EXTERNAL FACTORS INFLUENCING SLAB
AVALANCHES
There are many factors to consider in
evaluating whether or not a slab avalanche might occur in a certain area.
The following is a summary of just a few external factors to consider and
a brief description of how they affect the physics of the snow pack to
increase or decrease stability. Information presented here is extracted
from a combination of sources (University of Colorado IBS 1975; Perla and
Martinelli 1976; Daffern 1983; Fredston and Fesler 1994; Graydon and Hanson
1997).
Wind: Wind can increase the danger of avalanche by "windloading" leeward slopes, which can increase the shear stress or cause a weak layer to fail due to the additional weight. Wind blown slopes also provide a sliding, or bed layer, for any new snow that accumulates on top.
Wind: Wind can increase the danger of avalanche by "windloading" leeward slopes, which can increase the shear stress or cause a weak layer to fail due to the additional weight. Wind blown slopes also provide a sliding, or bed layer, for any new snow that accumulates on top.
Figure from Fredston and Fesler 1994
Precipitation: New snow loads can contribute
additional stress in the downslope direction and increase the chances for
slope failure. Heavy snowfall during a short time period is especially
dangerous, causing a rapid increase in unstable downward forces.
Temperature: In general, increasing temperatures lead to unstable conditions, especially in the first days of thawing when bonding strength is initially weakened. Conversely, several consecutive days of above freezing temperatures can lead to stable slopes where avalanches have not occurred due to consolidation of layers and elimination of weak layers that might fail.
Humans: By far the greatest contributing factor to disasters involving avalanches, humans often make mistakes in judgement or let their egos prevail. Beyond this obvious problem, the additional force created by a human on an unstable slope is often enough to create more stress than there is slope strength, resulting in slope failure. The picture below shows how not to cross a dangerous slope. If the slope slides everyone will be caught, and the additional weight of all the skiers together is much larger than the additional weight of one skier crossing at a time.
Temperature: In general, increasing temperatures lead to unstable conditions, especially in the first days of thawing when bonding strength is initially weakened. Conversely, several consecutive days of above freezing temperatures can lead to stable slopes where avalanches have not occurred due to consolidation of layers and elimination of weak layers that might fail.
Humans: By far the greatest contributing factor to disasters involving avalanches, humans often make mistakes in judgement or let their egos prevail. Beyond this obvious problem, the additional force created by a human on an unstable slope is often enough to create more stress than there is slope strength, resulting in slope failure. The picture below shows how not to cross a dangerous slope. If the slope slides everyone will be caught, and the additional weight of all the skiers together is much larger than the additional weight of one skier crossing at a time.
Photo from Daffern 1983