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<br />. <br /> <br />. <br /> <br />. <br /> <br />An assessment was made of the likelihood that the PMF would not have occurred by chance In the <br />study area, The probability that the PMF would not have occurred in Elkhead River Basin in 10,000 <br />years is 1.183x1 (}02, and the probability that the PMF would not have occurred in the 4,200 mi2 study <br />area in northwestem Colorado during the past 10,000 years is 3,3x1(}37. Conversely, the probability <br />that the PMF is expected to have occurred is 0.9817 in the Elkhead Creek Basin and essentially is 1 for <br />the PMF to occur at least once within the 4,200 mj2 study area, These probabilities help demonstrate <br />that paleoflood evidence for the PMF-based floods should have been identified if not in the Elkhead <br />River Basin, then in at least one stream in northwestem Colorado. Thus, it raises further question as to <br />the magnitude of the PMF estimate for Elkhead Reservoir, <br /> <br />Flood-frequency relations, which incorporated the paleoflood data, were developed for the selected <br />gaged and ungaged sites in northwestem Colorado, including Elkhead Creek. The 1 OO-yr flood is about <br />3,000 f13ls at Elkhead Dam. Extrapolation of flood-frequency relations for extreme floods are provided to <br />help place extreme floods In perspective with today's understanding of flood hydrology (National <br />Research Council, 1985; Jarrett and Costa, 1988). The recurrence interval for the maximum paleofloods <br />on Elkhead Creek ranges from about 1 ,000 to 10,000 years, The 1 O,OOO-yr flood estimate for Elkhead <br />Creek is about 6,500 1t3/s at Elkhead Dam. The PMF of 36,052 1t3/s (Ayres Incorporated, written <br />commun" 1996) has a recurrence interval in excess of a million years, The maximum paleoflood of 4,750 <br />1t3/s in at least 10,000 years at Elkhead Dam is 13 percent of the PMF estimate, <br /> <br />Large differences between the PMP/PMF-based hydrology and interdisciplinary paleoflood hydrology <br />for the Elkhead Creek study and numerous studies in other rivers in Colorado and the westem United <br />States indicate the need to continue to improve the understanding of flood hydrometeorology. The most <br />likely causes of excessive PMPIPMF estimates in the Colorado Rocky Mountains are: (1) transposition <br />of distant extreme rainstonns; (2) precipitation-maximization techniques; (3) remoteness from moisture <br />sources; and (4) overestimation of the areal extent of extreme rainstonns, New insight of the <br />hydrometeorology of flooding gained from interdisciplinary paleoflood investigations improves f100d- <br />frequency relations and provides Infonnation that can be used by dam-safety engineers and managers <br />to help make a risk-based approach for hydrologic aspects of dam safety. <br /> <br />3 <br />