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<br />Hydraulic analyses for Boxelder Creek and Cooper Slough were conducted by Anderson <br />Consulting Engineers, Inc. (Reference 75). Water surface profiles were developed using <br />HEC-RAS. A combination of field survey data and orthophoto based topographic mapping <br />(prepared at a 2-foot contour interval) was used as the base information for defining cross <br />section geometry and for delineating floodplains. For Boxelder Creek and Cooper Slough, <br />Manning's n-values generally range from 0.035 to 0.060 for the channels and from 0.045 <br />to 0.070 for the overbanks. <br /> <br />Eight study reaches located within the Boxelder Creek/Cooper Slough basins required <br />boundary conditions. During the hydraulic analyses, twelve splitflows were identified <br />within the Boxelder Creek basin, and two splitflows were identified within the Cooper <br />Slough basin. Also, many shallow flooding areas exist adjacent to the floodplains for both <br />basins. <br /> <br />Cross-sectional data for Cache La Poudre River, including overbanks, for the backwater <br />analyses were obtained by field survey. All bridges were surveyed to obtain elevation data <br />and structural geometry. The land-use and hydraulic-roughness data were also obtained by <br />field surveys. <br /> <br />Water-surface elevations for floods of the selected recurrence intervals were computed <br />using the USACE HEC-2 step-backwater program (Reference 22). Starting water-surface <br />elevations for the Cache La Poudre River at the Larimer-Weld County line were obtained <br />from a concurrent study in Weld County (Reference 40). <br /> <br />Channel and overbank hydraulic roughness factors (Manning's "nn) used in the hydraulic <br />computations were determined using engineering judgment and were based on field <br />observations of the stream and flood plain areas and the descriptions presented in standard <br />engineering references (Reference 41). The channel "n" value for the Cache La Poudre <br />River was estimated as 0.035 throughout the study reach, and the overbank "n" values <br />ranged from 0.043 to 0.100. The hydraulic analyses for this study were based on <br />unobstructed flow. <br /> <br />During the analysis, several divided-flow and split-flow reaches were identified. The most <br />prominent split flows occur near the upstream end of the study reach, just downstream of <br />Horsetooth Road. For the 2-, 1-, and 0.2-percent annual chance flood events, water splits <br />out over the right overbank and becomes separated from the main flow by the embankment <br />for Interstate Highway 25 (1-25). <br /> <br />The divided-flow path extends southerly along the west side of 1-25, before crossing 1-25 <br />and returning to the main flow path. There are two locations where water splits away from <br />the divided flow path and returns to the main path. <br /> <br />A third flow path occurs just north of County Road 36E. This path is caused by flow <br />escaping from the 1-25 divided-flow path as it returns to the main channel. <br /> <br />At the downstream end of the study reach, another split-flow situation is present. Water <br />splits from the left overbank over the Larimer-Weld County Line Road. This split is <br />affected by backwater conditions, so the weir coefficient for the split has been greatly <br /> <br />30 <br />