Part B Avalanches

1. General

The two main causes of avalanches are the weight of large amounts of accumulated snow, and steep slopes that exceed the cohesive forces within the snowpack or between the snowpack and ground. These two elements combined can produce an avalanche.

• Terrain. Ground surface conditions have considerable effect upon snow in contact. A broken, serrated, or boulder-strewn surface provides a good anchor for a snowpack. In other conditions, this surface can provide dangerous stress concentrations in the snowpack. Slides breaking off at ground level do occur but are unlikely. Smooth, even slopes of bare earth, solid rock, or grass favor massive ground-level avalanches, typical of the high alpine zone.

Contours of a mountain influence the avalanche. Barriers such as terraces, talus, basins, and outcrops either divert the moving snow or give it room to spread out and lose momentum. Gullies collect and channelize the descending snow, making favorable slide paths, which must be avoided (Figure 1-1). Ridges lying parallel to the slide path are normally secure.

The convex portion (Figure 1-2) of a slope is more likely to avalanche because the snow layers settling upon it are placed under tension. Avalanches usually fracture at the sharpest point on the curve, increase to full speed instantly, and pulverize rapidly. Snow is under compression on the concave portion (Figure 1-3) of a slope.

The terrain features and grade are both important. Slides are not always likely on steeper slopes. Vertical faces do not hold enough snow to avalanche. Most slab releases occur from 30 to 45 degrees. Grades below 30 degrees are of less concern for dry snow slides unless an avalanche is induced by unusual circumstances. Grades between 30 and 45 degrees are most critical for dry slab avalanches. Snow does not normally accumulate in large quantities on steeper slopes. Slush-flows often occur on shallow slopes, especially where the ground is impermeable in the spring.

Shallow Avalanches

The most dangerous slopes are those that have a convex portion above the tree line where tensile stresses can develop and have a concave portion below where the flow of snow can accelerate. The upper portion is usually the starting zone. The dimensions of the slope (length and width) determine the size of the snow slide and amount of destruction.

The most dangerous slopes are those that have a convex portion above the tree line where tensile stresses can develop and have a concave portion below where the flow of snow can accelerate. The upper portion is usually the starting zone. The dimensions of the slope (length and width) determine the size of the snow slide and amount of destruction.

Convex Avalanche
Figure 1-2, Convex Portion

• Slope Steepness. Slopes as gentle as 15 degrees have avalanches under the right conditions. The majority of avalanches start on slopes between 25 and 60 degrees. Above the 60 degrees line they are too steep to build up significant amounts of snow, however, any slope above 30 degrees should be evaluated for stability before movement over snow. The slope angle can be estimated by using an inclinometer, which may be constructed from two equal-length ski poles. One pole is marked at exact center and the other is left unmarked. To determine if a slope is greater or less than 30 degrees, the unmarked pole is mated to the mid point of the marked pole at a 90-degree angle. By placing the tip of the marked pole on the ground, a determination can be made by observing the full-length pole as it is oriented up the fall line.

Slope profile. Dangerous slab avalanches are more likely to occur on convex slopes, but may also occur on concave slopes. Concave slopes are usually more stable than convex slopes and, therefore, safer. This does not mean avalanches do not occur on concave slopes. Not only do avalanches occur on concave slopes, but they may be triggered from the flat ground below the slope.

Slope aspect. Snow on north-facing slopes is more likely to slide in midwinter. South-facing slopes are more dangerous in the spring and on warm sunny days. Slopes on the windward side are more stable than leeward slopes.

Slope aspect. Snow on north-facing slopes is more likely to slide in midwinter. South-facing slopes are more dangerous in the spring and on warm sunny days. Slopes on the windward side are more stable than leeward slopes.

Sound Concave Slopes

Vegetation. Vegetation, except grass, has a restraining effect on avalanches. The existence of heavy forest cover indicates that slides in that location are rare, however, it is a mistake to consider all forested areas as safe. Forested areas, while difficult to move through, can usually be assured to be a safe route. Slopes where the timber has been destroyed by fire are potentially good for snow slides. Climax avalanches in heavy snow years often destroy forested areas that have grown up since the last major slide.

