Snow Avalanche Activity in Central Spitsbergen, Past and Present

A project funded by the University Courses on Svalbard (UNIS) 2000-2005

Ole Humlum, UNIS, Department of Geology, Svalbard, Norway

Background

Snow avalanches are active geomorphic agents of erosion and have been a source of natural disasters as long as man has lived in cold-climate high relief areas. A common feature of many mountainous regions, avalanches may fall wherever snow is deposited on slopes steeper than about 30 degrees. Small avalanches, or sluffs, run in high numbers each winter, while the larger avalanches, which may encompass slopes 200-1500 m wide and millions of tons of snow, fall less frequent but inflict most destruction. Such hazard has been familiar to inhabitants of the Alps and Scandinavia for many centuries, while it is a more recent experience in other parts of the world.

The right circumstances for avalanche activity sometimes consist of 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 basis with high friction beneath the snow. On the other hand, even a shallow snow layer can slide from the mountainside if poorly bonded.

Types of avalanches

The wide variety of snow avalanche origin, nature of motion, and size reflects the 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.

Small loose snow avalanches. This type of avalanche generally occurs at the surface in new snow or wet spring snow. This type of avalanche (sluff) often begins at a point and spreads out as it moves downslope. Loose snow avalanches seldom entrain enough snow to bury a person deeply and the chief danger from this type of avalanche is from being pushed over a cliff or rock band.

Big loose snow avalanches. Avalanches composed of dry snow usually generate a dust cloud as the sliding snow is whirled into the air by and is called powder snow avalanches. Their speed may exceed 200 km/hr. Such 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 slope. Such an avalanche originates at a point, growing wider as it sweeps up more snow in its descent. The demarcation line between sliding and undisturbed snow is diffuse, especially in dry snow. Under certain 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 inflict heavy destruction well beyond the normal bounds of the avalanche track.

Figure 1. Car hit by the wind blast associated with a large loose snow avalanche and subsequently folded around a three.

Slab avalanches represents the most dangerous type of all snow avalanches. The slab is difficult to see and avoid. It will often allow a person to travel well out onto it before rupture takes place. Slab avalanches originate in snow with sufficient internal cohesion to enable one or several snow layers to react mechanically as a single entity.  A common source of weakness is depth hoar formed in the early winter snow cover. This provides very poor support for subsequent snowfalls. Even thin layers of depth hoar, surface hoar, or graupel can also provide a fragile bond (good lubricating layer) when sandwiched between stronger layers.  Another frequent cause of slab avalanching is an ice layer or crust which provides a smooth sliding surface with low friction. Crusts formed by refreezing following rain offer especially poor anchorage to subsequently deposited snow layers.  A slab avalanche breaks free along a characteristic fracture line (see photo below), 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. 

Slab avalanche released late April in upper Fardalen, central Spitsbergen, following a brief period of thaw. A well defined fracture line (70-100 cm high) delimits the slide from stable snow above. Photo 2002.04.24.

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, unpredictable in behavior and provide most of the winter avalanche hazard.

Figure 2. Skier releasing slab avalanche.

Cornice fall avalanches occur when cornices break loose from the lee side of ridges or mountain plateaus. Cornices form when prevailing winds remove snow from slopes or plateaus and deposit it in a leeside position. The snow that forms cornices is very dense and hard, yet can be extremely fragile. It is often difficult to determine from the mountain top where the solid bedrock ends and the overhanging cornice is not supported from below. This type of avalanche is easily avoided by staying back from the peak of ridges or the rim of plateaus, but can be deadly if the victim tumbles downhill amid massive chunks of snow which often trigger secondary slab avalanches as they pass. The photo at the top of this page illustrates cornice fall avalanches above Nybyen, late March 2000.

Wet snow avalanches usually occur during spring, but may also occur during short spells of thaw in midwinter. They move more slowly than dry snow avalanches and seldom are accompanied by dust clouds. However, 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. Wet snow avalanches are generated by intrusion of percolating liquid water (rain or snow melt) in the snow cover.  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. A warm,  windy and overcast day may produce more melting (and avalanche activity) than sunshine and cloudless skies.

