Section 3. Description of the Problem
As I have already said, avalanche force is a kind of dynamics. We can also define dynamics as a branch of mechanics concerned with the forces that cause objects to move. We know that there are some forces acting on an avalanche even if we did not know physics. The Newton´s first law of motion states: „An object at rest remains at rest, and an object in motion continues to stay in motion with constant velocity unless the object experiences a net external force.“ (Izaak Newton). This tells us that if an avalanche starts forming and accelerating, it will only stop untill it hits somekind of impediment or the bottom of the mountain. That is why avalanche is so harmful for people. Someone who is standing in the center of a slope has not got a chance of running away. The avalanche´s velocity increases when the trajectory increases so the lower you are standing, the worst it is because by the time it gets to you, its acceleration is too high.
Section 4. Research of Problem
Firstly, I will start speaking about the driving force and motion resistance. After fracture and release of a slab the snow accelerates downslope as an avalanche. This slab is broken up into a sliding, tumbling, and bounding movement. If it is on a big mountain, the avalanche slowly turns into a flow. Two opposite sets of forces form with or against the flowing avalanche. One is the driving force(F) which is parallel to the slope. Second is the resisting force(R) which is composed of several different forces which are against F preventing the avalanche from accelerating indefinitely. Avalanche will accelerate as long as F is bigger than R because by this a net force will form down the slope. The condition of F having to be bigger than R is only true at the starting zone. Here acceleration occurs because the slope is steep and F is large. The flowing mass can also increase due to the fact that additional unstable snow will be pushed and conjucted into the avalanche. Constant velocity starts when F equals R. This condition occurs after the starting zone all the way to the botton of the avalanche path. This zone is usually called the track. In fact, the velocity even here changes due to the entering of new snow, deposition and topographic irregulaties but just slightly. Deceleration occurs in therunout zone where R becomes greater than F. Here the gradient is reduced as well as F. Now, the underlying snow pack is stable and the kinetic energy is dissipated.
Next, I will tell you something about the resisting forces on their own. Talking about the resisting forces, there are five important factors contributing to frictional resistance which are:
R1: Sliding friction between the avalanche and the underlying snow or ground.
R2: Internal dynamic shear resistance due to collisions and momentum exchange between particles and blocks of snow.
R3: Turbulent friction within the snow/air suspension.
R4: Shear between the avalanche and the surrounding air.
R5: Fluid-dynamic drag at the front of the avalanche.
To find out acceleration on an avalanche, we have to know these resisting forces. The formula for this is listed in the documentation. The importance of the individual resisting forces varies within the avalanche and depends on the type of avalanching snow.
Lastly, I will teach you somethings about avalanche impact. Avalanches can produce very large dynamic forces on objects. High velocity, low density, dry snow avalanches may flow over or around objects. This produces a fluid-dynamic stagnation pressure which is calculated by P = 1/2 pV2. Here P is pressure, p is the avalanche density and V is avalanche velocity. Total force on the object consists of „drag and uplift“ forces which act perpendiculary or parallely to the flow direction and upward on the object(which may for example be a hut in the mountains). Denser, slower moving avalanches will not engulf an object. Some of the avalanche´s mass rest against the object and some is deviated. The equation for this is P = pV2. Because big defferences exist between the mechanics of impact of diffenent types of avalanches, someone must decide which type s likely to occus in that kind of area.
Section 5. Documentation
The driving force(F) and the resisting forces(R) act on a moving avalanche to determine its acceleration and maximum velocity.
Parts of an avalanche path
Formula for counting the acceleration of an avalanche
Hard slab avalanche flow
Slow slab avalanche flow
Impact of a dry snow or powder avalanche may produce
both drag and lift forces on an object.
Impact of a dense, wet snow avalanche.
Works Cited
Leaf, Charles. "references." avalanche dynamics. 1998-2001. WestWide Avalanche Network. 14 Jan 2009 <http://www.avalanche.org/~moonstone/zoning/avalanche%20dynamics.htm>.
Mers, Arthur. "avalanche dynamics." avalanche dynamics. 1976. Guidelines and methods for detailed snow avalanche hazard investigations. 14 Jan 2009 <http://www.avalanche.org/~moonstone/zoning/avalanche%20dynamics.htm>.
Voellmy, A.. "avalanche dynamics." avalanche dynamics. 1955. Uber die Zerstorungskraft von Lawinen. Schweiz. Bauzeitung. 14 Jan 2009 <http://www.avalanche.org/~moonstone/zoning/avalanche%20dynamics.htm>.
by
katerinacechmankova (student)
