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i <br />3.0 HYDROLOGIC ANALYSIS <br />3.1 Existing Data <br />For locations where existing detailed hydrologic analyses were available, those analyses were used <br />to obtain peak 50 -year and 100 -year flows. For other locations, statistical analyses of stream <br />gages and existing regional hydrologic analyses were examined. As is explained in the next <br />paragraph, those analyses were found to be inconsistent with the existing adopted analyses and <br />were not used for this report. <br />A draft floodplain study being jointly prepared by the U.S. Army Corps of Engineers and the <br />CWCB and published FEMA studies provided 50 -year and 100 -year flows for much of the study <br />reach. Corps of Engineers flows were available for the Colorado River from Rifle, Colorado, at <br />the upstream limit of this study, downstream to DeBeque Canyon. FEMA flows were available <br />for the Colorado River from Palisade, Colorado downstream to Fruita. The USGS performed <br />frequency analyses on gage locations near DeBeque, near Cameo and near the stateline. The <br />USGS gage analysis values were lower than the Corps values by 7 to 10 percent. In order to <br />maintain consistency, the Corps values were selected for this study. <br />3.2 New Hydrologic Information <br />The remaining hydrologic points can be divided into two groups: Group 1) Three hydrologic <br />points were located within reaches that had already been studied by detailed methods, but there <br />weren't any published flow values for those specific points; Group 2) Two points were in <br />reaches downstream of the Sato Hydrology Report, so they had not previously been analyzed. <br />Hydrologic analyses of the 50 -year and 100 -year flood flows for the hydrologic points in Group <br />1 were performed through interpolation of known flow values. Since detailed information already <br />existed, the flows were not used to compute flood depths and no changes were made to the <br />detailed floodplain maps. (The flow values are provided in this report simply for information <br />purposes.) <br />The known values for points near the Group 1 points were entered into a curve fitting computer <br />program which selects an equation that provides a close description of the plotted curve. Flow <br />values for the three Group 1 locations on the Colorado River (river miles 80 -75, 42 -45 and 25- <br />28) were calculated. A similar procedure involving extrapolation instead of interpolation was <br />used for the hydrologic points in Group 2 (river miles 0 -10 and 10 -12). The curve fitting <br />program computed the flows at these locations by using the corresponding drainage areas at each <br />' location. Table 3 summarizes the data at the selected hydrologic locations. The equation for the <br />Colorado River below Rifle is: <br />Q =k *A` <br />where: Q = 50 -year or 100 -year flow; <br />A = drainage area; <br />k = a constant; and <br />c = an exponent <br />1 10 <br />