Reconstruction of the 2023 South Lhonak Lake outburst flood and modelling future scenarios in the Sikkim Himalaya

On October 4, 2023, a catastrophic Glacial Lake Outburst Flood (GLOF) occurred from South Lhonak Lake, located in Sikkim, Himalaya. This resulted in large-scale destruction, which included the failure of the 1200 MW Teesta III dam. In addition to the GLOF, simultaneous occurrence of heavy rainfall exacerbated the flooding across the state. Therefore, based on remote sensing data, this study aims to reconstruct this extreme flood event to assess flood discharge contributions from both GLOF and rainfall using hydrodynamic and hydrological modelling using HEC-RAS and HEC-HMS, respectively. The simulation of the actual GLOF event revealed peak discharges of ~7355 m3/s at Chungthang and ~8282 m3/s at Gazoldoba. The calibration of hydrological simulation agreed with available observed streamflow data at Gazoldoba showing the peak discharge of ~8252 m3/s, obtaining accuracy of 78%. Additionally, three potential future re-outburst scenarios of South Lhonak Lake were also modelled: (1) considering its current size, (2) triggered by a GLOF from the upstream North Lhonak Lake, and (3) considering potential future lake expansions. Correspondingly, the modelled peak discharges at Chungthang for these scenarios were 8826 ± 1015 m3/s, 10817 ± 1244 m3/s, and 12385 ± 1424 m3/s, respectively. Thus, this study underscores the utility of integrated models in capturing the complex meteorological and hydrological characteristics of extreme flooding events driven by multiple hazards. The findings offer critical insights that are essential for developing effective flood risk management and mitigation strategies in glacial lake-dominated regions.