OCR Text |
Show only in exceptional cases does a slide descend over bare terrain. The optimum surface for slide movement is an ice crust, however insignificant its thickness. In the early part of the winter 1938- 39, we observed avalanches of loose windblown snow which had slid over 2 mm ice crusts, which in turn overlay loose snow. The ice crust often remained intact after the slide. Even light icing over of the snowcover creates favorable slide conditions. With the particularly destructive avalanches of December 5, 1935, the loose windblown snow deposited the day before on snowcover sheeted with ice by a thaw all came down. It should be noted, however, that an ice crust right next the ground, and conforming to uneven relief, is dangerous ( conducive to slides) only on even snow slopes, whereas on broken slopes the avalanche has to make way by leveling protrusions and heights and filling depressions. Where the relief is quite uniform, the snow is loose, and slide movement is not too fast, an avalanche produces little effect on the sub- slide snow. In an artificially induced avalanche on Mount Apatit, on February 12, 1936, chunks of dense snow up to 25 m3 in volume and weighing over 10 tons each slid down over a 20 cm layer of loose snow of density 0.15 and compressed it to a thickness of only 8- 10 cm. On April 22, 1936, on the northwest slope of Mount Ukspor, an avalanche of wet granular snow of density near 0.60 descended over a i m layer of the same type snow of density 0.49 without compressing it, just taking off the top 2- 3 cm. A 1000 m3 avalanche of loose windblown snow descended Mount Ukspor on December 19, 1933 in a narrow stream only 4 m wide, passing over loose snowcover 120- 140 cm in thickness. After the avalanche, the sub- slide snow was found compressed to a thickness of 80- 100 cm and a density of 0.22. Where the sub- slide snow consists of several layers not too firmly bound together, the avalanche may take one or more of them with it, thus forming steps similar to those left behind on break- away. Each of these layers is usually torn away separately, forming stairs of one- layer steps; less often several layers are torn away together, forming stairs of multilayered steps. Clearly, such sub- slide stairs are observed more often with avalanches of heavy snow than with those of loose snow. The less the snow density, the faster the avalanche movement. Avalanches of wet, oversaturated snow are an exception, however; these may also travel with great speed. Large avalanches move faster than small ones of the same type. However, the speed of a small avalanche of loose snow is greater than that of a large avalanche of heavy snow. On the other hand, avalanching loose snow decreases in speed sooner than heavy snow and, where volume is not too great, stops on a degree of slope that would keep a larger slide moving. The primary slide of an avalanche is straight down the steepest portion of a slope; but as speed increases, the avalanche can maintain within wide limits its initial heading, even where this does not coincide with the grade. On its primary plunge the avalanche passes over protrusions and hollows several meters wide without filling these latter. Meeting obstacles like a mountain stream, the descending snow passes over or divides around them in separate streams which later reunite, giving a " dead" or stopped avalanche the appearance of a turbulent river instantaneously frozen. The heavier the snow, the greater is the tendency of the avalanche movement to comply with grade and relief features. Even so, - 35- |