetness index, proximity of slopes to the highway, proximity of slopes to the stream) through ensemble GIS-based models (Shannon Entropy, Information Value, and Frequency Ratio Assessment). Comparative landslide hazard evaluation (LHE) was performed for pre-2018 (before highway expansion) and LHE post-2018 when the national highway expansion started. The southward-oriented slope units with an inclination > 45°, concave curvature, and proximity of 130 m from the highway stretch have shown more association with landslide pixels. Also, the total landslide pixels have shown a considerable increase from 11391 (up to 2018) to 17999 (post-2018), which mostly fall along National Highway 108. We deciphered the dominance of litho-structural factors that contribute to the extremely brittle nature of the rock slopes in the Dharasu region based on field and remote sensing studies.