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<br />4,0 <br /> <br />HYDRAULIC ANALYSIS <br /> <br />4.1 BascMaoDinl! <br /> <br />Base mapping selected for the delineation of the 50-year and IOO-year floodplains on the <br />Colorado River cOllSisted 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 />contolU" 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 SO and lOO-year floodplain boundaries cannot, in most cases, <br />bcsho\VilllSseparate. <br /> <br />4.2 Stream Reaches <br /> <br />Prdiminar)' 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 refined once cross-sections were ploned. The width <br />c1assitications selected are based on the distance from one side of the river to the other for <br />ground at 15 feet above the low Wllter level. those classificalionsare: <br /> <br />Broad <br />Medium <br />Narrow <br />Canyon <br /> <br />(>0.5 mile '.Vide); <br />(0.2 - 0.5 mile '.Vide); <br />(0.1 _ 0.2 mile wide); <br />(<0.1 mile '.Vide). <br /> <br />Based on this criteria, th~re were 12 stream reaches classified for a total of 93 rivet miles which <br />is an average of7.8 miles per reach. Of those 12 reaches. 6 had a1rcady been studied by detailed <br />methods. The 12 strcamrcaches are listed in Table 4 and they are shul.'.1lon Fit:uTe 6. <br /> <br />14 <br /> <br />TABLE 4 <br />COLORADO RIVER REACH CLASSlflCATIONS <br /> <br />REACH NUMBER REACH IN MAP MILES CHANNEL "'lD111 <br /> ABOVE UTAH STATE LINE AND SLOPE <br />, . 86.5-93 MEDlUM,16'IMI <br />2 . 84.5_86,S NARROW, 16'/MI <br />, . 59.84.5 MEDIUM,12'1M1 <br />, . 57.59 NARROW, 10'IM1 <br />, 50.57 CANYON, 1l,IM] <br />, 47.S.50 MEDIUM, 10'1M] <br />, 4547.5 CANYON. to'IM! <br />. . 4445 NARROW. 13'IM] <br />, , 17_44 MEDIUM.II'IMI <br />W 12.17 CANYON,5'IMI <br />U 10.12 NARROW, TiM! <br />" .,0 CANYON. TfMl <br /> <br />. <br /> <br />Th.",ruoh.,w.,.alrl:WyWdiedasparlofdollliltdfl,lOdplainotudie', Reache, 1_4..< included;nUl< <br />ongoing Corp, of Eng;"".n/CWCB study and Rc..he, 8 .nd 9 arc included in FEMA ",udies, The finding' <br />oflh<l..der.oiled,.pomwer<incorpora\ed;ntl1i"tudy, <br /> <br />" <br />