Snow Avalanches - Environmental Geomechanics - page 42

(LWC < 0 . 5%) does not significantly affect the mechanical properties of snow.
However, heavy rain induces a rapid and noticeable increase in LWC, which results
in a drop in the shear stress strength. This situation leads to widespread avalanche
activity (wet snow avalanches) [CON 93].
- Snowpack structure . A given snowpack results from the successive snowfalls.
The stability of the resulting layer structure depends a great deal on the bonds between
layers and their cohesion. For instance, heterogeneous snowpacks, made up of weak
and stiff layers, are more unstable than homogenous snowpacks [SCH 03].
2.1.3. Avalanche motion
It is very common and helpful to consider two limiting cases of avalanches
depending on the flow features [ANC 96, DEQ 73]:
- The flowing avalanche (avalanche coulante, Fliesslawine, valanga radente): a
flowing avalanche is an avalanche with a high-density core at the bottom. Trajectory
is dictated by the relief. The flow depth does not generally exceed a few meters (see
Figure 2.2). The typical mean velocity ranges from 5 to 25m/s. On average, the density
is fairly high, generally ranging from 150 to 500 kg/m 3 .
- The airborne avalanche (avalanche en aérosol, Staublawine, valanga
nubiforme): it is a very rapid flow of a snow cloud, in which most of the snow
particles are suspended in the ambient air by turbulence (see Figure 2.3). Relief
usually has weak influence on this aerial flow. Typically, for the flow depth, mean
velocity, and mean density, the order of magnitude is 10-100m, 50-100m/s, and
5-50 kg/m 3 , respectively.
Figure 2.2. Flowing avalanche impacting a wing-shaped structure in the Lauratet
experimental site, France (courtesy of O. Marco, Cemagref)