OCR Text |
Show over 30° f i. e., after reaching a velocity of about 20 m/ sec, the surface avalanche can, in dry cold weather, finally, partially or completely, take the form of a powder avalanche and in this way reach significantly higher velocities . Mixed forms of surface and powder avalanches are frequent, their effects are evaluated by investigation of the borderline cases. B. Powder Avalanches Avalanches, which have originated through the sliding of loose snow, promote the whirling up of snow and, with appropriate conditions of the same, the formation of powder avalanches, especially in dry cold weather. Also the naturally deposited snow layer can be completely or partially carried along in the turbulent flow by the avalanche traveling over it. Whirling up of loose snow occurs as soon as the vertical component of the turbulent velocity fluctuation becomes greater than the velocity of fall of the snow particle, which increases with particle size. From materials handling technology it is known that solid particles can be entrained in a horizontal air stream if its velocity is greater than about twice the free- fall velocity. of the particles. Since the velocity of fall of snow particles reaches only 1 or 2 m/ sec, the whirling up of dry fine powder snow must certainly begin after a very short running time of the avalanche, while recrystallized depth hoar requires a longer running time for this. The greater the flow velocity the sooner the formation of the turbulent layer and the mixing by air flow results. Fig. 23. Velocity measurements on powder avalanches as well as surface and ground avalanches, by Dr. M. Oeschlin, Canton Forester of Uri, Altdorf. Ordinate - Avalanche velocity m/ sec Abcissa - Slope inclination f Staublawinen - Powder avalanches Oberlawinen - Surface avalanches Grundlawinen - MGround avalanches Minimalwerte fur Grundlawinen - minimum value for ground avalanches 110 100 j , 30 ^ so • 6 | eo c 5 50 so krn/ h - wo + S -. o x 6 - 300 - 200 - 100 - ' Mc X , faublaw berlawi rundlah inert nen inert • Fa X * I Ic X -^ Ib +/ + 3| + 13 7 2 + + « + 5 t + • 2 • 4 • 2x 1* V. JY I fur > 6 ( lb la X GrundU v- k- si i ,.. alwerte winen 10° 20° 30° 40° 50° 60^ Hangneigung y 26 |