An avalanche is the most destructive and dangerous event that can occur in a ski area. The torrent of snow that obliterates everything in its path is the ultimate natural disaster in a mountain region. The famed St. Bernard dogs of the Swiss Alps acquired their legendary reputation from the rescue of travelers caught in snow falls and avalanches. Death from exposure, injuries, or suffocation are a common fate for a skier caught in an avalanche.
Avalanches have a measure of predictability in that they can only occur on certain types of mountain terrain. An avalanche will rarely occur on a slope that is angled at less than 30°; these events similarly almost never occur on slopes greater than 45°, as it difficult for snow to accumulate in sufficient volumes on a steeper angle to precipitate a descent. As avalanches typically occur year after year in the same places, the force of the avalanche creates a well-defined path on a mountainside. New avalanche sites are often inadvertently created through the removal of forest cover for development or road construction.
A large volume of snow on a mountain side is not the sole cause of an avalanche. The type of snow, the layers of snow that may be created by a series of snowfalls, and the degree of adhesion between each layer of snow are all factors. Avalanches will take one of two forms: movement that involves loose snow, the less serious type of avalanche, and those that are known as a slab advance, created when various layers of the season's snowfall adhere and create a single wall of snow moving down a slope. The slab advance avalanche is the most destructive form, commonly extending over hundreds of feet/meters in width.
An avalanche can move along the terrain in three distinct fashions. The snow can move through the air, as occurs when the avalanche moves over a cliff or rock outcropping. Avalanches most often move along the ground; in some circumstances, the movement may be a combination of ground and air.
An avalanche may be triggered by natural and artificial, or external, means; avalanches are often
To control the ski area from the consequences of an avalanche, the first preventative step is to ensure that avalanche forecasting can be performed with some measure of precision. The forecast will include daily examinations of the current snow cover, including its type and its apparent density as well as meteorological forecasts.
Avalanche control has two essential components: the modification of the terrain where the avalanche is expected to occur prior to the ski season, and physically redirecting the snow once it has fallen and accumulated in a potentially dangerous fashion.
Terrain modification may entail a number of physical changes to the avalanche area. These steps are taken after the terrain has been carefully assessed and the most common avalanche pathways have been mapped. A common modification is the erection of snow sheds in the vicinity of road ways, buildings, or other structures that potentially fall in the path of an avalanche. A snow shed is commonly constructed of concrete, to withstand the force of thousands of pounds of advancing snow. The shed is not designed to stop the avalanche so much as to deflect the advancing snow away from the protected structure or feature.
Steps may also be taken to alter the physical terrain through earthmoving or the placement of barriers to divert the path of a future avalanche. A mountainside will often be divided into segments for the purpose of avalanche control, to prevent one avalanche from triggering companion slides on adjacent ski areas. Lightweight but strong metal fencing is often placed high on a mountainside to prevent the formation of the snow slabs that cause the most avalanche damage.
Once the snow has fallen and begins to accumulate in a manner that may precipitate an avalanche, the danger may be controlled by a number of artificial means. Through avalanche forecasting, the fracture zone, the place on the snow surface where the slab is most likely to break off from the existing snow cover, is capable of being identified. Explosives can be placed in the fracture line to trigger an artificial and more manageable event; mortar shells are sometimes fired from a distance into the fracture line to reduce the risk associated with a person physically skiing to the desired area for this purpose. A technique known as ski-cutting is also employed, where ski patrollers, working in pairs, use their skis to ski along the area of the fracture line and cause a limited and predictable avalanche.