Bruce Jamieson

Evaluation of the shear frame test for weak snowpack layers

Abstract The shear frame allows testing of thin weak snowpack layers that are often critical for slab avalanche release. A shear metal frame with an area of 0.01–0.05 m2 is used to grip the snow a few mm above a buried weak snowpack layer. Using a force gauge, the frame is pulled until a fracture occurs in the weak layer within 1 s. The strength is calculated from the maximum force divided by the area of the frame. Finite-element studies show that the shear stress in the weak layer is concentrated below the cross-members that subdivide the frame and where the weak layer is notched at the front and back of the frame. Placing the bottom of the frame in the weak layer increases the stress concentrations, and results in significantly lower strength measurements than placing the bottom of the frame a few mm above the weak layer. Based on over 800 sets of 7–12 tests in western Canada, coefficients of variation average 14% and 18% from level study plots and avalanche start zones, respectively. Consequently,sets of 12 tests typically yield a precision of the mean of 10% with 95% confidence, which is sufficient for monitoring of strength change of weak layers over time in study plots. With consistent technique, there is no significant difference in mean strength measurements obtained by different experienced shear frame operators using the same approximate loading rate and technique for placing the frame. Although fracture surfaces are usually planar, only one of eleven shapes of non-planar fracture surfaces showed significantly different strength compared to planar fracture surfaces. For weak layers thick enough for density measurements, the shear strength is plotted against density and grain form. From these data, empirical equations are determined to estimate the shear strength of weak snowpack layers.

Snow Avalanche Hazard in Canada - a Review

Snow avalanches affect recreation, transportation, resource industries and property. During the 1990s an average of 12.5 persons per year were killed in avalanches in Canada. The snow avalanche hazard has affected people and facilities in B.C, Alberta, Yukon, NWT, Nunavut, Ontario, Quebec and Newfoundland. Avalanche risk may be voluntary, for example skiing and snowmobiling, or involuntary, for example public transportation corridors. A worst-case avalanche scenario is most likely to occur in the Western Cordillera, resulting from a single large-scale weather pattern, where a cold period resulting in the development of a weak layer in the snowpack is followed by a series of major mid-winter storms. Emergency preparedness for avalanches is most advanced in western Canada. New education and information initiatives in Quebec and Newfoundland are aimed at improving preparedness there. Current research is focused on avalanche forecasting, weather forecasting for avalanche prediction, avalanche failure characteristics, forestry and avalanches and geomorphology and avalanches. An important area of future research is the impact of climate change on avalanches, particularly in northern Canada.

Forecasting for deep slab avalanches

Abstract Avalanches released by deep weak layers are known to be difficult to forecast. This study considers the predictive merit of weather and snowpack data for avalanches that released throughout two winters on layers of faceted crystals. These layers formed above rain crusts in November 1996 and in November 1997 in the Columbia Mountains of western Canada. This study focuses on the first winter, during which the facet layer released an estimated 500,000 tonnes of snow in 700 dry slab avalanches. The facet–crust combinations were the result of a cold air mass cooling a layer of dry snow on top of a rain-wetted layer. The resulting temperature gradient in the dry snow formed a relatively weak layer of facets on top of a hard crust. By early January 1997, the faceted layer from the first winter was buried 1–2.5 m below the surface in many starting zones in the North Columbia Mountains. Most avalanches occurred during or within 2 days of loading by snowfall or wind transported snow. Increases in air temperature over 4–5 days correlated with increased avalanche activity. The effects of warming and cooling on slab stability are discussed, but for thick slabs, current theories do not explain observations of decreased stability. We argue that the fractures that release natural deep slab avalanches may be initiated where the slab is locally thin. Based on rank correlations, the highest ranked predictors of natural avalanches include previous avalanche activity, accumulated snowfall over several days, changes in air temperature over 4–5 days, snowpack properties including a shear frame stability index, and the difference in hardness between the facet layer and the crust.

Snow Avalanche Hazards and Management in Canada: Challenges and Progress

Avalanche impacts in Canada, including fatalities, are summarized for residential and public areas, as well as roads, ski areas, backcountry recreation, and resource industries. Methods for managing avalanche hazard, which include defence structures, zoning, forecasting and explosive control, are outlined. Problems with current avalanche hazard management are identified and progress at solving these problems is identified with an emphasis on residential areas, backcountry recreation and resource industries.

Snowpack factors associated with strength changes of buried surface hoar layers

Every winter in North America, failures in layers of buried surface hoar (frost) release many slab avalanches, some of which kill recreationists. Some surface hoar layers stabilize within a week of burial and others require a month or more. Little is known about whether snowpack factors such as crystal size, snowpack depth, slab thickness, load, temperature and temperature gradient are associated with strength changes of these layers. We tested buried surface hoar layers once or twice per week at study sites in the Columbia Mountains from 1994 to 1998, and measured over 300 changes in shear strength. We assess the factors associated with the rate of strength change using rank correlations. The factors are ranked to identify which are most relevant for forecasting changes in strength. Useful predictors include the total snowpack depth, the maximum grain size, and slab depth. The correlations between these predictors and the measured rate of strength change are discussed in terms of physical processes. We illustrate the predictive potential of combined factors by comparing measured values of the rate of strength change with fitted values from a regression tree.

