Quantifying degradation of the Imja Lake moraine dam with fused InSAR and SAR feature tracking time series

Abstract. Glacial lake outburst flood (GLOF) hazards are often tied to the structural properties of the moraines that dam glacial lakes. Traditional investigations of moraine dam structure and degradation involve costly and logistically challenging in situ geophysical and repeat topographic surveys, which can only be performed for a small number of sites. We developed a scalable satellite remote sensing approach using interferometric synthetic aperture radar (InSAR), InSAR coherence, and SAR feature tracking to precisely measure moraine dam surface displacement and map the extent of buried ice. We combined time series from ascending and descending Sentinel-1 orbits to investigate vertical and horizontal surface displacement for the period 2017–2024 with ∼12 d temporal sampling. We applied our approach to quantify degradation of the Imja Lake moraine dam in the Everest Region of Nepal. We validated our SAR-based displacement measurements using 3D displacement measurements from very-high-resolution satellite stereo digital elevation models that span the study period. We found that a 0.3 km2 area of the moraine dam has cumulatively subsided ∼90 cm over the 7 year study period. Seasonal change in InSAR coherence provides evidence for buried ice throughout the moraine dam. We observed consistent downward and eastward displacement throughout the colder months, which we attribute to ice flow. The magnitude of downward vertical surface velocity increases in the warmer months, likely due to melting of buried ice. Our observations provide new insights into the timing and magnitude of the processes that control moraine dam evolution, with broader implications for regional GLOF hazard assessment and mitigation.