A process-based approach to estimate point snow instability

Abstract

Abstract. Snow instability data provide information about the mechanical state of the snow cover and are essential for forecasting snow avalanches. So far, direct observations of instability (recent avalanches, shooting cracks or whumpf sounds) are complemented with field tests such as the rutschblock test, since no measurement method for instability exists. We propose a new approach based on snow mechanical properties derived from the snow micropenetrometer that takes into account the two essential processes during dry-snow avalanche release: failure initiation and crack propagation. To estimate the propensity of failure initiation we define a stress-based failure criterion, whereas the propensity of crack propagation is described by the critical cut length as obtained with a propagation saw test. The input parameters include layer thickness, snow density, effective elastic modulus, strength and specific fracture energy of the weak layer -all derived from the penetration-force signal acquired with the snow micro-penetrometer. Both instability measures were validated with independent field data and correlated well with results from field tests. Comparisons with observed signs of instability clearly indicated that a snowpack is only prone to avalanche if the two separate conditions for failure initiation and crack propagation are fulfilled. To our knowledge, this is the first time that an objective method for estimating snow instability has been proposed. The approach can either be used directly based on field measurements with the snow micro-penetrometer, or be implemented in numerical snow cover models. With an objective measure of instability at hand, the problem of spatial variations of instability and its causes can now be tackled.

Snow fracture in relation to slab avalanche release: critical state for the onset of crack propagation

“…However, at the slope scale, 3-D effects, slope-transverse propagation, terrain, and snowpack variability (Schweizer et al, 2008;Gaume et al, 2015a) might make the process even more complex. Nevertheless, it was shown that the critical crack length correlates very well with signs of instability (Reuter et al, 2015). In particular, they showed that no signs of instability were recorded for a c > 0.4 m while whumpfs, cracks, and avalanches were observed for a c < 0.4 m. Hence, our new model of critical crack length can be of major importance in view of avalanche forecasting.…”

“…In this paper, the multilayered character of the slab was not accounted for, for clarity reasons since the phenomenon is already very complex. However, the elastic moduli of the slab layers have a very important influence on slab deformation and thus on the critical crack length (Reuter et al, 2015). For the comparison with the experiments, the elastic modulus was computed from the average slab density.…”

“…It includes the average slab density ρ, slab thickness D, slope angle ψ, and WL thickness D wl . The WL specific fracture energy w f was evaluated from the penetration resistance of the weak layer using the SMP according to Reuter et al (2015) and ranges from 0.07 to 2.9 J m −2 . Reuter et al (2015) showed a good correlation between the SMP-derived w f and that derived using particle tracking and the anticrack model .…”

“…The WL specific fracture energy w f was evaluated from the penetration resistance of the weak layer using the SMP according to Reuter et al (2015) and ranges from 0.07 to 2.9 J m −2 . Reuter et al (2015) showed a good correlation between the SMP-derived w f and that derived using particle tracking and the anticrack model . The shear strength τ p of the WL was not measured but we used the mixed-mode shear-compression failure envelope defined by Reiweger et al (2015) based on laboratory experiments.…”

“…Presently, it can be estimated using three existing methods: (i) through PTV or finite element analysis of the PST (Sigrist and Schweizer, 2007;van Herwijnen et al, 2010Schweizer et al, 2011), (ii) from snow micro-penetrometer (SMP) measurements (Schneebeli et al, 1999) by integrating the penetration resistance over the thickness of the WL (Reuter et al, 2015), and (iii) from X-ray computer tomography-based (CT) microstructural models (LeBaron and Miller, 2014). Depending on the method, estimates of the WL specific fracture energy can differ by as much as 2 orders of magnitude, resulting in widely different values of the critical crack length.…”