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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 ele- vations of floods of the selected recurrence intervals along each stream studied in detail, Water-surface elevations of floods of the selected recurrence in- tervals were computed through the use of the COE HEC-2 water-sur- face profile' computer program (Reference 20). Starting water- surface elevations for Lefthand Creek, Dry Creek No.1, and Spring Gulch correspond to the computed water-surface elevations for the St. Vrain Creek at the confluence of the three streams. The flooding in Loomiller Basin is in the form of sheet runoff, in which velocities are low, in depths less than 1.0 foot. The 50-, 100-, and 500-year floods along Dry Creek NO, 1 are not contained by the Dry Creek No. 1 channel, Excess flows for these frequencies will be directed north to St. Vrain Creek along most of the study reach. The water-surface profiles for this stream were therefore computed by manual calcu-lation methods using con- trol sections at culverts to reflect this condition. Detailed cross section data for the St. Vrain Creek were obtained from the COE and supplemented with additional cross sections taken from maps at a scale of 1:4,800, with a contour interval of 2 feet, also prepared by the COE (Reference 6). Detailed cross sections for Lefthand Creek, Dry Creek NO.1, and Spring Gulch were field surveyed in September 1975. The cross sections were located at close intervals above and below bridges and culverts in order to accurately compute backwater effects at these structures. USGS topographic mapping enlarged to ~ scale of 1:6,000, with a contour interval of 10 feet, was used to, supplement field-survey data (Ref- erence 21). Locations of selected cross sections used in the hydraulic anal- yses are shown on the Flood prof~les (Exhibit 1). For stream seg- ments for which a floodway was cpmputed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 2), Channel roughness factors (Manni!lg's "n") for these computations were assigned on the basis of field inspection of the flood plain areas and engineering judgment. 'Bridge geometry and elevation information was obtained from the Colorado State Highway Depart- ment and the City of Longmont, when available, and measured in the field. The hydraulic analyses for this study were revised to include in- formation presented in flood plain information reports for Lefthand Creek, Dry Creek No, 1, and St. Vrain Creek (References 1-4). 14