| OCR Text |
Show agrees with existing observations better than with the complicated results of the flow theory. The total resistance of a prism of square horizontal projection is not changed by rotation about its vertical axis. The total pressure on a circular cylinder of radius r is: ( 27') P= ( y rv2/ 2g) J sin2p = ( it / 2) ( r y v2/ 2g) If an avalanche of width B flows around an obstacle of width b, or if barriers of total width b stand in front of a structure of width B, a loss of energy is brought about by the obstacle or the barriers. According to the momentum equation per unit of time it amounts to: P = ( m/ dt) ( v - v2) = bh y v2/ 2g = Bh ( Y /^ v - v The loss in velocity is A v = v . b/ 2B If a third of the width of a structure is protected by barriers, accordingly only about 70% of the pressure bears on the structure. The reduction of the range of avalanches as a result of installation of obstacles can be calculated from Equations ( 3), ( 5), and ( 23). In and near the damming zone of avalanches small obstacles often indeed suffice to offset the thrust force of avalanches behind themselves, as for example the locomotive in front of the Dalaas rail station ( see the first part of this report). Figure 29 shows how a loose snow avalanche, which fell through a forested slope without damaging the timber, lost so much energy in demolishing a brick hut that the wooden fence behind the hut remained intact, while it was destroyed on both sides thereof. The width of the intact zone is explainable by the surfaces of discontinuity spreading, with shooting flow, behind the obstacle. In the avalanche catastrophe of 1951 in Andermatt the Hotel Danioth was only slightly damaged because the ground avalanche had lost most of its energy through the prior destruction of two houses. sM Fig. 29. Dissipation of energy of a loose snow avalanche which has fallen thru a forested slope, by the destruction of a masonry hut, behind which the wooden fence remains intact. 43 |
