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<br />interval T. expresses the risk. Thus, there is a 1 percent chance that <br />the 100-year flood will be exceeded in a given year. This statement, <br />however, ignores the considerable risk that a rare event will occur <br />during the lifetime of a structure. The frequency curve can also be <br />used to estimate the probability of a flood exceedance during a specified <br />time period. For instance, there is a 50 percent chance that the flood <br />with annual exceedance probability of 1 percent will be exceeded one or <br />more times in the next 70 years. <br />Procedures for making these calculations are described in Appendix <br />10 and can be found in most standard hydrology texts or in (19) and <br />(20). <br /> <br />c. Expected Probability <br />The expected probability is defined as the average of the true <br />probabilities of all magnitude estimates for any specified flood frequency <br />that might be made from successive samples of a specified size [(8), <br />(21)]. It represents a measure of the central tendency of the spread <br />between the confidence limits. <br />The study conducted for the Work Group (8) and summarized in <br />Appendix 14 indicates that adjustments [(21, (22)] for the normal distri- <br />bution are approximately correct for frequency curves computed using the <br />statistical procedures described in this guide. Therefore, the committee <br />recommends that if an expected probability adjustment is made. published <br />adjustments applicable to the normal distribution be used. It would be <br />the final step in the frequency analysis. It must be documented as to <br />whether or not the expected probability adjustment is made. If curves <br />are plotted. they must be appropriately labeled. <br />It should be recognized when using the expected probability adjust- <br />ment that such adjustments are an attempt to incorporate the effects of <br />uncertainty in application of the curve. The basic flood frequency <br />curve without expected probability is the curve used in computation of <br />confidence limits and risk and in obtaining weighted averages of indepen- <br />dent estimates of flood frequency discharge. <br /> <br />22 <br />