OCR Text |
Show falls of rock and ice which will start avalanches on the slopes below, And if snowfields lie above, the descent over a cliff will naturally give a slide a maximum boost in velocity. Cirques are ideal accumulation zones for wind-driven snow. If their exposure is favorable they are natural reflectors where sun action will be far above general temperatures thus leading to the release of sun slides. Gullies collect and channelize snow descending from above. By increasing the friction, they retard a slide. But if the slide is big and fast enough to overcome this effect, it will issue from the gully with explosive force. Every competent ski mountaineer knows that a gully is the poorest possible place to travel. Ridges lying parallel to the slide path, on the other hand, are relatively secure and generally offer much better traveling conditions. i * Terraces, talus slopes, and basins, because of their easier grade are effective avalanche barriers. They give the moving snow room to spread out and lose its momentum. But as the winter progresses snowfall, wind action, and successive slides gradually obliterate them. Ground surface conditions have considerable effect beyond the snow in contact. A broken serrated terrain, boulder-strewn and brushy, will provide a good anchorage for the snowpack as a whole. Slides breaking off at ground level are unlikely. The description is typical of Alta where ground level slides seldom occur. Smooth, even slopes, grass-covered or of bare earth and rock, favor the violent and destructive ground level avalanche typical of the Alps. Vegetation of any kind except grass has a restraining effect on avalanches. Tree cover is the most important. Alta was quite heavily forested when the miners arrived. As they removed the trees for fuel and mine timbers, avalanches became proportionately worse. Under national forest administration natural reforestation has made good progress with noticeable effect on the slide hazard. It is interesting to note that the Swiss avalanche researchers consider reforestation to be one of the most important parts of the avalanche hazard control program. Recent studies carried out in Switzerland indicate that trees or any rounded obstruction are a more effective avalanche barrier than a solid wall. It is a mistake, however, to regard a forested area as safe. Two of the major slides of the winter' of 19^7^2, Coal Pit and Argenta, originated in locations with fairly good timber cover. The wild snow powder flowed down among the trees like sand or water without damage to them or noticeable . hindrance to the avalanches in their initial stages. Nevertheless, forest cover is highly beneficial. It cannot prevent avalanches under extreme conditions, but it is an effective barrier. It helps indirectly to lower avalanche hazard by promoting the growth of other vegetation, by breaking up the even flow of wind and thus the formation of snowslab, and by providing relatively safe routes for ski travel. -10- I * |