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Avalanche
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An avalanche is a rapid flow of snow down a slope, from either natural triggers or human activity. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope; however avalanches are always initiated in snow, are primarily composed of flowing snow, and are distinct from mudslides, rock slides, rock avalanches, and serac collapses from an icefall.
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An avalanche is a rapid flow of snow down a slope, from either natural triggers or human activity. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope; however avalanches are always initiated in snow, are primarily composed of flowing snow, and are distinct from mudslides, rock slides, rock avalanches, and serac collapses from an icefall. In mountainous terrain avalanches are among the most serious objective hazards to life and property, with their destructive capability resulting from their potential to carry an enormous mass of snow rapidly over large distances.
Avalanches are classified by their morphological characteristics, and are rated by either their destructive potential, or the mass of the downward flowing snow. Some of the morphological characteristics used to classify avalanches include the type of snow involved, the nature of the failure, the sliding surface, the propagation mechanism of the failure, the trigger of the avalanche, the slope angle, direction, and elevation. Avalanche size, mass, and destructive potential are rated on logarithmic magnitude scales, typically made up of 4 to 7 categories, with the precise definition of the categories depending on the observation system or forecast region.
Formation and Occurrences
Avalanches only occur when the stress on the snow exceeds the shear, ductile, and tensile strength either within the snow pack or at the contact of the base of the snow pack with the ground. A number of the forces acting on a snow pack can be readily determined, for example the weight of the snow is straightforward to calculate, however it is very difficult to know the shear, ductile, and tensile strength within the snow pack or with the ground. These strengths vary with the type of snow crystal and the bonding between them. The thermo-mechanical properties of the snow crystals in turn depend on the local conditions they have experienced such as temperature and humidity. One of the aims of avalanche research is to develop and validate computer models that can describe the time evolution of snow packs and predict the shear yield stress. A complicating factor is the large spatial variability that is typical.
Classification and Terminology
All avalanches share common elements: a trigger which causes the avalanche, a start zone from which the avalanche originates, a slide path along which the avalanche flows, a run out where the avalanche comes to rest, and a debris deposit which is the accumulated mass of the avalanched snow once it has come to rest. As well avalanches have a failure layer that propagates the failure and the bed surface along which the snow initially slides, in most avalanches the failure layer and the bed surface are the same. Additionally slab avalanches have a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a shallow staunch fracture at the bottom of the start zone. The crown and flank fractures are vertical walls in the snow delineating the snow that was entrained in the avalanche from the snow that remained on the slope.
The nature of the failure of the snow pack is used to morphologically classify the avalanche. Slab avalanches are generated when an additional load causes a brittle failure of a slab that is bridging a weak snow layer; this failure is propagated through fracture formation in the bridging slab. Loose snow, point release, and isothermal avalanches are generated when an stress causes a shear failure in a weak interface, either within the snow pack, or at the base. When the failure occurs at the base they are known as full depth avalanches. Spin drift avalanches occur when wind lifted snow is funneled into a steep drainage from above the drainage.
Loose snow avalanches occur in freshly fallen snow that has a lower density and are most common on steeper terrain. In fresh, loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usually forming a teardrop appearance. This is in contrast to a slab avalanche.
Slab avalanches account for around 90% of avalanche-related fatalities, and occur when there is a strong, cohesive layer of snow known as a slab. These are usually formed when falling snow is deposited by the wind on a lee slope, or when loose ground snow is transported elsewhere. When there is a failure in a weak layer, a fracture very rapidly propagates so that a large area, that can be hundreds of meters in extent and several meters thick, starts moving almost instantaneously.
A third starting type is a wet snow avalanche or isothermal avalanche, which occurs when the snow pack becomes saturated by water. These tend to also start and spread out from a point. When the percentage of water is very high they are known as slush flows and they can move on very shallow slopes.
Among the largest and most powerful of avalanches, powder snow avalanches can exceed speeds of 300 km/h, and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even up hill for short distances. A powder snow avalanches is a powder cloud that forms when an avalanche accelerates over an abrupt change in slope, such as a cliff band, causing the snow to mix with air. This turbulent suspension of snow particles then flows as a gravity current.
Terrain
Terrain affects avalanche occurrence and development through three factors: First terrain affects the evolution of the snow pack by determining the meteorological exposure of the snow pack. Second, terrain affects the stability of the snow pack, through the geometry and ground composition of the slope. Third, the down slope features of the terrain affects the path and consequences of a flowing avalanche.
