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Radar reveals the inside of a powder avalanche

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Several muffled detonations shake the Vallée de la Sionne above Arbaz, a village in Canton Valais, on 3 February 2015. The snow slab at 2700 meters altitude starts to slide. Within seconds an avalanche forms, its core soon disappearing in a powder cloud of suspended snow. SLF researchers released the avalanche artificially in the cordoned-off test area.

What happens under the powder cloud?

To calculate how far avalanches flow and what dynamic processes play a role, researchers need to be able to understand the flow behavior of avalanches as precisely as possible. While the test avalanche is nearing the valley floor, Anselm Köhler and Martin Hiller from SLF are sitting with Jim McElwaine from Durham University in the observation bunker underneath the avalanche slope. Their gaze moves back and forth between the reinforced tank-glass bulls-eye window, which allows a distorted view of the avalanche slope, and their computer screens. There they can see the data measurements of the radar antennas mounted on the outer wall of the bunker. The radar system enables them to see through the dust cloud and watch how the denser flowing part of the avalanche moves.

Understanding the dynamics of avalanches better

Anselm, who is analyzing the radar measurements as part of his doctorate, explains, “We have been able to verify directly, for the first time, what avalanche researchers have long suspected: large avalanches consist of many individual so-called ‘surges’, whose velocities we can measure.” Secondarily released avalanches may develop into large surges, which move inside the original avalanche and develop their own dynamics. Smaller surges of denser snow may also form, some of which flow faster than the main mass of the avalanche. They may break through the front of the avalanche proper, where they will be slowed down quickly. The researchers attribute the different flow velocities in the avalanche to differences in friction. These new insights should help to develop physical models of avalanches further by taking their complex flow behavior better into account. (Martin Heggli, Diagonal 1/17)