avalanches

avalanches Avalanches may be defined quite simply as the rapid downslope movement of earth, ice, rock, or snow. This broad definition incorporates a variety of phenomena that are typically classified on the basis of the constituent material. However, while debris, ice, and rock avalanches are all important mass-movement processes, attention is focused here upon snow avalanches, which are popularly considered to be synonymous with the term ‘avalanche’ and are of much greater significance as a natural hazard than other types of avalanche. Examples of major avalanche disasters in the twentieth century include the accident in 1910 at Rogers Pass, British Columbia, Canada, where 62 railway workers were killed while clearing the track of previous avalanche debris; the winter of 1950–1 in Switzerland, which left 279 people dead and 285 injured; and the 1995 avalanche in Flateyri, Iceland, which resulted in 20 fatalities.

Snow avalanches are commonly classified on the basis of release type and free-water content. Loose snow avalanches are released from a localized area or point and in general involve snow only at or near the surface. Small loose avalanches are known as sluffs, and in general loose avalanches are smaller than slab avalanches. The latter occur when an approximately rectangular block of snow with a depth of up to the full depth of the snowpack is released and moves downslope. Dry avalanches contain little or no free water at the time of fracture, although it is not unusual to detect some moisture in the snow immediately after deposition. This results from melting induced by frictional heat, which is generated between the avalanche and the sliding surface and by collisions between moving grains.

At the other end of the moisture-content scale, slush avalanches are the flow of a partially or totally saturated snow cover. Wet snow avalanches represent intermediate moisture contents. Wet and slush events are more common towards the end of the winter when temperatures begin to rise. Slush avalanches are particularly common at high latitudes (for example, in Norway), where the sudden return of direct radiation from the Sun promotes intense melting. Wet and slush avalanches commonly incorporate debris from the ground underlying the snowpack; in consequence, they produce dirty deposits that are of interest to geomorphologists studying rates of mountain erosion. It is, however, the dry avalanche, and in particular the dry slab avalanche, that is of primary concern to anyone evaluating the importance of avalanches as a natural hazard.

Dry slab avalanches tend to be released on slopes with angles between 25° and 55°. The majority of slab avalanches have a fracture depth of less than 1 m, although major avalanches will have depths much greater than this: for example, the Flateyri avalanche mentioned above had a maximum fracture depth of 3.7 m. Dry slabs are released when the snowpack fractures, that is, when a catastrophic failure occurs. The state of failure is reached when the downslope component of the weight of the snowpack above the eventual failure plane is approximately equivalent to the shear strength of this failure plane. Thus, the first fracture occurs in a plane approximately parallel to the snow surface. It then propagates both across-slope and upslope. A tensile fracture develops from the bed towards the surface and then spreads laterally, forming the crown of the avalanche, a face perpendicular to the failure plane that is usually clearly observable after the event. Very shortly after the formation of the crown, the flanks and the lower fracture (the stauchwall) are created. The slab now begins to move rapidly downslope, usually obliterating the stauchwall in the process.

The most important trigger for natural slab avalanches is the addition of new snow, either by direct precipitation or by wind-driven drifting. Increase in weight due to rainfall is an important cause of wet slab formation. The impact of falling snow from avalanches released above or from cornice collapse is another important natural trigger. Artificial triggers include release by explosives as part of the active control of avalanche-prone slopes and skier loading.

The avalanche failure plane is commonly much weaker than the layers of snow above and below. These weak layers can be remarkably persistent and widespread. A weak layer that formed in mid-November 1996 in British Columbia, Canada was responsible for avalanching in the Rocky, Columbia, and Coast Mountain ranges, including one avalanche 6 months after the layer formed. Weak layers can be formed by a variety of processes. Most weak layers form at or near the surface and are buried by subsequent snowfall. Crusts are an important type of weak layer that can form when water on the surface of the snow (originating from rainfall or melting) freezes. Fresh snow bonds poorly to these layers, promoting instability. If the temperature gradient in the snowpack locally exceeds 10 °C m⁻¹ it is possible for snow crystals to change into more angular forms called faceted crystals. These crystals bond poorly to one another and also form weak layers. Faceted crystals may also be associated with crusts. An important type of faceted crystal is known as depth hoar. These large, cup-shaped crystals can result in deep-seated instability in the snowpack and hence large avalanches.

Various defence strategies are commonly employed to protect people from avalanches. Artificial release can prevent large avalanches from taking place. Supporting structures in the starting zone of the avalanche path help to reduce the chance of release. Fences placed above the starting zone can prevent snow from drifting into leeward basins, thus reducing drift loading. Structures are also used in the runout zone of the avalanche path, where the avalanche begins to decelerate. Deflectors steer the snow away from regions of concern, while dams act as a barrier to the flow. Mounds of earth may also be used to slow down the avalanche and reduce its runout distance. The prediction of avalanches and of their runout distances are important areas of current research aimed at improving the effectiveness of avalanche-protection strategies.

Christopher J. Keylock

Bibliography

McClung, D. M. and and Schaerer, P. A. (1993) The avalanche handbook. The Mountaineers, Seattle.