Deformation Dynamics and Hazard of Slow-Moving Landslides: The Joshimath Event, Uttarakhand Himalaya

Abstract

In early 2023, Joshimath in the Uttarakhand Himalaya, India, became a major concern due to widespread ground deformation and building damage. As a densely populated town on an unstable hillslope, it is crucial to assess ground movement patterns and associated risks. Towards this objective, we used satellite imagery, field surveys, and numerical simulations to study the Joshimath landslide from 2017 to 2023, evaluating its causes and future risks. Our Interferometric Synthetic Aperture Radar (InSAR) measurements revealed that the Joshimath landslide began moving slowly in 2018, with significant acceleration after heavy rainfall in October 2022, leading to extensive surface damage. InSAR analysis also identified two other landslide areas, Hailang and Kalpeshwar, with similar deformation rates. Our land use change analysis showed a marked decrease in forest canopy from 2000 to 2022, mainly due to urban expansion and deforestation. These factors likely contributed to the landslide's destabilization in 2018 and 2022. We have also performed numerical modeling which shows that deforestation reduces slope stability due to decrease in tree-root cohesion. Using the D-Claw package, we simulated potential damage scenarios for a hypothetical runaway failure, suggesting that a sudden failure could severely threaten the Tapovan Vishnugad Hydropower Project (TVHP) downstream. Our findings indicate that significant canopy cover loss in Joshimath and nearby areas has directly impacted hydrological conditions and the mechanical strength of the landslide, increasing instability and surface displacements. The methods used in this study could be applied to similar landslides for hazard assessment and mitigation.