OCR Text |
Show Part II Technique CHAPTER 3. BASIC CAUSES OF AVALANCHES There are only two fundamental causes of avalanches: terrain and climate; in everyday terms, plenty of snow and a mountain for it to slide on. These two, acting together, can produce an avalanche without any other assistance. Terrain The study of terrain for the avalanche observer has two phases: administrative and technical. A winter sports center is a good example of terrain analysis from the administrative standpoint since it wraps up practically all the problems in one package. ( Figure 7). Normally, a ski area falls into three rather distinct divisions: ( l) The traffic area- highway and parking; ( 2) The lift- served ski area- that part of it which can be reached by the skiers without climbing; ( 3) The touring area. Improvements logically tie in, lodges with the traffic area, lifts with the ski area. Terrain analysis, technical and administrative, influences the layout of improvements for any sort of use and the avalanche protection measures to be employed. Again in combination with climate, it will determine whether or not the development justifies the time, trouble and expense of guarding it from hazard. All the infinitely varied features of mountainous country are important in technical terrain analysis. Ground surface conditions have considerable effect beyond the snow in contact. A broken, serrated terrain, boulder- strewn will provide a good anchorage for the snowpack as a whole. Slides breaking off at ground level are unlikely. Smooth, even slopes of bare earth, solid rock or shale favor the massive ground- level avalanches typical of the high alpine zone. The contours of a mountain influence the hazard. ( Figure 8). Terraces, talus slopes, basins and outcrops are effective barriers. They either divert the moving snow or give it room to spread out and lose its momentum on an easier grade. They are gradually obliterated by snowfall, wind action and successive slides as the winter progresses. Cirques are ideal accumulation zones for wind- driven snow. If their exposures are favorable, they make natural reflectors where sun action will be far above the general level thus promoting sun slides. Gullies collect and channelize the descending snow and thus make favorite slidepaths. By increasing the friction they retard a slide somewhat, but this effect is less important than the fact that a slide issuing from a gully does so with explosive force. Every competent ski mountaineer knows that a gully is the poorest place to travel. Ridges lying parallel to the slidepath, on the other hand, are relatively-secure and generally offer much better traveling conditions otherwise. - 21 - |