OCR Text |
Show area are too insignificant to be able to bring large stones valleyward. Thus, rocks lying upon grass, or overturned with mossy side down, are unmistakable signs of avalanching. Locations safe from avalanches at the beginning of winter may be dangerous toward the end because of the smoothing over of uneven relief features by the accumulated snowcover. Sites are known where 1000 m3 avalanches fail to reach the ( RR) right of way in November- December, where in February- March slides only 300 m3 in volume cover the tracks. An important factor in delimiting avalanche danger zones is the " angle of avalanche friction" or the angle between the horizon and the upper edge of the break- away, measured from the forefront of the avalanche dump. The heavier the snow and the thicker the critical snow bed or layer, the smaller the angle needed for avalanche break- away, depending on avalanche volume and type. The " friction angles" for several score avalanches observed, predominantly of medium size, varied between 14° and 28°. The largest avalanche possible at a given site can be calculated on the basis of the snow collection area. It should be noted that the " angle of avalanche friction" is not the same as the friction angle for snow. As observations on artificial avalanches show, the coefficient of friction for snow is not constant, changing with avalanche movement. Methods of Defense Against Snow Avalanches Many projects have been put forward for protection against avalanches in the Kirovsk area. Several of these protective measures have already been put into practice, and the data gained from our observations permit some generalizations regarding relative merits and drawbacks. To prevent accumulation of snow at critical sites would seem to be the surest way of avoiding avalanches, but this poses problems of great technical difficulty. Afforestation of the higher mountainsides to prevent the descent of snow to lower critical sites is impossible because these upper slopes are generally well above timberline. In 1936 an attempt was made to block the transfer of snow to critical sites down slope by setting up several rows of obstructive snow fences, like those used along railroad tracks, on the plateau of Mount Ukspor. The great amount of snow retained by these fences probably decreased the number and volume of avalanches, but did not eliminate them entirely, being incapable of preventing snow transfer from the remainder of the plateau and from the slopes. Thousands of snow fences would have to be set up to give really adequate protection here, a measure most difficult to carry out. Moreover, such snow fences are really no guarantee against heavy drifting during windstorms. In certain rare cases, however, a relatively few snow- retaining fences or barriers can produce positive results. At Kilometer 20 on the ( Kirovsk) branch line, on the steep slope of a moraine, we observed that collapsing snow cornices had been setting off avalanches that threatened the railroad track. To cope with this, a line of retainer fences was set up, after which the cornice formation ceased. Where there is no possibility of avoiding snow accumulation, the next most logical step is to try to prevent avalanching itself. An avalanche already - 60- |