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<br />Data Sources <br /> <br />- <br />35 <br /> <br /> <br />relevant to estimating the likelihood of future floods. If this assumption cannot be <br />made, the relative usefulness of particular data on past floods depends on the actual <br />distribution of floods in time, the age of the data, the length of the planning horizon, <br />and the exceedance probabilities of interest If we had a correct mathematical model <br />of the variation of floods in time i.e., an alternative to the independent and identically <br />distributed model we could estimate the parameters of that model to appropriately <br />weight data from past floods. Unfortunately, we only have a very general <br />understanding of how floods vary in time, and must rely heavily on judgment <br />Where paleoflood information is inconsistent with modern flood data (i.e., a <br />systematic flood record), the judgment might be not to use the paleoflood data in the <br />flood frequency analysis. As we will see in Chapter 3, the American River provides <br />such a case. <br />Even if American floods are assumed to be independent and identically <br />distributed, the nature of the USBR paleoflood data somewhat limits its utility for <br />flood planning and management In particular, these data consist of levels (and <br />hence flows) that have not been exceeded in the last 1,500 to 3,500 years. There is <br />little direct information about the magnitude and frequency of the smaller floods that <br />are of most interest to flood management in Sacramento--floods that occur every <br />100 to 200 hundred years. While it is true that the use of non-exceedance data in a <br />flood frequency analysis can improve the estimation of the exceedance probabilities <br />of smaller flows, the value of the data critically depends on whether the assumed <br />frequency distribution is correct for flows up to the non-exceedance flow. As we <br />shall see in Chapter 3, although our "best" log-Pearson type III model of the <br />American River 3-day flows provides a good fit to the systematic and historic data, it <br />does not appear to provide an adequate model for significantly larger flows. <br />Clearly there are potential problems associated with the use of the USBR <br />paleoflood information to estimate exceedance probabilities and flood quantiles for <br />the American River. Consequently, it was decided to not use this information to <br />estimate the committee's recommended flood frequency relationship for the <br />American River. <br /> <br />Envelope Curves <br /> <br />Meyer (1994) developed an envelope curve for peak flood discharges in <br />California based on the highest recorded peak discharges from 1,296 gaging stations <br />(Figure 2.7). For drainage areas greater than 1 square mile, the envelope curve is <br />defmed by <br /> <br />Q = 10,500AI.13(AoS + 5 t1.37 <br /> <br />where A is the drainage area in square miles and Q is the envelope discharge in cfs. <br />For the American River at Fair Oaks the value of Q is 267,000 cfs. <br />For several reasons, Meyer's envelope curve is of limited usefulness in <br />estimating the flood frequency distribution for the American River. One potential <br />problem is that the curve does not include data from floods that occurred during <br />