| OCR Text |
Show millions of hectares. After deducting unproductive surfaces and the forested area there are still 664' 280 hectares for which zoning plans must be prepared. The plans elaborated up to now comprehend a surface of 29' 480 hectares or 4% of the total. The rest namely 96% have to be treated yet. I I. Concept and Description of the Avalanche Zone An avalanche zone can generally be subdivided into three sections: a starting zone, a track and a runout- damage zone. Since the building authorities need to know only the conditions of the inhabited areas and potential construction sites, which are mostly located in the runout zone, the starting zone and the track are ordinarily not included in zone planning. However, workers compiling the avalanche zoning map must also judge the starting zone and the track. It is from the characteristics of these areas that the size, runout distance and frequency of expected slides can be estimated. If the starting zone is uniform, chances are that the slide is released over the full extent of the zone, which results in a large avalanche. If it is separated by rock outcroppings, forest patches and ridges, small slides will frequently release, but the resulting avalanches will be small and harmless. Slopes with an inclination of 60% or more can generally be considered avalanche starting zones; however, a slide will break loose only under certain weather conditions. The most important factor which is necessary to release an avalanche is heavy snowfall. A large amount of snowfall will result in new snow avalanches that can be dangerous for inhabited areas. Under conditions of heavy snowfall, the rapidly increasing new snow layer cannot settle and stabilize enough to bond with the old layer of snow or the ground below it, so that after a given amount of time the new snow layer will slide off as an avalanche. Steep slopes with approximately 30cm of new snowfall represent a local slide danger. If the snowfall accumulates to about 50cm, the expected avalanches will be larger, but if it accumulates to about 70cm, avalanches release in areas that seldom experience them. An important factor in judging avalanche areas are slopes with an inclination of between 55% and 60%. Such slopes do not release immediately under heavy snowfal1, a 1 lowing for exceptionally large avalanches, as was demonstrated in the winter of 1950/ 51. Along with the new snow avalanches which usually occur in mid- winter and which are composed of dry and light snow, spring and ground avalanches should be mentioned. Their release is effected by warm and clear weather and by strong solar radiation or by the " Fohn", all factors which cause the entire meltwater- saturated snow layer to slide along the ground. The wet and heavy snow moves relatively slow and generally follows a well- known slide path. Ground avalanches are especially important to transportation and communication links, such as railroads and highways. On slopes and in gullies with an inclination of about 30%, avalanches will not ordinarily stop in their downward movement. Slopes with such angles must still be counted as part of the track. The average inclination for most avalanche paths that extend for a long distance is between 40% and 70%. The |
