Unveiling the catastrophic landslide-induced flash flood in Teesta River, Sikkim: insight from South Lhonak Glacial Lake

The Teesta basin within the Sikkim Himalaya encapsulates a number of glacial lakes, prominently featuring the expansive and swiftly evolving South Lhonak Lake. The recent South Lhonak Lake Glacial Lake Outburst Flood (GLOF) in Sikkim has triggered a major flash flood in the river. Satellite imagery acquired by the National Remote Sensing Centre revealed that a massive landslide caused a sudden surge in the water levels of the glacial lake, eventually leading to the breach of the moraine dam. The catastrophic GLOF event led to the collapse of the Chungthang Dam, located 65.92 km downstream of the glacier lake, resulting in widespread human casualties and substantial infrastructural devastation. This paper presents a comprehensive analysis, simulation, and reconstruction of the entire GLOF process chain. A key innovation of this study is the consideration of cascading failure effects, including the landslide, South Lhonak Lake breach, and subsequent Chungthang Dam breach. Additionally, this study introduces the application of photogrammetric techniques for the first time to accurately calculate the landslide volume and the corresponding volume of displaced water. The GLOF process chain encompasses the volume of debris falling into the lake, impulse-wave initiation, moraine dam overtopping, moraine dam breaching, and flood propagation. Photogrammetric techniques are applied using pre- and post-event high-resolution stereo-satellite images for estimating the landslide volume and the volume of water displaced due to the reduction in water level. Numerical simulations from the study show the generation of a significant impulse wave, leading to the overtopping of the moraine dam with an exceptionally high overtopping discharge of 15,850 m³/s. The estimated peak discharge due to the moraine dam breach is 12,564 m³/s. The findings indicate that the collapse of the Chungthang Dam generated two distinct peak discharges of 14,673 m³/s and 10,282 m³/s, respectively. The results are illustrated across different sites and validated by comparison with high-resolution satellite imagery and field data. The study demonstrates an integrative scientific methodology that includes photogrammetric techniques, sophisticated numerical modeling, and dam break analysis in simulating the landslide-induced GLOF.