Investigation of the flash flood event caused by a massive rock–ice avalanche in the Himalayan river valleys of Rishiganga and Dhauliganga, Uttarakhand, through hydrodynamic modeling perspectives

The Indian Himalayan Region features climatically sensitive and extremely rugged topography. In recent years, risks to human settlements, infrastructure, and the mountain ecosystem have increased manifold because of frequent occurrences of natural hazards. In this context, the Chamoli flash flood event that occurred on February 7, 2021, in Uttarakhand was an extraordinary event causing unprecedented destruction in Rishiganga and Dhauliganga river valleys. This study attempts to simulate the event from the source up to 40.8 km downstream using a hydrodynamic modeling approach. HEC-RAS software was employed to model a hypothesized storage breach of 26.4 × 106 m3 at the source location, generating a peak inflow of 12,761.88 m3/s. The breach was simulated as an unsteady flow with a computational interval of 5 s and a mapping interval of 2 min for 6 h. The model-generated peak discharge values range between 7908.8 and 7975.26 m3s−1 near Rishiganga HEP and between 5779.53 and 5957.46 m3s−1 near Tapovan HEP. Also, flow depths at the above locations were 19.85 and 18.15 m, respectively. The flow velocities were 6.92 and 3.86 m/s, respectively. The model output shows good agreement with the extent and height of actual debris assessed using systematic pre- and post-event analysis of high-resolution satellite datasets. The presented modeling and damage assessment approach may be applied in dynamic mountain ecosystems where population and infrastructure growth require continuous evaluation of hazards. Furthermore, this study proposes implementable recommendations highlighting key issues of specific inadequacies and unpreparedness in the remote high mountain regions of Uttarakhand and elsewhere.