For a slope to generate an avalanche it must be simultaneously capable of retaining snow, and allowing snow to accelerate once set in motion. The angle of the slope that can hold snow depends on the ductile and shear strength of the snow, which is determined by the temperature and moisture content of the snow. Drier and colder snow, with lower ductile and shear strength, will only bond to lower angle slopes; while wet and warm snow, with higher ductile and shear strength, can bound to very steep surfaces. In particular, in coastal mountains, such as the Cordillera del Paine region of Patagonia, deep snow packs collect on vertical, and overhanging, rock faces. The angle of slope that can allow moving snow to accelerate depends on the shear strength of the snow. Snow that has been water saturated to the point of slush can accelerate on shallow angled terrain; while a cohesive snow pack will not accelerate on very steep slopes, such as the typical snow pack in the Chugach Mountains of Alaska.
The snow pack on slopes with sunny exposures are strongly influenced by sunshine. Daily cycles of mild thawing and refreezing can stabilize the snow pack by promoting settlement, strong freeze thaw cycles will result in the formation of surface crusts during the night, and the formation of unstable isothermal snow during the day. Slopes in the lee of a ridge or other wind obstacle accumulate more snow and are more likely to include pockets of abnormally deep snow, wind slabs, and cornices, all of which, when disturbed, may trigger an avalanche. Conversely a windward slope will be bare of snow.
The start zone of an avalanche must be steep enough to allow snow to accelerate once set in motion, additionally convex slopes are less stable than concave slopes, because of the disparity between the tensile strength of snow layers and their compressive strength. The composition and structure of the ground surface beneath the snow pack influences the stability of the snow pack, either being a source of strength or weakness. Vegetation, such as heavy timber, can anchor a snow pack; however, boulders and sparsely distributed vegetation will create weak areas deep within the snow pack, through the formation of strong temperature gradients. Full-depth avalanches (avalanches that sweep a slope virtually clean of snow cover) are more common on slopes with smooth ground cover, such as grass or rock slabs.
Avalanches follow drainages down slope, frequently sharing drainage features with summertime watersheds. At and below tree line these drainages are well defined by vegetation boundaries where the avalanches have prevented the growth of large vegetation. Engineered drainages, such as the avalanche dam on Mount Stephen in Kicking Horse Pass, have been constructed to protect people and property, by redirecting the flow of avalanches. Deep debris deposits from avalanches will collect in catchments at the terminus of a run out, such as gullies and river beds, .
Slopes flatter than 25 degrees or steeper than 60 degrees typically have a lower incidence of avalanche involvement, likewise slopes with windward and sunny exposure have a lower incidence of avalanche involvement . Human triggered avalanches have the greatest incidence when the snow's angle of repose is between 35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. But when the incidence of human triggered avalanches are normalized by the rates of recreational use hazard increases uniformly with slope angle, and no significant difference in hazard for a given exposure direction can be found. The rule of thumb is: A slope that is flat enough to hold snow but steep enough to ski has the potential to generate an avalanche, regardless of the angle.
Snow structure and characteristics
The snow pack is composed of deposition layers of snow that are accumulated over time. The deposition layers are stratified parallel to the ground surface on which the snow falls. Each deposition layer indicates a distinct meteorological condition during which the snow was accumulated. Once deposited a snow layer will continue to evolve and develop under the influence of the meteorological conditions that prevail after deposition.
For an avalanche to occur, it is necessary that a snow pack have a weak layer (or instability) below a slab of cohesive snow. In practice the mechanical and structural determinants of snow pack stability are not directly observable outside of laboratories, thus the more easily observed properties of the snow layers (e.g. penetration resistance, grain size, grain type, temperature) are used as proxy measurements of the mechanical properties of the snow (e.g. tensile strength, friction coefficients, shear strength, and ductile strength). This results in two principle sources of uncertainty in determining snow pack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snow pack vary widely within small areas and time scales, resulting in an inability to extrapolate point observations of snow layers. Second, the understanding of the relationship between the readily observable snow pack characteristics and the snow pack's critical mechanical properties has not been completely developed.
While the deterministic relationship between snow pack characteristics and snow pack stability is still a matter of ongoing scientific study, there is a growing empirical understanding of the snow composition and deposition characteristics that influence the likelihood of an avalanche. Observation and experience has shown that newly fallen snow requires time to bond with the snow layers beneath it, especially if the new snow falls during very cold and dry conditions. Shallower snow, that can lay above or around boulders, plants, and other discontinuities in the slope, will weaken from the presence of a stronger temperature gradient. Larger and more angular snow crystals are an indicator of weaker bonds within the snow pack, because the sintering process that forms bonds within the snow pack will also cause the snow crystals to become smaller and rounder. Consolidated snow is less likely to slough than either loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of slab avalanches, and will mask persistent instabilities within a snow pack. The empirical understanding of the factors influencing snow stability only place broad predictive bounds on the stability of the snow, consequently a conservative use of avalanche terrain, well within the recommended guidelines of the local avalanche forecasts and bulletins, is always recommended.
