OCR Text |
Show direction and run out distance of an avalanche in the area of deposit is determined by the form, direction and slope of the lower area of the track. According to their type and by the corresponding shape of the track, avalanches can be thrown out of their path and can follow totally different directions. In one instance, it could be observed that the advanced fronts of ground avalanches and dust avalanches ( a snow- air mixture) were deposited 400m apart, even though they released from the same zone. The different dynamics of two types of avalanches result in a much larger area of deposition than is observed with only one type of avalanche. The possibility of building on alluvial fans below the mouth of gullies must often be considered. Avalanches are channeled in the gullies and enter, at the beginning of the outrun, a relatively flat and wide terrain. Retardation often begins at the gully mouth. The outrun distance of large avalanches is long, despite the small inclination of the slope. At inclinations of 10% to 15%, outrun distances of 500m to 1000m are observed. On a steep and irregular track, dry snow avalanches develop into powder avalanches of respectable size. The resulting impact can lead to destruction at places which cannot be reached by the sliding snow. In narrow valleys, the air blast can reach far up the opposite slopes. As a result of unusual weather conditions, avalanches can occur at places where it was never considered possible. A combination of low temperature and intense snowfall can lead to an unusually loose snow layer that can even release in forested areas. Mountain forests in general and especially larch forests show thin open stands which are not able to prevent the starting of avalanches. The avalanches from January 10- 12, 1954, which especially affected the Grosse Walsertal in Austria are a case in point. Schilcher said following the event: " From the southern and partially from the northern valley slopes of the Grosse Walsertal, loose snow and dust avalanches of all sizes spread over 50% of the total area. The release areas existed along the length of the valley and were scattered in a southwesterly direction at any height between 800m and 1850m and were located in open country as well as in forests." Such possibilities must be strongly considered, since sunny ridges located under steep mountain forests make ideal locations for summer houses. Another local source of danger is gliding snow. This is the slow and not often noticeable movement of the snowpack along the ground. It is observed that these phenomena occur most often on steep mowing meadows in the snowy foothills of the Alps. However, more gentle snow slopes are also subject to dangerous movement if they are water saturated. Early and heavy snowfall results in pronounced " glide winters." Recent examples are the winters of 1952/ 53 and 1965/ 66, in which many buildings were damaged. The return interval of individual avalanches is the source of most difficulties for a sensible approach to an avalanche zone designation. We mean avalanches that are infrequent and extreme. It is these avalanches that are most dangerous to inhabited terrain, since they generally sweep large areas. In this respect, peculiar opinions have been expressed. For example, an |