An avalanche (also called a snowslide or snowslip) is a sudden, drastic flow of snow down a slope, occurring when either natural triggers, such as loading from new snow or rain, or artificial triggers, such as snowmobilers, explosives or backcountry skiers, overload the snowpack. The influence of gravity on the accumulated weight of newly fallen uncompacted snow or on thawing older snow leads to avalanches which may be triggered by earthquakes, gunshots and the movements of animals. Avalanches are most common during winter or spring but glacier movements may cause ice avalanches during summer. Avalanches cause loss of life and can destroy settlements, roads, railways and forests. 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. Avalanches are primarily composed of flowing snow, and are distinct from mudslides, rock slides, and serac collapses on an icefall. Avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snowpack. 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, slope aspect, and elevation. The size of an avalanche, its mass and its destructive potential are rated on a logarithmic scale, typically of 5 categories, with the precise definition of the categories depending on the observation system or geographic region in which the avalanche occurs.
Formation and classification
Most of the time, avalanches are caused by external stress on the snowpack; natural events are not random or spontaneous events. Natural triggers of avalanches include additional precipitation, rapid warming, rock fall, ice fall, and other impulse loads; however, even when environmental conditions are consistent, the seasonal snowpack will evolve over time and develop stresses, often from the downslope creep of the snowpack. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work. The triggering stress usually causes an avalanche at the location where force is directly applied to the snowpack (local trigger), but can in some cases cause avalanche formation at a different location nearby (remote trigger). Remotely triggered avalanches occur when a disturbance is transmitted from one location in the snowpack to another location in the snowpack. Small avalanches sometimes trigger much larger avalanches: for example, a small avalanche may apply significant overburden pressure to the snowpack, disturbing deeper weaknesses, and a larger avalanche may form as a result. This phenomenon is referred to as "stepping down".
A number of the forces acting on a snowpack can be readily determined. For example, there is little problem in calculating the weight of the snow, which provides information about the load on a weak layer. However, other factors are much more difficult to determine. It is very difficult to estimate the shear, ductile and tensile strengths within the snowpack or relative to the ground below. These strengths vary with the hardness of the snow, type of snow crystal, the number of bonds per unit volume, and the strength of contact interfaces between the layers.[1] The thermo-mechanical properties of the snow crystals in turn depend on the local conditions such as ambient air temperature that control moisture transport inside the snowpack. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time.[2] A complicating factor is the chaotic interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, and layering of the seasonal snowpack.
Classifications
The nature of the failure of the snowpack is used to morphologically classify the avalanche. To this point, there are two main types of avalanches: loose snow avalanches and slab avalanches, and either type of avalanche can involve dry or wet snow. For this reason, professionals refer to avalanches as "dry loose snow avalanches", "wet loose snow avalanches", "dry slab avalanches", and "wet slab avalanches". The primary distinction between wet and dry avalanches is the presence of liquid water in the snow at the time of avalanche formation.[1]
Loose snow avalanches
Loose snow avalanches, most common in steeper terrain, often occur in freshly fallen, low-density surface snow, or in older surface snow that has been softened by strong solar radiation. In loose snow avalanches, the release usually starts at a point and the avalanche gradually widens as it travels down the slope and entrains more snow. The characteristic shape of a loose snow avalanche is usually described as resembling a teardrop.[1] Large, loose snow avalanches may cause slab avalanches
Slab avalanches
Slab avalanches form frequently in new snow, wind deposited snow, and, less frequently, in old snow, and have the characteristic appearance of a block of snow cut out from its surroundings by fractures. Elements of slab avalanches include the following: a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a fracture at the bottom called the stauchwall. 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.
Slab avalanches, which account for around 90% of avalanche-related fatalities, form when the application of dynamic forces causes catastrophic structural failure inside a weakness below a slab of snow. Energy for fracture propagation is provided by gravity as the slab falls onto the weak layer. This cascade of failures causes one layer of snow to delaminate from the layer of snow below, enabling gravity to pull the delaminated slab downhill.[3] Fracture propagation can be widespread, sometimes traveling for hundreds of meters, and in some cases kilometers, and can involve snow depths ranging from 10 centimeters to five or six metres. Avalanches that form when the failure occurs between the base of the snowpack and the ground are known as full depth slab avalanches.
