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<br />I <br />I <br />I <br /> <br />Are!l5 platas Is on plate 2. Representative cross sections are <br />Illustratedonp,,,t-esI5through!7. Thecrossse<;tlonsshow <br />ground elevations across the valley on both sides of the channel <br />and the depths of overbank flooding. Reference points, coinciding <br />with the locations of the cross sections and pertinent planImetric <br />features are shown on the flooded Areas and profiles for correla- <br />tion between the drawings. Teble 7 provides data at reference <br />point locatIons and Includes dlst!lnce along the m81n ell!!n"ol cen- <br />terline; elevations at the streambed; elevations of the IO-year, <br />50-year, IOO-year, and SOO-year floods; and discharge magnitude of <br />the IO-yo,,"", 50-year, lOG-yea,..., !!nd SOO-year floods. <br /> <br />ColoradO Water COnservation Board in cooperation with logan County. <br />These maps. now used for admlnlstratl~e purposes, are on file and <br />a~allable for public Inspection at the Logan County PlannIng and <br />amlng Department, Logan County Courthouse, Sterling, Colorado. <br /> <br />I <br /> <br />~ reference line Is shown on the flooded Areas for the <br />South Platte Rl~er. This line Is used for reference only and does <br />not necessarily coincide ~Ith the existing stream channel e~en <br />though It may be referred to as such In this report. <br /> <br />The Flooded Areas show the area that would be flooded by <br />the 100-year and 500-year floods. The flood limits were located <br />at each c~ss section and the Inter~enlng flood outlines were <br />drawn based upon detailed topographic mapping, engineering judgment, <br />and f leld obser~atlons. It I s, ho...e~er, possible that rrore or less <br />flooding should be shown On the Flooded Areas. For II specific <br />situatIon, where more detailed accuracy of a flooded area is required, <br />the flood limits Clln be more accurately establiShed by determining <br />tha water surfaca ele~atlon from the prafll es or reference table <br />and then locatIng that ele~atlon by sur~ey On the flood plain. <br /> <br />The flood ele~atlons shown on the profile and In table <br />7 apply IlIterallyfrom"thechannel over.-.:>s"tOfthetloodplain <br />width. Rolldcl'"Osslngs and other topogrllphlc fellturescan alter <br />the lateral flood elevlltlons. Depending on whether these features <br />dl~ert or block flows, the flood ele~atlons lit the edge of the <br />flood plain may be higher or lower than lit "the channel. <br /> <br />Flood plaIn widths ~ary from One area to another through- <br />out the study reach. The 100-ye1lr flood plain In the Sterling <br />area resulting from the South Pr1ltte RI~er and Pawnee Creek ~lIrles <br />from II minimum near 4,500 feet to a maximum near 8,000 feet with <br />an a~erage of 7,000 feet. <br /> <br />The detailed topographic mapping utlllled to assist In <br />delineating theoutll/'leof the 100-yearand 500-yellr floods con- <br />sists of maps co~erlng the South Platte RI~er ha~lng 11 scale of <br />I Inch equllls 200 feet with a 2-foot contour Inter~1I1 IInd M0pS <br />co~ering " portion of the P",wnee Creek o~erf 10'" area southwest of <br />Sterling hll~1 ng " sc"le of Inch equals 100 feet "nd a conto~r <br />Inter~a I of 2 feet. These maps were prepar..d In 1977 by 8le~lns <br />Mapping Company of Den~er, Colorado under contract with the <br /> <br />Plates 8 - 12 show floodIng north IInd west from U.S. <br />Hlgh...ays6and 138. floodwaters In this area are generally <br />shallow, flowlnq lit ~arlous depths and In Irregular flow pllttBrnS <br />depending upon the topography. Due to this Irregularity of flo..., <br />f load depths are di fflcul t to pred Ict. Kowe~er, based UpOn limited <br />sur~ey data available In "this area tor thIs study, the IOO-year <br />flood Is astlm"ted to a~erage 2 feet In depth In the arell deslg- <br />natBd as "subJect to 100-yearshllllow tl()()dlng". Thlsdeptn Is <br /> <br />" <br /> <br />31 <br />