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Mr. Clay Kimmi <br />December 9, 2008 <br />Page 2 <br />MODELING ASSUMPTIONS <br />Study Reach <br />The downstream study limit for this analysis is Weld County Road (WCR) 18 located at the <br />north section line of Section 36. The study reach extends upstream for approximately 4,600 feet <br />along the channel of the South Platte River. A total of 10 cross sections were modeled at <br />locations illustrated in Figure 2. <br />100-year Flow Rate <br />A steady-state peak discharge of 29,000 cubic feet per second (cfs) was modeled as <br />representative of a 100-year flood event. This flow rate was obtained from the October 1978 <br />Flood Insurance Study (FIS) for the Town of Fort Lupton (Attachment B). The downstream <br />study limit for the FIS is located approximately one-half mile upstream of this current study for <br />the Fort Lupton Sand and Gravel Mine. <br />Pre-Mining Topography <br />A topographic base map with 2-foot contour intervals was developed from an April 7, 2008 <br />aerial survey (Figure 3). An estimation of pre-mining topography was made by altering this base <br />map. Contours representing disturbed areas of stockpiles or mining were removed from the map. <br />Also, depression contours were removed from the topographic map because these natural or <br />man-made depressions result in ineffective flow below the ground surface. Removing these <br />contours has the same effect in HEC-RAS modeling as creating ineffective flow areas below the <br />ground surface. Contours in the vicinity of the East Lateral of the Lupton Bottom Ditch were <br />preserved because this ditch alignment has not been disturbed by the mining activity. The <br />resulting pre-mining topography is presented in Figure 4. <br />Existing Topography <br />For HEC-RAS modeling of existing topographic conditions, it was necessary to eliminate the <br />ineffective flow in natural and man-made depressions. The resulting topographic map for <br />modeling existing conditions is presented in Figure 5. <br />Manning's Roughness Values <br />A Manning's roughness value of 0.035 was used to represent both channel and overbank <br />portions of the modeled cross sections. This n value is appropriate for channels with moderate <br />meandering, overbank fields with moderate to tall grass, and overbank mined areas with <br />stockpiles and berms. Although a lower n value of 0.020 can be used to represent overbank flow <br />over the surface of a submerged gravel pit, the n value of 0.035 was conservatively used in these <br />regions also.