Weather
Avalanches can only occur in a standing snow pack. Typically winter seasons and high altitudes have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a snow pack. The evolution of the snow pack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snow pack. Among the critical factors controlling snow pack evolution are: heating by the sun, radiational cooling, vertical temperature gradients in standing snow, snowfall amounts, and snow types. Generally, mild winter weather will promote the settlement and stabilization of the snow pack; and conversely very cold, windy, or hot weather will weaken the snow pack.
At temperatures close to the freezing point of water, or during times of moderate solar radiation, a gentle freeze-thaw cycle will take place. The melting and refreezing of water in the snow strengthens the snow pack during the freezing phase and weakens it during the thawing phase. A rapid rise in temperature, to a point significantly above the freezing point of water, may cause a slope to avalanche, especially in the spring.
Persistent cold temperatures can either prevent the snow from stabilizing or destabilize a snow pack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snow pack is close to freezing; unless the snow pack is standing on glaciated terrain, in which case the temperature at the base of the snow pack can be significantly below freezing. When a temperature gradient greater than 10oC change per vertical meter of snow is sustained for more than a day depth hoar will form in the snow pack, through the thermal transport of moisture away from the depth hoar along the temperature gradient, from bottom to top. This layer of depth hoar becomes a persistent weakness in the snow pack, characterized by faceted grains forming either above or below crusts and slabs. When a slab lying on top of this persistent weakness is loaded by a force above the tensile and ductile strength of the slab and the shear strength of the persistent weak layer, the persistent weak layer will fail and generate an avalanche.
Any wind stronger than a light breeze can contribute to a rapid accumulation of snow on sheltered slopes downwind. Wind pressure at a favorable angle can stabilize other slopes. A "wind slab" is a particularly fragile and brittle structure which is heavily-loaded and poorly-bonded to its underlayment. Even on a clear day, wind can quickly shift the snow load on a slope. This can occur in two ways: by top-loading and by cross-loading. Top-loading occurs when wind deposits snow perpendicular to the fall-line on a slope; cross-loading occurs when wind deposits snow parallel to the fall-line. When a wind blows over the top of a mountain, the leeward, or downwind, side of the mountain experiences top-loading, from the top to the bottom of that lee slope. When the wind blows across a ridge that leads up the mountain, the leeward side of the ridge is subject to cross-loading. Cross-loaded wind-slabs are usually difficult to identify visually.
Snowstorms and rainstorms are important contributors to avalanche danger. Heavy snowfall will cause instability in the existing snow pack, both because of the additional weight and because the new snow has insufficient time to bond to underlying snow layers. Rain has a similar effect. In the short-term, rain causes instability because, like a heavy snowfall, it imposes an additional load on the snow pack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snow pack together. Most avalanches happen during or soon after a storm.
Daytime exposure to sunlight will rapidly destabilize the upper layers of a snow pack. Sunlight reduces the sintering, or necking, between snow grains. During clear nights, the snow pack can strengthen, or tighten, through the process of long-wave radiative cooling. When the night air is significantly cooler than the snow pack, the heat stored in the snow is re-radiated into the atmosphere.
Triggers
Avalanches are always caused by an external stress on the snow pack, they are not random or spontaneous events. Natural triggers of avalanches include additional precipitation, radiative and convective heating, rock fall, ice fall, and other sudden impacts; however, even a snow pack held at a constant temperature, pressure, and humidity will evolve over time and develop stresses, often from the downslope creep of the snow pack. Human triggers of avalanches include skiers, snowmobiles, and controlled explosive work. The triggering stress load can be either localized to the failure point, or remote. Localized triggers of avalanches are typified by point releases from solar heated rocks. Remotely triggered avalanches occur when a tensile stress wave is transmitted through the slab to the start zone, once the stress wave reaches the start zone a fracture initiates and propagates the failure. Of exceptional note is that avalanches can not only entrain additional snow within the failing slab, but can also, given the sufficient accumulation of overburden due to a smaller avalanche, step down and trigger deeper slab instabilities that would be more resilient against smaller stresses. The triggering of avalanches is an example of critical phenomena.
Dynamics
When an avalanche occurs, as the snow slides down the slope any slab present begins to fragment into increasingly smaller tumbling fragments. If the fragments become small enough the avalanche takes on the characteristics of a fluid. When sufficiently fine particles are present they can become airborne and, given a sufficient quantity of airborne snow, this portion of the avalanche can become separated from the bulk of the avalanche and travel a greater distance as a powder snow avalanche. Scientific studies using radar, following the 1999 Galtür avalanche disaster, confirmed suspicions that a saltation layer forms between the surface and the airborne components of an avalanche, which can also separate from the bulk of the avalanche.
