Laserfiche WebLink
GANSIEGIz I.IT.IA\ & ASSOCIATES <br />Hhdro;eolo,ical and Environmental Consultants <br />Studv Area and River Reach <br />The study area and river reach along the South Platte River are shown on Figures 2 and 3 <br />and extend approximately 4,500 feet upstream along the channel from WCR 18 and <br />approximately 1,500 feet upstream past the Bachofer property. A total of 7 cross- <br />sections (presented on Figures 4 and 5) extend from approximately the Lupton Bottom <br />Ditch on the west side of the study area to the east side of the river. The eastern extent of <br />the cross-sections varies from 300 to 1800 feet beyond the South Platte River channel. <br />Model Flow Rates <br />The model was used to conduct a flood plain analysis for the 5-, 10-, and 100 -year design <br />events. The 100 -year flood event (29,000 cfs) was used as the baseline flood event as <br />previously modeled by Deere and Ault in their Flood Hazard Development Permit #355 <br />submitted to Weld County December 9, 2008 (D&A, 2008). This peak discharge value <br />was used in the Flood Insurance Study (FIS) of the Fort Lupton area to generate the <br />existence and severity of flood hazards. The results of the study delineated the extent of <br />the 100 year event. As part of the FIS, hydrologic flood plain analyses for recurrence <br />intervals of 10, 50, 100, and 500 years were also performed. These recurrence intervals <br />correspond to 10,000, 22,000, 29,000 and 52,000 cfs, respectively. <br />In order to determine the 5 -year event for this modeling study, a flood frequency curve <br />was created with the recurrence intervals and discharge values from the FIS data. The <br />plot of the flood frequency curve is shown below with a best fit regression line. <br />Discharge for the 5 -year event was calculated from the regression equation on the plot <br />below to be 7,900 cfs. This peak discharge value is slightly above the value recorded at <br />the Fort Lupton Station , which currently floods the Bachofer property. <br />Flood Frequency Curve for South Platte River by Fort Lupton <br />100,000 <br />U <br />1.0 <br />1,000 <br />i <br />i <br />y= 0.421x+3.602 . <br />rx = -.992 <br />10 100 1000 <br />Recurence Interval (years) <br />-4- <br />