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<br />39 <br /> <br /> <br />3 <br /> <br />Flood Frequency Estimates for the American River <br /> <br />INTRODUCTION <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 determination of the design flows <br />from specified exceedance probabilities, as well as the expected benefits associated <br />with alternative flood risk management proposals. These considerations are critical <br />for the American River, where billions of dollars of propetty are at risk from <br />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. While the American River flood record at Fair Oaks <br />is almost 100 years in length, there is a goal of providing flood projection for at least <br />the flood that has a chance of 1 in 200 of being exceeded in any year. This requires <br />extrapolation beyond the data, as well as smoothing of the empirical frequency curve <br />to obtain a more consistent and reliable estimate of the 100-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 l7-B (IAC\VD, 1982) were established to provide consistency <br />in the federal flood risk management process. In estimating a flood frequency <br />distribution for the American River, the committee believect it was desirable to <br />follow the spirit of these guidelines, although not necessarily the exact letter. The <br />committee based its estimation on the log-Pearson type III distribution, as specified <br />in Bulletin l7-B. With only a traditional systematic gaged record, we employed the <br />conventional log-space method of moments recommended by Bulletin 17-B. When <br />additional historical flood information is included or some peaks are censored, the <br />Expected Moments Algorithm is used as the generalization of the conventional log <br />space method of moments method. The Expected Moments Algorithm, developed <br />well after the publication of Bulletin 17-B, makes more effective use of historical and <br />paleoflood information than does the weighted moments method recommended by <br />Bulletin 17-B for use with historical information. <br />This chapter is organized as follows. An overview of the basic approach of <br />Bulletin l7-B is followed by a discussion of recent innovations in flood frequency <br />analysis that post-date Bulletin l7-B but are nevertheless consistent with its <br /> <br />- <br />