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Show while the house standing directly behind, somewhat set back and not extending into the avalanche path, remained completely unscathed. For small values of Ah Equation ( I*) applies: ( 1*) w ~ [ g h] 1/ 2 "" 9 for example for h = 2m, and g= 10 m/ sec f w~' 4.5 m/ sec, i. e., as a rule the flow of avalanches is shooting. The velocity of fanning out across the direction of flow of the avalanche is likewise determined, if the terrain is plane in this direction, by the momentum principle: If Ah is the flow height of an avalanche ( Boden-lawine)* over the natural snow layer of height, h, equation ( 1) likewise applies to the transverse propagation velocity. However, if it is a surface avalanche ( Oberlawine) of flow height, h\ oh the natural snow layer, then the mass overflowing across the side boundary per unit of time is y/ g . h' w and the equation of motion shows that Y h' 2 = Y h' w • w " 2 g from which ( 2) w= [( g hf) / 2 ] 1 / 2 The material moved transversely is replaced from the mass of snow flowing in the longitudinal direction. In terrain plane perpendicular to the direction of flow, the velocity of avalanches can be deduced from their shape of spreading with a knowledge of the flow height. For instance, this would be possible by photogrammetric interpretation of Figure 21. Here the widening of the avalanches is significant at first, diminishes with increasing velocity, and increases again toward the deposition cone. As soon as the flow of velocity of avalanches subsides to a magnitude similar to the velocity of lateral spreading, even relatively unimportant irregularities in the terrain can cause a division of the avalanche cone into different tongues ( Figure 18). When the replacement of material in the longitudinal direction is no longer sufficient to maintain the velocity of lateral expansions ( Equations 1 and 2), a flattening occurs on the edges which causes a further reduction in the velocity there; this phenomenon of dying out produces the familiar tongue-shaped snow deposits of avalanches. * Translator's Note: Bodenlawine - an avalanche to a resistant underlying layer, motion primarily on the ground. 19 |