In mid-January, three people lost their lives in avalanches in the backcountry of western Canada while a further seven people were caught in an in-bounds avalanche at the Fernie Alpine Resort and survived.
One fatality occurred when a group of skiers were involved in an avalanche in Kokanee Glacier Provincial Park while an earlier incident in Burstall Pass in Alberta's Kananaskis Country resulted in two fatalities. These incidents drive home the fact that avalanches pose a real and present danger to people undertaking outdoor activities during the winter.
While there are several different kinds of avalanches, the type that most concerns the average backcountry user are slab avalanches. Slab avalanches occur when a weak layer buried in the snowpack collapses, allowing a cohesive plate of snow to slide on top of a deeper layer.
The initiation of a slab avalanche is a matter of physics - it occurs when the weight of the cohesive slab overcomes the strength of the buried weak layer while the effect of gravity pulling downward on the slope simultaneously overcomes the bonds of the slab to the surrounding snowpack.
This creates the classic image of an independent slab of snow flowing freely down a slope and breaking up as it moves, leaving behind a visible fracture line or "crown" at the top of the slide.
A slab avalanche can be triggered by natural events such as heavy snowfall, wind loading, warm temperatures or cornice collapse. The movement of people on a slope may also create enough stress on a weak layer to trigger an avalanche.
According to Cam Campbell, a public avalanche forecaster with the Canadian Avalanche Association, the winter so far in the Coast Mountains has brought "several medium to large storms with associated storm snow and wind slab avalanche activity."
Also of interest are weak layers in the upper snow pack, predominately in the Duffey Lake/Southern Chilcotin areas, which have the potential to result in large, step-down avalanches. These occur when the weight of an avalanche already in motion triggers an even deeper instability further down the slope.
Other areas are plagued with what Campbell describes as "low-probability and high-consequence, deep persistent slab avalanche problems."
Strong and weak layers in the snowpack develop as a result of both weather and snowpack conditions in a particular location. Weak layers may include buried surface hoar, layers that develop through freeze-thaw cycles and layers composed of faceted snow crystals. Examples of strong layers of snow include wind-packed new snow, dense new snow and snow deposited near 0C (32F).
Depending on the influence of climate and exposure to the sun, layers that start out strong can potentially weaken, while weak layers can also become more stable. Given that conditions change from slope to slope over time, avalanche forecasting is an incredibly complex task. Campbell agrees, "There could be dozens of different weather and snowpack factors at play at any given time."
It is possible to reduce the risk of being caught in an avalanche. For more information on avalanche safety including current avalanche bulletins, avalanche safety training and backcountry trip planning, visit the Canadian Avalanche Association website at www.avalanche.ca
