Snow mechanics and avalanche formation: field experiments on the dynamic response of the snow cover

Abstract

Knowledge about snow mechanics and snow avalanche formation forms the basis of any hazard mitigation measures. The crucial point is the snow stability. The most relevant mechanical properties -the compressive, tensile and shear strength of the individual snow layers within the snow cover -vary substantially in space and time. Among other things the strength of the snow layers depends strongly on the state of stress and the strain rate. The evaluation of the stability of the snow cover is hence a difficult task involving many extrapolations.To gain insight in the release mechanism of slab avalanches triggered by skiers, the skier's impact is measured with a load cell at different depths within the snow cover and for different snow conditions. The study focused on the effects of the dynamic loading and of the damping by snow compaction. In accordance with earlier finite-element (FE) calculations the results show the importance of the depth of the weak layer or interface and the snow conditions, especially the sublayering.In order to directly measure the impact force and to study the snow properties in more detail, a new instrument, called rammrutsch was developed. It combines the properties of the rutschblock with the defined impact properties of the rammsonde. The mechanical properties are determined using (i) the impact energy of the rammrutsch and (ii) the deformations of the snow cover measured with accelerometers and digital image processing of video sequences. The new method is well suited to detect and to measure the mechanicalprocesses and properties of the fracturing layers. The duration of one test is around 10 minutes and the method seems appropriate for determining the spatial variability of the snow cover. A series of experiments in a forest opening showed a clear difference in the snow stability between sites below trees and ones in the free field of the opening.

Snow instability evaluation: calculating the skier-induced stress in a multi-layered snowpack

“…In particular, slab hardness influences the stress distribution below a skier, e.g. low values are found below hard slabs (Schweizer et al, 1995;Camponovo and Schweizer, 1997;Schweizer and Camponovo, 2001;Thumlert and Jamieson, 2014). Furthermore, the hardness of the substratum may also play a significant role, hard layers (such as crusts) just below the weak layer act as stress concentrators in the weak layer (van Herwijnen and Jamieson, 2007;Habermann et al, 2008).…”

Routing of canopy drip in the snowpack below a spruce crown

“…Dripping meltwater forms meltwater channels in the snowpack which, after refreezing, are assumed to stabilize the snowpack. Experiments determining snowpack stability below and immediately outside the area projected by the crown of larch trees have shown that such snowpacks have greater tensile strength (Schweizer et al, 1995). A further dierence to the snowpack in open areas is that organic materials CCC 0885±6087/99/010049±10$17Á50 like lichens, needles and little twigs may be deposited on to the surface and integrated in the snowpack.…”

Field measurements of sintering after fracture of snowpack weak layers

“…Slope angles vary from 34° in the upper part of the sampled area to 25° at the bottom. We conducted 24 stability measurements using the rammrutsch test [ Schweizer et al , 1995] in an 18 m × 18 m area of the slope (Figure 1). The rammrutsch test progressively loads an isolated 30 cm × 30 cm column of snow by dropping a 1 kg weight from increasing heights onto a plate placed on the column.…”