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An avalanche is a mass of snow and/or ice that breaks away from the side of a mountain and falls at great speed. Once the many tonnes of snow and ice that form an avalanche are set in motion it is impossible to stop, control or predict the path of an avalanche.
Avalanches can be specifically defined as mass movement hazards and the reason for this is obvious - movement is the fundamental reason for the hazard to humans. However, avalanches can be caused by other natural hazards such as earthquakes or volcanoes. Therefore they can come under the same classification as the hazards that initiated the movement.
A snow avalanche results due to the opposing forces acting upon snow on an incline. This incline is almost always a mountain or volcano, as both criteria are satisfied - the conditions needed for snow, and an incline. The force acting down the incline is the weight of the snow due to gravity whilst the opposing force is the limiting friction between snow and mountain. Any changes or fluctuations in these forces can cause the snow to begin to move down the slope - the start of an avalanche. Swiss legend has it that the chime of a bell, the crack of a whip or the beating of a bird's wings can trigger an avalanche such is the fine balance between the opposing forces.
Avalanches occur when the downward force becomes so great that it is larger than the upward slope. As the downward force is weight due to gravity an increase in mass (or weight due to gravity) can increase the danger of an avalanche. Over a winter much snow will fall on a mountain and the density of the snow can increase from 100kgm-3 to 400kgm-3. This means that while a mountainside may not look as if it has more snow on it, it can have a greater mass. This means that the majority of avalanches are in the spring (for Northern Hemisphere slopes). A second cause of avalanches occurring more in the spring is the increase in temperature. As the snow increases in temperature snow begins to melt and meltwater can cause the reduction of the friction acting upwards on the snow to decrease and this once again increases the likelihood of an avalanche occurring.
The most important triggers of an avalanche are heavy snowfall - due to increased mass -, rain or thaw - decreasing friction-, or an increase in the dynamic loading - meaning a moving mass such as a skier. Further for an avalanche to occur the slope the snow is on must be steep enough to make it slide. Therefore frequency is related to the angle of slope the snow is on. Most avalanches occur on slopes of 30 - 45 degrees as angles below 20 degrees are not steep enough, while 60 degrees means that not enough snow will accumulate.
Because of these factors avalanches occur regularly in mountain ranges during the springtime for each range respectively. Of course as I have already said it does not take much to start an avalanche and so they can occur at any time and so people should always be wary when on snow covered mountains.
Once again all of these factors combine to restrict where avalanches occur. The criteria of snow and a slope must be satisfied and so this generally limits avalanches to the major mountain ranges of the world such as the Alps, Rockies, Himalayas and Andes. However avalanches can occur anywhere where snow is on a slope.
If humans are in the vicinity of an avalanche they can cause catastrophic damage. In an avalanche upwards of 20 million cubic metres of snow and ice can hurtle down a slope at up to 200mph, almost twice that of the maximum speed of a person falling freely through the air. The impact of this huge mass of material can vary greatly from 0.1 tons per square metre up to huge pressures of 145 tone per square metre, which have been measured in monster avalanches in Japan. This table illustrates this:
| Impact Pressure (Tons per Square Metre) |
Potential Damage |
| 0.1 | Windows Broken |
| 3.0 | Destroys Wooden Frame House |
| 10.0 | Uproots Mature Tree |
| 100.0 | Moves Reinforced Structures |
In addition to the direct impact, avalanches may exert upward and downward forces that can lift trains and large trucks.
It is obvious from this that anything in the way of even a small avalanche does not have much chance of survival. However it must be remembered that an avalanche is only a hazard if humans are present and avalanches can often occur high in the mountains where there is nobody present for it to affect. This can mean that living near an area prone to avalanches can be dangerous.
Avalanches are an extremely dangerous threat for anyone on the snowy slopes and especially tourists, for example skiers or hikers, who may not have knowledge of precautions. Therefore slopes are often closed at dangerous times and education is essential.
All avalanches follow an avalanche path that has three elements: the starting zone where the snow initially breaks away, the track or actual path of the avalanche, and the runout zone where the snow decelerates and stops.
There are two main types of avalanche 'loose snow' and 'pure', and 'slab avalanches'. The former arises from low-density snow that behaves like sand. It only takes a small amount of snow to start sliding and an avalanche quickly follows. A 'slab' avalanche occurs when a layer of high-density snow breaks away from a weaker underlying layer, leaving a sharp fracture line. These are the most dangerous type of avalanche as large amounts of energy are released as slabs of up to 10,000 square metres by 10 metres deep can occur.
