| OCR Text |
Show decrease in strength ( cohesion) thus caused can be enough to start the avalanche without the additional load represented by drifted snow. Also wind pressure on the snowcover, especially the pressure of strong gusty winds, can upset its equilibrium. Paulke mentions that in the Alps avalanches often occur on slopes covered with high grass. His explanation is that such grass provides a good slide ( slippage) surface. It is far more likely that the avalanches cited are caused by the looseness and consequent structural weakness of snow covering high grass. As it moves, an entire avalanching snowcover does not slide over a grassy surface, but rather over one of icy snow which forms on top of the grass. By cutting or burning off grass cover, the danger of avalanches due to this cause can be lessened. There is no basis for the idea that coarse grains of underlying snow act as roller bearings for the snow masses on top of them. These grains are so fragile that they are immediately crushed and compressed to form the slick icy crust covering the ground. The same sort of ice crust forms on the lower surface of the avalanching snow blocks. No real evidence of snow grains acting as roller bearings has yet come to light. Now and then " lump snow" avalanches are observed in which a relatively thin upper layer of dense snow slides over a layer of loose, fine granular snow produced, most likely, by the sudden collapse or cave- in of such fine granular snow. Mostly, however, the collapse of solid snow occurs as a result of loading, as after a snowfall. Fragments of solid snow, " snow planks", float in the loose avalanche snow which is not compressed into chunks and blocks even in large-scale slides. This is why the terminal deposit can take the form of a flattened cone with " pour- over," like the dump of avalanches of dense snow. C. Avalanches Caused by Melting. 5) Moist snow: During heavy thaws, moist loose snow lets go and avalanches in whole sections which then break up and continue down as a stream of partially knurled snowballs. Moist snow avalanches break- away in a step. Their dumps take the form of a lumpy cone with " pour- over", the avalanche material thrown ahead of the dump over untouched snow as the avalanche dies. The avalanche material consists of dense balls or masses measuring up to 0.5 m and capable of supporting a man's weight. Despite their small volume, a few hundred m3, moist snow avalanches are destructive; they break off trees and leave a deposit of dense, heavy snow difficult to remove. Such avalanches are infrequent in our area, being observed only four times. They always originate in gullies, occurring with increases in air temperature to 3° C. 6) " Knurled snow": On clear spring days, with light freezing, balls or chunks of partly melted snow drop from sun- heated rocks and are knurled by rolling thin layers of snow on themselves. Streams of these knurled balls flow down steep slopes leaving a track. The fragments broken off these balls knurl themselves in turn, and the total volume of knurled balls increases rapidly. Within a few meters of the point of impact, i. e., of the first snow mass to drop, tens of them fan out over the slope and cascade over precipices, etc., but stop at still - 49- |
