PS-InSAR based Slope Deformation Monitoring in the Bhagirathi Valley, Uttarakhand Himalaya

Slow-moving, deep-seated landslides represent a significantly underestimated geologic hazard, incurring huge economic loss and persistent long-term risk to communities annually. Further, they have the potential to evolve into catastrophic events, which necessitates continuous monitoring to better understand their dynamics, minimize potential losses, and implement appropriate mitigation measures. The present study aims at understanding the dynamics of the slow-moving slopes housing villages such as Bhatwari, Raithal, and Barsu in the Bhagirathi Valley, Uttarakhand Himalaya, by means of PS-InSAR techniques. A total of 129 ascending-pass and 114 descending-pass scenes of Sentinel-1, from January-2021 up to March-2025, have been utilized to estimate slope velocities along the radar line-of-sight (LOS) for each pass, using open-source tools such as ISCE and StaMPS.  Further, these LOS velocities were decomposed to obtain vertical (up-down) and horizontal (east-west) velocities. The results reveal that Raithal (elevation ~2150 m), on middle of the slope, is subsiding at ~3 mm/year with an eastward movement of ~5 mm/year. Bhatwari (1650 m), on the lower slope, shows eastward creep at ~4 mm/year and upliftment at ~2 mm/year, suggesting rotational landslide activity. Barsu (2262 m), situated at a slope ~3 km upstream, exhibits eastward movement at ~6 mm/year and subsidence at ~3 mm/year. Field investigations corroborate these findings, revealing features such as scarps, cracks, tilted structures, disrupted roads, and longitudinal and transverse ponds. The persistent creeping suggests the potential for sudden slope failure during heavy rainfall or earthquakes, which may dam the Bhagirathi River, and the impoundment may further trigger cascading downstream hazards. Therefore, there is a need for a comprehensive investigation integrating the PS results with the slope stability analysis that assesses the role of geology, rainfall, and earthquakes. This integration shall assist in estimating the risk posed by the failure and further help in mitigation planning.