Among the largest and most powerful of avalanches, dry slab 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 uphill for short distances. A powder snow avalanche is a turbulent cloud of snow and air that forms when an avalanche travels over an abrupt change in slope angle, such as a cliff band. Powder snow avalanches may also form when the powder cloud of a dry slab avalanche continues moving after the core of the avalanche has stopped.[1]
There are two main types of slab avalanches, "soft slab avalanches", and "hard slab avalanches". Both types of avalanches are denoted by debris morphology[1]: the debris from a soft slab avalanche is highly granular, resembling a slurry of snowballs and ice grain paste, and the debris from a hard slab avalanche is angular, often featuring pieces of the original slab that did not break up during descent. Avalanches that descend significant vertical or horizontal distances may create debris that is not suitable for classification purposes.
Terrain, snowpack, weather
Doug Fesler and Jill Fredston developed a conceptual model of the three primary elements of avalanches: terrain, weather, and snowpack. Terrain describes the places where avalanches occur, weather describes the meteorological conditions that create the snowpack, and snowpack describes the structural characteristics of snow that make avalanche formation possible.[1][4]
TerrainAvalanche formation requires a slope where snow can accumulate, yet has enough steepness for the snow to accelerate once set in motion by the combination of mechanical failure (of the snowpack) and gravity. The angle of the slope that can hold snow, called the angle of repose, depends on a variety of factors such as crystal form and moisture content. Some forms of drier and colder snow will only stick to lower angle slopes; while wet and warm snow can bond to very steep surfaces. In particular, in coatstal mountains, such as the Cordillera del Paine region of Patagonia, deep snowpacks collect on vertical, and overhanging, rock faces. The angle of slope that can allow moving snow to accelerate depends on a variety of factors such as the snow's shear strength, which is itself dependent upon crystal form, and the configuration of layers and inter-layer interfaces.
The snowpack on slopes with sunny exposures is strongly influenced by sunshine. Diurnal cycles of thawing and refreezing can stabilize the snowpack by promoting settlement. Strong freeze thaw cycles result in the formation of surface crusts during the night, and the formation of unstable surface 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 deep snow, wind slabs, and cornices, all of which, when disturbed, may result in avalanche formation. Conversely the snowpack on a windward slope is often much shallower than on lee slopes.
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 snowpack influences the stability of the snowpack, either being a source of strength or weakness. Avalanches are unlikely to form in very thick forests, however boulders and sparsely distributed vegetation can create weak areas deep within the snowpack, 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.
Generally speaking, avalanches follow drainages down slope, frequently sharing drainage features with summertime watersheds. At and below tree line, avalanche paths through drainages are well defined by vegetation boundaries called trim lines, which occur where avalanches have removed trees and prevented regrowth 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. 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 human-triggered avalanches are most frequent, 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.[5] 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.
Avalanche paths
Avalanche paths in alpine terrain may be poorly-defined because of limited vegetation. Below treeline, avalanche paths are often delineated by vegetative trim lines created by past avalanches.
Avalanches and avalanche paths share common elements: a start zone where the avalanche originates, a track along which the avalanche flows, and a runout zone where the avalanche comes to rest. The debris deposit is the accumulated mass of the avalanched snow once it has come to rest in the runout zone. For the image at left, many small avalanches form in this avalanche path every year, but most of these avalanches do not run the full vertical or horizontal length of the path. The frequency with which avalanches form in a given area is known as the return period.
Snowpack structure and characteristics
The snowpack is composed of ground-parallel layers that accumulate over the winter. Each layer contains ice grains that are representative of the distinct meteorological conditions during which the snow formed and was deposited. Once deposited, a snow layer continues to evolve under the influence of the meteorological conditions that prevail after deposition.
For an avalanche to occur, it is necessary that a snowpack have a weak layer (or instability) below a slab of cohesive snow. In practice the formal mechanical and structural factors related to snowpack instability 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 index measurements of the mechanical properties of the snow (e.g. tensile strength, friction coefficients, shear strength, and ductile strength). This results in two principal sources of uncertainty in determining snowpack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snowpack vary widely within small areas and time scales, resulting in significant difficulty extrapolating point observations of snow layers across different scales of space and time. Second, the relationship between readily observable snowpack characteristics and the snowpack's critical mechanical properties has not been completely developed.
