Pressure Makes Snowpack More Unstable

The new data could help to further improve avalanche forecasting and an understanding of how and why avalanches occur.

Weak layers in the snowpack are often the cause of slab avalanches.

The forces that cause these weak layers to break and thus lead to avalanches are still disputed in avalanche research.

Since the 1970s, two opinions have clashed on this issue.

Theory One

Additional pressure from above, for example from a thicker or heavier snowpack, makes the weak layer more stable.

According to this theory, more pressure from above requires stronger so-called shear forces, which pull the snow down the slope to cause a fracture.

Theory Two

According to the other theory, it is precisely this additional pressure from above that brings the fragile layer closer to collapse, so that even a lower shear load is sufficient to trigger a fracture.

The new experiments now support this theory.

Avalanche in Tignes. Image c/o David Morgan.

For the study, the researchers analysed 63 samples of natural weak layers in the cold laboratory.

The snow samples came from the Davos in the Graubünden region and were subjected to combined compressive and shear forces using a specially developed test apparatus.

The samples consisted of snow-covered surface frost.

In the laboratory, the researchers simulated the forces acting on the snow cover on a slope: the vertical normal force and the parallel shear force.

They grow especially well at the Flüelabach stream in Davos: SLF PhD student Jakob

A high-speed camera recorded the exact moment at which the layers broke.

Captured by the high-speed camera: when does the weak layer break? (Video: Jakob Schöttner / SLF

Jakob Schöttner, who is studying for a PhD in snow mechanics at the SLF, concludes: “Pressure doesn’t make the snowpack more stable; rather, both forces together lead to failure of the weak layer.”

The results of the study have been published in the scientific journal Geophysical Research Letters.

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