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<br />Executive Sumnuuy <br /> <br />3 <br /> <br /> <br />changes over the past century in atmospheric composition or in global land use <br />patterns, which have changed the climate forcing and the hydroclimatic response on <br />regional scales in recent decades. In this regard the committee notes that its <br />understanding of climate variability suggests that (a) the uncertainty of flood <br />frequency estimates is higher than that indicated by the usual statistical criteria, (b) <br />climatic regime shifts may---slowly or abruptly-significantly affect the local flood <br />frequency curve for protracted periods, and (c) at this time, given the limited <br />understanding of the low frequency climate-flood connection, the traditional <br />approach to flood frequency estimation entails a tradeoff between potential bias and <br />variance. Bias arises from the use of long periods of record that are more likely to <br />include time periods during which flood risk is different from that during the <br />immediate planning period. On the other hand, longer periods of record allow the <br />construction of risk estimators with less variance due to the larger sample with which <br />the estimators are constructed. <br /> <br />FLOOD FREQUENCY ANALYSIS <br /> <br />Effective planning and design of flood risk management projects require <br />accurate estimates of flood risk. Such estimates allow a quantitative balancing of <br />flood control efforts and the resultant benefits, and also enhance the credibility of <br />floodplain development restrictions. They allow detennination of the flows <br />associated with specified exceedance probabilities, as well as the expected benefits <br />associated with alternative flood risk management proposals. These considerations <br />are critical for the American River, where billions of dollars of property are at risk <br />due to flooding. <br />Fitting a continuous mathematical distribution to data sets yields a compact <br />and smoothed representation of the flood frequency distribution revealed by the <br />available data, and a systematic procedure for extrapolation to flood discharges larger <br />than those historically observed. Whereas the American River flood record at Fair <br />Oaks is almost 100 years in length, there is a goal of providing flood protection for at <br />least the flood that has a chance of I in 200 of being exceeded in any year. This <br />requires extrapolation beyond the data, as well as smoothing the empirical frequency <br />curve to obtain a more consistent and reliable estimate of the I DO-year flood. <br />A variety of distribution functions and estimation methods are available for <br />estimating a flood frequency distribution. The guidelines for frequency analysis <br />presented in Bulletin 17-B were established to provide consistency in the federal <br />flood risk management process. In estimating a flood frequency distribution for the <br />American River, the committee believed it was desirable to follow the spirit of these <br />guidelines, although not necessarily the exact letter. The committee based its <br />estimation on the log-Pearson type III distribution, as specified in Bulletin 17-B. <br />With only a traditional systematic gaged record, the report employs the conventional <br />log-space method of moments, as recommended by Bulletin 17-B. When additional <br />historical flood infonnation is included or some peaks are censored, the Expected <br />Moments Algorithm (EMA) is used as the generalization of the conventional log- <br /> <br />- <br />