Monitoring post-GLOF moraine dynamics at South Lhonak lake using satellite radars

The South Lhonak Lake (SLL) Glacial Lake Outburst Flood (GLOF) cascade event of 3-4 October 2023 triggered widespread devastation across Sikkim and the downstream region of Bangladesh, causing significant loss of lives and property. The post-disaster research shows that the GLOF event was triggered by a moraine failure, creating tsunami waves in the lake, eventually leading to the breach of the frontal moraine. Despite partial drainage of the lake in the 2023 event, the hazard potential of the lake needs further investigation. This makes it extremely important to continuously monitor the surrounding regions to identify unstable slopes that can potentially fail and impact the lake. The present study utilises a Sentinel-1 Small Baseline Subset (SBAS) workflow performed in the ASF OpenSARLab environment to analyse the condition of the moraines post-SLL disaster. Post-disaster analysis spanning October 2023 to September 2025 reveals continued moraine instability, characterised by an actively deforming zone along the right flank of the failed zone. This region shows a maximum LOS displacement rate of approximately -4 cm yr^-1, with a maximum cumulative LOS displacement reaching around -6 cm in the ascending track and -5 cm in the descending track. The results indicate persistent post-failure deformation and ongoing slope instability in the moraines of South Lhonak. The study provides a critical insight into the temporal behaviour of moraine slopes. This study aimed at strengthening the disaster management strategies by integrating satellite-based deformation monitoring for early warning and risk reduction measures.