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Avalanche formation

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Whether or not an avalanche releases at a particular location depends primarily on the structure of the snowpack. We investigate the snow properties and processes at work in the formation of an avalanche and so help to improve avalanche forecasting.

 

Avalanches are a significant natural hazard in snow-covered mountains, including the densely populated Alpine region. They can endanger settlements, transport routes and people spending time in the mountains in winter.

Whether and how avalanches release depends mainly on the structure of the snowpack. Does it consist of many different layers? Are these layers thick or thin? Extensive and continuous or interrupted? Solid or weak? There are many influencing factors at work, some of which are difficult to measure. We investigate how exactly avalanches form in order to improve avalanche forecasting and help snow sports enthusiasts to assess avalanche danger themselves.

Avalanche formation

Loose snow avalanches start from a single point and form when snow is not well bonded. In very steep terrain, as individual snow particles become loose, roll downwards and bump into more and more particles, they form an inverted-V-shaped avalanche that starts from a single point and gradually becomes bigger. Because loose snow avalanchesusually carry less snow and travel more slowly than slab avalanches, they are also less dangerous.

 

 

 

Slab avalanches are characterised by the simultaneous release of a cohesive snow layer (slab). Slab avalanches are usually bigger (a typical skier avalanche is on average 50 m wide, 150-200 m long and 50 cm thick) and reach speeds of 50-100 km/h.

 

 

 

Slab avalanches can only form when the snowpack comprises multiple layers of snow. A layer with only a few weak bonds between the ice crystals is known as a 'weak layer'. If a weak layer is overloaded (during snowfall, for example), the initial breaking of individual bonds at a weak point in the layer can set in motion a whole series of similar local fractures. If the damage caused is large enough, a fracture occurs which suddenly and rapidly begins to propagate within the weak layer, parallel to the slope – like a house of cards that starts to wobble at a certain point and then collapses. If the slope is steeper than around 30°, the slab slides downhill at ever-faster speed on the fractured weak layer: a slab avalanche has occurred.

 

 

 

Wet-snow avalanches are usually triggered naturally, most often by a big increase in temperature. Meltwater or occasionally rainwater penetrating the snowpack weakens the bonds between the snow crystals, thereby destabilising layers in which the water accumulates. Both loose snow avalanches and slab avalanches can consist of wet snow.

Experiments and models

The aim of our research is to better understand the processes that take place before, during and after avalanche release. We use field measurements, laboratory experiments and computer models.

In field experiments, we measure the characteristics of the natural snowpack. This includes for example:

  • filming a fracture as it propagates inside a weak layer using high-speed cameras;
  • determining the water content of the snowpack from the ground using radar systems;
  • measuring the stratification and variability of individual snow layers using the SnowMicroPen, a high-resolution snow sampler.

In the cold laboratory, we work under controlled conditions, for example carrying out load tests on artificially created weak layers.

The results of measurements and experiments are used, among other things, as input for numerical simulations that help us to better understand physical processes, or to optimise the SNOWPACK model used by warning services in a number of countries as a tool for assessing avalanche danger.

 

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