Forecasting shear strength and skier-triggered avalanches for buried surface hoar layers

Abstract

In the Columbia Mountains of western Canada, skier-triggered slab avalanches on weak layers of buried surface hoar occur mostly within the first 10 to 20 days after a layer is buried (e.g. Chalmers and Jamieson, 2001), and the transition of such layers from unstable to stable conditions is difficult for avalanche professionals to forecast. Schleiss and Schleiss (1970) introduced a snow Stability Ratio (Canadian Avalanche Association, 2002) based on shear frame tests of the weak layer, which has been used since c. 1960 at the Mount Fidelity study plot to extrapolate snowpack stability for natural avalanches in the Rogers Pass highway corridor. Fohn (1987) developed a stability index for skier triggering that was refined by Jamieson and Johnston (1998). When the index is based on shear frame tests in a study plot and extrapolated to surrounding avalanche slopes (average slope angle of 38°), the index is denoted Sk38 (Jamieson, 1995). Jamieson (1995) converted Sk38 into an equivalent rutschblock score RBcalc. Over the winters 1995-2001, over 70 layers of buried surface hoar were monitored at study sites in the Columbia Mountains, using snow profiles and shear frame tests of surface hoar layers. The snowpack observations of this study were consistent with the guidelines of the Canadian Avalanche Association (2002), except for the thickness of a buried surface hoar layer, which was measured to the nearest millimetre. These sites were selected to be representative of the snowpack in surrounding avalanche starting zones. Within approximately 100 km of two sites, Mt. Fidelity and Mt. St. Anne, skiertriggered avalanches and the critical weak layers were often reported by surrounding helicopter skiing operations (Figure 1). The objectives of this study were: 1. to develop an empirical model that could be used to forecast the shear strength (Σ) of buried surface hoar layers in the Columbia Mountains, and 2. to assess the stability index RBcalc, based on forecast strength of surface hoar layers, with skiertriggered avalanche activity from the region. Forecasting shear strength and skier-triggered avalanches for buried surface hoar layers