Dimensions of dry snow slab avalanches from field measurements

Abstract

Dry slab avalanches release by a sequence of propagating fractures. In this paper, I provide field measurements of the important length parameters resulting from fractures for hundreds fallen slabs including depth, length, and width. The field data show wide variations in these parameters. Probability plots of all dimensions (length, width, and depth) suggest they approximately obey log normal probability density functions. Given slab dimensions, two applications are considered on the basis of the field data. These applications are: (1) Estimates of total fracture energy consumed around the perimeter of the slab are given, where it is presumed fracture is mostly mode I, and along the base (weak layer) of the slab, where shear fracture (mode II and III) is expected. For average characteristic dimensions, the analysis suggests that energy consumed on the perimeter is somewhat less than in the weak layer. Even though fracture energy around the perimeter is expected to be higher than in the weak layer, the larger area fractured in shear at the base of the slab compared to area fractured in tension around the perimeter results in a comparable amount of total energy needed around the perimeter. (2) Approximate estimates of avalanche mass for average characteristic dimensions based on slab depth D (the only length possibly known prior to avalanching) are made. The mass is considered related to destructive potential and simple guidelines are given to estimate mass in relation to D with validation by considering concurrent size and estimates of D by mountain guides.