South Lhonak Glacial System: Cascade Investigation Using Satellite Remote Sensing

In the Himalayas, the melting of glaciers leads to the creation of proglacial lakes. The expansion of these glacial lakes poses a significant risk of glacial lake outburst floods (GLOFs), a serious geomorphological hazard involving the sudden release of water from the glacial lake. The Teesta basin in the Sikkim Himalaya is home to numerous glacial lakes in the high-altitude glacierized region, including one of the largest and fastest-growing, South Lhonak Lake (SLL). The SLL is a moraine-dammed glacial lake in Sikkim's northern district (27˚54.741’ N and 88 ˚11.857’E). Situated at an elevation of 5200 msl (mean sea level), the lake is east-west elongated and located in the tongue of the South Lhonak glacier. The date of October 4, 2023, marks a significant event in the state of Sikkim. Investigation reports reveal a severe outburst flood triggered due to the breach of the moraine dam embankment around SLL. This wreaked havoc in the northeastern state, causing substantial damage and loss, especially along the Lachen and Chungthang regions downstream. The present study attempts to investigate the geometric changes in the lake between 2015 and 2023 using C-band Sentinel-1 Synthetic Aperture Radar (SAR) datasets. The glacial region is studied using the land surface temperature (LST) into a moisture index obtained from Landsat-8 Operational Land Imager (OLI) imagery. The lake’s geometric information (length and areal coverage) is acquired through a coupled automated manual delineation approach on the Google Earth engine platform. Analysing the backscatter information of Sentinel-1 datasets shows that the stretch of the lake has substantially increased by 575m (reaching up to 2.946 km) in 2023 before the outburst. Its average annual rate of expansion is observed to be 0.05 km² in the last 8 years (2015 to 2022). The lake volume is calculated using the well-established empirical equation for South Lhonak Lake using lake area. The average volume of water in the lake from 2015 till 2023 (before the GLOF) is observed to be 102.4 million m³, which is 55.59% more than the lake volume in 2014-2016 (65.8 million m³). After the event, there is approximately a 79.44% decrease in the lake volume, a 59% decrease in the lake area, and a 48.40% decrease in lake length. The largest change in the area of the lake is observed between 2020 and 2021. Apart from the geometric changes, the moisture index has shown a monotonous increment since 2018, suggesting enhanced melting of the South Lhonak glacier, which can be attributed as one of the important parameters for the formation of such a dangerous glacial lake. The application of geospatial technology in this research can offer valuable insights into the changes occurring in glacial systems in the Himalayas. Implementing such investigative protocols is crucial for comprehending the development patterns of moraine-dammed glacial lakes, thereby aiding in the formulation of effective mitigation strategies.