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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 />4.0 HYDRAULIC ANALYSIS <br /> <br />4.1 Base Mapping <br /> <br />Base mapping selected for the delineation of the 50-year and 100-year floodplains on the <br />Colorado River consisted of 7.5 minute U.S.G.S. topographic quadrangle maps. A total of 22 <br />quadrangle maps were used in compiling the base maps for the Colorado River study reach. The <br />contour interval for the maps were either 20 or 40 feet (see Figure 8). Due to the small scale <br />of the base maps (1 "=2000'), the 50 and 100-year floodplain boundaries cannot, in most cases, <br />be shown as separate. <br /> <br />4.2 Stream Reaches <br /> <br />Preliminary work on the floodplain mapping of the Colorado River included the classification of <br />reaches within the study area. The reaches were classified by estimating stream floodplain <br />widths. The width classifications were refmed once cross-sections were plotted. The width <br />classifications selected are based on the distance from one side of the river to the other for <br />ground at 15 feet above the low water level. Those classifications are: <br /> <br />Broad <br />Medium <br />Narrow <br />Canyon <br /> <br />(>0.5 mile wide); <br />(0.2 - 0.5 mile wide); <br />(0.1 - 0.2 mile wide); <br />(<0.1 mile wide). <br /> <br />Based on this criteria, there were 12 stream reaches classified for a total of 93 river miles which <br />is an average of 7.8 miles per reach. Of those 12 reaches, 6 had already been studied by detailed <br />methods. The 12 stream reaches are listed in Table 4 and they are shown on Figure 6. <br /> <br />14 <br />