Rapid degradation of ground ice in a ventilated talus slope: Flüela Pass, Swiss Alps

The degradation of ground ice in talus deposits, which are important forms of debris storage in mountain environments, can cause mass movements, subsidence or instability of mountain infrastructure. Two 20 m deep boreholes instrumented with thermistors and located in a talus slope at 2400 m asl above a lake at Flüela Pass in the eastern Swiss Alps allow the analysis of the evolution of ground temperature and permafrost thickness between 2003 and 2008. The occurrence of intra‐talus ventilation can be discerned due to thermal anomalies within a layer of coarse blocks with large voids at around 15 m depth near the base of the slope and at 10 m depth mid‐slope. Intra‐talus ventilation is particularly effective in winter when subzero air temperatures coincide with snow depths less than 50 cm and reverses to a gravity discharge of intra‐talus air when air temperatures become positive. Despite the lack of annual variability in active layer depth, rapid permafrost degradation and thinning of ground ice (from 7 m thickness to 3.5 m in 4 years) is currently occurring. It is attributed to latent heat transfer from the moving air stream, caused by the flux of vapour from the relatively warm lake water table at 20 m depth and its condensation at the base of the permafrost body. Copyright © 2009 John Wiley & Sons, Ltd.