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Show A common problem in selecting sites for mountain construction or development is determining the probability of avalanche damage. Prudent planning dictates that sites should be completely free of avalanche danger if at all possible. This should be the inflexible standard for buildings and lodges in recreation developments, but a slight exposure to avalanche hazard is sometimes acceptable for ski lifts or parking areas. In the case of mining construction or other industrial enterprises where the character of hazard exposure can be more strictly controlled, a more substantial risk of damage may sometimes be acceptable. This risk is known as encounter probability. The problem normally is not posed by large avalanches which run frequently ( annually or oftener). These present such obvious prospects of repeated damage and destruction that they must either be avoided entirely or else defended or eliminated by what may be prohibitively expensive construction. More often a proposed site lies within or adjacent to a slide path where normal avalanche activity is limited and non- destructive, but which shows evidence of infrequent avalanches of potentially destructive proportions. The situation is similar to that posed by other geophysical hazards earthquakes, floods, tidal waves, hurricanes which recur in destructive size at long and irregular intervals. The concept of a " 20- year flood" or " 100- year flood" is familiar to the hydrologic engineer. The avalanche specialist is faced with the similar problem of evaluating prospective damage from a " 20- year avalanche" or a " 100- year avalanche"- It often is undesirable or too costly to avoid completely the prospects of damage from an avalanche which may fail only once a century. In some circumstances, such as the exposure of a large number of people, the only acceptable hazard may be zero and there is |