Snow Stability Index

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Abstract
Field tests have been developed to measure the shear and tensile strengths of large volumes of snow. Basal shear strengths were measured across and down some slabs of snow, giving highly variable strengths. These measurements support the idea that the basal region of an avalanche may contain zones where the basal shear strength exceeded the gravitational shear stress (i.e. pinning areas) with weak zones between (deficit areas) where the shear strength was less than the gravitational shear stress. The slab tensile stresses induced by these deficit areas would become high if either the deficit length (down-slope) was large, or the deficit itself was large. Measurements of tensile strengths of slabs above weak layers, together with the down-slope gravitational stress of a snow slab, suggest that deficit lengths of only several metres are often sufficient to cause a local tensile failure. In some cases, this local failure may propagate across the remainder of the slope (depending on the pinning distribution) and cause an avalanche. We propose that themaximumlocal deficit, rather than the mean slope deficit of basal shear stress, and themaximumlength of the local deficit, are the first important parameters to consider when evaluating slope stability in the field, since the magnitude of these factors determine the probability of a local tensile failure.

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