Improving American River Flood Frequency Analyses (1999) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-8
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In February 199S, the USACE published a revised unregulated rain flood flow frequency analysis for the American River at Fair Oaks. The analysis produced a flood frequency curve that indicates that large floods are appreciably more likely than ~ Unregulated rain flood flow frequency analysis is conducted on annual peak flow data that have been corrected for the effects of upstream reservoir storage.
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Flood frequency analysis traditionally has been based on systematic streamflow or precipitation records, where use ofthe latter requires the application of precipitation runoff modeling. Flood frequency analysis is commonly based on the assumption that flood flows are independent and identically distributed random variables.
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Such estimates allow a quantitative balancing of flood control efforts and the resultant benefits, and also enhance the credibility of floodplain development restrictions. They allow determination of the flows associated with specified exceedance probabilities, as well as the expected benefits associated with alternative flood risk management proposals.
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Sensitivity analysis using the committee's recommended approach indicates that censoring below various flows with exceedance probabilities ranging from about 0.94 to 0.31 does not significantly affect the estimated distribution. in developing its recommended flood frequency distribution, the committee chose not to use the paleoflood information to compute a frequency curve for the American River.
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The committee did not have hme to develop a recommendation regarding extrapolation of the frequency distribution beyond the flow with an annual exceedance probability of 1 in 200. This is clearly an area in need of analysis.
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The worst consequence of falsely designating such flooUprone areas to be out of the regulatory floodplain would be a prolonged delay in solving acute flood problems, a delay that could have catastrophic results. Given the gross inequality of these two consequences, Me committee strongly recommends that authorities carefully consider the situation and the large uncertainties in the estimated 100-year floods, and attempt to develop a flood risk management strategy that addresses the significant risk of flooding in Sacramento.
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The impetus for such action is clear: rising property damages and loss of life; 30 years of experience with the National Flood insurance Program; aging federal policy and technical guidance; improvements in scientific methods of computing and modeling; emergence of understanding of paleohydrologic and climate variability issues; and a growing data base and availability of information. Virtually all these issues have arisen in the Sacramento case, and can be expected to arise in others as well.
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