Process-based assessment of conditioning factors, triggers, and flood dynamics of the October 2023 South Lhonak glacial lake outburst flood (GLOF)

Abstract

On October 4, 2023, South Lhonak Lake (SLL) in the Sikkim Himalaya catastrophically breached, generating flash floods in the Teesta Valley that caused ~70 fatalities and severe infrastructure damage, including destruction of the 1200 MW Teesta-III hydropower project. While previous studies emphasize breach hydraulics or downstream impacts, process-based understanding of preconditioning and compound triggers remains limited. This study develops an integrated GLOF framework to identify multiple GLOF triggers by combining geodetic mass balance of the parent glacier, glacier-melt sensitivity analysis, identifying meteorological changes, and performing 2D hydrodynamic GLOF simulation. The analysis shows a 127% expansion in the SLL area from 2000 to 2023 and volume with significant mass loss of the parent glacier. Additionally, the high-resolution meteorological data indicate that intense precipitation (during October 1–3, 2023) coincided with elevated temperature anomalies, amplifying meltwater input to SLL and destabilized the loose moraine and glacier body. Due to this, destabilization of the moraine (estimated at ~3.2 × 10⁷ m³) together with ice-calving-induced impulse waves likely contributed to a ~91 m breach, resulting in a peak discharge of ~18,000 m³/s within ~30 min at just near to lake outlet. Simulations reconstruct a maximum flood depth of 23.95 m (cross section at Chungthang) and a maximum velocity of 26.51 m/s (cross section near the lake outlet) along the ~68-km simulated profile, with markedly different values at these two specific cross sections (13.79 m depth and 26.51 m/s at the lake outlet and 23.95 m depth and 4.53 m/s at Chungthang), emphasizing evolving glacier lake feedbacks as critical GLOF precursors.