Ascertaining potential debris-flow landslides and associated hazards, NW Himalaya, India

Alaknanda River valley in the NW Himalaya has been subjected to frequent landslides of different types and sizes owing to its geographical position and topography that receive extreme rainfall from Indian Summer Monsoon and Western Disturbance. Such landslides, mainly debris flow, pose a growing risk to the rapidly growing human population and result in huge sediment influx into river valleys that contribute to fluvial regime changes.

In this study, we have simulated runout characteristics of four debris-flow landslides based on inputs from field observation and high-resolution satellite imagery analysis. These landslides are situated along the river valley and accommodate villages/towns near the crown/toe of landslides. Hillslopes constituting these landslides are mostly made up of schist/quartzite/gneissic rock mass, and regional thrust fault passes in the vicinity, contributing to shearing. A Voellmy-Salm fluid-flow continuum model was used to perform debris flow simulations. Frictional, turbulence, and cohesion parameters are probabilistic to eliminate uncertainty, caused mainly by selective input values.

Results indicated that debris flow from these landslide slopes might reach up to 250 m upstream and downstream along the river during extreme rainfall events, contributing to sediment influx, with a flow velocity and height of 5-10 m/sec and 6-13 m, respectively. Further, the human population residing near the crown/toe of these landslides might be subjected to ground instability and subsequent failure due to shear strength loss. Such predictive studies contribute to the effective evaluation of growing hazards associated with hillslopes during extreme rainfall events. 

Keywords: Debris flow; Rainfall; Himalaya