Physics of Avalanches


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Avalanches are just one of many natural disasters that kill people every year. The difference to Alaskans is that this disaster hits a little closer to home than most. The fear of hurricane and tornado are not existent in my region, and even earthquakes are seen more as a little shake then any threat. Physics plays a huge part in avalanches, thermodynamics, statics, and gravity are just a few elements of these disasters. The map above confirms that Alaska is at high risk for avalanche. As a result of this threat, learning about avalanches is a necessity when recreating or working in high avalanche areas.

In order to better understand Avalanches, it makes sense to first learn about what avalanches are compose of, snow. Snow forms when atmospheric conditions cause water vapor to condense. However, it is obvious that all snow doesn't have the same structure.

The density of fresh snowfall is dependent on both the kind of snow crystal and the air temperature. In cold, calm conditions the snowfall is the lightest. While in warmer climates, where graupel and needle crystals fall, the snowfall is the densest.

When looking at a cross section of snowfall it is often evident what weather conditions formed the cross section. This is due to the different densities and structures of the snow layers. The bonds between snow layers are large factor in avalanches. If there is a weak bond between layers, the top layer can easily slide off the bottom layer. When this happens it is called a slab avalanche. Other layering characteristic will create other avalanches and hazards, such as ice avalanches and cornices.

Whenever traveling in avalanche country it is important to be aware of your environment. Steep gullies and wide plains are perfect paths for avalanches. Ridges and unconformities in the terrain may help to slow down an avalanches speed. The severity of an avalanche is directly related to the terrain in which it happens.

One of the most important factors to consider is the slope of the mountain or hill. Most avalanches occur between 20 and 50 degrees like the diagram above shows. However, the largest avalanches occur between 30 and 45 degrees, and the areas which have the greatest frequency of avalanches are between 35 and 40 degrees.

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This is due to the fact that this angle allows the most snow to collect at the least stable angle.

Another important feature to consider when working or playing in places where avalanches are a definite possibility are anchors. Anchors are features of the terrain which hold the snow in one place. A good example is the picture below on the left. The route with many trees is labeled "best route" because the trees help hold the snow in place. Although an avalanche could still go through this area, the trees would slow it down, if not stop it. Also an avalanche is less likely to start in a densely forested area.

Another essential anchor to look for when choosing your path are rocky areas. Rocks, like trees, help hold the snow in place. Obviously an avalanche probably won't start on a rocky area. However, if an avalanche is coming down the mountain, it will take the path of least resistance meaning that a rocky ridge will probably be avoided by the avalanche. The picture above to the right illustrates the path an avalanche would probably take.

The two main avalanches are loose snow and slab avalanches. These two types of avalanches are distinguished by the snow condition at the origin. However, sometimes classifications must be modified on long avalanches, due to the fact that snow conditions vary throughout the avalanche. All avalanches have a great potential energy, that can be determined by their height and the force of an avalanche is simply F= ma. This means that if a small avalanche contains 1000 kg of snow, it has a force of 9810 Newtons.

Loose snow avalanches usually start at a point or small area and expand as they move. They are a result of snow that has been deposited at a steeper angle then the snow’s natural angle of repose. The most dangerous loose snow avalanches are made of wet snow. This is because the wet snow is denser and has a greater destructive force even though the velocities may be low. The diagram shows the differences between loose snow and slab avalanches.

Slab avalanches are the most dangerous type of avalanche. They are the largest source of winter hazards, and most are triggered by the victims. Slab avalanches form in almost all types of snow. Wind is an important factor to these avalanches, because it causes and unstable slab. However, wind alone will not cause a slab avalanche.

Gravity and the strength of the bonds between snow layers are important in slab avalanches. This is because gravity is the force which is pulling the slab down the mountain. When the force of gravity is greater than the bond between layers the gravity causes the top layer to separate and slide down the mountain.

The diagram below gives a good idea as to how a slab avalanche moves. It is easy to see how after the avalanche is triggered there is start zone, which moves down the "track" and finally ends in a debris toe. As the avalanche moves down the mountain kinetic energy increases while the potential energy decreases.

Cornices are an interesting snow feature which can often be found in snow territory. Although their impact is often not a great as an avalanche, they can be just as deadly if they are not approached correctly.

Windblown snow often forms cornices on ridge crests and the sides of gullies. The snow builds out horizontally, and when they break off it may be much farther back then their overhanging edge. The diagram below shows how the wind can form a wave shaped cornice. When cornices break off they might cause large slides, especially when they hit the snow pillow (see diagram) below. Along with causing large slides, they can obviously be deadly if they fall directly on a person.

Ice avalanches are a hazard that exists in glacial areas. They are caused by the collapse of unstable ice blocks from a steep or overhanging part of a glacier. Due to the fact that they are part of a glacier, ice avalanches can have large amounts of rock in them. Ice avalanches are quite dangerous because they can travel long distances and they are usually unpredictable.

There is no specific time of day or season in which ice avalanches occur. If an ice avalanche occurs near the ocean, it can cause surge waves or tsunamis. When traveling in areas with an ice avalanche risk, move as quickly as possible to minimize exposure time. The picture below is an example of an ice avalanche.

Whenever traveling in areas where there is an avalanche risk it is always a good idea to take 10-20 minutes to test the snow conditions. To begin with choose a test area that has similar slope, elevation, and snow conditions of the area you will be traveling in. First, dig a test pit that is four to five feet deep and wide enough to work in. Pay attention to weaknesses between layers of snow, and make sure that the snow above the test pit isn't disturbed. Most human triggered avalanches are within four to five feet of the surface.

Stratigraphy test:
The stratigraphy test is used to identify weak bonds between layers. To perform this test, use a mitten, hat, or a whisk broom to brush away loose snow in your snowpit. By brushing away loose snow layers, the once flat surface will show the snow history and identify weak bonds between layers. Ridged and raised layers in the snowpit, are layers that have the potential of becoming part of a slab avalanche. The picture below illustrates what this test might show.

Resistance test:
The resistance test simply identifies potential slab layers. Run a credit card, saw, or anything straight through the snow. Note any resistance that is felt. By combining this information with the straigraphy test, you now have a more information as to where the weak bonds are.

Hardness test:
Hardness tests combined with the information above can provide additional information as to the strength of the snow layers. The chart below gives classifications of hardness. In general strong layers are hard, while soft layers are weak. However, this is not always the case. Additional test can be performed to confirm. For more information on testing snow stability you can go to the following web site: http://www.uoregon.edu/~opp/snow/avalanche/stability_eval.htm.



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