Cold-to-warm flow regime transition in snow avalanches

Abstract

Large avalanches usually encounter different snow conditions along their track. When they release as slab avalanches comprising cold snow, they can subsequently develop into powder snow avalanches entraining snow as they move down the mountain. Typically, this entrained snow will be cold (T < −1 • C) at high elevations near the surface, but warm (T > −1 • C) at lower elevations or deeper in the snowpack. The intake of warm snow is believed to be of major importance to increase the temperature of the snow composition in the avalanche and eventually cause a flow regime transition. Measurements of flow regime transitions are performed at the Vallée de la Sionne avalanche test site in Switzerland using two different radar systems. The data are then combined with snow temperatures calculated with the snow cover model SNOWPACK. We define transitions as complete when the deposit at runout is characterized only by warm snow or as partial if there is a warm flow regime, but the farthest deposit is characterized by cold snow. We introduce a transition index F t , based on the runout of cold and warm flow regimes, as a measure to quantify the transition type. Finally, we parameterize the snow cover temperature along the avalanche track by the altitude H s , which represents the point where the average temperature of the uppermost 0.5 m changes from cold to warm. We find that F t is related to the snow cover properties, i.e. approximately proportional to H s . Thus, the flow regime in the runout area and the type of transition can be predicted by knowing the snow cover temperature distribution. We find that, if H s is more than 500 m above the valley floor for the path geometry of Vallée de la Sionne, entrainment of warm surface snow leads to a complete flow regime transition and the runout area is reached by only warm flow regimes. Such knowledge is of great impor-tance since the impact pressure and the effectiveness of protection measures are greatly dependent on the flow regime.

Decoupling the Role of Inertia, Friction, and Cohesion in Dense Granular Avalanche Pressure Build‐up on Obstacles

“…For low velocities up to 10 m/s, the scatter of the simulated data is in the same range as the scatter of the measured data. The large scatter of the measurements for velocities higher than 10 m/s is very likely to be caused by the variability of the snow properties in the avalanches (Köhler et al, 2018b). The scatter in the simulations is smaller because many snow properties are kept constant (Table 1).…”

“…This is, however, a very coarse distinction as Köhler et al (2018a) have recently shown that even for the same topography, the flow regimes can be manifold. It is assumed that snow properties, which are mechanically and thermodynamically very sensitive, are a main contributor to this diversity of flow behaviors as well as an important influence on avalanche-obstacle interaction behavior, which is not yet fully understood (Köhler et al, 2018b;Steinkogler et al, 2015). The problem of snow avalanche impact pressure has been investigated for some decades in field experiments (Gauer & Kristensen, 2016;Sovilla et al, 2008b;Thibert et al, 2015) and small-scale chute experiments (Hauksson et al, 2007;Moriguchi et al, 2009;Salm, 1964).…”

Angle of repose experiments with snow: role of grain shape and cohesion

“…On larger scales, cohesion and its variation with temperature have been addressed by friction and strength measurements (Roch, 1966), or more recently in the context of avalanche rheology (Steinkogler and others, 2015; Köhler and others, 2018). In snow, cohesion is effectively mediated by sintering as a temperature and time-dependent process.…”

The Avalanche of Les Fonts d’Arinsal (Andorra): An Example of a Pure Powder, Dry Snow Avalanche

“…In general, a cold and light snow cover typically fluidises [17]. Since the surface (i.e., new snow) is cold, a PSA maintains its dynamics from surface entrainment; as it entrains warmer snow, its dynamics (i.e., the flow regime) changes, and transition to a denser flow occurs as soon as the temperature of the flowing snow reaches −1 • C and above [39].…”

“…Köhler et al [39] observed that nearly every large powder snow avalanche in VdlS has undergone a partial transition to denser flows, evolving from a cold regime flow (involving cold snow >−1 • C) to a warm regime (involving warm snow >−1 • C). In all their GEODAR data, acquired over seven years and covering 20 PSAs, only one large PSA without a clear partial transition occurred.…”