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<br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in <br />the community were carried out' to provide estimates of the <br />elevations of floods of the se.lected recurrence intervals <br />along each stream studied in the community. <br /> <br />Water-surface elevations of floods of the selected <br />recurrence intervals were computed through use of the <br />U. S. Army Corps of Engineers .HEC-2 step-backwater <br />computer program (Reference 12). Starting elevations <br />for the tributaries to the South Platte River were taken <br />from the flood profile elevation of the corresponding <br />flood frequency on the South Platte River. In many cases, <br />this control elevation shifted upstream to bridges or <br />culverts along Santa Fe Drive and/or the parallel rail- <br />road structures. . <br /> <br />Lee Gulch and Little Creek cross sections were field- <br />surveyed and were located at close intervals above and <br />below bridges and culverts in order to compute the <br />effects of backwater. For Big Dry Creek and Slaughter- <br />house Gulch, cross sections Were taken from detailed <br />topographic maps (References 4 and 13). Detailed mapping <br />of the South Platte was secured from the U. S. Army Corps <br />of Engineers. The U. S. Geological Survey topographic <br />mapping at a scale of 1:24,000, with a contour interval <br />of 10 feet, was used with the 'field survey data (Refer- <br />ence 14). <br /> <br />Locations of selected cross sections used in the hydraulic <br />computations are shown on the Floodway Maps (Exhibit 2). <br />Channel roughness factors (Manning's On") for these <br />computations were assigned on the basis of field inspec- <br />tion of the flood plain areas ,and engineering judgment. <br />Likewise, bridge and culvert qoefficients used in <br />computations were assigned on:the basis of field inspec- <br />tions of structures and engin4ering judgment. <br /> <br />Flood Profiles were drawn showing computed water-surface <br />elevations to an accuracy of 0.5 foot for floods of the <br />selected recurrence intervals (Exhibit 1). <br /> <br />All elevations are referenced'to the National Geodetic <br />Vertical Datum of 1929 (NGVD). Elevation reference <br />marks used in the study are s~own on the maps. <br /> <br />The hydraulic analyses completed in this study did not <br />account for probable debris blockage of bridges and <br />culverts. The potential for blockage exists, and plan- <br />ning in or near the flood plain should recognize the <br />limitation of this data if structures become inoperable <br />and fail. <br /> <br />15 <br />