Item: Density and friction measurements in a flowing dry snow avalanche
Title: Density and friction measurements in a flowing dry snow avalanche
Proceedings: Proceedings of the 1996 International Snow Science Workshop, Banff, Canada
Authors: K.J. Burrell, J.D. Dent, M.Y. Louge, D.S. Schmidt, E.E. Adams, T.G. Jazbutis, Department of Civil Engineering, Montana State University, Bozeman, MT 59717, Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853
Abstract: Using the Revolving Door avalanche facility near the Bridger Bowl Ski Area (Dent et al1994), measurements of density and dynamic friction in a flowing avalanche were made. Corresponding flow depths were concurrently measured using a floating arm with a small skid that rode on the surface of the snow. Measurements of density were made using two devices; a capacitance probe (Louge et aI1996), and calibrated optical sensors that serve to measure flow velocity. The capacitance probe was constructed to measure the dialectric constant of any material that passes in front of it. Through a calibration procedure, the dialectric constant of a given type of snow can be related very accurately to the density of that snow. The capacitance probe was used in one avalanche last winter to determine the density of the snow at 1 cm and 6 cm depths within a 1.5 m deep avalanche. In addition, optical sensors were used to measure the reflectance of the snow as it passed the sensors. The reflectance measured is determined in part by the density of the snow. By proper calibration, the signals from the optical sensors can be related to the snow density. Signals from adjacent optical sensors are also cross-correlated to determine snow velocity. Last winter density and velocity measurements were made in several slides at various depths using these optical sensors. The results indicate that snow density is largest at the bottom of an avalanche, upwards of 400 Kg/ m3 , and in the first few centimeters from the bottom, falls off rapidly to about 300 Kg/m3 . The density then continues to decrease slowly to about 250 Kg/m3 at the surface of the avalanche. The dynamic friction coefficient at the base of two avalanches was found by measuring the shear and normal forces on a roughened 23 cm x 28 cm aluminum plate mounted flush to the running surface. Deflections of the plate were measured using strain gauges, from which the forces on the plate could be found. The ratio of the shear force to the normal force on the plate provides a measure of the friction coefficient at the base of the slide. Of the two slides measured, the drier and deeper slide had a lower coefficient of friction which allowed it to travel faster and farther.
Keywords: density, friction, dry snow, runout
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