While the deterministic relationship between snowpack characteristics and snowpack 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. If ambient air temperatures are cold enough, shallow snow above or around boulders, plants, and other discontinuities in the slope, weakens from rapid crystal growth that occurs in the presence of a critical temperature gradient. Large, angular snow crystals are an indicator weak snow, because such crystals have fewer bonds per unit volume than small, rounded crystals that pack tightly together. Consolidated snow is less likely to slough than loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of slab avalanches, and persistent instabilities within the snowpack can hide below well-consolidated surface layers. Uncertainty associated with the empirical understanding of the factors influencing snow stability leads most professional avalanche workers to recommend conservative use of avalanche terrain relative to current snowpack instability.
Weather
Avalanches can only occur in a standing snowpack. Typically winter seasons at high latitudes, high altitudes, or both, have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a seasonal snowpack. Continentality, reflected by the distance from the moderating effects of oceans, is another important factor.[6] The evolution of the snowpack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snowpack. Among the critical factors controlling snowpack 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 snowpack; and conversely very cold, windy, or hot weather will weaken the snowpack.
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 snowpack 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 avalanche formation at any time of year.
Persistent cold temperatures can either prevent new snow from stabilizing or destabilize the existing snowpack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snowpack is usually around °C, and the ambient air temperature can be much colder. When a temperature gradient greater than 10 °C change per vertical meter of snow is sustained for more than a day, angular crystals called depth hoar or facets begin forming in the snowpack because of rapid moisture transport along the temperature gradient. These angular crystals, which bond poorly to one another and the surrounding snow, often become a persistent weakness in the snowpack. When a slab lying on top of a persistent weakness is loaded by a force greater than the strength of the slab and persistent weak layer, the persistent weak layer can 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 slab forms quickly and, if present, weaker snow below the slab may not have time to adjust to the new load. Even on a clear day, wind can quickly load a slope with snow by blowing snow from one place to another. Top-loading occurs when wind deposits snow from the top of a slope; cross-loading occurs when wind deposits snow parallel to the slope. 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 snowpack, 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 snowpack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snowpack together. Most avalanches happen during or soon after a storm.
Daytime exposure to sunlight will rapidly destabilize the upper layers of the snowpack if the sunlight is strong enough to melt the snow, thereby reducing its hardness. During clear nights, the snowpack can re-freeze when ambient air temperatures fall below freezing, through the process of long-wave radiative cooling, or both. Radiative heat loss occurs when the night air is significantly cooler than the snowpack, and the heat stored in the snow is re-radiated into the atmosphere.
Dynamics
When a slab avalanche forms, the slab disintegrates into increasingly smaller fragments as the snow travels downhill. If the fragments become small enough the outer layer of the avalanche, called a saltation layer, 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.[7] Scientific studies using radar, following the 1999 Galtür avalanche disaster, confirmed the hypothesis 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.[8]
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.[9]
Human involvement
Prevention
Preventative measures are employed in areas where avalanches pose a significant threat to people, such as ski resorts and mountain towns, roads and railways. There are several ways to prevent avalanches and lessen their power and destruction; active preventative measures reduce the likelihood and size of avalanches by disrupting the structure of the snowpack; passive measures reinforce and stabilize the snowpack in situ. The simplest active measure is by repeatedly traveling on a snowpack as snow accumulates; this can be by means of boot-packing, ski-cutting, or machine grooming. Explosives are used extensively to prevent avalanches, by triggering smaller avalanches that break down instabilities in the snowpack, and removing over burden that can result in larger avalanches. Explosive charges are delivered by a number of methods including hand tossed charges, helicopter dropped bombs, Gazex concussion lines, and ballistic projectiles launched by air cannons and artillery. Passive preventive systems such as 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.
To mitigate the effect 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. In addition to industrially manufactured barriers, landscaped barriers, called avalanche dams 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. Finally, along transportation corridors, large shelters, called snow sheds, can be built directly in the slide path of an avalanche to protect traffic from avalanches.
