Wet-snow failure mechanics

Wet-snow avalanches are responsible for half of the victims caused by spontaneous avalanche activity and may produce considerable damage to infrastructure since their destructive potential is high. Still though, wet-snow avalanches are notoriously difficult to forecast because their release depends on a complex interaction of water percolation and snow stratigraphy. The mechanical properties governing the release are poorly understood. Exact characterisation of the mechanical properties, however, is a prerequisite for reliable snow stability evaluation under field conditions as well as for stability modelling, which in term is important for a correct avalanche danger assessment. In view of the predicted climate change we expect a higher proportion of wet-snow avalanche and therefore the importance of correctly forecasting wet- snow avalanches will increase. Accordingly, the overall aim of the current proposal is to improve our understanding of wet-snow failure mechanics. We believe that fracture initiation and propagation in a wet snowpack may behave similar to dry-snow slab avalanche release, but is altered by the amount of water within the weak layer- slab configuration. The amount of water within the snowpack will strongly influence both the elastic properties of the slab and/or the strength of the weak layer. If changes within the elastic properties of the slab dominate the failure, fracture mechanics will be very similar to dry-snow slab avalanche release, i.e. a mixed-mode failure. However, if changes within the weak layer, i.e. loss of strength, represents the failure process best, then wet-snow avalanche formation will be better described with gradual weakening than a mixed-mode crack failure. In order to test these hypotheses, we will establish experimental procedures to measure the relevant fracture mechanical parameters in wet snow. This will include recurrent propagation- saw tests, shear frame measurements and highly resolved penetration measurements. In addition, we will use a finite-element model to assess which of the above-mentioned failure processes dominates. From the obtained results it will be possible to define requirements for improving wet-snow avalanche forecasting through coupling of the finite element model and a numerical snow cover model. Our results will considerably increase the quality of avalanche danger assessments of local and national forecasting services.