Driving a (non-airborne) avalanche is the component of the avalanche's weight parallel to the slope; as the avalanche progresses any unstable snow in its path will tend to become incorporated, so increasing the overall weight. This force will increase as the steepness of the slope increases, and diminish as the slope flattens. Resisting this are a number of components that are thought to interact with each other: the friction between the avalanche and the surface beneath; friction between the air and snow within the fluid; fluid-dynamic drag at the leading edge of the avalanche; shear resistance between the avalanche and the air through which it is passing, and shear resistance between the fragments within the avalanche itself. An avalanche will continue to accelerate until the resistance exceeds the forward force.
Modelling
Attempts to model avalanche behaviour date from the early 20th century, notably the work of Professor Lagotala in preparation for the 1924 Winter Olympics in Chamonix. His method was developed by A. Voellmy and popularised following the publication in 1955 of his Ueber die Zerstoerunskraft von Lawinen (On the Destructive Force of Avalanches).
Voellmy used a simple empirical formula, treating an avalanche as a sliding block of snow moving with a drag force that was proportional to the square of the speed of its flow:
He and others subsequently derived other formulae that take other factors into account, with the Voellmy-Salm-Gubler and the Perla-Cheng-McClung models becoming most widely used as simple tools to model flowing (as opposed to powder snow) avalanches.
Since the 1990s many more sophisticated models have been developed. In Europe much of the recent work was carried out as part of the SATSIE (Avalanche Studies and Model Validation in Europe) research project supported by the European Commission which produced the leading-edge MN2L model, now in use with the Service Réstitution Terrains en Montagne (Mountain Rescue Service) in
France, and D2FRAM (Dynamical Two-Flow-Regime Avalanche Model), which was still undergoing validation as of 2007.
Avalanche avoidance
Due to the complexity of the subject, winter travelling in the backcountry (off-piste) is never 100% safe. Good avalanche safety is a continuous process, including route selection and examination of the snowpack, weather conditions, and human factors. Several well-known good habits can also minimize the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings heeded. Never follow in the tracks of others without your own evaluations; snow conditions are almost certain to have changed since they were made. Observe the terrain and note obvious avalanche paths where vegetation is missing or damaged, where there are few surface anchors, and below cornices or ice formations. Avoid traveling below others who might trigger an avalanche.
Prevention
There are several ways to prevent avalanches and lessen their power and destruction. They are employed in areas where avalanches pose a significant threat to people, such as ski resorts and mountain towns, roads and railways. Explosives are used extensively to prevent avalanches, especially at ski resorts where other methods are often impractical. Explosive charges are used to trigger small avalanches before enough snow can build up to cause a large avalanche. Snow fences and light walls can be used to direct the placement of snow. Snow builds up around the fence, especially the side that faces the prevailing winds. Downwind of the fence, snow buildup is lessened. This is caused by the loss of snow at the fence that would have been deposited and the pickup of the snow that is already there by the wind, which was depleted of snow at the fence. When there is a sufficient density of trees, they can greatly reduce the strength of avalanches. They hold snow in place and when there is an avalanche, the impact of the snow against the trees slows it down. Trees can either be planted or they can be conserved, such as in the building of a ski resort, to reduce the strength of avalanches.
Artificial barriers can be very effective in reducing avalanche damage. There are several types. One kind of barrier (snow net) uses a net strung between poles that are anchored by guy wires in addition to their foundations. These barriers are similar to those used for rockslides. Another type of barrier is a rigid fence like structure (snow fence) and may be constructed of steel, wood or pre-stressed concrete. They usually have gaps between the beams and are built perpendicular to the slope, with reinforcing beams on the downhill side. Rigid barriers are often considered unsightly, especially when many rows must be built. They are also expensive and vulnerable to damage from falling rocks in the warmer months. Finally, there are barriers that stop or deflect avalanches with their weight and strength. These barriers are made out of concrete, rocks or earth. They are usually placed right above the structure, road or railway that they are trying to protect, although they can also be used to channel avalanches into other barriers. Occasionally, earth mounds are placed in the avalanche's path to slow it down.