Human survival and rescue
Notable avalanches
Two avalanches occurred in March 1910 in the Cascade and Selkirk Mountain ranges; On March 1 the Wellington avalanche killed 96 in Washington State, United States. Three days later 62 railroad workers were killed in the Rogers Pass avalanche in British Columbia, Canada.
During World War I, an estimated 40,000 to 80,000 soldiers died as a result of avalanches during the mountain campaign in the Alps at the Austrian-Italian front, many of which were caused by artillery fire.[15][16] Some 10,000 men, from both sides, lost their lives in avalanches in December 1916.[17] However, it is very doubtful avalanches were used deliberately at the tactical level as weapons; more likely they were simply a side effect to shelling enemy troops, occasionally adding to the toll taken by the artillery. Avalanche prediction is nearly impossible; forecasters can only assert the conditions, terrain and relative likelihood of slides with the help of detailed weather reports and from localized snowpack observation. It would be almost impossible to predict avalanche conditions many miles behind enemy lines, making it impossible to intentionally target a slope at risk for avalanches. Also, high priority targets received continual shelling and would be unable to build up enough unstable snow to form devastating avalanches, effectively imitating the avalanche prevention programs at ski resorts.
In the northern hemisphere winter of 1950-1951 approximately 649 avalanches were recorded in a three month period throughout the Alps in Austria, France, Switzerland, Italy and Germany. This series of avalanches killed around 265 humans and was termed the Winter of Terror.
In 1993, the Bayburt Üzengili avalanche killed 60 individuals in Üzengili in the province of Bayburt, Turkey.
A large avalanche in Montroc, France, in 1999, 300,000 cubic metres of snow slid on a 30° slope, achieving a speed of 100 km/h (60 mph). It killed 12 people in their chalets under 100,000 tons of snow, 5 meters (15 ft) deep. The mayor of Chamonix was convicted of second-degree murder for not evacuating the area, but received a suspended sentence.[18]
The small Austrian village of Galtür was hit by the Galtür avalanche in 1999. The village was thought to be in a safe zone but the avalanche was exceptionally large and flowed into the village. Thirty-one people died.
A mountain climbing camp on Lenin Peak, in what is now Kyrgyzstan, was wiped out in 1990 when an earthquake triggered a large avalanche that overran the camp.[19] Forty-three climbers were killed.[20]
An avalanche in the Siachen glacier in the Himalaya mountains buried at least 124 Pakistani soldiers and 11 civilians in April 2012.[21]
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, slope aspect, and elevation. The size of an avalanche, its mass and its destructive potential are rated on a logarithmic scale, typically of 5 categories, with the precise definition of the categories depending on the observation system or geographic region in which the avalanche occurs.
Formation and classification
Most of the time, avalanches are caused by external stress on the snowpack; natural events are not random or spontaneous events. Natural triggers of avalanches include additional precipitation, rapid warming, rock fall, ice fall, and other impulse loads; however, even when environmental conditions are consistent, the seasonal snowpack will evolve over time and develop stresses, often from the downslope creep of the snowpack. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work. The triggering stress usually causes an avalanche at the location where force is directly applied to the snowpack (local trigger), but can in some cases cause avalanche formation at a different location nearby (remote trigger). Remotely triggered avalanches occur when a disturbance is transmitted from one location in the snowpack to another location in the snowpack. Small avalanches sometimes trigger much larger avalanches: for example, a small avalanche may apply significant overburden pressure to the snowpack, disturbing deeper weaknesses, and a larger avalanche may form as a result. This phenomenon is referred to as "stepping down".
A number of the forces acting on a snowpack can be readily determined. For example, there is little problem in calculating the weight of the snow, which provides information about the load on a weak layer. However, other factors are much more difficult to determine. It is very difficult to estimate the shear, ductile and tensile strengths within the snowpack or relative to the ground below. These strengths vary with the hardness of the snow, type of snow crystal, the number of bonds per unit volume, and the strength of contact interfaces between the layers.[1] The thermo-mechanical properties of the snow crystals in turn depend on the local conditions such as ambient air temperature that control moisture transport inside the snowpack. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time.[2] A complicating factor is the chaotic interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, and layering of the seasonal snowpack.
