Figure 1 - uploaded by Francois Louchet
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(a) A slab on a weak layer may be schematised as a series of blocks linked by elastic/brittle springs, lying on a collapsible house of cards; (b) the skier's load may collapse part of the house of cards (basal crack); (c) driven by either the skier's action or the snow weight, the basal crack may extend; (d) when the extension of the basal crack is large enough, the weight of the hung part of the slab initiates a crown crack at the top, resulting usually in the avalanche release.
Source publication
day before. This is part of his job: he is a mountain guide (IFMGA), and a ski patroller in charge of avalanche control. He has been working for about twenty years in the ski resort just above the village. The access to the firing spots is easy, first taking the ski lift, and then crossing the plateau that directly leads above the slopes. The bowl...
Context in source publication
Context 1
... the case of accidental or of artificial triggerings, both the cohesive slab and the weak layer are elastic/brittle bodies: they may deform elastically under stress, and fail in a brittle way if the stress exceeds a threshold value. The system can therefore be schematized as in Figure 1. The elastic/brittle slab is represented as a series of blocks linked by brittle springs, that can extend or contract depending on the stress they experience, or split into parts if the stress exceeds a threshold value. ...
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Citations
... Slab release can be described as a first approximation by the succession of four main steps [Louchet and Duclos (2006)]: i) nucleation of a basal crack, ii) expansion of the basal crack, iii) nucleation of a crown crack, and iv) expansion of the crown crack. Simple calculations can be made on this basis within the assumption of a shear disturbance in the weak layer separating an homogeneous slab from an homogeneous substrate [e.g. ...
Modelling avalanche release is a long lasting challenge. Despite a general agreement on the basic mechanisms responsible for avalanche release, deterministic models suffer from a lack of reliable data due to spatial and temporal variability of snow cover properties. On the other hand, field observations reveal that starting zone sizes are organized into power law statistical distributions characterized by a universal exponent. Yet, statistical approaches developed so far, that essentially are binary cellular automata, only consider the shear failure of the weak layer, and cannot take into account slab rupture. As a consequence, they cannot reproduce the observed power law exponent. This is why the present model is a two-threshold multi-state cellular automaton, that incorporates both the shear failure of the weak layer and the rupture of the slab. It reproduces field data on statistical distributions of starting zones of snow slab avalanches, but also of other gravitational failures. It can be used to model blast-triggered and skier triggered avalanche, or to provide initial conditions in avalanche flow simulations in particular slopes. Possible applications of the automaton to educational purposes may be contemplated.
... In a similar way, the slab is connected to older snow by elastic/brittle bonds (the weak layer), represented as some kind of flexible (i.e. elastic) and brittle flat house of cards, that may fail and collapse if the stress it experiences is large enough [Louchet F., Duclos A., 2006]. ...
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 2 d 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... 1. INTRODUCTION After a more comprehensive theory of slab avalanche release in terms of statistical physics, and based on an original cellular automaton [Faillettaz 2004, 2006], the present paper aims at "merging theory and practice", through a simple analysis of the basic processes responsible for initiation and expansion of fractured zones due to an external (artificial) action (skier or avalanche control), and of how they may be influenced by the various characteristics of the snow cover. Each of the four steps leading to avalanche triggering are discussed and illustrated by figures. We subsequently report field observations that support such mechanisms, and finally suggest a few lessons that may be deduced from both the theory and the practice.
