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<br />the confluence with the Arkansas River (3+30) <br />limit is approximately 70 feet upstream of <br />Atchison Topeka and Santa Fe Railroad (98+89) <br /> <br />and the upper <br />the abandoned <br /> <br />Roughness coefficients (Manning 's "n") used in the hydraulic <br />analysis were determined using engineering judgement and based <br />on field observations, orthophoto topographic mapping, and <br />ground level photography of the channel and flood plain areas. <br />Factors affecting the determination of the Manning's "n" <br />include channel material, channel irregularity, obstructions, <br />vegetation, and the degree of channel meandering. The <br />roughness value used for the main channel was 0.045, and for <br />the flood plain areas the roughness values range from 0.035 to <br />0.15. The selected roughness coefficients are referenced in <br />tables presented in the handbook qpen Channel Hydraulics <br />(Reference 13). <br /> <br />The recommended values for the contraction and expansion <br />coefficients for gradual transitions are 0.1 and 0.3, <br />respecti vely. When the change in effective cross-sectional <br />area is abrupt and increased losses occur, such as at bridges <br />or culverts, the recommended contraction and expansion <br />coefficients are 0.3 and 0.5, respectively. <br /> <br />The computer program HEC-RAS (Reference 12) was used to <br />develop the 10-, 2-, 1-, and 0.2-percent-chance water surface <br />elevations for Oak Creek. The computer analysis uses the <br />mixed flow regime method which computes both supercritical and <br />subcritical solutions. For the supercritical flow regime, the <br />model starts with critical at the upstream cross section. The <br />subcritical flow regime uses the downstream cross section and <br />the slope-area method for computing the water surface <br />elevations for the 10. 0-, 2.0-, 1.0-, and O. 2-percent-chance <br />floods. Although Oak Creek is supercritical, the bridge or <br />cuI vert crossings are subcri tical and the backwater affects <br />several cross sections upstream of the structures. <br /> <br />The effective floodway for Oak Creek has been removed from the <br />FIRM as a result of this restudy. No new floodways have been <br />calculated for the following flooding sources in the City of <br />Florence: Arkansas River, Oak Creek, Oak Creek Right Overbank, <br />and Minnequa Canal. <br /> <br />This revision relied on mapping developed in 1977 by Nelson, <br />Haley, Patterson, and Quirk, Inc., for the Comprehensive <br />Drainage and Flood Study for Florence, Colorado, <br />(Reference 14). Nelson, Haley, Patterson, and Quirk mapped a <br />portion of the revised study area using aerial photography to <br />produce orthophoto topographic mapping at a scale of 1"=100' <br />with 2-foot contour intervals and showing all planimetric <br />features such as streets and major structures. This mapping <br />provided by the City of Florence covers the area that ends <br />approximately 800 feet upstream of Minnequa Canal. <br />Consequently, the FIS study reach extends approximately <br />three-quarter miles upstream of the limit of the orthophoto <br />topographic mapping coverage provided by the City. For the <br />reach of Oak Creek that lies beyond the mapping coverage of <br /> <br />17 <br />