Improving American River Flood Frequency Analyses (1999) / Chapter Skim
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5 Summary and Recommendations
5 Summary and Recommendations
Pages 101-106
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From page 101... ...
The committee estimated the regional skew by averaging the sample skew of the log three-day flow series from seven rivers on the west slope of the central Sierra Nevada. The Expected Moments Algorithm matches log space sample and population moments, and hence is consistent win Bulletin 17B.
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From page 102... ...
Use of the paleoflood data implies that the log skew is much more negative, and as a result when the paleoflood data is used with the systematic and historical data, the resulting f~tted log-Pearson type III distribution does not provide an adequate description of the flood flow frequency relationships for floods with exceedance probabilities from 0.5 up to and beyond 0.002. BEYOND BULLET~ 17-B While its preferred estimate of the frequency distribution of three-day rain flood flows on the American River is consistent with the systematic and historical data, the committee is uncomfortable with extrapolating it much beyond the flow with an exceedance probability of 0.005.
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From page 103... ...
Using the estimated distribution of average basin precipitation and a simple regression model of the rainfall-runoff relationship, the committee estimated a ~ree-day flow distribution that flattens in response to the constraint imposed by precipitation. While this estimated distribution is based on incomplete data and simplifying assumptions, the general approach should be explored as a potential method of extrapolating the flood frequency distribution.
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From page 104... ...
Given the gross inequality of these two consequences, the committee strongly recommends that authorities consider the situation carefully and the large uncertainties in the estimated lOO-year floods, and attempt to develop a flood risk management strategy that addresses the significant risk of flooding in Sacramento. RESEARCII NEEDS Flood frequency analysis has been practiced for nearly a century and has seen significant developments in both technological and sociopolitical contexts.
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From page 105... ...
As was pointed out by the NRC Committee on Flood Risk Management in the American River Basin, the need for future research and issue resolution should not be used as an excuse for inaction now. While that committee's comment was directed specifically to the American River situation, this committee believes that the ongoing needs and opportunities being experienced by Sacramento suggest that the time is ripe to begin to seriously reassess policy and strategies for flood risk assessment and management not only for the Sacramento case but for the nation as a whole.
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From page 106... ...
(3) A decision analytic framework that uses information as to the uncertainty of the flood frequency estimates explicitly in the analysis of the design level of floor} protection is also needed.
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