Multiple drivers of the recent South Lhonak glacial lake outburst flood in Sikkim Himalaya and its aftermath on Teesta River Valley

Abstract

Glacial lake outburst floods (GLOFs) are the most severe cryospheric hazard in the ‘Third Pole’ region, encompassing the Tibetan Plateau and surrounding areas including the Himalayas, Hindu Kush, Kunlun, and Tianshan mountains. Understanding the proper response of glaciers to the current situation of global warming is vital because of their role as a water source in the Asian region. Numerous glacial lakes are formed in the higher Himalayan areas due to the contemporary increase in global temperature. The upper part of the Teesta Basin, Sikkim hosts several glacial lakes including one of the largest and fastest growing South Lhonak Lake (5200 m from the mean sea level). Recently, a devastating GLOF event occurred in South Lhonak Lake after the breaching of moraine dams on midnight of October 3, 2023. This disastrous GLOF event collapsed the Chungthang Dam, located approximately 65 km downstream of the lake and accelerated extensive casualties along with infrastructural damages. It is identified that; the impact of cloudburst may be a significant triggering factor behind this event. The satellite imagery and digital elevation models also revealed that a sudden collapse of lateral moraine eventually produced an impulse wave which accelerated the breaching process. Additionally, this study also combined with advanced remote sensing applications. Satellite imageries indicate a huge reduction of the lake area after the GLOF event (1.66 km² before the GLOF event and 0.63 km² after the GLOF). The overtopping volume of the water has been estimated as approximately 106,400 $m^3$, with a duration of 12.78 s. The peak discharge during overtopping touched approximately 16,651.02 cumecs, indicating the maximum flow rate during the phase. The results have been validated by the high-resolution satellite data across various sites.