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Show Due to the concentration of railroads, highways and habitations in areas of hazard, avalanche barriers are extensively used as a protective measure in the Alps. The European researchers have explored the field thoroughly and much theoretical and technical information is available. The use of barriers is less common in the United States. The most notable example is the system of snowsheds which protects many of our railroads in alpine terrain. There are a few snowsheds guarding highways. The snowshed is an example of the diversion type of barrier which has several adaptations. Diversion walls and piers seek to confine the avalanche to a certain path, or turn it away from the object to be protected. Austrian researchers have developed a variation which they call avalanche breakers. These are mounds of earth, earth and stone, or concrete resembling the pillboxes and tank obstructions of the last war. A series of these mounds are strategically placed in a slidepath, generally in a transition zone. Without directly resisting the force of an avalanche, they check its momentum, increase the friction, set up cross currents in the moving snow and cause it to spread out. Even if the mounds themselves are buried by earlier snowfall and avalanches, the great heap of snow and debris continues to act as a barrier. The avalanche breakers are much cheaper to build and maintain than continuous barriers and are reported to be equally effective. There are many hazardous locations in this country affecting highways particularly where barriers of this type could be constructed at reasonable cost. The diversion type of avalanche barrier is passive in its effect. It does nothing to reduce the size of the avalanche or prevent it at the source. Another category of barriers seeks to stabilize the snow in place. Terraces, walls, fences, mounds, pillars and even thickets of posts are set up in the accumulation zone. Primarily these barriers reduce the hazard by providing better anchorage for the snowpack. They also modify the snow profile and do something to inhibit the formation of very large, continuous layers of slab. The " cat- tracks" at Sun Valley, Idaho, are an example in the United States of this type of barrier. In the long run, reforestation is the most effective form of stabilization barrier. Timberline is at a comparatively low elevation in the Alps and determined efforts are made there to extend it to higher levels by plantations of trees often in connection with constructed barriers. Natural reforestation has the same effect. The process can be seen at work in the vicinity of Alta and other onetime mining camps. The careless logging practices of the miners often backfired on them in the shape of man- made avalanche paths. Many of these are gradually becoming inactive as natural reforestation progresses. There is one other form of stabilization barrier, still in the experimental stage, but so promising that discussion here is warranted. It attempts to stabilize particular snowfields indirectly by modifying the natural wind action. This is a very intriguing conception, to use wind, the most important contributory avalanche factor, as a stabilizing - 72 - |
