New Research Shows Avalanches Are Starting Faster

New experiments provide insights for avalanche forecasting and risk management and confirm a recently formulated theory.

• High speed: Cracks in the snowpack propagate faster than previously thought – namely at a speed of more than 100 metres per second.

• New insights for models: The results may help to improve avalanche size and hazard assessment forecasts.

• Extensive field research: Experiments in Davos back up theory – further studies on natural avalanches needed.

This year the WSL Institute for Snow and Avalanche Research (SLF) has received reports of hundreds of ‘whumpfs’ – a sound indicating a collapse in the snowpack.

They are signs of a critical avalanche situation involving a weak snowpack.

A ‘whumpf ‘is where a skier causes a fracture in a weak layer of the snow, which within seconds can propagate as a crack across the terrain.

If the crack reaches steep terrain this may then trigger an avalanche.

Avalanche in Pongau, Austria. Image c/o Pongau Mountain Rescue Service

Previously, it was assumed that such cracks propagate at speeds of between 20 and 80 metres per second.

The new research concludes the propagation speed increased from 50 to 130 metres per second.

SLF researchers have now demonstrated that cracks propagate faster than expected, even exceeding the previously assumed theoretical boundaries for this process in snow.

SLF scientist Bastian Bergfeld has been trying to initiate cracks and, ideally, trigger and measure avalanches.

Avalanche researcher Bastian Bergfeld during an experiment in Davos. (Photo: Roman Oester / SLF)

His latest research revealed that cracks in weak layers propagate slowly at first.

However, once they reach a critical distance of between five and six metres, the speed builds up, even exceeding the theoretical boundaries.

The observed acceleration is probably due to the force of gravity on the slope.

“Basically, the crack in the weak layer propagates faster than was explicable by previous models,” explains Bergfeld.

Bergfeld’s experiments confirm the findings from the computer models.

He also sees a need for further research because his avalanches happened under controlled conditions, but the situation is different out in the field.

Every slope has its own particular characteristics.

“We don’t yet know how often this rapid crack propagation occurs in the natural environment and what role the properties of the snowpack play in this process,” he says.

This means the implications of the new insights for avalanche formation are not yet completely clear.

Avalanche in Davos.Image c/o Graubunden Cantonal Police.

How effectively a crack propagates in a weak layer determines, for example, how big an avalanche will be.

It could therefore be that rapid cracks are less likely to lead to crack arrest which would tend to make avalanches bigger.

If the conditions for rapid crack propagation were better understood the expected avalanche size could be better estimated. .

The size of an avalanche is crucial not only for working out the danger level in the avalanche bulletin, but also for assessing the risk to property and structures as well as people.

See here for the full article from The Swiss Institute for Snow and Avalanche Research, SLF.

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