SnowFlurry 4 2017/18 | Why you can't trigger sliding snow avalanches

A real winter in large parts of the Alps again at last! In many regions, the thick snow cover and regular snowfalls mean that there are only a few, rather weak layers in the old snow. Sliding snow avalanches are a seriously increasing danger, especially in the snowiest regions with a smooth ground surface. They usually occur spontaneously, it is practically impossible to trigger them and blasting them does not help at all. You shouldn't spend time under sliding snow. We hear these phrases over and over again - but what is behind them?

Mechanics & theory

Sliding snow avalanches descend from the ground to the snowpack due to a loss of friction. This means that they belong to the "base avalanche" category, just like dry slab avalanches that slide along the ground and wet slab avalanches in spring. BUT: sliding snow avalanches have a different triggering mechanism than slab avalanches.

The friction between the ground and the snowpack is reduced on the one hand by the moistening of the snow layer directly at the boundary to the ground and on the other hand by the inherently low roughness of the ground. This is why they mainly come loose on meadow slopes and rock slabs.

Sliding snow avalanches do not occur due to a break in a weak layer like slab avalanches. Sliding snow avalanches slide down to the ground surface (without prior breakage!) simply due to the loss of friction in the snowpack. It is almost impossible to predict when they will fall. Because no weak layer breaks in a sliding snow avalanche, it is practically impossible to trigger sliding snow avalanches. The tensile fracture of a sliding snow avalanche, the sliding snow mouth, is not the same as the fracture within a weak layer. A slab avalanche also has a tensile fracture, but this only occurs after the slab has broken and slid off.

A slab avalanche that slides off the ground is also not the same as a gliding snow avalanche. In a slab avalanche that slides along the ground, there is a weak layer in the snow directly above the ground surface. A fracture is initiated in this weak layer, which propagates. Because the snow crystals break apart and are no longer connected, a snow slab detaches from the entire snowpack and subsequently slides down to the ground, provided there is no further layer of snow underneath.

However, only if gravity can overcome the friction that has now arisen between the detached snow slab and the ground (roughly from a slope inclination of around 30°). A slab avalanche, in which there are further layers of snow below the weak layer, slides on the next layer of snow below the first weak layer.

Comparisons, comparisons

Comparisons from everyday (mountaineering) life are usually more useful than theoretical blah-blah. Sliding snow avalanches flow in a similar way to glacier ice. Cracks and crevasses open up. The tensile crack, the mouth, opens exactly at the boundary between non-gliding (or slower gliding) and (faster) gliding snow cover. Similar to a bergschrund: The bergschrund is a special form of crevasse: A so-called rim crevasse forms directly between rock and ice, whereas a bergschrund opens up between ice and ice in steep, glaciated walls. The ice above the bergschrund is frozen solid, it does not slide, the ice below the bergschrund slides slowly downwards (in the case of a glacier, the term "flowing" is used instead of "sliding"). A crack forms between the two, the bergschrund.

Summary

Sliding snow avalanches are triggered by a loss of friction between the ground surface and the snowpack and only slide down without breaking.

Snow slab avalanches are triggered by a break in a weak layer. This fracture separates the snowpack into two parts. The part above the fracture then slides down as a slab avalanche. In a sliding snow avalanche, the snowpack does not split into two parts. The entire snowpack shifts on the ground. At most, sliding snow avalanches can be triggered by cutting off the "foot" (this is the damming wall when the sliding movement has already begun and a mouth opens at the same time) or by "pushing" the edge of the crack below the tension crack.

The fracture in a weak layer required for the slab avalanche can be generated by additional load, but the friction loss required for the sliding snow avalanche cannot. The amplification of the friction loss from above on the sliding snow slab by an additional load is negligible.

Thus, it is also practically impossible to learn to assess sliding snow avalanches with the help of snow profiles.

Overall, very little is still understood about sliding snow avalanches. The SLF has a research focus here. Highly recommended as further reading!

Note: Gliding snow avalanches, slab avalanches and loose snow avalanches are like uphill racing skis, freeride skis and downhill racing skis - part of the same category but completely different.