High‐resolution LiDAR and stereo‐DEMs for debris flow analysis in the Western Ghats, India: sediment volume computation and run‐out modeling

Computation of sediment volume and accurate modeling of run‐out using high‐resolution digital elevation models (DEMs) are essential for assessing debris flow hazard. In this study, we used stereo‐derived DEMs coupled with field observations and high‐resolution LiDAR DEM to compute the depleted and accumulated sediment volume, and interpret the initiation phase of mobilized sediment from one of the catastrophic debris flows of the Western Ghats in India, the 1.2 km long Pettimudi debris flow of 2020 that killed over 70 people. Over 180 field‐observed measurements were used to rectify the stereo‐DEM, which was affected by dense vegetation. Accordingly, the depth of depletion over the scarp region was corrected from an overestimated value of ~25 m to a more accurate value of 7.08 m. The volume estimated using the DEM of Difference (DoD) of rectified DEMs shows that this debris flow progressively entrained and mobilized 17 × 10 4 m 3 of sediments. The Rapid Mass Movement Simulation (RAMMS) tool was used to simulate flow characteristics under conditions similar to those in the field. The high‐resolution LiDAR DEM enabled us to validate debris flow simulations and provided an unprecedented record of their development and evolution at sub‐second time resolution.