Identifying avalanche tipping points
An avalanche occurs when a surface layer of fresh snow comes loose and slides downhill, entraining more snow on its way. Understanding how once-stationary particles start moving en masse is a critical and difficult challenge for predicting avalanche behavior.
Now Nico Gray at the University of Manchester and colleagues have used a laboratory model to demonstrate how inclination, snow depth, terrain, and the mass budget of sliding snow contribute to avalanche evolution. Their mountainside analogue was 1.5 m long, with the snowy surface represented by a layer of irregular, micron-scale silicon carbide grains. The release of additional grains onto the slope triggered the controlled avalanches.
After many trials, the researchers identified three general behaviors. For a thick existing granular layer situated on a steep (35.2o) slope, the laboratory avalanche grew rapidly by entraining more material at its front than it deposited behind, and it left a deep channel in its wake (left image). On a steep slope with just a thin layer of preexisting sediment, the avalanche left behind a shallow trough surrounded by ridges; had the track been longer, the avalanche would have continued indefinitely. On a shallower (34.1o) slope, the avalanche petered out. Gray and colleagues found that those results could be explained by combining two previously published models of friction and rheology. The models offer insight into the critical range of thicknesses, viscosities, and slope angles that determines whether the snow on a slope remains static or moves throughout its entire depth. The findings could help scientists better identify avalanche-prone slopes, like the one in the right image at Bad Gastein, Austria, that would benefit from friction-increasing actions such as planting vegetation.
Gray’s team then analyzed particle-laden environments where snow never falls. Recent Lunar Reconnaissance Orbiter images of the Moon show sites where crater sides have collapsed and left erosion–deposition features that resemble those in the small-scale experiments. The team suggested that the same friction and rheology models could illuminate how they formed. (A. N. Edwards et al., J. Fluid Mech. 823, 278, 2017 ; B. P. Kokelaar et al., J. Geophys. Res. Planets, in press, doi:10.1002/2017JE005320 .)