Towards a field test for fracture propagation propensity in weak snowpack layers

Slab avalanche release requires fracture initiation and propagation in a weak snowpack layer. While field tests of weak-layer strength are useful for fracture initiation, the challenge remains to find a verified field test for fracture propagation. We introduce the two current versions of a field test for fracture propagation propensity, and report results of testing conducted in the Columbia Mountains of British Columbia, Canada, during the winter of 2005. By extending the column of a stability test approximately 3 m in the downslope direction, the test method allows for the development of a flexural wave in the slab, and thereby maintains the contribution of this wave and the associated weak-layer collapse to the fracture process. Fracture lengths collected on a day and location where the propagation propensity of the snowpack was locally high show a bimodal distribution, with approximately 50% of observed fractures similar to those collected in stable snowpacks, and approximately 50% with much longer fracture lengths.

Extrapolating the skier stability of buried surface hoar layers from study plot measurements

Abstract Buried layers of surface hoar pose a challenge to avalanche forecasters in many areas, partly because some layers stabilise quickly and others remain unstable for a month or more. This paper relates the measurements of two surface hoar layers in a study plot, one buried 30 December 1999 and the other buried 21 February 2000, to skier-triggered slab avalanches within 100 km of the study plot in the Columbia Mountains of western Canada. The two surface hoar layers were monitored at a tree-line study slope at Rogers Pass every 4 to 8 days until the end of March 2000. Physical properties of the slab (load, thickness, hardness profile), and weak layer (shear strength, temperature, temperature gradient, crystal size, crystal form) were observed. Approximately once every 2 weeks, the weak layers were photographed in the pit wall to document their texture. On the same days, disaggregated crystals from the weak layers were photographed on a crystal screen. The February 21 layer, which initially consisted of 4–6-mm crystals, was loaded more slowly by snowfall, gained strength and stability more slowly, yielded initially lower stability indices and released many more skier-triggered avalanches than the December 30 layer, which initially consisted of larger, 10–20-mm crystals. Critical study plot values of load, shear strength, and stability are compared with critical values measured adjacent to over 50 skier-triggered slab avalanches. The shear strength of the weak layer, calculated skier stability index Sk38, layer thickness, and load on the weak layer show potential predictive value for the stabilisation of buried surface hoar layers. While the time series of photographs of separated crystals shows distinct changes, the time series of photographs of the buried surface hoar layers in situ reveals little useful information on textural changes other than thinning of the layers.

Meteorological forecasting variables associated with skier-triggered dry slab avalanches

Abstract A variety of stability, snowpack and meteorological variables, as well as previous avalanche activity are typically used to forecast the potential for skier-triggered avalanches. However, the relative importance of, and the interaction between, the various variables used to forecast for skier-triggered avalanches have received little attention. This study analyzes the statistical influence of 16 simple meteorological variables, 14 calculated or elaborated variables, and 2 variables for previous skier-triggered avalanche activity at a helicopter skiing operation in the Columbia Mountains of British Columbia, Canada. Forecasting variables are individually assessed using rank correlations to identify the variables most relevant for forecasting the potential for skier-triggered slab avalanches on the regional scale. The variables showing the strongest forecasting potential include: the largest size class of skier-triggered avalanche over the previous one and two days, the 24-h snowfall, the 24-h precipitation, the cumulative storm snow, the height of the snowpack, and the number of days since December 1. The physical processes that relate these variables to skier-triggered avalanches are discussed. The predictive potential of combined forecasting variables is assessed using a multi-variate classification tree model. This model is verified using the last two years of data that was excluded from development of the tree model. The model correctly predicts relatively large avalanches approximately two-thirds of the days for the last two years of the dataset.

Statistical avalanche-runout estimation for short slopes in Canada

Abstract To develop avalanche runout models for short slopes, field measurements were made at 48 short-slope avalanche paths located in the Coast, Columbia and Rocky Mountains of western Canada, and at several paths in eastern Canada. Field studies included detailed topographic surveys and estimation of the extreme runout position in each path. A statistical runout model was developed using the runout ratio method, for which runout ratios from the four mountain ranges are well fit by an extreme-value type I (Gumbel) distribution when the β point is defined at the uppermost point where the slope is 24°. A second model was developed by regressing the α angle for the extreme runout position on numerous terrain variables. This regression model uses three predictor variables that can be easily measured in the field or on topographic maps. Length-scale effects were noted in both models, but are more pronounced in the runout ratio model. A comparison of models developed using the two methods shows that the runout ratio model estimates more conservative (longer) runout distances than the regression model for most threshold probabilities. Data from 13 additional paths from Switzerland and Québec, Canada, are used to test the models.

Heat flow from wet to dry snowpack layers and associated faceting

Abstract Layers of faceted crystals adjacent to crusts form the failure layers for some unexpected dry-slab avalanches. This paper focuses on the case of facets that form when dry snow overlies wet snow. From a basic equation for heat flow in solids, the approximate freezing time of the wet layer is derived. Seven experiments are described in which a wet layer was placed between two dry-snowlayers in a cold laboratory. Measured freezing times are comparable to the freezing times from the approximate solution assuming that latent heat from the irreducible water content flowed up. In four of the experiments, evidence of faceting was observed at the base of the upper dry snow layer within 5 hours and before the wet layer froze. In all seven experiments faceting was observed in the upper dry layer after the wet layer froze. Simulations performed with the snow-cover model SNOWPACK yield freezing times that agree reasonably with the approximate solution and allow the influence of various parameters on the results to be explored. In addition, simulated temperatures and grain evolution are compared with observations, showing good agreement.