Comptes Rendus
no. 1
The structure of powder snow avalanches
[Structure des avalanches en aérosol]
Betty Sovilla1  ; Jim N. McElwaine2  ; Michel Y. Louge3
1 WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 11, 7260 Davos Dorf, Switzerland
2 Department of Earth Sciences, Durham University, Durham DH1 3LE, United Kingdom
3 Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, United States
Comptes Rendus. Physique, Volume 16 (2015) no. 1, pp. 97-104.

Les avalanches en aérosol peuvent atteindre plusieurs centaines de mètres de hauteur et dévaler à des vitesses étonnantes. Cette revue peint une image générique de ces avalanches à partir de données obtenues sur le site de la vallée de la Sionne, en Suisse. Ces données incluent l'épaisseur du manteau neigeux en fonction du temps, observée par un radar enfoui et, à plusieurs hauteurs sur un pylône, des pressions d'impact provenant de cellules de charge, des mesures de pression de l'air, des vitesses de particules par l'intermédiaire de capteurs optiques et, enfin, la densité de l'aérosol et la taille de ses amas, par une sonde à capacité. Les avalanches en aérosol comprennent des zones distinctes, avec stratification de la densité moyenne. À leur front, les pressions d'impact fluctuent, mais s'affaiblissent avec la hauteur, alors que les profils verticaux de vitesse évoluent rapidement le long de l'écoulement. Ceci suggère que des couches de neige légère, froide et sans cohésion font éruption en créant une région frontale turbulente, hétérogène et recirculante. Sur des centaines de mètres derrière le front, la stratification de l'aérosol s'accroît avec la déposition de particules suspendues. Un écoulement de surface à épaisseur croissante s'établit alors, tandis que des éléments plus chauds et plus profonds du manteau neigeux sont entraînés. Vers la queue de l'avalanche, les profils verticaux s'uniformisent, la pression d'impact et ses fluctuations diminuent, alors que l'écoulement s'affine et que l'érosion du manteau neigeux devient négligeable.

Powder snow avalanches (PSAs) can be hundreds of metres high and descend at astonishing speeds. This review paints a composite picture of PSAs from data acquired at the Vallée de la Sionne test site in Switzerland, including time-histories of snow cover thickness from buried Radar and, at several elevations on a pylon, impact pressures from load cells, air pressure, particle velocity from optical sensors, and cloud density and particle cluster size from capacitance probes. PSAs feature distinct flow regions with stratification in mean density. At the head, highly fluctuating impact pressures weaken with elevation, while vertical velocity profiles evolve rapidly along the flow, suggesting that surface snow layers of light, cold, cohesionless snow erupt into a turbulent, inhomogeneous, recirculating frontal cloud region. For hundreds of metres behind the head, cloud stratification sharpens with the deposition of suspended cloud particles, while a denser basal flow of increasing thickness forms as deeper, warmer and heavier parts of the weakened snow cover are entrained. Toward the tail, vertical velocity profiles are more uniform, impact pressures become lower and steadier as the flow becomes thinner, and snow pack entrainment is negligible.

Publié le :
DOI : 10.1016/j.crhy.2014.11.005
Keywords: Snow entrainment, Avalanche impact pressure, Avalanche velocity, Avalanche density, Mixed avalanches, Powder snow avalanches
Mot clés : Avalanches mixtes, Avalanches en aérosol
Betty Sovilla 1 ; Jim N. McElwaine 2 ; Michel Y. Louge 3

1 WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 11, 7260 Davos Dorf, Switzerland
2 Department of Earth Sciences, Durham University, Durham DH1 3LE, United Kingdom
3 Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, United States 
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Betty Sovilla; Jim N. McElwaine; Michel Y. Louge. The structure of powder snow avalanches. Comptes Rendus. Physique, Volume 16 (2015) no. 1, pp. 97-104. doi : 10.1016/j.crhy.2014.11.005. https://comptes-rendus.academie-sciences.fr/physique/articles/10.1016/j.crhy.2014.11.005/

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