Loose snow avalanchesIn loose snow avalanches, a small volume (less than 1 m3) of loose snow separates from the surface. As the initial mass descends, it gathers more snow, producing an ever-widening, triangular track.
AvalancheAn avalanche is a rapid, downslope movement of SNOW, ICE, water, rock, SOIL and vegetation, mixed in various proportions. Snow or ice is the major component, which differs from a LANDSLIDE or ROCKSLIDE).
TypesSnow avalanches are classified as either "loose" or "slab," referring to the initial condition of the sliding snow.
Loose snow avalanches
In loose snow avalanches, a small volume (less than 1 m3) of loose snow separates from the surface. As the initial mass descends, it gathers more snow, producing an ever-widening, triangular track. Loose snow avalanches may also be called "point releases," because they start from a point and fan out. In general, loose snow avalanches tend to be smaller and less dangerous than slab avalanches.
Slab avalanches occur when a cohesive, stiff, planar snow slab becomes detached from the underlying snowpack and slides downslope. Slabs can be as little as 0.1 m thick, or may involve the entire snowpack of several metres in depth. Thicker slabs tend to release over larger areas, in many cases exceeding 10 000 m2 (or 1 hectare). Slab avalanches are more hazardous than loose snow avalanches because of the large amount of snow set in motion at once. The starting zone of a slab avalanche has distinctly defined fracture surfaces: bed, crown and flank surfaces, and staunchwall (the downslope boundary of the slab). The most important of these is the flat basal bed or "sliding" surface, where a thinner, weaker layer buried in the snowpack fails and allows the overlying slab to become detached.
Avalanches can occur anywhere with steep slopes and unstable snow. Even very small slopes (less than ten metres high) may produce avalanches under certain conditions.
Loose snow avalanches occur due to the loss of cohesion of soft, near surface snow. This may occur spontaneously due to warming, insolation (exposure to the sun's energy), or loading by storm snow; or may be triggered by an external disturbance such as a person or animal passing over a slope.
Slab avalanches release when a shear or collapse fracture occurs in an underlying weak layer. That fracture is first triggered, then spreads up, down, or across the slope. External forces such as wind, the weight of snowfall, humans, animals, and vehicles may cause overloading, triggering a fracture in the weak layer. It may also occur spontaneously because of a change in the mechanical properties of the snowpack (such as slab stiffness or density) due to rapid temperature changes or other factors.
The avalanche size classification scheme used in Canada ranks avalanches on a scale of 1 to 5. Size 1 avalanches are relatively harmless to people. Size 2 avalanches are large enough to bury, injure or kill a person. Size 3 avalanches are large enough to destroy a car or small building, or break a few trees. Size 4 avalanches could destroy a railway car, large truck or building, or up to four hectares of forest. The largest avalanches (Size 5) could destroy a village or up to 40 hectares of forest.
While the smallest avalanches (Size 1) travel relatively short distances (ten metres) and have an impact pressure of one kiloPascal (kPa), Size 5 avalanches can travel up to 3000 metres, include large volumes of snow (up to 100 000 m3) and impact with great pressure (100 kPa). Fortunately, the frequency of avalanches decreases as their size increases: size 1 avalanches are very common, and size 5 avalanches occur very rarely.
Avalanche Areas in Canada
Large and small avalanches are very common in the mountains of British Columbia, Alberta and Yukon during the winter. Significant avalanche hazard also exists in Québec, Newfoundland and Labrador, Nunavut and the Northwest Territories. Avalanches are exceedingly rare in the other provinces, although one fatal avalanche did occur in Toronto, Ont.
Professional avalanche forecasters rely on avalanche, snow and weather observations in order to predict or forecast avalanches. Recent avalanches and other signs of unstable snow, as well as heavy precipitation, rising temperatures and strong winds are usually associated with increased avalanche hazard.
