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<br /> <br />'l <br />V <br />" _/ <br /> <br />.' <br /> <br />Sacramento area. Evaluations of alternatives continue. At the time this report was <br />being prepared, two major flood control approaches are under serious consideration. <br />One option is for the construction of a $1 billion, 500-foot-high flood retention ("chy") <br />dam upstream at Auburn. Funding for similar plans has been repeatedly rejected at <br />the federal level for decades, largely due to high costs and environmental issues. <br />Alternatively, a "stepped release" option seeks to raise and reinforce levees <br />downstream from the Auburn site and to increase the outflow capacity of the existing <br />Folsom Dam. To add considerable complication to the technically and politically <br />difficult decision process, in January 1997 the American River experienced a major <br />flood, nearly as large as and hydrologically similar to the "flood of record" that <br />occurred just II years before in 1986. <br /> <br />Flood Flow Frequency Relationships <br /> <br />The occurrence of the 1997 flood suggested that it may be necessary to <br />recompute flood flow frequency relationships for the American River at Sacramento. <br />This second major flood in the past II years of 93 years of hydrologic history has <br />significant implications for the flood risk management decision process. Simply put, <br />climatic and hydrologic conditions may be changing so that larger, more damaging <br />events would be expected to occur more frequently. If this is the case, residual flood <br />risks--under present or future conditions--would likely be greater than previously <br />thought <br />Perhaps not surprisingly, recalculation of the flow frequency relationships bas <br />proven controversial. Occurrence of the 1997 flood has brought into question many <br />issues of technical methodology (e.g., consideration .of the paleoflood record, <br />hydrometeorological non-stationarity, the validity of prescribed statistical approaches, <br />and consideration of detenninistic precipitation runoff modeling) that should be <br />considered before further assessment of flood risk and consideration of alternatives for <br />risk management can effectively proceed. Revised flood flow frequency relationships <br />fonn the underpinnings of all future planning and must be realistic and professionally <br />defensible in order to avoid controversy, to the maximum extent possible, and <br />minimize uncertainties and errors in the decision process. <br /> <br />Hydrologic Risk and Uncertainty Analysis <br /> <br />The timing of the 1997 flood coincides with new thinking and methods in the <br />field of hydrologic risk and uncertainty assessment. In 1994, the USACE adopted <br />new risk and uncertainty analysis procedures for project evaluation that explicitly <br />include uncertainties of hydrology, hydraulics, and economics of project planning. <br />The primary advance in these new methods is that uncertainty is quantified and <br />incorporated in project analysis (NRC, 1995). The U.S. Congress also recently <br />commissioned a study of the USACE's risk-based analysis, which is just underway <br />and is being carried out by a new committee organized by the WSW. The WSW <br />1995 American River study, the 1997 floods, and the concurrent WSW study on risk- <br />based analysis all suggest that a case study on updating flow frequency and other <br />hydrologicihydraulic parameters in the American River basin is particularly timely. <br /> <br />- <br />12 <br /> <br />Improving American River Flood Frequency Analyses <br />