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Snow glide and full-depth avalanche occurrence, Cascade Mountains, British Columbia Clarke, Jennifer A.


Snow glide is the translational slip of the entire snow pack over a sloping ground surface. It is thought that rapid rates of snow glide precede the release of full-depth avalanches. The nature of avalanches that release at the ground makes them difficult to predict and difficult to control using explosives. The aim of this research is to determine the relationship between rapid snow glide and full-depth avalanche occurrence and to examine climate factors affecting both processes. Data collected from an instrumented site along the Coquihalla Highway in the Cascade Mountains of British Columbia were used for analysis during two winter seasons (1992-93, 1993-94). Glide is influenced by the nature of the interaction between the roughness of the ground and the snow pack, and by the distribution of water at the interface. The presence of water at the interface affects the material properties of snow and the friction conditions. The impact of freewater on glide is influenced by the volume and rates of water input. Higher glide rates and fulldepth avalanche release are the almost immediate responses to contributions of free-water. The data show that the most significant contributor is rainfall, which is common in the study area throughout the winter season. The supply of free-water from snow melt due to radiative and thermal sources of energy become more significant in the spring. Water inputs increase the thickness of the saturated layer at the base of the snow pack, allowing greater amplitudes of roughness to be overcome. By drowning or partially drowning the roughness elements, a thin film of water reduces the shear resistance of the snow pack to downslope movement. Inputs of water at rates higher than transmissions rates will increase pore pressures and decrease shear stress encouraging further downslope movement. Failure of the snow pack at the ground is translational, most often occurring 12-24 hours after a rainfall event, but sometimes much later when avalanche release would not be expected. Although there is no threshold glide velocity associated with avalanche release, it can be concluded that snow glide is a good indicator of active periods of full-depth avalanche occurrence. However, results from this study show that rainfall rates and snow melt rates may be more accurate predictors of avalanche occurrence in the study area.

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