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Show III. ON THE DYNAMICS OF AVALANCHES. The following explanations are collected as an approximate survey of the area to be investigated, yet they are in good agreement with the present observations. It is necessary, but not sufficient, for the formation of avalanches that a part of the sliding layer, as a result of gradient changes, damming effects, or sliding surfaces running diagonally to the movement, be pushed over the stable snow layer and, with or without the underlying snow, attain a considerable velocity. Frequently avalanches are caused by cornices, snow drifts, and snow accumulations, for example at the foot of steep slopes or bands of rock. An essential fundamental for each avalanche investigation is knowledge of the following quantities: YQ: average density of the naturally deposited snow in the sliding layer, see Table 2 for numerical values. Y : average density of the flowing snow h : vertically measured, total depth of the naturally deposited snow, or the depth of the sliding layer ty : gradient in the release zone of the avalanche \| ru: gradient in the effects zone If possible: A longitudinal profile of the avalanche path. TABLE 2 Density of Snow Loose new snow 30 to 100 kg/ m Felted or wind packed new snow 50 " 300 Dry, granular, settled old snow 200 " 450 Wet, settled old snow 400 " 600 '' Firn snow 500 " 800 1. The Phenomenon of Flow of Snow The first movement or the breaking away of the snow is induced by a slow creeping or by the tensions caused thereby. The laminar flow exists, according to its definition, as long as the upper surface layer of the snow remains intact. This can be the case up to significant distances in slides of wet snow in which the intact snow layer is compressed into waves ( Schneetuch, blanket of snow). However in other instances the laminar flow is confined to the initial stage of avalanche formation ( Figures 31 and 32). 17 |