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<br />Flood Frequency Estimates for the American River <br /> <br />45 <br /> <br /> <br />log-Pearson type III distribution; (2) has been shown to be relatively efficient; and (3) <br />is consistent in principle with Bulletin l7-B. It was also decided to use EMA to <br />explore various low censoring limits. <br /> <br />ANALYSIS OF AMERICAN RIVER DATA <br /> <br />The committee used the following data to explore estimation of the <br />probability distribution of three-day rain floods on the American River at Folsom <br />Dam (although not all of the data were used to estimate its recommended <br />distribution): <br /> <br />. annual maximum average three-day rain flood discharges for the period <br />1905-1997, as reconstructed by the USACE; <br />. the estimated peak of the 1862 flood (265,000 cfs), assumed to be the <br />largest instantaneous peak flood discharge since 1848; <br />. paleoflood information from the U.S. Bureau of Reclamation (i.e., non- <br />exceedance of 300,000-400,000 cfs during the last 1,500-3,500 years). <br />. the skew map from Bulletin 17-B; <br />. estimated log skews for maximum annual three-day rain flood discharges <br />from the Feather River at Oreville, Yuba River at Maryville, Mokelumne River, <br />Stanislaus River, Tuolumne River, and Merced River; and <br />. two PMF estimates for the American River at Folsom Dam (three-day <br />average flows of 401,000 cfs and 485,000 cfs). <br /> <br />Estimation of Average Three-Day Flows from Instantaneous Peak Flows <br /> <br />Use of the historical and paleoflood data required that a relationship be <br />developed between instantaneous peak discharge and maximum three-day average <br />discharge. This relationship was derived from a log-log linear regression with the <br />observed three-day maximurn as the dependent and the instantaneous peaks as the <br />independent variables. . <br />The instantaneous peak flows corresponding to Fair Oaks (below Folsom) <br />were obtained from the USACE, Sacramento District. For the period water years <br />(WY) 1905-1955 these are, with certain exceptions, identical to USGS annual peaks <br />at Fair Oaks. The exceptions occur in years for which the USGS peak of record is <br />either unknown (WY 1918), a snowmelt, as distinct from a rainfall event (1910, <br />1912, 1913, 1929 and 1933); or when the maximum three-day discharge is associated <br />with an event other than that which produced the instantaneous maximum (1908, <br />1914, 1915, 1916, 1937, 1941, 1946). In the period since 1956, estimates of <br />unregulated instantaneous maxima are generally not available, although the USACE <br />and others have estimated peak flows for 1956-86, and for 1997. It must be assumed <br />that the magnitudes of reconstructed peak flows are known with Jess precision than <br />are gaged flows. <br />A log-log ordinary least squares (OLS) regression was first estimated using <br />the 38 measured, rainfall-generated instantaneous peaks (Qp) and corresponding <br />- <br />