Rock and snow avalanche at Flüela Wisshorn

Over 250 000 cubic metres of rock collapsed shortly after midnight on March 19 in the Northwest flank of Flüela Wisshorn (3085 m asl, Davos, Grisons). The impact of the rock debris triggered a very large slab avalanche. The rock and snow avalanches were partly mixed. Whereas the blocky rock debris was deposited in the glacial cirque at the foot of the Wisshorn, the snow avalanche travelled further downslope. The dirty avalanche, containing small stones and soil almost reached the closed Flüelapass road, near the Wägerhütta at Carlimatta (Fig. 1). This event was therefore a process chain, consisting of a rock slope failure and a snow avalanche.

The exact timing of the event at 00:00:27 am was determined from seismic data registered by the Swiss Seismological Service and by SLF. The volume of the rock slope failure was measured by SLF researchers, using drone-based photogrammetry (Fig. 2). The rock slope failure was likely induced by the geological structure of the slope and by former glacial erosion at the base of the slope. Although there is most likely permafrost in the detachment area, very little ice was visible after the rock slope failed.