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Inside an avalanche

14 February 2006

Researchers from the Environmental Hydraulics Laboratory at the EPFL (Ecole Polytechnique Fédérale de Lausanne) have built a laboratory-scale avalanche simulator and are using it to build a complex fluid model that will be able to follow the dynamic, non-equilibrium flow characteristic of heavy snow avalanches.  Coupled with terrain-based testing and analysis in the Sionne Valley of Switzerland, this physics-based model will be an important step forward in our understanding of natural disasters.

The winter blanket of snow covering the Alps is stunningly beautiful– and incredibly dangerous. In 2004-2005, 26 people died in avalanches in Switzerland alone. The victims range from occasional snow-boarders catching some powder off-piste to backcountry ski guides with years of experience. In this mountainous country, avalanches also pose a serious public danger. They can bury people in their homes, cut off access roads or even flatten whole villages. Scientists have put great effort into trying to understand the physical mechanisms at work in avalanches, particularly in the domain of fluid mechanics, in an attempt to improve our ability to predict and manage avalanche danger. But progress is limited because the computer models that simulate complex fluid movement are still quite rudimentary.

 
EPFL professor Christophe Ancey, an expert in rheology, or flow phenomena, is working to improve that situation. His team is building an installation that will generate avalanches in the comfort of the laboratory. Unlike natural avalanches, no two of which are alike, and all of which are quite uncomfortable in scale and force, all the variables involved in these slides can be controlled and the same avalanche can be studied repeatedly. The simulation data will be used to construct a new numerical model capable of describing the avalanche's dynamic behavior.
 
The laboratory system Ancey is developing is based on the"dam-break" concept, in which a viscous fluid is poured onto a steeply inclined plane. The blue ooze flowing down the slope may not look like snow, but it deforms in the same way an avalanche does, and shares the same physics – the highly complex, non-equilibrium, non-linear flow that is characteristic of heavy snow and mud. “No existing numerical model can reproduce what's happening in even this simple setup," explains Ancey."As a first step, we need to be able to reproduce what we observe, and with a model that takes only hours, not days, to run."
 
The model Ancey constructs from the simulation data will be tested against reality in the Sionne Valley in the Swiss Alps. There, in an avalanche-prone area, the Swiss Federal Institute for the Study of Snow and Avalanches has set up an amazing measurement station. A narrow wedge steel construction, a 20m tall pylon and a bunker equipped with a variety of sensors, video and Doppler radar have been placed in an avalanche track. As an avalanche pounds down the mountainside, this equipment will collect a wide range of data. Ancey's model will predict the progression of this same avalanche, and the comparison with the data will reveal how well his model captured reality.
 
In this second, reality-testing phase of his project, Ancey will collaborate with EPFL Professor Eduardo Charbon, who has developed inexpensive new-generation sensors that can be placed across the snow surface before the avalanche is triggered. These sensors will track the velocity inside the avalanche, something that has never been possible before. The data can then be analyzed and used to further improve Ancey's dynamic model.
 
“It is important to change the way we understand, anticipate and manage natural hazards," explains Ancey."Several elements interact to cause an accident or a natural disaster. We often think that we can calibrate the models we use for prediction using data from past events. But if we really want to be able to predict what could happen, say in a scenario involving climate change, we must be able to understand and explain the physics behind natural phenomena, instead of just describing them. That's what we're doing in this research program."
 
In the long run, Ancey's laboratory research, combined with the terrain-based testing and analysis, will be used to develop a diagnostic system that can help decision-makers better predict and prepare for these kinds of natural disasters. Important political decision-making, such as approving development in avalanche or flood-prone areas, requires accurate risk assessment. And the physics-based approach being developed by Ancey will ultimately provide decision-makers with a tool they can confidently use to evaluate the risk of natural disaster in a variety of potential scenarios.
 
Launched in 2004, this project is financed by the EPFL and the Swiss National Science Foundation, as part of the latter's National Center of Competence in Research in Mobile Information and Communication Systems.

journaliste: Florence Luy