SLAB AVALANCHE TRIGGERING: A COMBINATION OF FOUR BASIC PHENOMENA IN SERIES.

ABSTRACT: There is still a significant gap between models describing slab avalanche triggering mechanisms and practical field applications in terms of risk prediction and mitigation. The present paper aims at "merging theory and practice", through a simple analysis i) of the basic processes responsible for initiation and expansion of fractured zones, and ii) of how they may be influenced by the various characteristics of the snow cover. Slab avalanche triggering results indeed from four main steps occurring in series: i) nucleation of a collapsed zone in the weak layer, ii) expansion of this collapsed zone, iii) nucleation of a crown fracture, and iv) expansion of this crown fracture, leading to avalanche release. If a single of these steps is missing, avalanche triggering cannot occur. This is why many avalanches are not observed to occur, even if most of triggering criteria seem to be fulfilled. The occurrence of any of these four steps is controlled by simple physical rules, which are described and discussed. More particularly, the 2d step may occur in two very different modes, resulting in either small or very large starting zones. The transition between these two modes is very sensitive to both slab and weak layer properties and to the way skiers choose to cross the slope. Such a strong sensitivity of the expansion mode of the collapsed zone to these parameters is partly responsible for the large variability of avalanche sizes and locations. It accounts for instance for the possible and unpredicted occurrence of huge avalanches at places where much smaller ones are usually observed, or for a sudden release only after several skiers have successively (and safely!) traveled through the area, or for remote triggering events, etc..