2011-07-11_PERMIT FILE - M2011001 (12)

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undisturbed. For reclamation, there are no proposed hydraulic components such as conveyance channels, rundown channels, culverts, grade control structures, or detention outlets. The pond will be constructed with an emergency spillway to maintain the normal pool elevation. 5.0 EXISTING DRAINAGE CHARACTERISTICS The existing drainage conditions at the Iverson Mine site are presented in the Historic Drainage Plan presented in Appendix D. The site was delineated into two basins, H -1 and H- 2 and was analyzed with two design points, DP 1 and DP2. DP 1 is meant to represent a majority of the site at one point along the Cache la Poudre River. In reality, existing conditions do not collect at one point on the river. Site topography drains continuously along the south bank of the river. The peak runoff results for the 100 -year storm events are discussed below. The following is a more detailed description of each existing drainage basin. Calculations are presented in Appendix D. Design Point 1 Basin H -1 contains approximately 81.38 acres of undeveloped, agricultural land and therefore has an imperviousness of 2 percent. Basin H -1 has an average slope of 0.6 percent and storm water drains from southwest to northeast as overland flow and returns to the Cache la Poudre River as such. Based on the runoff equation in the USDCM, the runoff coefficient was calculated to be 0.442 for the 100 -year storm. A time of concentration of 111 minutes would result in a 100 -year rainfall intensity of 1.83 inches per hour. Applying the rational method produced a peak runoff of 65.79 cfs. Design Point 2 Basin H -2 contains approximately 23.43 acres of undeveloped, agricultural land and therefore has an imperviousness of 2 percent. Basin H -2 has an average slope of 0.9 percent. Storm water drains from the southwest to northeast and sheetflows off the property to the east to wetlands adjacent to the Cache la Poudre River. Based on the runoff equation in the USDCM, the runoff coefficient was calculated to be 0.409 for the 100 -year storm. A time of concentration of 83 minutes would result in a 100 -year rainfall intensity of 2.24 inches per hour. Applying the rational method produced a peak runoff of 21.50 cfs. 6.0 MINING DRAINAGE CHARACTERISTICS Mining for the Iverson Mine site will occur in one phase. Mining operations either will not change or will significantly reduce the drainage area and historical runoff leaving the site from each historical basin. The mining phase will incorporate the use of temporary topsoil and overburden stockpiles around the perimeter of the mining area. A conveyor will be used to move mined materials to the processing plant. Lafarge anticipates mining and reclaiming the site in approximately 4 -5 years; however, the rate of mining and overall life of the mine is dependent upon demand and market conditions. Design Point 1 Basin M -1 has an area of 29.15 acres of undeveloped, agricultural land and therefore has an imperviousness of 2 percent and will drain to design point DP1. No mining activities are proposed for this basin and therefore the imperviousness will remain 2 percent. Based on the -3 Final Drainage Report Iverson Mine February 2011 P:\2351 lU33-23511-10002 \Deliverables \Drainage Report \Drainage Report.doc