S N O W  A V A L A N C H E
their characteristics, forecasting and control

by Edward R. LaChapelle
Avalanche Hazard Forecaster
Wasatch National Forest, Utah
U.S. Department of Agriculture

Introduction

Snow avalanches are active geological agents of erosion and have been a source of natural disasters as long as man has dwelled in the mountains. A common feature of mountainous terrain throughout the temperate and arctic regions of the earth, they may fall wherever snow is deposited on slopes steeper than about 30 degrees. Small avalanches, or sluffs, run in uncounted numbers each winter, while the larger avalanches, which may encompass slopes a mile or more wide and millions of tons of snow, fall infrequently but inflict most of the destruction. The avalanche hazard arises whenever man and his works are exposed to sliding snow. Such hazard has been familiar to inhabitants of the Alps and Scandinavia for many centuries, while it is a more recent development in other parts of the world.

Avalanches run in the same paths year after year,the danger zones often being well known in normal circumstances. Exceptional weather at intervals of many years may produce exceptional avalanches which overrun their normal paths and even break new ones where none existed for centuries. Unwise timber removal in alpine terrain can also create avalanches where none existed before. Given exceptional snow conditions, even short slopes like the walls of a ravine can become dangerous. Snow avalanche may fall anywhere that enough snow is deposited in the right circumstances on an inclined surface.

These right circumstances sometimes consist of abnormally large snowfalls, but not always so. Avalanches find their genesis in snow cover structural weaknesses which are often induced by internal changes. A large overburden of snow alone may not result in avalanching if it is anchored to a solid underlayer. On the other hand, even a shallow snow layer can slide from the mountainside if poorly bonded. The snow avalanche is a complex problem in mechanical stability which can best be understood in terms of the physical processes taking place in the changeable winter snow and the dependence of these on temperature.


Types of Avalanches and Their Characteristics

The wide variety of snow avalanche origin,, nature of motion, and size reflects the highly changeable nature of snow. The fundamental classification of avalanches is based on conditions prevailing at the point of origin, or the release zone. There are two basic types, loose snow and slab avalanches. Each is subdivided according to whether snow involved is dry, damp or wet, whether the slide originates in a surface layer or involves the whole snow cover (slides to the ground), and whether the motion is on the ground, in the air,
or mixed.
Fig. 1 The classification of snow avalanches


                 
Loose snow avalanches form in snow with little internal cohesion among individual snow crystals. When such snow lies in a state of unstable equilibrium on a slope steeper than its natural angle of repose, a slight disturbance sets progressively more and more snow in downhill motion. If enough momentum is generated, the sliding snow may run out onto level ground, or even ascend an opposite valley wall. Such an avalanche originates at a point, growing wider as it sweeps up more snow in its descent. The demarcation between sliding and undisturbed snow is diffuse, especially in dry snow.

Three processes commonly leave snow in a state of unstable equilibrium on a slope steeper than its natural angle of repose: (1) Deposition of stellar or dendritic crystals with little or no wind, (2) reduction of internal cohesion among crystals by metamorphism, and (3) reduction of internal cohesion by intrusion of liquid water. Though very numerous, most dry loose snow avalanches are small and few achieve sufficient size to cause damage. With advent of spring melting, wet loose snow avalanches also are common. Most of the latter, too, are small, but they are more likely to develop occasional destructive size, especially when confined to gulleys.

Slab avalanches originate in snow with sufficient internal cohesion to enable a snow layer, or layers, to react mechanically as a single entity. The degree of this required cohesion may range from very slight in fresh, new snow (soft slab) to very high in hard, winddrifted snow (hard slab), according to circumstances of layer attachment to the external environment. A slab avalanche breaks free along a characteristic fracture line, a sharp division of sliding from stable snow whose face stands perpendicular to the slope. The entire surface of unstable snow is set in motion at the same time. A slab release may take place across an entire mountainside, with the fracture racing from slope to slope to release adjacent or even distant slide paths. The mechanical conditions leading to slab avalanche formation are found in a wide variety of snow types, both new and old, dry and wet. They may be induced by the nature of snow deposition (wind drifting is the prime agent of slab formation),, or by internal metamorphism.

Slab avalanches are often dangerous and unpredictable in behavior. Providing most of the winter avalanche hazard, they axe the primary object of avalanche defense and control measures.

