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Show SNOW MECHANICS Theoretical snow mechanics is the province of basic research, and the advanced student is referred particularly to Swiss publications, some of which are listed in the bibliography. The only work in this field being done in the United States is under the auspices of the Snow, Ice and Permafrost Research Establishment, Corps of Engineers. We are concerned here with the practical aspects of avalanche mechanics in the field; in other words, how they start and how they behave while in motion. The avalanche is not a mysterious force which erupts spontaneously and without rhyme or reason. On the contrary, avalanches obey natural laws. If this were not the case it would be impossible to operate ski areas of concentrated use in the alpine zones with any acceptable margin of safety. Snow is a plastic substance which is continually being pulled downhill by the force of gravity. When gravity overcomes the cohesion of the snow, a fracture occurs and this is usually followed by a slide. Basically then, the stability of snow on a slope is determined by a simple relationship: cohesion opposed by the force of gravity. The force of gravity can be calculated as follows: If G is the weight of the snow and X is the angle of the slope, then the force of gravity, T, equals: G sin X. Further, if Bs represents the cohesion of the snow then, S, stability, equals Bs, or Bs_ If S is 1 or less, there should be an T G sin X. avalanche, theoretically. The elusive element in this formula is Bs, the cohesion. In any snowpack, two types of cohesion are involved: internal and external. Internal cohesion is the manner in which the particles cling to each other within the layer. It is strong, for instance, in crystalline snow due to the interlocking of the branches; weak in granular and pellet forms which have fewer points of contact. Internal cohesion drops when sublimation and thawing transform the particles into simpler forms with fewer projections. The end product of this process is the cup crystal found in old snow layers, which has little cohesion. At the same time that metamorphosis is reducing cohesion, settling increases it by packing the particles closer together. Normally this process is more rapid than loss of cohesion through metamorphosis and each snow layer becomes progressively more stable with time. If every snowpack were homogeneous, with the same cohesion throughout, it would not be difficult to calculate the avalanche hazard. Unfortunately snowpacks are not homogeneous. Cohesion does vary, and markedly, from layer to layer. Different types of show have different reactions to metamorphosis and settlement. The calculation of Bs, therefore, becomes a very complex problem depending upon variables which are difficult to observe. - 42 - |