| OCR Text |
Show 7 The biggest difficulty in applying these figures to evaluation of avalanche hazard is determining average values of return intervals. The historical record in North America simply is not long enough to provide data from which averages can be calculated for avalanche return intervals greater than two or three decades. The data that are available for a given avalanche path, whether from historical records or inferred from indirect evidence, usually give at best the length of a single return interval. In many cases there is evidence of a major avalanche at some time in the past which has not recurred since. Evidence from tree growth can usually establish when the avalanche last fell, so all that is then known is a minimum possible length of a single return interval. If this is all there is to go on, it will have to serve. But encounter probabilities based on uncertain return intervals obviously will themselves be uncertain. Experienced judgment on the part of the avalanche specialist can take into account terrain, climate and character of the avalanche path, thus placing the estimate of return intervals on a better basis than guesswork. Such improvement in accuracy is real and often considerable, but it is essentially subjective and thus difficult to account for in statistical analysis. This approach to estimating risks from avalanche hazard does not offer advice on whether risks should be taken. This question is properly one to be answered by administrative or operational decision. What is available is a rational method of determining just what the risk might be. The decision of whether to assume it can then proceed on a sounder basis. Example No. 1. An avalanche of unusual size destroyed five houses at Twin Lakes, Colorado, in 1962. Historical records showed that a previous avalanche of this size had fallen at this site in 1882. The houses had been constructed in the avalanche |
