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SUMMARY REPORT <br /> This application is for a Town of Windsor Floodplain Development Permit for the SerFer Pit Project, <br /> which is located at the southeast corner of the intersection of County Road 13 and State Highway <br /> 392, in the NE Y4 of Section 24, Township 6 North, Range 68 West, Larimer County, Colorado. The <br /> applicant proposes to mine alluvial sand and gravel with an end use of lined water storage. This <br /> application and the supporting information are to investigate the impacts of the proposed mining <br /> operations on the floodplain and floodway water surface elevations. <br /> The SerFer Pit Property is approximately 45 acres and is currently used for general agricultural <br /> purposes. The property is bordered by the Cache la Poudre River to the south,County Road 13 to <br /> the east,State Highway 392 to the north and open space to the west. A vicinity map is attached for <br /> your convenience. <br /> The SerFer Property is located on map number 0869C1202F of Larimer County,CO Flood Insurance <br /> Rate Map (FIRM) dated December 19, 2006. The FIRM shows the property area within a Zone AE, <br /> consequently base flood elevations (BFE's) are determined. The area also shows a floodway <br /> delineated. A copy of the FIRM with the property identified is attached. <br /> The Larimer County Flood Insurance Study(FIS)was revised in November 1993 to include some of <br /> the additional split flows along the Cache la Poudre River. The FIS was revised again December 19, <br /> 2006 in order to change the Base Flood elevations from the NGVD 29 vertical datum to the NAVD 88 <br /> vertical datum. However, the modifications to the FIS were not reflected in the HEC-2 hydraulic <br /> modeling related to the study and as a result, the HEC-2 Model remains in the NGVD 29 vertical <br /> datum. The vertical difference between the datums in the area of the site was determined to be <br /> Approximately 3.05 ft. This value was obtained from the National Geodetic Survey's (NGS) <br /> VERTCON-North American Vertical Datum Conversion Utility located on the NGS websites. <br /> Modeling Analysis <br /> The effective hydraulic model was obtained from Anderson Consulting at the suggestion of Larimer <br /> County's Floodplain Manager, Ed Woodward. The model was obtained for a stretch of river from <br /> Interstate 25 to the Town of Windsor. During discussions with Anderson, it was indicated that <br /> modeling east of County Road 13 has been completed but is not currently reflected on any FEMA <br /> maps. <br /> Because of limited existing information within the effective model, it was determined that a <br /> duplicate effective HEC-RAS Model would not reflect accurate results. Therefore, the best way to <br /> update the effective model would be to keep it in the HEC-2 format. A duplicate effective model was <br /> run in HEC-2. Printouts of the input and the output for the HEC-2 model are in the located in <br /> Attachment G of this document. <br /> After examination of the effective model's cross sections located on the property compared to <br /> topographical information for the site provided by the Town of Windsor,it was determined that the <br /> updated topography represented a more accurate portrayal of the existing conditions. As a result, <br /> s http://www.ngs.noaa.gov/TOOLS/Vertcon/vertcon.html <br /> Serfer Pit I Summary Report 1 <br />