Item: Effects of an early season "rain" crust on snow pack stability within the continental climate
Title: Effects of an early season "rain" crust on snow pack stability within the continental climate
Proceedings: 2002 International Snow Science Workshop, Penticton, British Columbia
Authors: Dan Moroz, Copper Mountain Ski Patrol, Copper Mountain, Colorado USA
Abstract: On January 6th and 7th, 2002, the central and northern mountains of Colorado, and Copper Mountain ski resort, experienced a "freezing rain" weather event. This deposited a 4 - 8 mm solid ice crust upon the surface ofan 80crn deep, mostly faceted snow pack. This layer affected snow pack stability throughout the season culminating in a large explosives triggered avalanche cycle starting on April 1, 2002 lasting through April 7,2002. This event involved open terrain which had 3 months of light to heavy skier compaction along with avalanche mitigation measures. Fracture line depths ranged from 1.25 to 2.5 m with widths exceeding 40 m. Aspects ranged from east to south with fracture line elevations from 3767 to 3700m with vertical fall exceeding 180m. Fifteen medium to large slides produced extensive debris fields with depths of4 to 7m. Differential creep rates within the upper layers of the snow pack (which occurred after a rapid warm up late March), was a major contributing factor. Water percolation within these layers produced interesting liquid conduits and ice layering evident at the fracture lines. A combination of multiple planer layering and vertical water flow contributing to differential creep stress between adjacent layers will be discussed. Finally, a 6-1Ocm faceted snow layer which was sandwiched between the "rain crust" and a melt freeze layer produced a very weak shear failure plane above the "rain crust" bed surface.
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Keywords: fracture, stability, snowpack, climate
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