Figure 3. Snow profile with top layers of new snow resting on a horizon of ice crystals with reduced interlocking ability due to their simple geometry. This kind of snow stratigraphy will often lead to an avalanche on slopes inclined more than about 30 degrees, as the top layers slide downslope on the weak layer below.

Mechanisms of avalanche release

Avalanches usually follow the same paths year after year and the risk zones are often well known in populated regions (see photo below). However, exceptional wind or precipitation at intervals may produce avalanches which overrun their normal paths or even follow completely new tracks. Removal of threes in alpine terrain can also create avalanche risk where none existed before. Given exceptional snow conditions, even short slopes like the walls of a ravine can become dangerous. Snow avalanche may occur anywhere enough snow is deposited on an inclined surface.

Figure 4. Avalanches often follow the almost same path from year to year. Local knowledge on high- and low risk zones may with considerable success be used to locate buildings and other structures in the landscape. The picture shows an example from Switzerland.

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 snow avalanches. Slopes steeper than 50-60 degrees usually do not acquire any significant snow layer due to their gradient.

Internal metamorphism or stress development may sometimes initiate snow rupture, but several avalanches are initiated by external triggers. An overload of new snow during a blizzard may dislodge an existing slab. Falling cornices or chunks of snow from trees are common natural triggers 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. Unintentional triggers are a major cause of accidents; most persons who fall victim to an avalanche trigger the slide which traps them.

Snow slowly settles as the internal metamorphism proceeds. On inclined surfaces 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 (bare soil, boulders, vegetation). 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.

Figure 5. Andermatt in southern central Switzerland. Avalanche defense structures (threes and iron fences) are established on the mountain slope above the houses, in order to provide artificial roughness to the terrain. 

Field indications of avalanche risk

The condition of snow stability is most readily inferred from direct examinations of snow cover structure. The techniques of such observations have been highly developed over the past thirty years, particularly in Switzerland. The standard techniques of snow pit investigation and the use of instruments as the ram penetrometer have served as the basis for avalanche risk studies. Today their application to avalanche forecasting, the original reason for their development, is also widespread. However, even without doing instrumental investigations, just by observing the certain features in the landscape, it is possible to avoid many avalanche risk situations. Such field indications is listed below. 

Previous avalanche activity on certain slopes represents a major clue to avalanche risk, but is nevertheless often overlooked.

Collapse and whoomp sound. Often, the combined weight of snow and people walking on a snowpack with an internal weak layer such as depth hoar or light density snow will cause the weakness to collapse, dropping the upper layer of snow a few centimeters. The resulting collapse sound ' "whoomp' may sound like  distant cannon fire. Sometimes, a person will actually be able to see or feel the surface of the snow layer drop several inches. If this happens to you on a steep slope you are entering an extreme danger zone. 

Hollow sounds. The wind can move huge quantities of snow, accumulating it into drifts and creating slabs. These deposits can be either soft or so hard that they can carry your weight. However, although the slab itself is very strong, it may be sitting on top of a weak layer deeper in the snow. Sometimes a poorly supported hard slab will make a hollow sound like a drum when you walk on it. Also in this case, you are entering a high risk zone.

Shooting cracks. If the snow layer is exposed to strong surface tension, it may fracture as a person walk across. These "shooting cracks" that spread out from your toes are not a problem in gently sloping terrain, but indicate that steeper slopes presumably are suspect. The deeper and longer the cracks, the greater the potential instability.

Loose snow present in trees or on steep surfaces. Avalanche hazard is usually greatest during the first 24-48 hours following a storm or blizzard. The layer of snow will tend to adjust to the weight of new snow with time. If trees (not especially relevant on Svalbard) or other steep surfaces (houses, rock steps, boulders, etc.) are still holding much loose snow after a storm, presumably the snow on the ground has not compacted and settled, either.

Figure 6. Cornice formed in upper Vandledningsdalen, late June 2000. The snow accumulated by snow drift across the mountain plateau by prevalent SE winter winds. The height from the valley bottom to the mountain plateau above is about 70 m.

Cornices. These features form on the lee side of ridges or along plateau rims and indicate that winds have been moving great quantities of snow. Much deeper snow should be expected -- and consequently, increased avalanche hazard -- below a cornice than on the upwind, scoured side of the ridge or mountain plateau.

