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Show 8 path during the interim, three of them quite recently. The average age of the other two at the time of destruction was about 40 years. To what risk had they been exposed? Here the return interval and estimated life are sufficiently long that there is very little difference between the figures given by Table 1 and 2. Lacking an average, we must use instead the single return interval of 80 years. Judging from climate and terrain, this is a reasonable approximation. The time unit is years and the estimated ( in this case actual) life is 40 years. From Table 1 the encounter probability is 0.395- The two older houses experienced approximately two chances out of five of destruction. If their normal life anyway is 40 years, they were exposed to this same encounter probability from the day they were built, although the figures would not have been available to calculate it at that time. These figures tell, in other words, that they experienced quite a good chance of eventual destruction at this site. Example No. 2 Construction of a major ski development has been proposed in an area of known avalanche activity. For the purpose of utilizing available private real estate, the developers wish to locate the main center of lodges, ski shops and other buildings adjacent to two major avalanche paths. Historical records show that avalanches large enough to cause damage to the proposed development fell at this site in I878, 1880, 19,06' and 1955- Others may have fallen between 1906 and 1955, but there are no records. Return intervals from the available data are 2, 26 and 49 years. The average is 25- 2/ 3 years. Estimated life of the development is 25 years. |