Scenario-based 1D hydrodynamic modelling of glacial lake outburst floods in the Western Indian Himalaya

Glacial Lake Outburst Floods (GLOFs) are emerging as a significant hazard in high-mountain regions due to accelerated glacier retreat and lake expansion resulting from climate warming. The present study employs a one-dimensional hydrodynamic modelling framework to simulate the propagation of GLOF and downstream flood characteristics for the Gepan Gath Lake–Chandra Basin in the Western Indian Himalayas. The selected study area represents one of the most rapidly evolving and hazard-prone glacial lake settings in the region. Unsteady flow simulations are performed using the HEC-RAS hydraulic model to route scenario-based GLOF hydrographs along the downstream river corridor. Breach outflow hydrographs have been generated using plausible combinations of lake volume and dam failure mechanisms, and are applied as upstream boundary conditions. River geometry is represented through cross-sections extracted from the ALOS PALSAR digital elevation model and supporting geospatial datasets. The simulations capture the temporal and spatial evolution of discharge and water surface elevation along the river network under multiple GLOF scenarios. Results indicate rapid flood wave propagation in steep upstream reaches, followed by attenuation and lateral spreading in wider downstream valleys. Peak discharge, inundation depth, and flood arrival time exhibit strong spatial variability, primarily governed by valley morphology and hydraulic connectivity. The modelling outputs enable identification of critical downstream impact zones and provide first-order estimates of exposure to GLOF hazards. This study demonstrates that 1D hydrodynamic modeling using HEC-RAS, combined with remotely sensed terrain data, provides an efficient and robust approach for regional-scale GLOF hazard assessment, supporting the design of early warning systems and disaster risk reduction planning in data-scarce Himalayan environments.