2008-07-10_PERMIT FILE - M2008006 (3)

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1 SUMMARY REPORT ' This application is for a Town of Windsor Floodplain Development Permit for the SerFer Pit Project, which is located at the southeast'corner of the intersection of County Road 13 and State Highway ' 392, in the NE 1/4 of Section 24, Township 6 North, Range 68 West, Larimer County, Colorado. The applicant proposes to mine alluvial sand and gravel with an end use of lined water storage. This application and the supporting information are to investigate the impacts of the proposed mining operations on the floodplain and floodway water surface elevations. The SerFer Pit Property is approximately 45 acres and is currently used for general agricultural purposes. The property is bordered by the Cache la Poudre River to the south, County Road 13 to the east, State Highway 392 to the north and open space to the west. A vicinity map is attached for your convenience. The SerFer Property is located on map number 0869C1202F of Larimer County, CO Flood Insurance Rate Map (FIRM) dated December 19, 2006. The FIRM shows the property area within a Zone AE, consequently base flood elevations (BFE's) are determined. The area also shows a floodway delineated. A copy of the FIRM with the property identified is attached. i The Larimer County Flood Insurance Study (FIS) was revised in November 1993 to include some of the additional split flows along the Cache la Poudre River. The FIS was revised again December 19, 2006 in order to change the Base Flood elevations from the NGVD 29 vertical datum to the NAVD 88 vertical datum. However, the modifications to the FIS were not reflected in the HEC-2 hydraulic modeling related to the study and as a result, the HEC-2 Model remains in the NGVD 29 vertical datum. The vertical difference between the datums in the area of the site was determined to be ' Approximately 3.05 ft. This value was obtained from the National Geodetic Survey's (NGS) VERTCON - North American Vertical Datum Conversion Utility located on the NGS websitel. Modeling Analysis The effective hydraulic model was obtained from Anderson Consulting at the suggestion of Larimer County's Floodplain Manager, Ed Woodward. The model was obtained for a stretch of river from Interstate 25 to the Town of Windsor. During discussions with Anderson, it was indicated that modeling east of County Road 13 has been completed but is not currently reflected on any FEMA ' maps. Because of limited existing information within the effective model, it was determined that a duplicate effective HEC-RAS Model would not reflect accurate results. Therefore, the best way to r update the effective model would be to keep it in the HEC-2 format. A duplicate effective model was run in HEC-2. Printouts of the input and the output for the HEC-2 model are in the located in Attachment G of this document. After examination of the effective model's cross sections located on the property compared to topographical information for the site provided by the Town of Windsor, it was determined that the updated topography represented a more accurate portrayal of the existing conditions. As a result, http:.I A7\A7fA/ gs.noaa gQy/T00LS,.I.Vertco vertcon html Serfer Pit I Summary Report