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<br />CHAPTER IV DEBRIS-FLOW PROBABILITY <br /> <br />A. Hydro-Geo log ic Probability Re 18t io~s~iE.!~. <br />Regardless of the destructive potential ,md areal extent of any debris-flow <br />or flood event, the event dc)es UC)t necessarily constitute a significant <br /> <br />hazard unless the probability that people are exposed to it is sufficient ly <br /> <br />high. Flows reaching a developed area an average of once in 1000 years <br /> <br />would probably not be considered espe,oia11y hazardous, ,.hereas flows reaching <br /> <br />the same area once very 100 years or less should be considered a significant <br /> <br />hazard. Therefore, in order to evaluate the hazard and m.!3ke recommendations <br /> <br />about land use and mitigation, it is necessary to estimate the annual probability <br /> <br />(or return period) of debris flows similar in size to those r!!cently observed. <br /> <br />A reciprocal relationship exi~sts between annual probability, P, and <br /> <br />return period, T, such that P equals liT. <br /> <br />The return period is merely <br /> <br />a statement of probability and implies nothing about the actual distribution <br /> <br />of debris-flow events through time. The fact that an event has just occurred <br /> <br />does not reduce the probability that it will occur again soon. In order <br /> <br />for this to be true it is necessary for a drainage basin to be susceptible <br /> <br />to flows, as discussed in Chapter II. <br /> <br />Because the drainage basins are susceptible to future flows, it can <br /> <br />be assumed that a rainstorm of the intensity and duration of that which <br /> <br />occurred on July 24, 1977 would also trigger f,,,ture flows. <br /> <br />In this particular <br /> <br />- 29 - <br />