Exposure. Slopes facing the sun favor avalanches produced by thawing. Loose snow avalanches are more common on slopes opposite the sun. Cornices form along ridges and crests that lie at right angles to the prevailing wind. They can suddenly release and start an avalanche in the slope below. Leeward slopes are the most probable locations for overloads of wind-driven snow and formation of slab.

On the other hand, snow is transported from the wind-beaten slopes, and that which remains is packed and stabilized.

• Climate and Weather. In addition to the terrain factors, climate and weather are the other basic elements for the avalanche phenomenon.

Storms that deposit up to 2.5 cm (1 inch) of new snow each hour are common. About 80 percent of all avalanches occur as a direct result of the additional load deposited during a snowstorm. The remaining 20 percent are delayed action avalanches that occur for no obvious reason.

Temperature fluctuates widely and rapidly in the mountains. Prolonged spells of extremely low temperatures occur. There are occasional intrusions of warm air masses, usually in connection with a storm. Rainfall may occur in the coastal zones and create avalanche conditions. The temperature greatly affects the cohesion of snow; a decrease in temperature retards the settlement of the slab and increases the chances of forming a depth hoar layer on which the slab can glide. Wind action during storms in the mountains is strong. Its influence on snow is the most important of all the contributing factors. It transports snow from one exposure to another during storms and fair weather, thus promoting overloads on certain slopes. It also modifies the size and shape of snow particles.

2. Types of Avalanches

Avalanches may be classified according to the type of snow involved, manner of release, or size. Classification according to the type of snow involved is normally used.

All slides are divided into two general groups: loose-snow avalanches and slab avalanches.

• Loose-Snow Avalanches. An avalanche of loose snow always starts on the surface from a point or a narrow sector. From the starting point it grows like a fan, expanding in width and depth. The speed and nature of its development depend on whether the snow is dry, damp, or wet.

• Dry Loose-Snow Avalanches. These are composed of loose snow, possibly drifted but not wind-packed. They normally start at a point of origin and travel at high speeds on a gradually widening path, increasing in size as they descend. (Figures 1-4 and 1_-5 depict the enormous cloud of snow dust involved.) A dry loose-snow avalanche is always shallow at the start and depends on the snow it can pick up during its run for volume. Thus, a dry-snow avalanche of dangerous size can only occur on a long slide path, or from a large accumulation zone that funnels into a constricted outrun. Sometimes, heavy snowfall at low temperatures produces the phenomenon of the "wild snow" avalanche--formless masses pouring down the mountainside. They are actually avalanches of mixed air and snow. Windblast, a side effect of large, high-speed avalanches, is powerful enough to damage structures and endanger life outside the actual avalanche path. The hazard from loose-snow avalanches is soon over.

• Damp- and Wet-Slide Loose-Snow Avalanches. These resemble avalanches of dry snow with the same point of origin, gradually becoming wider. Their mass is many times greater than that of a dry avalanche, and they are therefore much more destructive. Being heavier and stickier, however, they develop more friction and travel at a slower rate.

The principal hazard of damp- and wet-snow avalanches is to fixed installations. Such avalanches are of comparatively low speed, causing them to stop suddenly when they lose momentum and to pile up in towering masses. This is in contrast to the dry slide, which tends to spread out like the splash of a wave.

Damp and wet slides (Figures 1-6 and 1-7) solidify immediately upon release from the pressure of motion, adding to the problem of rescue or clearing operations. Wet slides have the distinctive characteristic of channeling. The moving snow makes its own banks and flows between them like a river of slush, often in unexpected directions. The damp snowslides of midwinter are usually shallow. But the wet avalanches of spring, caused by deep thawing either from rain or prolonged temperatures above freezing, often involve enormous masses of snow and debris, and have tremendous destructive power. Slush flows are the extreme cases of wet slides.

Different Parts SnowflakeLoose Snow Avalanche
Figure 1-7. Wet Loose-Snow Avalanche

• Slab Avalanches. Windpacked snow called wind slab or snow slab causes more deaths than other avalanches and is equal to the wet-spring avalanche as a destroyer of property. Hard slab is usually the result of wind action on snow picked up from the surface. Soft slab is usually the result of wind action on falling snow. Wind slab, cornices, and climax avalanche are different types of slab avalanches.

• Wind Slab Avalanches. These avalanches behave entirely different from loose snow. They have the ability to retain their unstable character for days, weeks, or even months; thus, leading to the unexpected release of delayed-action avalanches. These are often triggered from minor causes such as a skier cutting a slope or from no observable cause at all.

