| OCR Text |
Show insofar as it influences snow weight. But in loose snow, where internal friction is minimal, snow thickness is an index of slope load. A thin layer of such ( loose) snow does not give way by crumbling; but when cover thickness exceeds certain limits, avalanching results. The greater the snowcover thickness, the less the slope grade required to start an avalanche. Slides of freshly fallen snow 10- 15 cm in thickness take place on grades of 50- 60° ; slides of snow 40 cm in thickness, on 50- 35° slopes. Dynamic load may be the impact of a falling snow cornice or rock, or, quite possibly, a particularly strong wind gust. All too great an importance is attached to collapsing cornices as a cause of avalanches. Such collapses are rarely observed, and then mostly during thaws. Even with thawing, cornices melt away as a whole, in place, and their outlines may be seen intact high on the mountains right through to summer, after the snow on the lower slopes has long since disappeared. The progressive drawing along by avalanches of deeper and deeper layers of sub- slide snow is also explained in terms of dynamic load. The clean, sharp breaks in the steps left behind by descending snow attest to the great speed of an avalanche. If the contrary were true, such steps would be abraded smooth or filled in with snow. The most frequent agency to break the bond in a bed of critical snow is another avalanche which destroys the snowcover support. Where several secondary gullies come together in one, an avalanche from the main gully will set off avalanching from the laterals. Thus, on the southwest slope of Mount Ukspor, avalanching from a main gully ( fourth grade) is often accompanied by slides from two or more laterals discharging into the trunk. An avalanche of March 6, 1938, on the east side of Mount Vudyavrchorr, moved down the bottom of a long deep gully and, by cutting away the snow on its sides, produced a second avalanche which covered the terminal deposit of the first. The composite volume was 75,000 m3, of which the second avalanche constituted one third. The passage of a skier or hiker across a critical slope may also break snow cohesion. On February 8, 1937, a hiker crossing a gully on the southwest slope of Mount Ukspor set off a 500 m3 avalanche. Here the loose windblown snow of density 0.16 broke away along the hiker's tracks and descended as a layer nearly 1 m in thickness. The author of this avalanche, who had already reached the other side of the gully, saved himself by hanging in a tree. The internal changes taking place in snow, scarcely evident externally, are the main cause of many avalanches. The decrease in snow stability during recrystallization and melting may be such that the cohesive force of the snow becomes insufficient to hold the snow on the slope, whereupon the snow, stable until such moment, simply lets go without any external impulse. It has been noted above that avalanches of windblown snow do not occur during or immediately following the windstorm, but a day or two later, or even later. The same is true of avalanching during still weather. The main avalanche period will extend over a month, although increase in snowcover for this period is not great and there is thus no overload of the slope. All this shows that the avalanches in these cases are due to changes internal to the snow and not to external factors. This consideration will be discussed in more detail below. In certain cases avalanches are caused by a change in condition of the - 43- |
