Snowpack properties associated with fracture initiation and propagation resulting in skier-triggered dry snow slab avalanches

Abstract

This study investigates snowpack properties associated with skier-triggered dry slab avalanches, with a particular view on snowpack conditions favoring fracture propagation. This was done by analyzing a data set of over 500 snow profiles observed next to skier-triggered slabs (including remotely triggered slab avalanches and whumpfs) and on skier-tested slopes that did not release a slab avalanche. The relation of the snowpack variables with fracture initiation and fracture propagation, both of which are required for skier-triggering, was investigated. Specific snowpack characteristics, including hardness difference and difference in crystal size across the failure layer, associated with skier-triggered dry slab avalanches were identified and the frequency of skier-triggering was determined. In order to assess snowpack variables favouring fracture propagation, variables from failure layers associated with skier-triggered slabs that were not remotely triggered and relatively small were contrasted with snowpack variables from failure layers of remotely triggered slab avalanches, whumpfs and relatively large slab avalanches. The properties of the slab overlying the weak layer, as well as the layer above the weak layer, were found to affect fracture propagation. Stiffer slabs were associated with large avalanches as well as whumpfs and remotely triggered avalanches. Furthermore, a correlation analysis of snowpack variables with the size and width of the investigated slab avalanches further accentuated the importance of these slab properties with regards to fracture propagation.

Introduction

The fracture sequence prior to the release of a human-triggered dry snow slab avalanche is well known. A person on the snow surface initiates a fracture in a buried weak layer or interface (fracture initiation). This fracture propagates outwards from the trigger point through the weak layer (fracture propagation), rendering the slab unstable and usually resulting in the release of a slab avalanche (Schweizer et al., 2003). Recent field observations have shown that fractures easily initiate in weak layers below skiers, without necessarily causing the release of a slab avalanche (van Herwijnen, 2005, van Herwijnen and Jamieson, 2005). This indicates that fractures must not only initiate, but also propagate in order to release an avalanche.

Several studies have examined the role of snowpack parameters in skier-triggered avalanches with increasing detail (Schweizer and Jamieson, 2001, Schweizer and Lütschg, 2001, Schweizer and Wiesinger, 2001, Schweizer and Jamieson, 2003). Based on the comparison of snow profiles from stable and unstable slopes, Schweizer and Jamieson (2003) showed that there are significant variables associated with instability, such as weak layer grain size and hardness and difference in hardness and grain size across the failure interface. Methods based on a threshold sum allow these variables to be combined to assess whether a snowpack structure is likely to cause skier-triggered avalanches (McCammon and Schweizer, 2002, Schweizer and Jamieson, 2007, Schweizer et al., 2006). While these variables are likely to affect slab avalanche release in terms of fracture initiation (i.e. stress transmission, stress concentration in the weak layer and strength of the weak layer), the relation between these snowpack variables and fracture propagation is less clear (Schweizer et al., 2006). In a recent review on snow avalanche formation, Schweizer et al. (2003) state that the properties of the overlying slab have to be considered, in particular for fracture propagation, without further explanation.

Recently, avalanche researchers have developed field tests that focus on fracture propagation (Gauthier and Jamieson, 2006, Sigrist, 2006, Simenhois and Birkeland, 2006). Preliminary results are encouraging; however, these proto-type tests are at the stage of development and have not yet been used to study the relation between snowpack parameters and fracture propagation.

An indirect way to study fracture propagation is by assessing snowpack properties associated with whumpfs and remotely triggered slab avalanches, since these events are generally associated with widespread fracture propagation. Jamieson and Johnston (1998a) compared snowpack properties from skier-triggered start zones with those from remote trigger points, defined as a site outside an avalanche start zone where a person initiates a fracture that propagates along a weak snowpack layer. They found that remote trigger points had deeper and denser slabs. These findings were confirmed by Johnson et al. (2000) who also identified significant differences in weak layer thickness and maximum crystal size. While these studies identified specific snowpack conditions associated with whumpfs and remotely triggered avalanches, there were only few data available. Furthermore, the significance of these variables with regards to fracture propagation was not studied and some important variables, such as hand hardness and crystal size difference, were not included in the analysis.

The goal of the present study is to assess snowpack variables associated with snowpack conditions favoring fracture propagation, thereby gaining a qualitative understanding of the propagation process. We analyzed a data set of over 500 snow profiles from skier-tested slopes, whumpfs and remotely triggered avalanches. Special attention was given to snowpack variables associated with whumpfs, remotely triggered slab avalanches and relatively large avalanches with a particular view on the propagation process.

The results presented in this article are part of a detailed statistical analysis of a much larger data set consisting of 1780 snow profiles (van Herwijnen, 2005), many of which were not on skier-tested slopes. In total, 19,722 snowpack layers were recorded and the data were analysed to determine typical distributions of snowpack parameters and correlations between various snowpack parameters. A detailed statistical comparison of stable and unstable snowpack characteristics with respect to skier-triggering was also performed. Despite some differences in methodology, the results from this latter statistical comparison were similar to those reported by Schweizer and Jamieson (2003). For the weak layer, the most significant variables were: depth, thickness, crystal type and size, and hand hardness. Unstable weak layers were generally shallow, thin, soft layers composed of relatively large crystals. For the slab, the most significant variables were: hand hardness, bridging (Schweizer and Jamieson, 2003) and difference in hand hardness between the slab and the weak layer. Slabs above unstable weak layers were relatively soft and the hand hardness difference with the weak layer was generally large. Finally, for the layers adjacent to the weak layer, the most significant variables were: thickness, density as well as difference in hand hardness and crystal size with the weak layer. The layers adjacent to an unstable weak layer were generally relatively thick, of lower density and differences in crystal size and hand hardness were relatively large. In this paper, a reanalysis of the data is presented to distinguish between factors favorable to fracture initiation and those for fracture propagation.