北アルプス黒部峡谷の雪崩

雪崩の発生頻度とそのエネルギー解析

抄録

The energy of the Kurobe avalanche is about 10⁸ watt in its momentary maximum while that of a typhoon visiting Japan is about 10¹⁶-10¹⁸. Therefore the former is very small compared with the latter. But the two energies differ in this that in the case of typhoon its energy is dispersed in a wide scope extending over 500-1, 000 kilometers in radius while in avalanche its energy is concentrated in a very narrow extent 30-50 meters in width and 200-500 meters in length. In the typhoon it scarecely happens that even in its momentary maximum wind speed goes beyond 70 meters per second while in the avalanche, as is shown in Table 1, it is quite simple that its blast passes the speed of sound (331m/sec. 0°C). This accounts for the destructive force of an avalanche as stated in (2 ₍₁₎ ) - (2 ₍₃₎ ) above.
A snow slide has hitherto been considered as somehow or other a natural fall of a snow mass. It goes without saying that an avalanche, if viewed generally, is caused by conversion of potential energy into kinetic energy and it is quite to say so. But the energy of avalanche cannot be fully analysed by such ideological explanation. An avalanche is not a simple falling of a snow mass by gravitation, but it is rather a flow of a fluid, a mixture of snow and ice. Accordingly it shows a meandering movement resembling that of flowing water and unlike a solid body it can pass freely through narrow openings. However, it must be considered that it is not a mere gravitational movement of a fluid, but that the air contained in the snow suffers, in the process of sliding down, adiabatic compression, which greatly increases its internal pressure to give driving impetus to the avalanche.
It must therefore be understood that an avalanche can cross the main stream 100 meters wide, instead of flowing into it, and rebounds at the foot of the mountain on the opposite bank comes from this that when the head of the avalanche reduces its speed the air contained in the snow that follows after, compressed adiabatically, increases its internal pressure to give driving force to the snow ahead. It has hitherto been argued that an avalanche jumping over a hill is the result of its first burying this side with snow and running over it. But no ! A big avalanche can skip over a hill.