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Show On the back side of a small obstacle with flow around all sides a suction up to the magnitude Y r- v^/ 2g <' 1/ 10 atmosphere can occur. The suction behind obstacles around which flow occurs is usually small. The avalanche misfortune at Goppenstein in 1908 also indicated this: of 30 persons sitting together in a very lightly constructed wooden building, the 18 survived who turned their backs toward the powder avalanche which struck unexpectedly, while the 12 casualties of the avalanche had turned their faces toward it and had suffocated [ 3, No. 2] . The upward pressure and thrust effects are a result of the snow movements accompanying damming; they can take effect vertically as well as in random side directions. Immediately upon impact the snow is deflected almost without loss of velocity, considering the short path length; friction likewise has only slight influence on the movement. According to Equation ( 24) wide avalanches at large obstacles produce the vertical damming height H times Y m/ Y max ( measured from the underlying surface). According to Bernoulli the vertical velocity at the height h* over the underlying surface reaches: ( 28) u = ( 2g ( H - h*)) 1 / 2 Continuity is maintained by the rolling back of the impounded snow, as is also observed in a stream of water. The specific upward pressure on projecting surfaces ( balconies, window lintels, and the like ) amounts to: ( 2 9 ) Pv = Y max u / 29 in which v is determined from Equation ( 22) for the impact pressure p ' max n resulting when the avalanche strikes. In addition even on moderately smooth wall surfaces frictional forces directed vertically or inclined upwards arise: per square meter, R = pU, in which li = Y / 1000 to Y / 2000. max max As has been shown in the first part of the present report, the vertical forces occurring on the impact of avalanches cause a good part of the severe destructive effect of avalanches since no buildings constructed in the usual manner are able to cope with such lifting forces. A constant or fluctuating deflection of the snow from its direction of flow by the angle 6 causes a total force P acting along the angle bisector on the convex side: ( 30) P = 2 Y ' h' ( v2/ g + h'/ 2) sin( 6/ 2) where h' denotes the flow height of the avalanche. This total pressure is 44 |