Persistent Scatterer Interferometry for early detection of Glacial Lake Outburst Floods: A case study of the 2024 Thyanbo GLOF near Thame, Nepal

There is a clear link between global warming and the increase in glacier melting, leading to the expansion of glacial lakes, often dammed by fragile moraines. Triggers such as heavy rainfall, earthquakes, landslides, avalanches, glacier breakoffs, or thawing permafrost can cause glacial lake outburst floods (GLOFs). These events result in moraine breaches, releasing flood waves of mud and debris that can cause significant damage and endanger populations. On August 16, 2024, a GLOF from the Thyanbo glacial lakes affected the village Thame, Nepal. This flood caused destruction of the local infrastructure, buildings and agricultural land, and displaced over 135 inhabitants. According to first investigations, it seems that an initial trigger originated from the upper glacial lake and overtopped its terminal moraine. This flood wave further ran into the lower glacial lake, which overtopped the terminal moraine and caused it to breach. All mentioned cascading incidents triggered the GLOF running downstream. As no in situ data is available, we used high-resolution optical as well as Synthetic Aperture Radar (SAR) remote sensing data to map the lakes dynamics and measured the ground deformations at the terminal moraines. To date, such analyses have been applied only to glacial lakes and terminal moraines without documented GLOF events, but not to systems affected by a previously occurred GLOF.

High-resolution PlanetScope multispectral images from 2019 to 2024 showed an expansion of the upper lake by 213.3 % before the event, followed by a loss of 25.9 %, while the lower lake just increased slightly by 2.8 % over the timeseries, but lost over 74.3 % of its area during the GLOF. The analysis showed that the upper terminal moraine has not eroded at all or only very slightly, whereas the lower moraine has largely eroded. Consequently, while the lower lake no longer represents a future hazard, the upper lake continues to pose a high risk.

SAR Sentinel-1 images from 2020 to 2024 were used to perform a Persistent Scatterer Interferometry (PSI) with the Stanford Method of Persistent Scatterers (StaMPS). By combining ascending and descending orbits, the vertical and horizontal movements of the resulting scatterers were deconstructed. As the GLOF was likely triggered by an external factor, no abrupt movements were detected in advance by the PSI. Nevertheless, significantly stronger vertical and horizontal subsidence was observed at the lower terminal moraine, reflecting its greater exposure to the GLOF relative to the upper moraine. The analysis demonstrated that, with certain improvements, remote-sensing data combined with PSI can be used to assess the overall stability of terminal moraines and enable meaningful comparisons between them.

Based on this case study, the methodology will be transferred to a regional approach in the Himalayas in a future study to contribute to a more comprehensive inventory of potentially dangerous glacial lakes by adding the parameter of terminal moraine stability, which has not yet been considered in depth.