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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Running/Box Elder Creek basin is about 4 miles (fig. 1). 'fhis narrow width (or basin <br />elongation), the small areal extent of extreme rainstorms in eastern Colorado (Follansbee and <br />Spiegel, 1937; Follansbee and Sawyer, 1948; Matthai, 1969; Snipes and others, 1974), and <br />small likelihood that a storm would move down the entire Box Elder Creek probably contribute <br />to the relatively small flooding. Converseiy, note that Jimmy Camp Creek (fig. 1) also is <br />elongated, but is a high TR basin south of the Palmer Divide and had a large flood of 124,000 <br />fP/s in the 54.3 mi2 basin in June 1965. <br /> <br />5. The envelope curve (fig. 4) of maximum flOOding can be used to estimate the hypothetical <br />maximum flood for streams in Cherry Creek basin (and Box Elder Creek). For Cherry Creek at <br />the Reservoir (fig. 4), the corresponding maximum flood is about 80,000 fl3/s. This <br />discharge compares favorably with the maximum paleoflood estimate of 74,000 fl3/s <br />determined for Cherry Creek near Melvin (06712500). Even using the envelope curve for <br />other Palmer Divide streams (fig. 4; these high-topographic relief stream are not believed to <br />be the same hydrologic region as Cherry Creek basin) for Cherry Creek basin, the maximum <br />potential flood is about 300,000 fl3/s (fig. 4). PMF values for selected streams in eastern <br />Colorado (Bullard, 1986) with an enveloping curve also are shown on figure 4. Generally, <br />PMF values exceed the envelope curve of maximum floods and paleofloods for Palmer Divide <br />streams by a factor of about 2.2. PMF values exceed the envelope curve for Cherry Creek by a <br />factor of about 8 at Cherry Creek Reservoir to a factor of about 15 near Franktown. The <br />maximum paleoflood of 74,000 fl3/s for Cherry Creek near Cherry Creek Reservoir is about <br />11 percent of the PMF of 662,000 fl3/s. <br /> <br />9 <br /> <br /> <br />Flood-Frequency Analysis <br /> <br />Flood-frequency relations for Cherry Creek developed using the recorded annual peak-flow data <br />near Franktown (06712000) and near Melvin (06712500) and incorporating the paleoflood <br />data into the frequency analyses are shown in figures 6 and 7. Rectangles (figs. 6 and 7) <br />bracket the estimated ranges of uncertainty of discharge and relative age of a paleoflood. The <br />historic-record length used in the LPIII analysis for the ages of paleofloods for Cherry Creek <br />ranged from 1,000 to 10,000 years based on relative-age dating criteria. Flood-frequency <br />relations for Cherry Creek (figs. 6 and 7), which use the shorter (conservative) historic- <br />record length of 1,000 years, indicate the 10,000-year flood ranges from about 44,000 fl3/s <br />(near Franktown) to about 75,000 fl3/s (near Melvin). Because paleoflood data document <br />maximum flooding in thousands of years, flood-frequency relations reflect any climatic or <br />land-use (agriculture or wildfires) changes during that time span. <br /> <br />Because no streamflow-gaging stations have been operated on Box Elder Creek, flood-frequency <br />relations (figs. 8 and 9) were developed by rainfall-runoff modeling (Water Resources <br />Consultants, 1979; U.S. Army Corps of Engineers, 1990; CH2MHILL, 1995); the CH2MHILL <br />modeling was done for the UDFCD. Paleoflood data can be used to help assess the reliability of <br />regional flood-frequency relations from rainfall-runoff modeling and regional-regression <br />equations. The age of the largest paleofloods along Box Elder Creek ranges from about 100 to <br />500 years near Watkins (1-70) and from about 500 to 2,000 years near Elizabeth; more <br />recent paleoflood data also are shown. <br />