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Show can still cause damage over large distances even on opposing slopes; not infrequently debris, trees, stones, and the like are observed thrown far uphill on the opposing slope ( for example avalanches at Gastein, St. Antonien, and Monstein, all in 1951). C. Avalanches to a Resistant Underlying Layer ( Bodenlawinen) When the snow is so compacted that no significant dust cloud occurs during movement, or in damp and wet snow, whose particles are held together by the surface tension of the water film between them, a densification of the snow occurs caused by the damming pressure of movement instead of a reduction in the average density due to disintegration and suspension accompanying movement. The sliding snow layer, on the average becoming heavier, during turbulent descent works itself into the underlying natural snow layer until finally a ground avalanche ( Grundlawine) is produced, which passes through the snow cover right to the ground like a snow plow and not infrequently carries along scree, trees and surface layers of earth, especially if the latter are soaked by thawing weather. The darker color of the ground avalanche ( Grundlawine) frequently observed ( Figure 17) arises from the mixing of the snow with scree, sod and pieces of soil. Ahead of the avalanche front the existing snow cover surges upward from a compression shock. If h' designates the front height of the avalanche, the average density of the natural snow cover surging upward immediately ahead is Y y - Y oh/ h'. In the compression wave preceding the avalanche the static pressure of the natural snow cover climbs to the damming pressure, this causes a rolling back of the avalanche front, whose velocity is reduced to the value given by Equation ( 12) if the value y is inserted in this equation in place of Y L. On the other hand the continuity of mass flow must be taken into consideration: if a longitudinal strip from the avalanche one meter wide is considered, then the mass Y hv flows from the natural snow cover in one second up to a moving reference plane whose velocity is v, that of the avalanche snow, while the mass Y n'v' flows away f r o m t n e reference plane. The average decrease in velocity behind the avalanche front is therefore: v' = ( Y oh/ Y h' > v and thus the effective thrust velocity of the avalanche front is: ( 15) w = v - v' = v[ 1 - Y on/ Yh'] The energy equation ( 12) ( with Y L ~ Yy) and the continuity equation ( 15) are satisfied simultaneously by natural adjustment of the flow height h': 1/ 2 , w= v/( l + ( Yo"/ Yh') = v( l - Yoh/ Yh') from which it follows that: 32 |