Avalanches composed of dry snow usually generate a dust cloud as part of the sliding snow is whirled into the air. Such slides, called powder snow avalanches, most frequently originate as soft slabs. Under favorable circumstances, enough snow crystals are mixed with the air to form an aerosol which behaves as a sharply bounded body of dense gas rushing down the slope ahead of the sliding snow. This wind blast can achieve high velocities to inflict heavy and capricious destruction well beyond the normal bounds of the avalanche path.

Wet snow avalanches move more slowly than dry ones and seldom are accompanied by dust clouds. Their higher snow density can lend them enormous destructive force in spite of lower velocities. As wet slides reach their deposition zones, the interaction of sliding and stagnated snow produces characteristic channeling.

Direct action avalanches fall as the immediate result of a single snow storm. They usually involve only the fresh snow. Climax avalanches are caused by a series of snow storms or a culmination of weather influences. Their fall is not necessarily associated with a given current storm or weather situation.


The Mechanism of Avalanche Release

Most avalanches of dangerous size originate on slope angles between 30 degrees and 45 degrees. They seldom occur below 30 degrees and hardly ever below 25 degrees. Above 45 degrees to 50 degrees sluffs and small avalanches are common, but snow seldom accumulates to sufficient depths to generate large slides.

Though internal metamorphism or stress development may sometimes initiate snow rupture, avalanches are often dislodged by external triggers. An overload of new snow may dislodge an existing slab. Falling cornices or chunks of snow from trees are common natural triggers, similar in action to the sunballs or snow wheels which frequently initiate wet slides. In the absence of external triggers; unstable snow may revert to stability with passage of time and no avalanche occurs. Artificial triggers in the form of mechanical disturbance may be intentionally introduced for control purposes. Unintentional triggers are a major cause of accidents; most skiers who fall victim to an avalanche trigger the slide which traps them.


Fig. 2 The fracture line of a slab avalanche, showing the sharp boundary between the stable
snow and that which slid away to the left.  Note blocks of the hard slab resting on the sliding
surface.


Slab avalanches fall when a well defined snow later breaks free and slides away. The sliding surface is usually the interface between distinguishable layers of snow which has been formed by variations in weather or snow deposition. In some cases the sliding surface may be the ground (entire snow cover avalanches). There often exists a lubricating layer of low shear strength which allows the slab to break free from the sliding surface. This lubricating layer may be generated by deposition of fragile crystal forms (e.g., surface hoar), internal metamorphism, or the intrusion of melt water. In some cases the lubricating layer is absent, instability being provided simply by a poor bond between snow slab and the sliding surface.

The primary instability develops when the component of force parallel to the slope due to the weight of the slab exceeds the shear strength of the bond to the underlayer (sliding surface). The situation is mechanically complex due to irregular attachment of the slab to stable snow or other anchorages at the head, toe,, and sides. In general., slab avalanche release occurs when one of these attachments is broken by a trigger; redistributed stresses then exceed the strength of the other bonds and an avalanche falls. Only part of the slab attachments may be weak, while others are strong. In this case a trigger will initiate fracturing in the snow but the slab remains in place and no avalanche falls.

Snow settles as destructive metamorphism proceeds, and on an inclined surface it also creeps downhill under the influence of gravity by internal plastic deformation and slip on the ground. Creep velocity varies with temperature., snow type, snow depth, slope inclination and profile., and ground cover. These variations from one zone of the snow cover to another develop creep stresses. The zones of creep tension are favorable locations for slab avalanche fracture lines; these commonly occur on convex profiles or at the head of open slopes where the snow cover first finds anchorage (trees, ridge top, etc.). A snow slab under tension may not only break free when triggered but shatter into blocks as well when the stress is relieved. In hard snow of high tensile strength this release may achieve almost explosive violence. Creep stresses are in large measure responsible for the dangerous and unpredictable character of slab avalanches.


Forecasting Snow Avalanches

Although the general features of snow instability are known,, many details of avalanche formation are not clearly understood. Forecasting snow avalanches is therefore largely an empirical art based on accumulated experience. Known physical and mechanical principles of snow behavior provide a qualitative understanding of avalanche origin, but quantitative extension of these principles to specific situations is difficult,, for nature presents too many variables to allow exact calculation of snow stress and strength variations with time. The precise time a given slope will avalanche cannot be predicted, but the general degrees of instability in a given area can be estimated with reasonable accuracy.