The wind slab avalanche combines great mass with high speed to produce maximum energy. It starts on a wide front with penetration in depth. The entire slab field-top, sides, and bottom releases almost at the same time. The place where the slab has broken away from the snowpack is always marked by an angular fracture line (Figure 1-8 depicts the area that it broke at the sharpest point of the curve on a convex slope) instead of a point, roughly following the contour. In a packed-snow avalanche, the main body of the slide reaches its maximum speed within seconds. Speeds of 100 kph are common. It exerts full destructive power from the place where it starts, whereas a loose-snow avalanche does not attain its greater momentum until the end of its run.

Based on the characteristic delayed release action previously stated, the slab avalanche is the most dangerous of all types. A series of slab avalanches may stabilize conditions only locally, leaving an adjacent slab as lethal as an unexploded shell. Wind slab found on a surface has a dull, chalky, nonreflecting appearance and has a hollow sound underfoot. Wind slab that is hard often settles with a crunching sound, which an experienced mountaineer recognizes as a danger signal. When movement is necessary across a snow slope, it is imperative that soldiers apply the principles of protective measures to break the slab in order to increase the safety of all individuals.

Slab Avalanche
Figure 1-3. Slab Fracture.

• Cornices. Cornices are snow formations allied to the slab. They build up on the lee side of crests and ridges, which lie at or near right angles to the wind. Occasionally they are straight-walled, but their characteristic shape is that of a breaking wave (Figure 1-9). The obvious hazard from cornices is due to the fractures of the overhang from simple overloading, or weakening due to temperature, rain, or sun erosion. These falling blocks are large enough to be dangerous by themselves. They may also release avalanches on the slopes below.

Three Types Avalanches

• Climax Avalanche. The climax avalanche is a special combination type. The distinguishing characteristic of this type avalanche is that it contains a large proportion of old snow and is caused by conditions that have developed over a period of time--at least one month and possibly an entire season. They occur infrequently because they require an unusual combination of favorable factors. Whenever they occur, the penetration of a climax fracture is always in great depth, usually to the ground. They travel farther and spread out wider than ordinary avalanches on the same slide path. On heavy snow years these avalanches may travel farther than the normal slide path, and destroy forests and structures.

• Ice Avalanches. These occur on the steep ice cascades of glaciers or under hanging glaciers and ice cliffs. Their location is usually obvious, as is the debris they produce, but it is hard to predict their occurrence. Unlike snow avalanches that are usually started by their victims, ice avalanche accidents usually result from a natural fall. Main causes are glacier movement and internal melt.

3. Avalanche Triggers.

A loose-snow slide usually occurs during or immediately after a storm before the new snow has had a chance to form into a strong layer. Avalanches may trigger due to internal changes or forces (such as the growth of a depth hoar), which release masses of snow without apparent cause.

There are four avalanche causes or triggers. They are:

• Overloading. Added weight by a storm is the most frequent cause of avalanches. New snow accumulates until the weight overcomes the strength at some depth. A combination of failures may occur such as collapse of a weak layer and the spread of a tension fracture. The overloading may occur just in the starting zone from wind deposition.

• Shearing. Slab avalanches are characterized by the shearing of snow along a line in the starting zone. The slab shears along the entire line almost at the same time. Some avalanche shearing can be caused by a ski path across a slope or by snowshoes. Occasionally, a slope with a shear (crack) across the fall line may occur, which has not yet avalanched. This slope may avalanche at the slightest disturbance, especially by overloading.

• Temperature. Most avalanches occur during the warmer midday. Rising temperatures can cause the top layers of a snowpack to become too heavy, causing overloading of these layers. The result may be the worst kind of avalanche--the wet snow slab avalanche. Storms starting with low temperatures and dry snow, followed by rising temperatures, are most likely to cause avalanches. The dry snow early in the storm forms a poor bond and does not have the strength to support the heavier snow deposited late in the storm.

• Vibration. Avalanches are often triggered by vibration. The vibration of a passing helicopter, operations of heavy equipment, explosions, other avalanches, or natural earth tremors have all been known to trigger avalanches.

