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<br />selected recurrence intervals along each of the flooding <br />sources. Users should be aware that flood elevations shown on <br />the FIRM represent rounded whole-foot elevations and may not <br />exactly reflect the elevations shown on the Flood Profiles or <br />in the Floodway Data tables in the FIS report. Flood <br />elevations shown on the FIRM are primarily intended for flood <br />insurance rating purposes. For construction and/or flood <br />plain management purposes, users are cautioned to use the <br />flood elevation data presented in this FIS in conjunction with <br />the data shown on the FIRM. <br /> <br />Water-surface elevations for the 10-, 50-, 100- and SOO-year <br />floods for Coal and Oak Creeks were computed using the U. S. <br />Army Corps of Engineers HEC-2 Water Surface Profile Program <br />(Reference 6). Cross sections for this analysis were obtained <br />from aerial orthophoto mapping flown in February 1977, coupled <br />with appropriate field verification (Reference 7). <br /> <br />Roughness factors (Manning's "n") were determined by field <br />inspection. Ground leve 1 photography aided in the selection <br />of roughness factors and aerial photography was used to <br />determine the limits of each roughness zone. Roughness values <br />for the main channel of Oak Creek ranged from 0.032 to <br />0.045 with flood plain values ranging from 0.030 to 0.080 for <br />all floods. Roughness values for the main channel of Coal <br />Creek range from 0.032 to 0.047, with flood plain roughness <br />values ranging from 0.040 to 0.090 for all floods. Roughness <br />values for the Arkansas River ranged from 0.032 in the channel <br />and 0.06 on the overbanks for all floods. Roughness values <br />for the East and West Coal Creek Overflows ranged from .016 <br />to .04. <br /> <br />The hydraulic analyses for the Arkansas River were determined <br />as part of the June 1982 Flood Plain Information Report <br />(Reference 8). Water-surface elevations were calculated using <br />the u.S. Army Corps of Engineers HEC-2 computer program <br />(Reference 6). <br /> <br />Starting water-surface elevations for Coal Creek East <br />Overflow, Coal Creek West Overflow, and Oak Creek were <br />determined from critical depth elevations at the confluence <br />wi th the Arkansas River. Starting water-surface elevations <br />for Coal Creek were obtained from the water-surface profile <br />elevations for the Arkansas River. <br /> <br />Flow overtops the channel banks of Coal Creek at the Denver <br />And Rio Grand Western Railroad bridge and the East Main Street <br />bridge and moves east and west from the main channel. A <br />di vided flow analysis was used on the railroad bridge and <br />other bridges within Florence to proportion the flow between <br />the channel and the overbanks of Coal Creek. Additional cross <br />sections were obtained and hydraulic computations made to <br />define the water-surface elevations along the east and west <br />overbank flow paths. Backwater computations were made for <br /> <br />-7- <br />