There are two basic methods of anticipating avalanche hazard. One is the examination of snow cover structure for patterns of weakness, particularly those leading to slab avalanches. This method finds its greatest success in forecasting climax slab avalanches caused by structural weaknesses which may be evolved over a period of time and by a variety of weather conditions. The second method is analysis of meteorological factors affecting snow depositions. The latter is now successful in forecasting direct action soft slab avalanches which run in fresh surface snow layers where structure is poorly differentiated. In practice the two methods overlap and both are used. Emphasis on one or the other depends on local climate,, snow type, and avalanche characteristics. Both apply principally to winter avalanches in dry snow; forecasting wet spring avalanches depends on knowledge of heat input to the snow surface as well as elements of the foregoing methods.


Fig. 3 Typical snow structure at the fracture line of a slab avalanche. 
Layers of new, partly metamorphosed and old snow are separated
from an icy crust by a thin layer of very fragile depth hoar crystals. 
The profile of ram resistance at right indicates low strenght in the
slab layer which slid away.

Snow cover structure is investigated directly by digging pits and examining the exposed stratigraphy. Snow temperature, density, strength properties, and crystal type are all important for determining stability. Time variations in these properties are examined by a succession of pits in a given study area, the result being plotted in a time profile. Indirect evidence on snow structure can be gathered by instruments probing from the surface. The most useful of these is the ran penetrometer which measures snow strength variations with depth by means of a pointed rod driven by a falling weight. Periodic observations at representative study plots (usually on level ground) are compared with snow profiles from actual avalanche fracture lines to determine and anticipate stability trends.

The basic structure leading to slab avalanche formation is a cohesive snow layer resting on a weak substratum which offers poor support or attachment. Actual combinations of slab layer and substratum strength vary widely. A heavy,, hard slab of great thickness may exert enough shear stress at its base to rupture a relatively strong supporting layer which would provide adequate anchorage for a lesser overburden. On the other hand, even shallow layers of soft, weak snow may break free as a slab avalanche if the substratum is sufficiently fragile. A common source of weakness is depth hoar formed in the early winter snow cover. This provides very poor support for subsequent snowfalls which often slide off fully developed depth hoar regardless of their individual character. Thin layers of depth hoar, surface hoar, or graupel can also provide a fragile bond (good lubricating layer) when sandwiched between stronger layers. The general process of constructive metamorphism always weakens snow layer strength and bonds; it may precipitate an avalanche long before recognizable depth hoar crystals actually appear. Another frequent cause of slab avalanching is an ice layer or crust which provides a smooth sliding surface. Crusts formed by refreezing following a rain storm offer especially poor anchorage to subsequently deposited snow layers. The bond between slab layer and a crust can be poor at low temperatures., while it rapidly gains strength if the interface is near the freezing point. Other patterns of snow stratigraphy also lead to slab avalanche formation in dry snow though these are the most important.

Soft slab avalanches usually run during or immediately after a storm. In motion they are similar to dry loose snow avalanches and sometimes are confused with the latter when they fall during poor visibility. The characteristic fracture line and initial motion as a cohesive layer is nevertheless present, identifying them as true slab avalanches. Observation of contributory weather factors before and during a snow storm provides the basis for forecasting this hazard situation. The depth and surface character of the existing snow base, established by previous storms, must be known. A deep snow cover favors avalanching by smoothing the terrain, while certain surface conditions such as a crust (see above) offer a good sliding surface. The new snow depth, type, and density also offer clues to stability. New snow layers more Wain 25-30 cm thick most frequently lead to soft slabs, with graupel end intermediate stages of rimed crystals the most favorable crystal type. New snow densities above about 0.12 g/cm-3 are a warning sign. (Very low new snow densities, 0.05 g/cm-3 or less., are usually associated with dry loose snow avalanches.). Settlement in the new snow Is a stabilizing factor.

Rising temperature during a storm accompanied by rising new snow density tends to cause avalanching,, while falling temperatures have the opposite effect. New snow precipitation intensity is a signifi cant factor, for it represents the rate at which the slopes are being overloaded. Values above 2.5 mm of water per hour warn of impending hazard. In practice this factor may not be measured directly; instead, new snow density and snowfall intensity are observed. The wind is also critically important, for soft slab avalanches seldom occur unless sustained average wind velocity exceeds 6 to 7 m/sec -1. The most reliable indicator of developing avalanche hazard is a sustained period of coincident high wind and high precipitation intensity.

