Item: Deep slab avalanche hazard forecasting and mitigation: the south face at big sky ski area
Title: Deep slab avalanche hazard forecasting and mitigation: the south face at big sky ski area
Proceedings: Proceedings of the 2006 International Snow Science Workshop, Telluride, Colorado
Authors: Scott Savage, Big Sky Snow Safety, Big Sky, MT
Abstract: In December 1995, the Big Sky Ski Area installed a tram to the summit of Lone Mountain, accessing over 130 hectares (320 acres) of avalanche terrain in an area historically known as the South Face. During the 2006-7 season, an additional 40 hectares (100 acres) of avalanche terrain on the South Face will be opened to the public. The South Face’s windy alpine setting, relatively large path size, and generally southeast aspect combine to present both interesting and significant avalanche hazard forecasting and mitigation issues. This paper focuses on observed deep slab avalanche activity on the South Face of Lone Mountain since 1995 and the implications that this activity has had on the ski area’s avalanche hazard forecasting and mitigation practices. The examination of a data set of 74 recorded deep slab avalanches (crown size >1.2m, hard slabs, failing on persistent weak layers or interfaces) highlights trends and themes in the weather and snowpack factors that contributed to the observed events. As would be expected, multi-day precipitation events and strong prevailing winds are important factors. Interestingly, every avalanche in the data set had either a crust or a hard ice layer as the bed surface. The past eleven years of experience on the South Face has helped dictate what tools and techniques Big Sky avalanche practitioners currently employ to evaluate and deal with potential deep slab instabilities. The dramatic spatial variability encountered on the South Face decreases the usefulness of study plots and data pits and forces us to rely more on hasty pits and probing. During hazard reduction work, 1-2 kg hand charges have generally been effective triggers for these relatively large hard slab avalanches. This paper does not offer any hard scientific theories or conclusions, but instead presents what avalanche practitioners at Big Sky have observed and learned about dealing with a challenging piece of avalanche terrain.
Keywords: deep slab instability, crusts, ice, facets, explosives, avalanche forecasting
Digital Abstract Not Available