Thermokarst processes as triggers of debris flows: A case study at Hüttekar Rock Glacier (Austrian Alps)

A cascading process including thermokarst lake outburst, debris flow initiation, and river blockage, hit a high mountain valley in the Austrian Alps during summer 2019. The rapid development of thermokarst features on an active rock glacier, including a lake with a water volume of approximately 166,000 m³ as well as a 350 m long drainage channel, most likely triggered the failure of ice-cemented debris within its front, with subsequent mobilization of roughly 50,000 m³ of sediment. This study explores the drivers of thermokarst evolution by tracking the lake development using satellite imagery and modeling its energy budget. We employ a simple balance model, assuming that the atmospheric energy input was efficiently transferred to the frozen rock glacier core through convection of lake water. This process provided sufficient melting energy to establish the thermokarst channel draining the lake within several hours. Our results highlight the need to account for thermokarst processes in hazard assessment studies involving permafrost-affected terrain.