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<br />Water-surface profiles for Cherry Creek utilized in this FrS were <br />obtained from a COE Flood Plain Information report (Reference 14), <br />and were developed using the COE HEC-2 step-backwater computer <br />program (Reference 25). The computations were based on channel <br />and flood plain conditions as represented by the survey data <br />gathered in April 1975 and as supplemented by later field investi- <br />gations. <br /> <br />Cross section data were obtained by field measurements except for <br />portions of East Plum and Happy Canyon Creeks, and Sellers, New- <br />lin, Baldwin, Sulphur, and Tallman Gulches. These areas were <br />photographed and mapped, and digitized cross sections were ob- <br />tained from the contour plans. All bridges, culverts, and other <br />structures were surveyed to obtain elevation data and structural <br />geometry. <br /> <br />Locations of selected cross sections used in the hydraulic ana- <br />lyses are shown on the Flood Profiles (Exhibit 1). For stream <br />segments for which a floodway was computed (Section 4.2), selected <br />cross section locations are also shown on the Flood Insurance Rate <br />Map (Exhibit 2). <br /> <br />Roughness coefficients (Manning's On") were estimated by field <br />investigation and from pictures of the stream and its flood plain <br />using USGS Water Supply Paper 1849 (Reference 26), Open-Channel <br />Hydraulics (Reference 27), and Handbook of Applied Hydraulics <br />(Reference 28). The On" values for Plum Creek, East Plum Creek, <br />West Plum Creek, Indian Creek, Hangmans Gulch, Sellers Gulch, <br />Unnamed Tributary to Sellers Gulch, and Carpenter Creek ranged <br />from 0.03 to 0.06 in the main channels and from 0.04 to 0.12 for <br />the overbanks. The On" values for Cherry Creek ranged from 0.04 <br />to 0.055 for the channel and from 0.061 to 0.095 for the over- <br />banks. For Happy Canyon Creek, Newlin Gulch, Baldwin Gulch, <br />Sulphur Gulch, Tallman Gulch, and Bayou Gulch, the "nn values <br />ranged from 0.03 to 0.05 for the main channels and from 0.03 to <br />0.12 for the overbanks. For the South Platte River, Horse Creek, <br />and West Creek, the .'n" values ranged from 0.03 to 0.045 for the <br />main channels and from 0.016 to 0.12 for the overbanks. <br /> <br />For Cherry Creek, the COE used critical depth to determine the <br />starting water-surface elevation. For every other stream studied <br />by detailed methods, the starting water-surface elevation was <br />determined using the stream slope at a starting valley cross <br />section. <br /> <br />The methods used in estimating elevations of streams studied by <br />approximate methods included direct use of UDFCD information, <br />calculation of normal depth from field cross sections with an <br /> <br />25 <br />