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Sacramento and the Struggle to Manage Flood Risk - 13 TECHNICAL ISSUES A.l'ID POLICY IMPLICATIONS In FebruaI)' 1998, the USACE published a revised unregulated rain flood flow frequency analysis for the American River at Fair Oaks (USACE, 1998)/ This revision "''as the fIrst since 1986, and was motivated by the occurrences in 1986 and 1997 of two major floods on the American River. As expected, the analysis produced a flood frequency curve that indicates that large floods are appreciably more likely than previously thought Based on the newly estimated 100-year flood discharge, the levees protecting Sacramento would no longer provide protection against the 100-year flood according to criteria' set by the Federal Emergency Management Agency-the effect of this essentially being "decertifIcation" of the existing levees to provide 100- year protection and new requirements for the purchase of flood insurance. The revised flow frequency relationships also reduce the estimated level of protection of the flood control alternatives that are currently being considered for Sacramento. USACE Approach In calculating the revised flood frequency analysis, the USACE used daily flow data collected at the Fair Oaks gage (USGS #11446500) that were adjusted for the impact of upstream reservoirs.' The adjustment consisted of adding the gaged mean daily flows to the daily change in storage at Folsom Lake and the lagged daily change in storage of the most signifIcant reservoirs in the upper American River basin (the latter accounting for about 90% of all storage in the upper basin). The change in storage for the upstream reservoirs was lagged by one day to account for travel time. For each year of record, the maximum rain-event flows for 1-, 3-, 5-, 7-, 10-, 15-, and 30-day durations were extracted (USACE, 1998). Spring snowmelt events were excluded to avoid mixing populations. Analyses In conducting flood frequency analysis on each of the maxirnmn flow series, the USACE was guided by Bulletin 17-B (IACWD, 1982). The mean, standard deviation, and coefficient of skewness of the logarithm of the flow series were computed from the flow series. (The 15-day and 30-day flows for 1977 were identifIed as outliers; but in order to avoid having the frequency curves cross, the flows were not censored.) Forthe I-day, 3-day, and 5-day series, the skews used were weighted averages of the sample and regional skews; for the remaining durations, sample skews were used directly. The regional skews were based on the skew map given in Plate I of Bulletin 17-B, and weighting was based on the mean square errors I Unregulated rain flood flow frequency analysis is conducted on annual peak flow data that have been corrected for the effects of reservoir storage and are associated with events that are primarily due to rainfall rather than snowmelt ] 44 CFR 65 of the Code of Federal Regulations. j Daily rather than instantaneous flows are critical to flood management decision on the American River because of the significant volume of upstream flood storage at Folsom Dam.