4. Avalanche Hazard Forecasting

It is impossible to predict the actual occurrence of an avalanche since they are affected by many factors or variables. An experienced mountaineer may conduct a reconnaissance of the area and can usually recognize the development of a hazardous situation in time to avoid the danger area.

The two basic causes of avalanches are terrain and climate. Terrain includes the slope angle, and climate includes the weight of the snow.

Factors which contribute to the avalanche hazard and have been identified by systematic studies are the depth and condition of the base; old snow surface; types of new snow;

snow density; snowfall intensity; precipitation intensity; settlement; wind action; and temperatures. These factors, which are subdivisions of climate and weather are discussed as follows:

• Depth and condition of the base. A 60-cm (24 inch) depth is sufficient to cover ground obstructions and to provide a smooth sliding base. Greater depths destroy such major natural barriers as terraces, gullies, outcrops, and clumps of small trees. If one of the power layers consists of highly faceted crystals, especially depth hoar, the slope is dangerous because it may easily fail on that layer. The presence of a layer of depth hoar or a thick layer of granular or sugar snow can be detected by an experienced observer by probing. Snow pits are commonly dug to observe the layering.

• Old snow surface. A loose snow surface promotes good cohesion with a fresh fall but allows deeper penetration of any avalanche that starts. A crusted or windpacked surface means poor cohesion with the new snow but may restrict the avalanche to the new layer.

• Types of new snow. Dry snow has little cohesion except for the mechanical interlocking of snowflakes. Thus, loose snow avalanches can easily occur. Wet snowflakes are cohesive and undergo rapid settlement to form a layer. When slightly damp, a strong slab may readily form under the action of wind.

• Snow density. Snow density is an indication of its strength and is commonly measured in snow pits. It is usually expressed as a fraction of the density of water, which is 1 g/cc. Dry, new snow can have a density as low as 0.04 g/cc; values up to 0.1 g/cc are common. Wet snowfall can reach densities of 0.14 g/cc. The density of each layer normally increases throughout the lifetime of a snow cover although the layer just over the ground can decrease.

• Snowfall intensity. When the snow accumulates at a rate of 2.5 cm (1 inch) or more per hour, the pack is growing faster than stabilizing forces (such as settlement) can take care of it. This sudden increase in load may fracture a slap beneath and result in a slide.

• Precipitation intensity. With a continuous precipitation intensity of .25 cm (3/32 inch) of water or more per hour and with wind action at effective levels, the avalanche hazard becomes critical when the total water precipitation reaches 2.54 cm (1 inch). This is one of the new methods used in avalanche forecasting. It requires interpretation by trained weather station personnel with special equipment.

• Settlement. Settlement of snow is continuous. With one exception, it is always a stabilizing factor. The exception is shrinkage of a loose snow layer away from a slab thus depriving it of support. In new snow, a settlement ratio less than 15 percent indicates that little consolidation is taking place; above 30 percent, stabilization is proceeding rapidly. Over a long period, ordinary snow layers shrink up to 90 percent, but slab layers may shrink no more than 60 percent. Abnormally low shrinkage in a layer indicates that a slab is forming.

• Wind action. Wind action is an important contributing factor. It overloads certain slopes at the expense of the others, it grinds snow crystals to simpler forms, and it constructs stable crust and brittle slab, often side by side. Warm wind (chinook of North Arnerica, and the foehn of Europe) is an effective thawing agent--even more effective than sunlight. An average velocity of 15 knots is the minimum effective level for wind action in building avalanche hazards.

• Temperatures. Air temperature determines the type of snow that falls. At temperatures below 15 degrees F the new snow is unstable, and does not settle to form a strong layer. The settlement is rapid above 28 degrees F. Sudden temperature changes can induce dangerous thermal stresses in a slab.

Avalanches obey mechanical laws that can be identified and evaluated by trained personnel with special equipment. While in the field, you must rely on the following factors:

• Terrain-grade 25 degrees or steeper anywhere in the avalanche track. (This is not an average grade.)

• Old snow depth-enough to cover ground obstructions.

• Surface crusted-normally only new snow will slide.

• Surface loose-good cohesion between layers but both old and new snow may slide.

• Snowfall intensity-2.5 cm (1 inch) per hour or more. This can be assumed when snowfall is heavy enough to restrict visibility from 100 to 200 meters (109 to 218 yards).

