Quantification of basal friction for technical and silvicultural glide-snow avalanche mitigation measures

A long-standing problem in avalanche engineering is to design defense structures and manage forest stands such that they can withstand the forces of the natural snow cover. In this way, glide-snow avalanches can be prevented. Ground friction plays a crucial role in this process. To verify existing guidelines, we collected data on the vegetation cover and terrain characteristics of 101 glide-snow release areas in Davos, Switzerland. We quantified the Coulomb friction parameter μm by applying a physical model that accounts for the dynamic forces of the moving snow in the stauch zone. We investigated the role of glide length, slope steepness and friction in avalanche release. Our calculations revealed that the slope angle and slab length for smooth slopes correspond to the technical guidelines for defense structure distances in Switzerland. Artificial defense structures, built in accordance with guidelines, prevent glide-snow avalanche releases, even when the terrain is smooth. Slopes over 40 m in length and 45° in steepness require a ground friction of μm = 0.7 corresponding to stumps or tree regeneration to ensure protection. Forest management guidelines that define maximum forest gap sizes to prevent glide-snow avalanche release neglect the role of surface roughness and therefore underestimate the danger on smooth slopes.