Comprehensive Study and Evaluation of Significant Landslides Applying Geospatial and InSAR Coherence Techniques in Garhwal Himalaya, India

Landslides are the common geo-environmental hazard in Himalayan terrain. In the Indian Himalayan terrain, landslide-related hazards have intensified in the past few decades as a result of human intervention in the geomorphological and geological equilibrium conditions. This paper investigates the occurrences and their associated factors (geomorphological, geological, human-related factors) and presents a complete geomorphological mapping of four selected landslides, i.e., landslide near the Budhakedar region (landslide-1/L1), landslide near the Saund region (landslide-2/L2), Sirobagad landslide zone (landslide-3/L3) and Rampur landslide near Sonprayag (landslide-4/L4). These slides are mainly reactivated landslides, which become active during the monsoon period. Rock strength analysis (using rebound values from a standard N-type Schmidt hammer) shows that the rebound value is around 30 in most of the cases, but in the case of landslide 2, the intact rocks have a rebound value near about 60–65. Landslides 1, 3 and 4 are rock-cum-debris slides dominated by gneiss-schist, phyllite and quartzite types of rock, respectively, and landslide 2 is mainly a rock slide. The grain size test shows that the soil is poorly graded, which is composed of gravel and sand in every case. In the case of slide 2 (rockslide), the soil is classified as gravelly soil. The landslide activeness has been detected based on interferometric coherence change detection analysis using single look complex (SLC) sentinel 1A imageries (pre-monsoon and post-monsoon). The coherence results enable us to detect surface deformation over time. The result based on the InSAR coherence technique also stipulated the surface deformation or reactivation of the slides after the effect of rainfall.