Rheology of dense snow flows:: Inferences from steady state chute-flow experiments

Good knowledge of snow rheology is useful for the mitigation of avalanches. However, experiments with snow are difficult and the few available data provide only a partial knowledge of snow flows. In this study we investigated the theological behavior of a dense flow of dry snow, which often occurs in real avalanches. To this end, we carried out systematic small-scale in situ flows down a flume with natural snow. Over three winters, we performed approximately 100 experiments with various slopes and flow discharges and we characterized them by measuring the velocity profile and basal stress. This data set, unique in its extent, allows us to identify various generic characteristics of dense flow of dry snow, which are found to differ from common fluids. We point out that snow flows develop as a very viscous upper thick layer over a much less viscous thin layer. We interpret this heterogeneity as a consequence of a shear-induced evolution of the snow microstructure that gives rise to different materials between the lower part made of single snow grains and the upper layer made of large aggregates. Finally, we show that a single constitutive law that describes dense flow of cohesionless grains can represent the behavior of each layer assuming different grain sizes. Beside its practical importance, this study on snow flow provides new insights into the rheological behavior of similar materials: the wide variety of cohesive granular materials such as humid sand, powders, and bituminous suspensions. (C) 2008 The Society of Rheology.