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<br />Statistical data, calculated by the log-Pearson Type III method for <br />stations with more than l2 years of record, were used in the regional <br />analysis. using logarithmic-probability 9raphs, the mean discharge <br />for each station was correlated to various basin parameters (i.e., <br />shape and size of drainage area, its exposure and permeability of <br />soil, prevailing winds, vegetative cover, and other natural char- <br />acteristics). A log-log plot of mean discharge versus drainage area <br />provided the highest correlation. <br /> <br />~' <br /> <br />'~ <br /> <br />~ <br /> <br />The computed standard deviation of each stream gaging station was <br />plotted on a regional map and lines weredra~ through points of <br />equal standard deviation. This map becam,e the basis for determining <br />the standard deviation for ungaged strea'ms. The skew coefficient <br />for ungaged areas was determined with averages of the skew coefficients <br />computed for neighboring stream gaging st:ations. <br /> <br />Peak <br />from <br />oped <br />both <br /> <br />flows in areas studied by approximat:e methods were determined <br />100-year peak flow envelope curves. These curves were devel- <br />by plotting the largest recorded peak flows in the region for <br />rain and snowmelt flood events against drainage area. <br /> <br />The drainage areas that were studied by detailed methods required <br />the use of the rainfall-runoff computer program HEC-l (Reference 2). <br />This program utilized input data such as unit hydrograph coefficients, <br />soil loss rates, rainfall depths and dis1:riblltion, and, when <br />necessary, routing coefficients to compute a flood hydrograph for a <br />given drainage area and area of storm cO'Terage. Peak flows from the <br />computed flood hydrographs were compared to values obtained from the <br />regional frequency curves, and adjustmen1:s were made to the computed <br />flood hydrographs to assure their consis1:ency with the frequency <br />curves. <br /> <br />The computational methods and techniques used are generally accepted <br />for hydrologic analyses and produced reasonable results for Garfield <br />County. Unexpected results were not encountered in carrying out the <br />hydrologic analyses for this Flood Insurance Study. Drainage-area, <br />peak-discharge relationships for the Colorado and Roaring Fork <br />Rivers and Alkali and Cattle Creeks are shown in Figure 2. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the county <br />were carried out to provide estimates of the elevations of floods <br />of the selected recurrence intervals along each stream se~ent <br />studied in detail. <br /> <br />? <br /> <br />... <br /> <br />,I.. <br /> <br />Except where noted otherwise, cross sections for backwater analyses <br />were field surveyed. Cross section data for the Colorado River at <br />Rifle were developed from topographic data furnished by the Colorado <br />Department of Highways (References 3 and 4). Additional cross <br /> <br />9 <br />