Improving American River Flood Frequency Analyses (1999) / Chapter Skim
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1 Sacramento and the Struggle to Manage Flood Risk
1 Sacramento and the Struggle to Manage Flood Risk
Pages 9-15
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at Auburn, which was ultimately recommended by the USACE. In response, Congress directed the USACE to reevaluate available flood control options and, at the same time, asked the USACE to engage the National Research Council as an independent advisor on these difficult studies.
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The report contains a variety of recommendations covering improved operations of existing dams in the upstream basins, the integrity and hy`draulic capacity of existing levees, statistical analysis of the historic flood record, better hydrologic monitoring in Me basin, ecological analysis of alternatives, risk management analysis and water resources planning approaches, and research needs. CURRENT PLA01NG EFFORTS AND CONTROVERSIES In March 1996, the USACE and its non-federal affiliates completed the Congressionally directed reevaluations of flood control options and submitted recommendations to Congress.
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that should be considered before farther assessment of flood risk and consideration of alternatives for risk management can effectively proceed. Revised flood flow frequency relationships form the underpinnings of all future planning and must be realistic and professionally defensible in order to avoid controversy, to the maximum extent possible, and minimize uncertainties and errors In the decision process.
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The regional skews were based on the skew map given in Plate 1 of Bulletin 17-B, and weighting was based on the mean square errors ' 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. 2 44 CFR 65 of the Code of Federal Regulations.
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Below is a list of the issues that the committee recognized as potentially critical, and that are addressed In this report: Information, · accuracy of the adjusted daily flows used In the flood frequency analysis; · failure of the USACE analysis to incorporate historical data or paleoflood · consistency of the results with probable maximum flood estimates, envelope curves of maximum flood discharges, and rainfall runoff modeling results; · use of the Bulletin 17-B map skew, given that the skew map is out of date and was developed for instantaneous flood discharges, not maximum daily flows; · the use ofthe expected probability correction; · adequacy of the log-Pearson type III distribution for modeling flood distributions over a wide range of exceedance probabilities; · adequacy of the Bulletin 1 7-B procedure for accounting for historical data; · the potential advantages of censoring the lower part of the distribution so that the estimation depends only on the largest floods; · the fact that the record from 1950 to the present has many more large floods than the 1905-1950 record; and · potential changes in flood probability due to global climate or regional change. The first two issues concern data used (or not used)
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. · Even if the underlying assumptions are reasonably correct, there are large standard errors in flood frequency analysis; even flood records 100 years in length have insufficient information to allow accurate estimates of quartiles such as the flood flow exceeded with a probability of I% in any year (lOO-year flood discharge)
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