• Precipitation intensity-.25 cm (3/32 inch) per hour of water or more, plus strong wind action. This can be assumed if snowfall intensity is 2.5 cm (1 inch) per hour and snow is damp or noticeably heavy for dry snow, or dry snow is granular or like pellets.

• Settlement-noticeably low. Watch the snow collars around the trees or posts.

• Wind-an average of 15 knots or higher. This can be assumed if snow is blown parallel or almost parallel to ground.

Temperature-any sudden change up or down. For example, a thawing temperature day and night for 36 hours; that is, no overnight freeze.

• Weak layers-if depth hoar or other weak layers are found, snow slopes are especially unstable.

5. Protective Measures

During missions in avalanche areas, protective measures can be used to reduce the avalanche hazard. Protective measures such as restrictions, control methods, and use of barriers may be adopted by units.

• Restrictions. Based upon terrain analysis and reconnaissance, areas that are considered hazardous may be placed "off limits" to all troops. This may affect only a few narrow avalanche paths, an entire valley, or several valleys, depending on the terrain and weather conditions. Enforcement of restrictions may be necessary even if the troops involved are required to conduct long and time-consuming detours. If an avalanche-prone slope must be crossed, the crossing should be made single-file with all possible precautions taken to track and rescue a victim if an avalanche should occur.

• Control Methods. In combat, individuals and units may be required to take risks and enter hazardous areas. The risks can be reduced by applying the following methods:

Ski packing. The snow settles on the dangerous slope. Constant use of the hazardous slide path area also prevents the snow from building up avalanche conditions. The work is performed by teams of expert skiers (two and three on each team). Great care, coordination between teams, and supervision must be exercised due to the danger of the work. If this is done early in the winter when the snow first accumulates, the packing will reduce the formation of depth hoar and the subsequent hazard when the snow cover is thicker. Ski cutting. The leading skier in the party, using an avalanche transmitter or avalanche cord, and sometimes belayed by other members of the party, skis across the slope in the starting zone to see if the ski cut will become a fracture and release an avalanche. If an avalanche is released below the cut, the skier must be ready to ski to the side. As with other control procedures, the lack of any avalanche activity does not prove that the slope is stable but only indicates a lower risk of hazard.

Testing by means of explosives. Under dangerous conditions, it may be safer to stabilize the snow by using hand-placed charges or hand grenades. The safest way to do this it to throw charges of about 5 pounds over a crest so that the charge lands on the slope 5 to 6 meters (16 to 20 feet) below the crest. The charge may then be safely detonated from behind the crest. Under some snow conditions, more powerful charges may be required. As a rule, from 5 to 10 pounds of TNT controls 40 meters (131 feet) of slope width. Huge cornices are blasted by digging charges into the snow along the probable line of fracture. Individuals digging holes and placing charges to test the snow must be belayed while working. Testing by artillery and infantry weapons. Carrying explosives and demolition equipment up mountain slopes in deep snow is hard work. An easier way to cause the snow to slide is to use artillery pieces. Due to the difficulty of moving artillery pieces off the road or over secondary mountain trails, recoilless rifles will suffice against avalanches.

Testing by aircraft. The use of artillery pieces as well as recoilless rifles is limited by their range. Avalanches may also be set off against lines of communications deep in enemy territory by use of aircraft. Rockets may be an effective means of releasing avalanches if they can be accurately fired into a starting zone. Small weapons (rifles) and highly dispersed energy sources (sonic booms) are not reliable.

• Use of Barriers. Lines of communications and fixed installations that are under avalanche threat can be protected by construction of avalanche barriers. Barriers can be formed by adding rocks and earth, concrete, or other similar materials to natural obstacles. This barrier absorbs some of the tremendous energy of the slide. Diversion walls and piers may also be used to divert the avalanche to a planned path or turn it away from the area to be protected. Because of the amount of effort involved, such work is normally undertaken only during static situations. Effective barriers must often be high (over 10 meters) (33 feet) because they can fill up with repeated slides during the winter.

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Responses

  • gordon
    When do avalanches occur?
    8 years ago
  • sinit
    How loose snow avalanches occur?
    8 years ago
  • Carita
    How a avalanche start?
    7 years ago
  • Rita
    How do avalanches happen?
    7 years ago
  • anni
    How are avalanches are made?
    7 years ago
  • melody
    What causes concave slopes?
    3 years ago

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