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<br />Paleollood Investigations for Cherry Creek basin, Eastern Colorado <br />Robcrt D, Jarrelt <br /> <br />Rcsearch Hydrologist, U,S, Geological Survcy, P.O. Box 25046, MS-412, Dcnver, CO 80n5: PH <br />(303) 236-6447; FAX (303) 236,5034: email: rjarrell@usgs,gov <br /> <br />Abstract <br /> <br />In 1950 when Cherry Creek dam, which is locatcd in Denvcr, Colomdo, was complcted, the design <br />flood was 5,126 m3!s. Two recent probable maximum flood (l'MF) eSlimates for the dam range from <br />14,840 to 18,750 m3!s demonstrate the uncertainly in estimating cXlrcme flooding in eastern <br />Colorado, PMF difference is due in part to a lack of exlreme rainfall and flood data in eastern <br />Colorado, A paleoflood study was conduCled to assist dam-safety officials in assessing the risk of <br />large floods in Cherry Creek basin, An envelope curve encompassing maximum contemporary <br />floods (19 sites) and paleofloods (99 sites) was developed for Cherry Creek basin streams; paleoflood <br />data reflecl maximum flooding during the last few hundred to many thousands of years, Maximum <br />paleofloods in Cherry Creek range from about 1.050 m3!s near Franktown (in about 5,000 to at leasl <br />10,000 years), about 2,100 m3!s near Melvin (in about 1.500 to 5,000 years), and about 2,270 m3Js at <br />Cherry Creek Reservoir (also in about 1,500 to 5,000 years), Flood-frequency relations for Cherry <br />Creek, which incorporate paleoflood data, indicale the IO,OOO-year flood (10-4 annual exceedence <br />probability) ranges from about J ,200 m3!s (near Franktown) to about 2,200 m3!s (near Melvin), PMF <br />estimates arc about six to eight times larger than paleofloods in Cherry Creek basin, Additional <br />research in flood hydrometeorology is needed to help dam safety officials evaluate potential safety <br />problems related to large floods in Cherry Creek basin, <br /> <br />Introduction <br /> <br />Extreme or rare floods, with annual exceedence probabilities of about 10,3 to 10-7, are of <br />continuing interest to the hydrologic and engineering communities for purposes of planning and <br />design of structures such as dams (National Research Council, 1988), The Bureau of Reclamation <br />began conducting risk assessment studies using paleoflood hydrology for many of its dams in the <br />mid-1990s (Ostenaa and Levish, 1995), The U,S, Army Corps of Engineers is implementing a risk <br />assessment method to evaluate potential safety problems for its more than 550 dams to aid decision- <br />makers in prioritizing inveslment decisions (Foster, 1999), Substantial uncertainty is associated with <br />estimating flood magnitude and frequency, particularly those of extreme floods, in the Rocky <br />Mountain region, which is due in large part to a lack of extreme flood data ( Hansen et aI., 1988; <br />Jarrell and Costa, 1988; Jarrett and Tomlinson, in press), <br /> <br />In 1950, Cherry Creek Reservoir (fig, l) was designed for a flood of 5,126 m3!s with a flood- <br />storage capacity of 2,06 x 108 m3 (U,S, Army Corps of Engineers, wrillen commun" 1997), <br />Recently, methods used to estimate the probable maximum precipitation (PMP) values were <br />developed for the Rocky Mountain region including most of eastern Colorado (Hansen et aI., 1988), <br />From these PMP values, the probable maximum flood (PMF) for Cherry Creek Reservoir was <br />estimated in 1993 by the U.S, Army Corps of Engineers to be 18,750 m3!s, thus, raising questions <br />about the safety of Cherry Creek Dam, Using older PMP methods developed for areas east of the <br />105th meridian (Schreiner and Riedel, 1978), which is about the longitude of Denver, another PMF <br />of 14,840 m3!s was estimated (U,S, Army Corps of Engineers, wrillen commun" 1997), The <br />estimated cost for proposed modifications of the Cherry Creek dam is as high as $250 million (V,S, <br />Army Corps of Engineers, written commun., 1997), Differences in flood estimates such as for <br />Cherry Creek demonstrate the importance of reducing the uncertainty in estimating the magnitude <br />and frequency of flooding in eastern Colorado. <br /> <br />The objective of the study was to estimate prior maximum flooding from evidence preserved in <br />Cherry Creek and its tributaries and to incorporate paleoflood data in flood-frequency analysis, <br />