Classifications
The nature of the failure of the snowpack is used to morphologically classify the avalanche. To this point, there are two main types of avalanches: loose snow avalanches and slab avalanches, and either type of avalanche can involve dry or wet snow. For this reason, professionals refer to avalanches as "dry loose snow avalanches", "wet loose snow avalanches", "dry slab avalanches", and "wet slab avalanches". The primary distinction between wet and dry avalanches is the presence of liquid water in the snow at the time of avalanche formation.[1]
Loose snow avalanches
Loose snow avalanches, most common in steeper terrain, often occur in freshly fallen, low-density surface snow, or in older surface snow that has been softened by strong solar radiation. In loose snow avalanches, the release usually starts at a point and the avalanche gradually widens as it travels down the slope and entrains more snow. The characteristic shape of a loose snow avalanche is usually described as resembling a teardrop.[1] Large, loose snow avalanches may cause slab avalanches
Slab avalanches
Slab avalanches form frequently in new snow, wind deposited snow, and, less frequently, in old snow, and have the characteristic appearance of a block of snow cut out from its surroundings by fractures. Elements of slab avalanches include the following: a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a fracture at the bottom called the stauchwall. 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.
Slab avalanches, which account for around 90% of avalanche-related fatalities, form when the application of dynamic forces causes catastrophic structural failure inside a weakness below a slab of snow. Energy for fracture propagation is provided by gravity as the slab falls onto the weak layer. This cascade of failures causes one layer of snow to delaminate from the layer of snow below, enabling gravity to pull the delaminated slab downhill.[3] Fracture propagation can be widespread, sometimes traveling for hundreds of meters, and in some cases kilometers, and can involve snow depths ranging from 10 centimeters to five or six metres. Avalanches that form when the failure occurs between the base of the snowpack and the ground are known as full depth slab avalanches.
Among the largest and most powerful of avalanches, dry slab 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 uphill for short distances. A powder snow avalanche is a turbulent cloud of snow and air that forms when an avalanche travels over an abrupt change in slope angle, such as a cliff band. Powder snow avalanches may also form when the powder cloud of a dry slab avalanche continues moving after the core of the avalanche has stopped.[1]
There are two main types of slab avalanches, "soft slab avalanches", and "hard slab avalanches". Both types of avalanches are denoted by debris morphology[1]: the debris from a soft slab avalanche is highly granular, resembling a slurry of snowballs and ice grain paste, and the debris from a hard slab avalanche is angular, often featuring pieces of the original slab that did not break up during descent. Avalanches that descend significant vertical or horizontal distances may create debris that is not suitable for classification purposes.
Terrain, snowpack, weather
Doug Fesler and Jill Fredston developed a conceptual model of the three primary elements of avalanches: terrain, weather, and snowpack. Terrain describes the places where avalanches occur, weather describes the meteorological conditions that create the snowpack, and snowpack describes the structural characteristics of snow that make avalanche formation possible.[1][4]
TerrainAvalanche formation requires a slope where snow can accumulate, yet has enough steepness for the snow to accelerate once set in motion by the combination of mechanical failure (of the snowpack) and gravity. The angle of the slope that can hold snow, called the angle of repose, depends on a variety of factors such as crystal form and moisture content. Some forms of drier and colder snow will only stick to lower angle slopes; while wet and warm snow can bond to very steep surfaces. In particular, in coatstal mountains, such as the Cordillera del Paine region of Patagonia, deep snowpacks collect on vertical, and overhanging, rock faces. The angle of slope that can allow moving snow to accelerate depends on a variety of factors such as the snow's shear strength, which is itself dependent upon crystal form, and the configuration of layers and inter-layer interfaces.
The snowpack on slopes with sunny exposures is strongly influenced by sunshine. Diurnal cycles of thawing and refreezing can stabilize the snowpack by promoting settlement. Strong freeze thaw cycles result in the formation of surface crusts during the night, and the formation of unstable surface 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 deep snow, wind slabs, and cornices, all of which, when disturbed, may result in avalanche formation. Conversely the snowpack on a windward slope is often much shallower than on lee slopes.
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 snowpack influences the stability of the snowpack, either being a source of strength or weakness. Avalanches are unlikely to form in very thick forests, however boulders and sparsely distributed vegetation can create weak areas deep within the snowpack, 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.