Wet snow avalanches are generated by intrusion of percolating liquid water (rain or snow melt) in the snow cover. The rapid temperature rise-quickly alters snow behavior, while the water itself reduces snow strength. Liquid water accumulating at an impervious crust provides an especially good lubricating layer for slab release. The most extensive wet snow avalanching occurs during winter rains or the first prolonged melt period in spring, when liquid water intrudes into previously subfreezing snow. Snow melt by solar radiation is the commonest source of wet snow avalanching and this is amenable to quantitative prediction. It is essential., though., that the total snow surface energy balance be considered in estimating amount of melt, for longwave radiation, vapor exchange and sensible heat from the air all play an important part. A warm., windy,, overcast day may produce more melting (and avalanche activity) than sunshine and cloudless skies.

Accuracy of formal forecasting procedures is enhanced by frequent field checks of snow stability. For this purpose small, accessible avalanche paths are sometimes chosen as sites for test skiing , where snow conditions are checked by actually trying to set the snow in motion. This technique is particularly useful in detecting incipient soft slab formation during storms. It is less effective (and more dangerous) on hard slabs formed by heavy wind drifting. Tests of the latter are usually restricted to blasting with high explosives.


Avalanche Control Techniques

Avalanche hazard can be mitigated or eliminated by the application of operational and engineering techniques. There are two fundamental methods of avalanche control: modification of terrain, and modification of the snow cover.

Terrain modification may deflect the sliding snow away from fixed facilities to be protected, or actually prevent the avalanche release. Examples of deflecting structures are snowsheds used to protect railways and highways. These must be strong enough to support the dynamic load of sliding snow; hence most modern snowsheds are built of reinforced concrete. Where sheds are impractical, the sliding snow can be diverted laterally by wedges, pylons, or diversion walls.

In favorable terrain the snow may be arrested by snow dams or catchment basins. Avalanches are also arrested in the outrun, or transition zone, of their paths by braking mounds conical earthen or masonary mounds four meters or more high which axe arranged in a pattern to break up the flowing snow into crosscurrents which internally dissipate its kinetic energy. All of the passive deflection structures act principally on snow sliding on the ground which may exert impact forces up to 50 tons/m2. They have less effect on the dust cloud accompanying a powder snow avalanche.

Active avalanche defense by terrain modification is achieved with supporting structures in the avalanche release zone. These are large walls, fences, or nets arranged to retain snow and prevent avalanches from falling. Their size and spacing are designed to (1) terrace the mountainside into discrete zones, each of which has snow deposited to a surface slope less than the mean, (2) break up the the continuity of the snow surface and prevent slab formation, and (3) support snow on the mountainside in small, manageable sections. These supporting structures, mostly massive fences in modern design, must be strong enough to support creep pressures reaching tons per square meter, while at the same time being light enough for economical transport and erection high on a mountainside. Another type of defense used in the release zone is the wind baffle, a wall or panel arranged to induce irregular wind drifting which breaks the continuity of snow slabs. They are not designed to withstand large creep pressures and are less effective than supporting structures.

Avalanche control by snow modification does not give the high degree of protection afforded by terrain modification but is much cheaper. It commonly is used to reduce the hazard to mobile entities, such as skiers or highway traffic, which may be removed during periods of danger. The commonest technique is artificial release, which brings down avalanches at a chosen safe time and inhibits formation of large avalanches by relieving slopes of their snow burden piecemeal in small ones. Slides on small paths are sometimes intentionally released by skiing, but the preferred method is the detonation of a brisant high explosive on the snow surface close to the expected fracture line. One kilogram of TNT or its equivalent is considered the minimum reliable charge. The charge may be placed by hand, but this can be difficult and is sometimes dangerous. Artillery shells, armed with superquick point detonating fuzes, are much more efficient, for a number of targets can quickly and safely be engaged from a single gun emplacement. Principal disadvantages of artillery are limitations to military or government use and possible damage from shrapnel dispersion. Mortars, light howitzers, and recoilless rifles have all been successfully used for avalanche control; the 75mm recoilless rifle is the most practical weapon for this purpose. Where frequent artificial release is undertaken to protect a ski area or highway,, a fixed artillery emplacement permits increased efficiency by blind firing during storms or at night. Artificial release cannot be effectively employed at random. It must be based on accurate appraisal of snow and weather conditions, and careful selection of targets.

Another snow modification technique is the application of mechanical disturbance to break up slab formation (especially soft slabs) and induce stabilization through age hardening. Skier traffic is the commonest available disturbance, while deliberate packing of the snow by foot or ski is sometimes used. Depth hoar can be satisfactorily stabilized only by intensive foot packing. Mechanical aids, such as oversnow vehicles, can seldom be used at the slope angles existing in avalanche release zones.

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