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L7 <br />1 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 some other locations, statistical analyses of <br />stream gages provided by the USGS were used to compute the 50 -year and 100 -year flows. <br />IPublished floodplain studies prepared by FEMA for Craig, Colorado provided 50 -year and 100 - <br />year flows at the upstream limit of this study, above the confluence with Fortification Creek. <br />They also provided flows below that confluence, a short distance downstream. The USGS <br />performed frequency analyses on gage locations at Maybell, near the middle of the study reach, <br />and at Deerlodge Park, at the downstream study limit. In addition, USGS analyses were available <br />for two gages on tributary streams. <br />3.2 New H,, d�gic Information <br />Hydrologic analyses of the 50 -year and 100 -year flood flows for the remainder of the hydrologic <br />points along the study reaches of the Yampa River were accomplished by interpolation of the <br />known flow values. The known values and corresponding drainage areas were entered into a <br />curve fitting computer program that selects equations which provide a close description of the <br />plotted curve. The curve fitting program computed the flows at the other hydrologic points on <br />the Yampa River locations by using the drainage areas at each location for which flows needed <br />to be calculated. <br />The graphical interpretation of the computed flow values on the Yampa River indicated a distinct <br />change in hydrology near the Maybell gage, at the mid -point of the study reach (see Figure 5). <br />For this reason, two separate sets of equation coefficients were obtained to define the hydrology <br />upstream and downstream of the Maybell gage. The form of the equations for the Yampa River <br />below Craig 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 />The form of the equation is the same for the entire study reach, but the coefficients are different <br />for the two portions. <br />1 <br />1 10 <br />