Generally speaking, avalanches follow drainages down slope, frequently sharing drainage features with summertime watersheds. At and below tree line, avalanche paths through drainages are well defined by vegetation boundaries called trim lines, which occur where avalanches have removed trees and prevented regrowth 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. 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 human-triggered avalanches are most frequent, 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.[5] 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.
Avalanche paths
Avalanche paths in alpine terrain may be poorly-defined because of limited vegetation. Below treeline, avalanche paths are often delineated by vegetative trim lines created by past avalanches.
Avalanches and avalanche paths share common elements: a start zone where the avalanche originates, a track along which the avalanche flows, and a runout zone where the avalanche comes to rest. The debris deposit is the accumulated mass of the avalanched snow once it has come to rest in the runout zone. For the image at left, many small avalanches form in this avalanche path every year, but most of these avalanches do not run the full vertical or horizontal length of the path. The frequency with which avalanches form in a given area is known as the return period.
Snowpack structure and characteristics
The snowpack is composed of ground-parallel layers that accumulate over the winter. Each layer contains ice grains that are representative of the distinct meteorological conditions during which the snow formed and was deposited. Once deposited, a snow layer continues to evolve under the influence of the meteorological conditions that prevail after deposition.
For an avalanche to occur, it is necessary that a snowpack have a weak layer (or instability) below a slab of cohesive snow. In practice the formal mechanical and structural factors related to snowpack instability 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 index measurements of the mechanical properties of the snow (e.g. tensile strength, friction coefficients, shear strength, and ductile strength). This results in two principal sources of uncertainty in determining snowpack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snowpack vary widely within small areas and time scales, resulting in significant difficulty extrapolating point observations of snow layers across different scales of space and time. Second, the relationship between readily observable snowpack characteristics and the snowpack's critical mechanical properties has not been completely developed.
While the deterministic relationship between snowpack characteristics and snowpack 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. If ambient air temperatures are cold enough, shallow snow above or around boulders, plants, and other discontinuities in the slope, weakens from rapid crystal growth that occurs in the presence of a critical temperature gradient. Large, angular snow crystals are an indicator weak snow, because such crystals have fewer bonds per unit volume than small, rounded crystals that pack tightly together. Consolidated snow is less likely to slough than loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of slab avalanches, and persistent instabilities within the snowpack can hide below well-consolidated surface layers. Uncertainty associated with the empirical understanding of the factors influencing snow stability leads most professional avalanche workers to recommend conservative use of avalanche terrain relative to current snowpack instability.
Weather
Avalanches can only occur in a standing snowpack. Typically winter seasons at high latitudes, high altitudes, or both, have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a seasonal snowpack. Continentality, reflected by the distance from the moderating effects of oceans, is another important factor.[6] The evolution of the snowpack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snowpack. Among the critical factors controlling snowpack 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 snowpack; and conversely very cold, windy, or hot weather will weaken the snowpack.
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 snowpack 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 avalanche formation at any time of year.
Persistent cold temperatures can either prevent new snow from stabilizing or destabilize the existing snowpack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snowpack is usually around °C, and the ambient air temperature can be much colder. When a temperature gradient greater than 10 °C change per vertical meter of snow is sustained for more than a day, angular crystals called depth hoar or facets begin forming in the snowpack because of rapid moisture transport along the temperature gradient. These angular crystals, which bond poorly to one another and the surrounding snow, often become a persistent weakness in the snowpack. When a slab lying on top of a persistent weakness is loaded by a force greater than the strength of the slab and persistent weak layer, the persistent weak layer can 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 slab forms quickly and, if present, weaker snow below the slab may not have time to adjust to the new load. Even on a clear day, wind can quickly load a slope with snow by blowing snow from one place to another. Top-loading occurs when wind deposits snow from the top of a slope; cross-loading occurs when wind deposits snow parallel to the slope. 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 snowpack, 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 snowpack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snowpack together. Most avalanches happen during or soon after a storm.
Daytime exposure to sunlight will rapidly destabilize the upper layers of the snowpack if the sunlight is strong enough to melt the snow, thereby reducing its hardness. During clear nights, the snowpack can re-freeze when ambient air temperatures fall below freezing, through the process of long-wave radiative cooling, or both. Radiative heat loss occurs when the night air is significantly cooler than the snowpack, and the heat stored in the snow is re-radiated into the atmosphere.
