2008-09-03_PERMIT FILE - M2008006 (2)

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t Flood Hazard and Drainage Studies Relevant to Site The entire Site is located within a Zone AE 100-yr floodway boundary, which is delineated on FEMA FIRM Map # 08069C 1202F. A copy of this map is attached in Appendix A for your reference. A floodplain development permit has been submitted to the Town of Windsor including HEC-2 modeling that shows that the proposed changes to the site will cause no rise in the floodway. HISTORIC DRAINAGE Major Basin The Site is located within the Timnath Reservoir Basin as defined in the Report. The basin is large but it appears that only a portion of the stormwater within the basin is routed through the Site. The Report indicates that during most storm events, off-site flows that are routed on to the Site enter through a 36" RCP under HWY 392. The 36" RCP has limited capacity because of sediment buildup. An existing drainage channel located on the Site conveys the off-site stormwater to the Poudre River. The Report also indicates that during a 100-yr event the limited capacity of the 36" RCP, along with other factors, will most likely cause stormwater to overtop HWY 392 and enter the Site along the northeast property line. Common practice for mining sites is to strip the topsoil from the site to expose the gravel reserves underneath. This practice effectively eliminates all runoff from the site because rainfall infiltrates almost immediately into the exposed sand and gravel deposit. Since all runoff from the Site will be retained within the mining pit, no additional developed basins are needed to attenuate stormwater flows. Once mining operations have begun, all stormwater will be retained within the gravel pits. The flows that are routed to the pit will be pumped out of the pit along with the dewatering operation back to their historic flow patterns. For areas where mining will not take place, the runoff patterns will not be altered from their historic conditions. Because of these conditions, a specific hydrologic analysis is not necessary for these portions of the Site. There is only one on-site historic basin designated as Basin A (please see the attached Existing Basin Delineation). This basin contains areas that are both off and on the mining site. The off- site basin runoff flows to the south and enters the project site from the north. All flows from the south and east do not enter or affect the Site. Basin A was assumed as two percent impervious, which is a conservative value for undeveloped areas. An NRCS soil type of C was used for these basins based on the predominant soil types on the Site. Sub-Basin and Site Drainage Basin A: 1 Basin A is approximately 38 acres and consists of areas on-site and off-site. Topography shows that a portion of HWY 392, CR 13, and a small portion of area to the west will drain on to the Site. The existing bank of the Poudre River was used as the downstream boundary for this basin. Final Drainage Plan and Report - SerFer Pit Applegate Windsor, CO 3 Group, January 2008