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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. Water-surface elevation of floods of the selected recurrence intervals were computed through the use of the CDE HEC-2 step- backwater computer program (Reference 38). Starting water-surface elevations for the tributaries of the South Platte River were taken from previously computed stage-discharge relationships when available. In many cases, control elevations were shifted upstream to bridges or culverts. 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. Detailed cross section data for Cottonwood Creek, Cherry Creek, Piney Creek, Murphy Creek, Coal Creek, Comanche Creek, Little Comanche Creek, West Bijou Creek, and Box Elder Creek were field surveyed and were located at close intervals above and below culverts in order to compute the effects of backwater. For Little Dry Creek, Big Dry Creek, and Sand Creek, cross sections were taken from detailed topographic maps (References 30 and 34). Detailed mapping of the South Platte River was secured from the CDE. The USGS topographic mapping, at a scale of 1: 24 ,000, wi th a contour interval of 10 feet, was used to supplement field survey data (Reference 29). Hydraulic analyses included in the Flood Insurance Studies for the incorporated communities of Aurora, Cherry Hill Village, Columbine Valley, Englewood, Greenwood Vi llage, Li ttleton, and Sheridan were incorporated into the restudy in their entirety with the exception of streams or portions of streams which were superseded by more up- to-date information (References 2, 3, and 5 through 9). Hydraulic analyses for portions of First Creek, Piney Creek, Murphy Creek, Lone Tree Creek, Happy Canyon Creek, Cottonwood Creek, Lee Gulch, and Little's Creek were taken from published UDFCD reports (References 10, 11, 12, 13, 14, 15, and 16). Additional hydraulic analyses from the various engineering reports discussed in Section 7.0 have been incorporated into the Arapahoe County restudy. Hydraulic analyses for portions of Big Dry Creek Tributary B, East Tributary to West Toll Gate Creek, First Creek, Sampson Gulch, and Senac Creek were performed using topographic maps at a scale of 1:24,000, with a contour interval of 10 feet (Reference 39). Field surveyed cross sections were used and normal-depth calculations were performed in order to obtain top widths at the selected cross sections. 22