The Canadian Avalanche Centre (CAC) and Canadian Avalanche Association (CAA)
The Canadian Avalanche Centre (CAC) is a non-government, not-for-profit organization that produces public avalanche bulletins and warnings for a number of geographic areas in western Canada. (Similar organizations provide the same service in Québec [Centre d'avalanche de La Haute-Gaspésie] and Yukon [Yukon Avalanche Association.]) While the format of these advisories may vary, they all provide information about avalanche activity and snowpack conditions, highlight primary concerns for the day, offer travel advice, and forecast weather conditions. Forecasters use a 5-level, internationally recognized danger scale that identifies the likelihood, size, distribution and expected consequences of avalanches, and provides advice regarding travel in avalanche terrain. The main users of the public bulletins are backcountry recreationists such as snowmobilers, skiers and snowboarders. In addition to providing public bulletins, the CAC runs avalanche training workshops and courses (including online instruction), and provides learning materials and support for recreational avalanche skills training. The CAC also encourages avalanche research and provides decision-support tools such as the "Avaluator," a system that helps recreationists determine the avalanche risk of a proposed trip.
The Canadian Avalanche Association (CAA) is the governing association for professional avalanche workers in Canada who are employed by ski hills, backcountry operations, and government and private companies working in avalanche areas. These professionals provide avalanche forecasting and control services tailored for particular locations or activities, using a similar forecasting process as their public counterparts.
Mitigation and Management
Avalanche risk may be mitigated by removing people or property from areas affected by avalanches, or by protecting elements at risk using engineered sheds, walls, berms and deflectors to absorb, dissipate or redirect moving avalanches. Land-use planners and engineers assess the risk to a given location and restrict development or specify mitigation measures to reduce risk to an acceptable level.
Often, active avalanche control is required to protect people and infrastructure that cannot be relocated or otherwise protected. Explosives--including hand-charges, artillery shells, and propane blasts--are applied to unstable slopes by professional avalanche workers in order to bring down smaller, controlled and predictable avalanches. During times of elevated avalanche hazard when passive or active control is not possible, work sites may be evacuated or transportation corridors may be closed to public or industrial traffic.
Economic ImpactWhether due to avalanche control work or because of unanticipated avalanches, the economic costs of closing a major highway or railway, or a worksite such as a mine, are significant to both the public and private sectors. In addition to the disruption of business, these costs include those associated with forecasting, control, mitigation and cleanup of avalanches. Actual cost estimates vary--and the necessary data is not always released or simply does not exist--so the total economic impact of these closures is not known precisely.
Accidents and Fatalities
Fatal avalanche accidents occur in Canada each winter: an average of 10-15 people die every year. Historical accidents typically involved workers at industrial sites; notable accidents occurred at Rogers Pass, BC, on 4 March 1910 (62 fatalities), near Britannia Mine, BC, on 22 March 1915 (57 fatalities) and at Granduc Mine, BC, on 18 February 1965 (26 fatalities). However, the victims of recent accidents tend to be recreational backcountry enthusiasts such as skiers or snowmobilers, as was the case in the Columbia Mountains of British Columbia on 20 January and 1 February of 2003 (with seven fatalities each incident). The tragedy at KANGIQSUALUJJUAQ, Qué, on the morning of 1 January 1999, illustrates the danger avalanches pose to communities: nine people (five of them children under the age of eight) were killed when snow smashed into a school where most of the small community were celebrating.
Victims of avalanche typically die of asphyxia or trauma sustained during the avalanche. The likelihood of surviving decreases rapidly after approximately ten minutes of burial. Most persons who knowingly travel in avalanche terrain wear radio-frequency transceivers, which broadcast a signal to allow searchers--using the same device--to locate them if they become buried. Conversely, any person wearing such a beacon can perform a search immediately after an avalanche occurs. This type of rescue is often the avalanche victim's best chance for survival, although people have been recovered alive after long burials by avalanche dog, systematic probing or other means.
Cam Campbell, Laura Bakermans, Bruce Jamieson, Chris Stethem, Current and Future Snow Avalanche Threats and Mitigation Measures in Canada (2007); Bruce Jamieson, Pascal Hageli, Dave Gauthier, Avalanche Accidents in Canada, Volume 5, 1996-2007 (2011); David McClung and Peter Schaerer, The Avalanche Handbook (first edition by R.I. Perla, 1976; 3rd edition 2006).