Recent wind activity. Pillows of freshly-deposited snow are often obvious in the winter landscape and their stability is very sensitive to various triggers. Take note of the wind direction during situations with significant snow drifting and consider lee-side slopes as risk zones for especially the first days to follow. It should, however, be observed that topography by channeling effects may exercise considerable influence on the local wind direction.

Confluence of valleys and ravines. If possible, areas immediately below the confluence of two or more valleys or ravines should be avoided or hastily crossed. Avalanches often release simultaneously in neighboring valleys and therefore will merge with each other in this area and suddenly achieve huge proportions.

Figure 7. Melting avalanche snow near Larsbreen, July 2000. Rock debris is released on the snow surface by melting, resulting in a high frequency of delicately balanced, perched boulders and stones.

Perched rock fragments. Avalanche snow often contains a lot of debris and rock fragments. As the snow melts during summer, this sediment is released and dumped upon the ground below. During summer time, avalanche risk areas therefore may be located by mapping the distribution of such debris (see figure above).

Figure 8. Avalanche boulder tongue in Endalen, Spitsbergen, Svalbard, August 2000. The avalanche track is seen as a light-colored deposit of debris, extending from the small valley in the upper right to the lower left corner of the photograph. The mountain rises to about 580 m asl., while the valley bottom is at about 200 m asl. This avalanche track extents all the way to the river plain, signaling even the valley bottom beyond to be within the avalanche risk zone during winter.

Avalanche boulder tongues. At sites where snow avalanches are frequent, a tongue-shaped deposits of loose, angular debris may accumulate through time (see figure above and below). Such large-scale landform may - at summer time - assist in mapping the extent of areas exposed to increased avalanche risk. Avalanche boulder tongues and associated landforms usually are the result of avalanche activity during long time spans and their internal stratigraphy may yield important palaeoclimatic information relating to past and present wind activity, dominant wind direction and precipitation.

Figure 9.  Three avalanche boulder tongues along the eastern valley slope below Larsbreen, near Longyearbyen, Svalbard. Part of the glaciers right lateral moraine from the Little Ice Age maximum extent is seen in the lower right part of the photo. Note the slightly fluted or striated surface appearance of the individual avalanche boulder tongues, testifying to modern avalanche activity. The top of the mountain is at about 500 m asl., while the foot of the mountain slope is at about 300 m asl. Seen towards ENE, August 1999.

Mapping of past and present avalanche activity in central Spitsbergen

Mapping of modern avalanche activity is carried out by means of field investigations and the use of automatic digital cameras. The mapping of past avalanche activity is done by means of geomorphological mapping of avalanche boulder tongues, avalanche deposits, etc., during summertime. Combined with stratigraphic investigations and dating of organic material contained in these deposits, information will be obtained on the timing of Holocene variations in wind- and precipitation regime in the Svalbard region. Also historical records on past avalanche activity represent a valuable source of information.

Figure 10.  Buildings on Haugen destroyed by a wet snow avalanche from Vandledningsdalen, June 1953. By this event 3 persons were killed and 30 other persons were injured. The old hospital was destroyed and several other buildings damaged. Another wet snow avalanche occurred in 1989, resulting in loss of property but no casualties. Avalanche defense structures have since been constructed at the mouth of Vandledningsdalen (see picture below).

Figure 11. Vandledningsdalen, July 2000. Avalanche defense structures (fences and a dam; indicated by yellow arrows and stippled line, respectively) have been constructed at the mouth of the valley.

Terrain model showing potential snow avalanche risk zones around Longyearbyen, viewed towards SW. This model only indicate potential release zones and potential run-out zones are not shown. In general, however, all areas below potential release zones should be considered as potential run-out zones. The diagram only shows large-scale terrain features and even small slope segments with a height difference of only 5 m (not shown in the diagram) may represent potential avalanche risk zones. The daily avalanche risk will depend upon recent and past wind direction, -strength and precipitation. Especially downwind (lee) slopes will be prone to increased snow avalanche risk. Following the onset of thaw, all steep slopes with a thick snow cover may rapidly become exposed to high avalanche risk. 

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