Laserfiche WebLink
<br />Peak discharge-dl"all1age ar....:a n:::lalionships tor South Pl.atte kiver, <br />Cache La Poudre River, Sheep, Ashcroft, and Eaton Draws, Tri-Area <br />Drainageway, Coal Creek, and the Big Thompson River are shown in <br />Summary of Discharges (Table 1). <br /> <br />Discharges for U.S. Highway 34 Levee Overflow Channel were obtained <br />using a weir flow equation, <br /> <br />Q CLH3/2 <br /> <br />where Q = <br /> C <br /> L = <br /> H = <br /> <br />Total flow over the weir <br />Coefficient of discharge <br />Effective length of weir controlling flow <br />Difference between the energy grade line elevation and <br />the roadway crest elevation <br /> <br />Values of H were determined by using the COE HEC-2 computer program <br />(Reference 15) where the flow was confined to the channel. The <br />100- and 500-year discharges ranged from 550 to 600 cfs and from <br />7,000 to 12,500 cfs, respectively. <br /> <br />The 10-, 50-, 100-, <br />Drainageway Overflow <br />respectively. <br /> <br />and 500-year discharges for the Tri-Area <br />Channel are 240, 780, 850, and 1,100 cfs, <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the county <br />were carried out to provide estimates of the elevations of floods <br />of the selected recurrence interva'ts along each stream studied in <br />the county. <br /> <br />Cross section data for the South Platte River were obtained from <br />field surveys done by the COE for the Special Flood Hazard <br />Information Report (Reference 5) and by field surveys performed by <br />the study contractor. All bridges were surveyed by the COE to <br />obtain elevation data and structural geometry. Roughness <br />coefficients (Manning's "n") were estimated by field inspection. <br />Channel "n" values ranged from 0.035 to 0.100. Water-surface <br />profiles were computed using the COE HEC-2 computer program <br />(Reference 12). Starting water-surface elevations for the South <br />Platte River were determined using the slope area method 2,000 feet <br />downstream of the study reach. <br /> <br />Cross sections for the Cache La Poudre River were taken from <br />topographic maps by COE for their Flood Plain Information Report <br />(Reference 4). All bridges were field surveyed to obtain elevation <br />data and structural geometry. Roughness coefficients were selected <br />by the COE. The channel "n" value was 0.040, and the overbank "n" <br />values ranged from 0.060 to 0.100. The 100-year water-surface <br />profile was computed by the COE (Reference 15). Profiles for the <br />10-, 50-, and 500-year floods were hand calculated using the step- <br /> <br />11 <br />