Safety in avalanche terrain
- Terrain management - Terrain management involves reducing the exposure of an individual to the risks of traveling in avalanche terrain by carefully selecting what areas of slopes to travel on. Features to be cognizant of include not under cutting slopes (removing the physical support of the snow pack), not traveling over convex rolls (areas where the snow pack is under tension), staying away from weaknesses like exposed rock, and avoiding areas of slopes that expose one to terrain traps (gulleys that can be filled in, cliffs over which one can be swept, or heavy timber into which one can be carried).
- Group management - Group management is the practice of reducing the risk of having a member of a group, or a whole group involved in an avalanche. Minimize the number of people on the slope, and maintain separation. Ideally one person should pass over the slope into an area protected from the avalanche hazard before the next one leaves protective cover. Route selection should also consider what dangers lie above and below the route, and the consequences of an unexpected avalanche (i.e., unlikely to occur, but deadly if it does). Stop or camp only in safe locations. Wear warm gear to delay hypothermia if buried. Plan escape routes. In determining the size of the group balance the hazard of not having enough people to effectively carry out a rescue with the risk of having too many members of the group to safely manage the risks. It is generally recommended not to travel alone, because there will be no-one to witness your burial and start the rescue. Additionally, avalanche risk increases with use; that is, the more a slope is disturbed by skiers, the more likely it is that an avalanche will occur. Most important of all practice good communication with in a group including clearly communicating the decisions about safe locations, escape routes, and slope choices, and having a clear understanding of every members skills in snow travel, avalanche rescue, and route finding.
- Risk Factor Awareness - Risk factor awareness in avalanche safety requires gathering and accounting for a wide range of information such as the meteorological history of the area, the current weather and snow conditions, and equally important the social and physical indicators of the group.
- Leadership - Leadership in avalanche terrain requires well defined decision making protocols that use the observed risk factors. These decision making frameworks are taught in a variety of courses provided by national avalanche resource centers in Europe and North America. Fundamental to leadership in avalanche terrain is honestly assessing and estimating the information that was ignored or overlooked. Recent research has shown that there are strong psychological and group dynamic determinants that lead to avalanche involvement.
Human survival and avalanche rescue
Even small avalanches are a serious danger to life, even with properly trained and equipped companions who avoid the avalanche. Between 55 and 65 percent of victims buried in the open are killed, and only 80 percent of the victims remaining on the surface survive. (McClung, p.177).
Research carried out in Italy based on 422 buried skiers indicates how the chances of survival drop:
- very rapidly from 92 percent within 15 minutes to only 30 percent after 35 minutes (victims die of suffocation)
- near zero after two hours (victims die of injuries or hypothermia)
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Consequently it is vital that everyone surviving an avalanche is used in an immediate search and rescue operation, rather than waiting for help to arrive. Additional help can be called once it can be determined if anyone is seriously injured or still remains unaccountable after the immediate search (i.e., after at least 30 minutes of searching). Even in a well equipped country such as France, it typically takes 45 minutes for a helicopter rescue team to arrive, by which time most of the victims are likely to have died.
In some cases avalanche victims are not located until spring thaw melts the snow, or even years later when objects emerge from a glacier.
Search and rescue equipment
Chances of a buried victim being found alive and rescued are increased when everyone in a group is carrying and using standard avalanche equipment, and have trained in how to use it. However, like a seat belt in a vehicle, using the right equipment does not justify exposing yourself to unnecessary risks with the hope that the equipment might save your life when it is needed. A beacon, shovel and probe is considered the minimum equipment to carry when exposing yourself to avalanche danger.
Avalanche cords
Using an avalanche cord is the oldest form of equipment — mainly used before beacons became available. The principle is simple. An approximately 10 meter long red cord (similar to parachute cord) is attached to the person in question's belt. While skiing, snowboarding, or walking the cord is dragged along behind the person. If the person gets buried in an avalanche, the light cord stays on top of the snow. Due to the color the cord is easily visible for rescue personnel. Typically the cord has iron markings every one meter that indicate the direction and length to the victim.
Beacons
Beacons — known as "beepers", peeps (pieps), ARVAs (Appareil de Recherche de Victimes en Avalanche, in French), LVS (Lawinen-Verschütteten-Suchgerät, Swiss German), avalanche transceivers, or various other trade names, are important for every member of the party. They emit a "beep" via 457 kHz radio signal in normal use, but may be switched to receive mode to locate a buried victim up to 80 meters away. Analog receivers provide audible beeps that rescuers interpret to estimate distance to a victim. To use the receiver effectively requires regular practice. Some older models of beepers operated on a different frequency (2.275 kHz ) and a group leader should ensure these are no longer in use.
Recent digital models also attempt to give visual indications of direction and distance to victims and require less practice to be useful. There are also passive transponder devices that can be inserted into equipment, but they require specialized search equipment that might only be found near an organized sports area.
Probes
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