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<br />',0 <br /> <br />HYDRAULIC ANALYSIS <br /> <br />4.1 BascMuoDinl! <br /> <br />Base mapping selected for the delineation of the 50-year and IOQ-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 "~20oo'), the 50 and loo-year floodplain boundaries cannot, in most cases, <br />bcsho\VilllSseparate. <br /> <br />4.2 Stream Reaches <br /> <br />Prdiminal)' work on the floodplain mapping of the Colorado River included the classification of <br />reaches within thc study area. The reaches were classified by eSlimaling stream floodplain <br />'Widths. The width classifications were refined once cross-scctiollS were ploned, The width <br />classifications selccted 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 rivet miles which <br />is an aVCfage of7.8 miles per reach. Of those 12 reaches. 6 had already been studied by detaikd <br />methods. The 12 stream reaches are listed in T4ble 4 and they are shu\,',1l on Fit:uTe 6. <br /> <br />14 <br /> <br />TABLE 4 <br />COLORADO RIVER REACH CLASSlflCATIONS <br /> <br />REACH NUMBER REACH IN MAP MILES CHANNEL "'lD1ll <br /> ABOVE UTAH STATE LiNE AND SLOPE <br />I . 86.5-93 MEDlUM,16'IMl <br />2 . 84.S_86.S NARROW, 16'/MI <br />, . 59-84.5 MEDIUM.12'1M1 <br />, . 57-59 NARROW, 10'IMI <br />, SO-57 CANYON, 1I,IM1 <br />, 41.5.50 MEDIUM, 10'fMl <br />T 4547.5 CANYON.10'iMl <br />. . 4445 NARROW. 13'IMl <br />, . l7_44 MEDIUM.ll'lMl <br />10 12.17 CANYON,5'IMI <br />II 10.12 NARROW, TfMl <br />12 >10 CANYON. TfMl <br /> <br />. <br /> <br />The",,,,aoh..werealread}Wdiedasparlofdolll;lcdfl,lOdpia;nstudie'. Reache, 1_4... inciudedin Ul< <br />ongoing Corp, of Enginnen/CWCB study and Rc..he, 8 .nd 9 are included in FEMA ",udi... The finding' <br />oflh<l..delaiiedr<pnmwer.incorpora\ed;ntl1i"tudy. <br /> <br />2S <br />