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<br />Sacramento and the Struggle to Manage Flood Risk <br /> <br />- <br />15 <br /> <br /> <br />underlying assumptions, the validity of which cannot be absolutely established (e.g., <br />the assumption that flood discharges are independent and identically distributed in <br />time). <br />. Even if the underlying assumptions are reasonably correct, there are large <br />standard errors in flood frequency analysis; even flood records 100 years in length <br />have insufficient infonnation to allow accurate estimates of quantiles such as the flood <br />flow exceeded with a probability of I % in any year (lOO-year flood discharge). <br />. The differences between our best estimate of flood quantiles (such as the <br />100-year flood discharge) and those of the USACE are small compared to the likely <br />uncertainties in the estimates. <br />. Critical policy decisions in the American River basin, such as certification <br />of the levees, Sacramento's floodplain status, and the adoption of flood mitigation <br />strategies, are extremely sensitive to the official estimates of flood probabilities and <br />quantiles. Hence, even though our best estimates are not significantly different from <br />those of the USACE in a statistical sense, the differences may have significant policy <br />implications. <br /> <br />The last issue is particularly important, and is discussed in Chapter 5. <br />