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3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments where a floodway was computed (Section 4.2), selecied cross section locations are also shown on the Flood Insurance Rate Map (Exhibit 2). Channel roughness factors (Manning's "n") for the hydraulic computations were assigned on the basis of field inspection of the floodplain areas and engineering judgment. Likewi se, bridge and culvert coefficients used in computations were assigned on the basis of field inspections of structures and engineering judgment. Flood profiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). Lee Gulch and Little5Creek cross sections were field-surveyed and located at close intervals above and below bridges and culverts in order to compute the effects of backwater. For Big Dry Creek and Slaughterhouse Gulch, cross sections were taken from detailed topographic maps (References 6 and 13). Oetai led mapping of the South Platte was secured from the COE. The USGS topographic mapping at a scale of 1:24,000, with a contour interval of 10 feet, was used with the field survey data (Reference 14). As a result of the channel improvement project for the reach of Little's Creek downstream from the Atchison, Topeka and Santa Fe and the Denver and Rio Grande Western railroads to its confluence wi th the South Plat te Ri ver, the IOO-year base flood would be contained wi thin the channel. The hydraul ic reanalysi s for this reach of Little's Creek was performed by Love and Associates, Inc., in January 1989 (Reference 28). Water-surface elevations of floods of the .sele-cted recurrenee intervals were computed through the use of the COE HEC-2 Water- Surfa~e Profile computer program (Reference 12). Starting elevations for the tributaries of South Platte River were taken from previously computed stage-discharge relationships when available. In many cases, control elevations were shifted upstream to bridges or .cui verts. Where no other information or control structures were available, the starting water-surface elevations were computed by the slope-area method option of the HEC-2 program. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 12