Avalanches

An avalanche is a process that incorporates snow or ice from a release area and transports it along an avalanche track as a sliding mass or as a turbulent snow and air mixture down to an accumulation area.

How does an avalanche develop?

The snow cover is not a steady body. The snow is continuously creeping downhill due to its ability for deformation. The weight of the snow cover is the driving force for this downward movement. Changes in slope angle cause velocity changes of the movement. This results in tensile or pressure strength in the transitional zones. When forces of two layers or parallel movements are working against each other then additional shear stress occurs. Additional stress in the release area, e.g. by a skier, changes the tensions within the snow cover and can cause the development of an initial crack. This crack develops with a velocity of 100 m/sec. At its lateral margin new secondary cracks develop. As soon as the slab avalanche is fully released by the secondary cracks it starts sliding and can reach up to 100 km/h. This describes a typical «skier avalanche».

Large-scale avalanches that cause damage in inhabited areas or to infrastructure are called «catastrophic avalanche» or «damaging avalanche». The «catastrophic avalanche» is in most cases a combined wet- and powder avalanche. The formation of a powder avalanche is favoured by a sudden increase of slope angle of the avalanche track or when the avalanche jumps over a precipice. The avalanche gets airborne and reaches velocities of 300 km/h as snow-air mixture due to reduced friction. The destructive force of avalanches is huge. The flowing avalanche carries trees and rocks away and erodes the snow cover down to the soil. Increased friction causes the avalanche to slow down. It stops in the accumulation area.

What type of damage can occur?

Over the past 20 years about 22 people per year lost their lives due to avalanches. Most of the avalanche victims are killed in free nature when performing snow sports. Avalanches cause major damage to buildings, infrastructure and forests. The avalanche hazard is classified according to a unified avalanche scheme all over Europe. The hazards are controlled by an integrated risk management approach.

Text: A. Schilling, Swiss Federal Institute for Snow and Avalanche Research, Davos