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<br />draft 3/20/98
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<br />Paleoflood investigations are now being incorporated into a variety of water-resources investigations
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<br />(Osterkamp and Costa, 1987; Jarrett and Costa, 1988; Levish et al., 1994; Carling and Grodek, 1994;
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<br />House et al., 1995; Wohl, 1995; Jarrett and Browning, in review; Jarrett, in review), particularly in
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<br />using paleoflood data for providing robust frequency estimates of extreme floods. Much effort has
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<br />been made during the past twenty years to incorporate paleoflood and historical data into flood-
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<br />frequency analysis (Stedinger and Cohn, 1986; Hirsch and Stedinger, 1987; Guo and Cunnane, 1991;
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<br />Frances et al., 1994; Cohn et al., 1997; O'Connell et al., 1998; England, 1998). In the past decade,
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<br />there has been a trend to incorporate risk-based assessments for design-flood hydrology (National
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<br />Research Council, 1985a & b, 1988; Interagency Committee on Water Data, 1986). In an evaluation
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<br />of extreme floods and use of the probable maximum flood (PMF) to determine design floods
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<br />(Cudworth, 1989), the Interagency Advisory Committee on Water Data (1986) raised a major concern
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<br />about the Nation's flood hydrology, whether computed PMFs are either dangerously small or
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<br />wastefully large. Therefore, they emphasize that accurately estimating the risk of extreme flooding is
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<br />important. Risk-based assessment, which primarily is based on paleoflood data, for spillway design or
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<br />adequacy of existing spillways are currently being developed in the dam-safety community,
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<br />particularly within the Bureau of Reclamation (Jarrett and Costa, 1988; Waythomas and Jarrett, 1994;
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<br />Levish et al., 1994; 1996; Ostenaa et al" 1997; Cohn et al., 1997; Salmon et al., 1997; O'Connell et
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<br />al., 1998; Jarrett, in review).
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<br />For example, a paleoflood study was conducted for the Bureau of Reclamation (Jarrett and Costa,
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<br />1988) for Olympus Dam in Estes Park, Colorado, on the Big Thompson River. Olympus Dam is
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<br />located at an elevation of 2,300 m, and the spillway was designed for a PMF of 637 m3/s. However,
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<br />the revised PMF is 2,380 m3/s and a spillway modification was estimated to cost $10 million (Bureau
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<br />of Reclamation, written commun., 1988). Paleoflood investigations (Jarrett and Costa, 1988)
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<br />indicated that the largest flood in the Big Thompson River upstream from Olympus Dam is 142 m3/s
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<br />(or 6 percent of the revised PMF) in at least 10,000 years. Jarrett (in review) summarized results
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<br />from ten paleoflood studies where PMF data also were available in the western United States; on
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<br />average, the maximum paleoflood is about 7 percent of the PMF value.
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