OCR Text |
Show been observed frequently, for example this has caused light structures to be pushed over ahead of the avalanche front. In many cases the effect of a very lightly loaded powder avalanche of great velocity preceding the main part of the avalanche is erroneously designated as air blast. For instance the opinion has been expressed that the victims of the 1951 Gastein avalanche were killed by the air blast, because the lung lesions typical of it were established by medical findings. The investigation of explosion catastrophies carried out earlier by the author has shown, however, that these phenomena first appear as a result of air blasts of about three atmospheres over pressure, which, as pointed out above, are never reached from the air blast of an avalanche. On the other hand this over pressure is in agreement with the highest damming pressure of powder avalanches. As a result of damming, the air escaping from the snow- air mixture and penetrating all openings reaches the same pressure as the damming pressure of the avalanche itself. The observation on suction effects of powder avalanches are mostly explainable by eddy effects. Probably the avalanche entrains air; however, from investigations of the turbulent mixing process it is concluded that the velocity of the air streaming in from the side can hardly reach 1/ 20 of the avalanche velocity. Significant suction formed behind the avalanche would automatically cause an aerodynamic match of the shape of the avalanche tail and the suction would in this way be further reduced. " The material is tricky" reads a motto handed down from Prof. W. Ritter. On the other hand suction effects are possible behind small obstacles completely overrun by powder avalanches of great velocity, which can reach at most ( 13a) p (-) = ( YL/ 2g) w2 £ w 2/ 16 Also portals of tunnels or galleries, or other openings in structures, can be overrun by a powder avalanche in such a way that they act similar to an injector [ n] . As a result of successive disintegration and aereation of the snow, and the resulting increase in height and decrease in the relative resistance to flow, powder avalanches reach astonishingly high velocities ( up to about 100 m/ sec). Their effect generally reaches the entire height of structures. If the powder avalanche occurs in a gully or ravine and is deflected, the upper part of the powder avalanche can flow further in the original direction. Powder avalanches frequently divide ( Figure 20). As a result of the large velocity powder avalanches or parts of them frequently leave the ground at breaks in the terrain and can shoot through the air in a free jet. Equation ( 10) shows in combination with Equation ( 9) that when a powder avalanche strikes level terrain ( ty -* 0), y can become very small and h' very large. The powder avalanche loses specific mass and velocity after dissipation of its kinetic energy, disintegrates gradually to clouds of snow, and finally the snow settles again. Until this occurs, however, powder avalanches 31 |