Dynamics
When a slab avalanche forms, the slab disintegrates into increasingly smaller fragments as the snow travels downhill. If the fragments become small enough the outer layer of the avalanche, called a saltation layer, 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.[7] Scientific studies using radar, following the 1999 Galtür avalanche disaster, confirmed the hypothesis 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.[8]
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.[9]
Human involvement
Prevention
Preventative measures are employed in areas where avalanches pose a significant threat to people, such as ski resorts and mountain towns, roads and railways. There are several ways to prevent avalanches and lessen their power and destruction; active preventative measures reduce the likelihood and size of avalanches by disrupting the structure of the snowpack; passive measures reinforce and stabilize the snowpack in situ. The simplest active measure is by repeatedly traveling on a snowpack as snow accumulates; this can be by means of boot-packing, ski-cutting, or machine grooming. Explosives are used extensively to prevent avalanches, by triggering smaller avalanches that break down instabilities in the snowpack, and removing over burden that can result in larger avalanches. Explosive charges are delivered by a number of methods including hand tossed charges, helicopter dropped bombs, Gazex concussion lines, and ballistic projectiles launched by air cannons and artillery. Passive preventive systems such as 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.
To mitigate the effect 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. In addition to industrially manufactured barriers, landscaped barriers, called avalanche dams 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. Finally, along transportation corridors, large shelters, called snow sheds, can be built directly in the slide path of an avalanche to protect traffic from avalanches.
[edit] Safety
- 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 snowpack), not traveling over convex rolls (areas where the snowpack 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.[5] Most important of all practice good communication within 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 rescue
Notable avalanches
Two avalanches occurred in March 1910 in the Cascade and Selkirk Mountain ranges; On March 1 the Wellington avalanche killed 96 in Washington State, United States. Three days later 62 railroad workers were killed in the Rogers Pass avalanche in British Columbia, Canada.
During World War I, an estimated 40,000 to 80,000 soldiers died as a result of avalanches during the mountain campaign in the Alps at the Austrian-Italian front, many of which were caused by artillery fire.[15][16] Some 10,000 men, from both sides, lost their lives in avalanches in December 1916.[17] However, it is very doubtful avalanches were used deliberately at the tactical level as weapons; more likely they were simply a side effect to shelling enemy troops, occasionally adding to the toll taken by the artillery. Avalanche prediction is nearly impossible; forecasters can only assert the conditions, terrain and relative likelihood of slides with the help of detailed weather reports and from localized snowpack observation. It would be almost impossible to predict avalanche conditions many miles behind enemy lines, making it impossible to intentionally target a slope at risk for avalanches. Also, high priority targets received continual shelling and would be unable to build up enough unstable snow to form devastating avalanches, effectively imitating the avalanche prevention programs at ski resorts.
In the northern hemisphere winter of 1950-1951 approximately 649 avalanches were recorded in a three month period throughout the Alps in Austria, France, Switzerland, Italy and Germany. This series of avalanches killed around 265 humans and was termed the Winter of Terror.
In 1993, the Bayburt Üzengili avalanche killed 60 individuals in Üzengili in the province of Bayburt, Turkey.
A large avalanche in Montroc, France, in 1999, 300,000 cubic metres of snow slid on a 30° slope, achieving a speed of 100 km/h (60 mph). It killed 12 people in their chalets under 100,000 tons of snow, 5 meters (15 ft) deep. The mayor of Chamonix was convicted of second-degree murder for not evacuating the area, but received a suspended sentence.[18]
The small Austrian village of Galtür was hit by the Galtür avalanche in 1999. The village was thought to be in a safe zone but the avalanche was exceptionally large and flowed into the village. Thirty-one people died.
A mountain climbing camp on Lenin Peak, in what is now Kyrgyzstan, was wiped out in 1990 when an earthquake triggered a large avalanche that overran the camp.[19] Forty-three climbers were killed.[20]
An avalanche in the Siachen glacier in the Himalaya mountains buried at least 124 Pakistani soldiers and 11 civilians in April 2012.[21]
No comments:
Post a Comment