Item: Creep and failure of alpine snow: measurements and observations
Title: Creep and failure of alpine snow: measurements and observations
Proceedings: Proceedings of the 1996 International Snow Science Workshop, Banff, Canada
Authors: H. Conway, s. Breyfogle, J.B. Johnson and C. Wilbour, University of Washington, Geophysics, Washington State Department of Transportation, US Army Cold Regions Research and Engineering Laboratory
Abstract: We investigated the creep behavior of alpine snow in an effort to help understand and predict the timing of avalanche release. Measurements of motion of glide shoes buried within a natural snowpack show strains within low density snow are typically large (often exceeding 70%). The rate of deformation increases with temperature and is especially rapid in the presence of liquid water. Creep rates decrease rapidly as the snow densifies. The slope parallel shearing component of motion is much smaller than expected from the usual constitutive assumptions for snow. Even when snow is first wetted and on slopes up to 36°, the resultant direction of motion is typically close to vertical. We explain this apparently anomalous behavior by considering the effects of metamorphic processes and "capillary strain" (when liquid water is present) which cause deformation independently of gravity. It is well known that avalanche activity usually increases at the onset of rain, long before liquid water has penetrated to depth. We discuss how capillary induced shrinkage at the surface might alter the distribution of stress through the slab sufficiently to cause existing zones of deficit (or "super weak spots") to extend in length. A rain induced surface alteration occurs rapidly over a wide region and has the potential to perturb all existing zones of deficit simultaneously, thereby increasing the possibility of slope failure. The analysis predicts slope failure is more likely if the overlying slab is thin and the stability is already Close to critical. Field observations of behavior at the onset of rain support this prediction.
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Keywords: rain-on-snow, creep, avalanches
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