| OCR Text |
Show critical snow bed does not cover too great an area. The greater the break- away area, the less the bond with the snow on the periphery, the bond with the surrounding snow being proportional to the increase in perimeter and the linkage with the underlying surface being proportional to the increase in area. This is why the forces holding a solid snow layer on a given area cannot hold the ( same) snow on a larger area under like conditions. In other words, the larger the critical area, the greater the possibility of avalanches. As a matter of fact, on even slopes with uniform snowcover, avalanches occur only where the critical area is large and they move at once down the whole slope, and not just a part of it. Moreover, the longer the slide slope, the less the grade needed to produce avalanching. Avalanches of solid snow come rarely from gullies as narrow as 10- 15 m, and then only from the largest part of the mouth. Loose snow bound with the same type of snow on a relatively small periphery can avalanche from a small area, explaining why avalanches of loose snow may be quite small. Avalanche break- away is sometimes attributed to overload of a slope with an increasing snow burden. If this were completely so, the greatest number of avalanches would occur in April, when snowcover reaches its maximum thickness. Observation shows, however, that the main avalanche period is from December to February, inclusive. The lack of a clear- cut correlation between snow height and avalanche occurrence is confirmed by snow soundings at avalanche sites. For example, there was avalanching from a gully on the south branch of Mount Aykuayventchorr during the first part of the winter of 1938- 39, when snowcover was 0.9- 2.5 m high. During the second part of the same winter, with an accumulated snowcover of 5 m, not a single avalanche occurred. Moreover, an increase in snowcover during a snow storm does not result in immediate avalanching; only after 24- 48 hours does sliding occur. This shows that snow load in itself is not always the cause of avalanching. Snow will detach and slip downward when the motive component of gravity exceeds the cohesive force of the snow. An avalanche may be caused by: increase in snowcover to a critical point, dynamic load, mechanical rupture of the bond with the surrounding snow, decrease in snow stability ( solidity) due to internal changes, change of condition in the underlying surface, and internal stresses. It should be emphasized that none of the above causes is isolated; an avalanche will usually be due to several causes, only one of which predominates. Quite a casual occurrence may be the direct impetus for an avalanche, for example, footsteps, the vibration from a passing train, even, as people say, a shout or shot. In the Chibin area the deposition of snow by a windstorm can provide this impetus; here nearly all avalanches of dry snow, except those caused by snowfall during still weather, occur during a windstorm or a short while later. Although windstorms are frequent, nearly every avalanche can be directly attributed to a storm of a few days before. Where avalanching takes place during a snowfall or windstorm, the increase in slope load can be clearly seen as the cause of avalanching of freshly fallen or loose windblown snow. In gullies where snow accumulation is an important factor, two and even three avalanches will occur with every windstorm. Snow thickness is of significance in the stability of bound snow only - 42- |
