The 2024 Thame GLOF, Khumbu Nepal - causes, consequences, and dynamics

Glacial lake outburst floods (GLOFs) are recognized as a major hazard in many mountainous regions of the world, and particularly in the Himalaya. Much of the efforts around GLOF mitigation and early warning in the Himalaya focuses on lakes classified as dangerous, which are generally large; however, even small glacial lakes can produce devastating floods. This was illustrated on 16 August 2024, when a glacial lake outburst flood struck the village of Thame, in the Khumbu region of Nepal. The GLOF originated from a cascade of two small lakes that had not previously been considered dangerous. We use a combination of seismic, remote sensing, meteorologic and gauge data, and field observations to examine the GLOF dynamics, impacts, and potential triggers. The combination of all the data suggests that a wet snow avalanche into the upper bedrock dammed lake was the most likely trigger of the GLOF. The resulting impulse wave overtopped the upper lake, sending a flow 650 m downstream to the lower lake, leading to a breach of the lower lake’s moraine dam. Overall, we estimate that ~4-5 x 10⁵ m³ of water was released from the two lakes. Before and after Pleiades and HMA DEMs reveal a complex pattern of erosion and deposition as the GLOF propagated down the Thame Khola valley. In the village of Thame, damage resulted from inundation, coarse sediment impacts, and erosion of a paleochannel passing through the village. Despite the small initial volume of the GLOF, impacts continued far downstream on the Dudh Koshi, including landslide damage to a key road bridge ~45 km downstream of the GLOF source. This GLOF highlights both the risk of small glacial lakes and the need to understand GLOF erosion and deposition dynamics in order to properly estimate hazard.