Once the snow has begun to move due to either of these two types the subsequent motion can also vary. Powder avalanches are formed of an aerosol of fine snow behaving like a dense gas. They flow in deep channels and are not influenced by objects in their path. Speeds are between 20 and 70 metres per second. Dry-flowing avalanches are formed of dry snow with particles ranging from powder grains to 20cm blocks. They are not easily deflected and travel at 15 to 60 metres per second. Wet-flowing avalanches occur mainly in spring and are composed of wet snow that is easily deflected by the terrain. Wet-flowing snow has a high average density and can cause great erosion despite slow speeds of 5 to 30 metres per second.
When avalanches occur near humans there is always the potential for death and destruction. Anyone on the slopes in the path of an avalanche has little, if any, chance of survival. Victims of avalanches can often be buried just a few feet below the surface and not escape, as the snow becomes extremely dense due to its fall.
In 1916 during the First World War, over 100 avalanches killed about 18,000 soldiers as they fought on the slopes.
It is not only those who are on the slopes when an avalanche occurs that are in danger. Whole villages can be consumed. In Switzerland, the country with the highest number of deaths due to avalanches in the world, many villages have been destroyed several times over.
One avalanche in an ELDC (Economically Less Developed Country) that caused catastrophic destruction was in Peru on the Nevado Huascaran. In 1970 an earthquake that measured 7.8 on the Richter scale sent approximately 50 million cubic metres of snow and ice to the bottom of the valley in 3 minutes. By the time the snow came to rest 10 miles from its starting zone it had killed around 18,000 people.
The largest avalanche in United States history occurred in 1910 in Wellington, Washington. A slag of rain-heavy snow resting on unstable powder snow broke loose from the side of the mountain into mountain railway stop, missing the hotel but ploughing into the railway depot. The 20ft high, half a mile long, quarter of a mile wide wall of snow slammed into passenger trains stranded there by the bad weather, picking them up and dropping them into a 150ft gorge. 26 people were rescued but 96 died.
In 1951 in Switzerland 240 people died and over 45,000 were trapped by a series of avalanches that swept through the Alps. Both of these hazards are examples in EMDC countries
Once an avalanche has occurred the first thing that is done is for rescue teams to be organised to hunt for survivors as the chance of surviving once buried is 50% in the first hour and this rapidly decreases. Only 5% of those people that rescuers find are alive. Search parties do not use high tech equipment to find survivors as it is often made helpless by the inaccessibility of the rescue area the dense packing of the snow. Sonar, radar, magnetometers, infrared detectors and even lasers cannot do as good a job as a rescue dog. A properly trained dog can search an area of over 100 square metres in as little as 20-30 minutes.
Also important after an avalanche is the clearing of roads. This is necessary to allow rescue services and supplies to reach the effected area quickly, as many people will be left out in the cold and open without food or shelter.
Perceptions of avalanches depend upon the location of the person's home. For those living in alpine and mountainous areas the threat of avalanches is real through most of the year and people living here are very aware of their existence. People here appreciate the destructive powers of an avalanche. However in recent times tourism has boomed in mountain area, and as the popularity of skiing, snowboarding, hiking, climbing and other alpine sports has increased, more outsiders are brought into avalanche risk areas. These people do not appreciate the need for caution when on slopes as much, or at all, as those living nearby. It is these people that put others in danger and who must be educated of the power of avalanches.
Avalanches are one natural hazard that can be relatively well prevented through the use of many techniques. As mentioned above many ski runs are closed when there is the danger of an avalanche. Education is also vital to reducing the risk.
A technique used to prevent the start of a major avalanche is artificial avalanche creation. These are good because areas can be closed so that gathering snow can be cleared from the slopes with minimum inconvenience in several smaller avalanches rather than one big, unexpected avalanche. Explosives or missiles fired from large guns can be used to trigger these artificial avalanches.
It is preferable though for the snowpack to remain intact. Therefore protective measures are also used to shield settlements, roads and electricity lines from moving snow. The wide variety of snow defences can be classified into:
Some of the structures used include fences, rakes, walls, wedges, avalanche gullies, avalanche sheds, deflectors and retardant mounds. The longest-term defence though is afforestation
The forecasting of avalanches is a practised skill. Trained local people work part-time at observation posts in the mountains transmitting reports and warning by radio. For the most reliable forecasts however a detailed diagnosis of the snowpack structure must be made with the emphasis on locating weak layers of snow. Local meteorological data is also an important input. These forecasts regularity depends on the amount of human activity in an area - for example ski and tourist resorts have daily forecasts. However, with all this said avalanche forecasting is not an exact science and cannot be totally relied upon.
Cyberspace Snow and Avalanche Center
Westwide Avalanche Network
Canadian Avalanche Association
Scottish Avalanche Infromation Service
American Association of Avalanche Proffesionals
Avalanche Links from About.com
ABC News