Saturday, July 21, 2012

Avalanche Forces

An avalanche is a sudden snowslide. An accumulation of snow, gravity, and a slope are all that's needed.

Avalanches are triggered by natural phenomena like new snow, or by man-made phenomena like snowmobilers, skiers, and gunshots.

Avalanches are classified by type as loose snowor slab avalanches, and by moisture content:
  • dry loose snow avalanche;
  • wet loose snow avalanche;
  • dry slab avalanche; and
  • wet slab avalanche.

A slab avalanche in Gallatin National Forest

Slab Physics

An idealized slab avalanche is expressed as up- and down-slope forces in a free body diagram (shown below).

By Jeff Levison, The Alaska Avalanche Information Center
The free body diagram has the following variables:
Gcenter of gravity
wweight of the slab
ffrictional force
Nnormal force
Fccompressive force from the down-slope snow
θslope angle

The stability of the slab is a balance between stress and strength.
stress = strength
That is, stability is a balance of the strength of the down-slope snow Fc and the frictional force f between slab snow and base snow against the stress caused by the slope-adjusted weight w sin θ of the slab:
w sin θ = Fc + f
The balance between stress and strength is often precarious.
Even the largest avalanche is triggered by small things.
Vernor Vinge
From the equation, one can imagine an imbalance triggered by:
  • More snow - which would add to the slab's weight w, or
  • A temperature change - which might lessen the frictional force f.

Word Origin

Avalanche is French word derived from the verb avaler meaning to descend or go down.
No snowflake in an avalanche ever feels responsible.
Computer Models

3-dimensional computer models can simulate avalanches. Computer models use topographic data and prevailing snow conditions. Such models are idealized, but serve an objective measure of avalanche risk.

An animation of the Salezer avalanche near Davos Dorf, Canton Grisons, Switzerland was compiled from the results of a RAMMS (Rapid Mass Movements) computer simulation.

Simulation of Salezer avalanche near Davos Dorf,
Canton Grisons, Switzerland.