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<br />reviewed and accepted by the Town of Windsor, FEMA, and the CWCB, <br />Results of the COE hydrology analysis are shown in Summary of <br />Discharges (Table 1). <br /> <br />Cross-sectional data, including overbanks, for the backwater <br />analyses were obtained by field survey. All bridges were surveyed <br />to obtain elevation data and structural geometry. The land-use and <br />hydraulic-roughness data were also obtained by field surveys. <br /> <br />Water-surface elevations for floods of the selected recurrence <br />intervals were computed using the COE HEC-2 step-backwater program <br />(Reference 41). Starting water-surface elevations for the Cache La <br />Poudre River were determined by normal depth calculations. <br /> <br />Channel and overbank hydraulic roughness factors (Manning IS "nil) <br />used in the hydraulic computations were determined using <br />engineering judgment and were based on field observations of the <br />stream and floodplain areas and the descriptions presented 1n <br />standard engineering references (Reference 42). The channel "n" <br />value for the Cache La Poudre River was estimated as 0.035 <br />throughout the study reach, and the overbank "n" values ranged from <br />0.043 to 0.110. The hydraulic analyses for this study were based on <br />unobstructed flow. All computed water-surface elevations are <br />referenced to the NGVD of 1929. <br /> <br />Four locations of divided flow were identified within the study <br />reach; at the Larimer/Weld county boundary, at Weld County Road 17, <br />at Colorado State Highway 257, and at the Kodak and Windsor <br />wastewater treatment plants, Each of these divided flow paths were <br />modeled separately using HEC-2. <br /> <br />Floodplain boundaries were delineated .using topographic maps at a <br />scale of 1:2,400 with a contour interval of 4 feet. These maps <br />were provided by the CWCB solely for mapping purposes (Reference <br />43). The date of the mapping is unknown. The hydraulic analysis <br />for the Cache La Poudre River in Weld County was based on surveyed <br />cross-sectional data. In locations where the contours on the <br />mapping do not agree with surveyed data, the floodplain was mapped <br />according to the surveyed data. <br /> <br />Computation of portions of the floodway in the study reach required <br />nonstandard solutions due to the numerous divided flow paths. In <br />location of divided flow, it was attempted to channel all of the <br />flow into the mainstem path, limiting the flow to the 100-year <br />floodplain. If the water-surface elevation increase was less than <br />the allowable surcharge, the main flow path was encroached by <br />proportional conveyance reduction. If the water-surface elevation <br />increase caused by the additional discharge was greater than the <br />allowable surcharge, then water was allowed to flow both along the <br />main flow path and the involved divided flow paths. The divided <br />flow paths were then encroached using proportional conveyance <br />reduction until the allowable surcharge was reached. The floodway <br />was then defined as the outer limits of the two paths. <br /> <br />46 <br />