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runoff equation in the USDCM, the runoff coefficient was calculated to be 0.454 for the 100 - <br />year storm. A time of concentration of 44 minutes would result in a 100 -year rainfall <br />intensity of 3.45 inches per hour. Applying the rational method produced a peak runoff of <br />45.70 cfs. Basin M -1 will drain as it did historically and travel by overland flow until it <br />reaches the Cache la Poudre River. During mining conditions, runoff to design point 1 will be <br />reduced from historical conditions from 65.79 cfs to 45.70 cfs. The drainage area <br />contributing to flow leaving the site reduces from the historical condition of 81.38 acres to a <br />mining condition of 29.15 acres. <br />Design Point 2 <br />Basin M -2 has an area of 8.44 acres and will drain to design point DP2. No mining activities <br />are proposed for this basin and therefore the imperviousness will remain 2 percent. The <br />runoff coefficient was calculated to be 0.473 for the 100 -year storm. A time of concentration <br />of 81 minutes would result in a 100 -year rainfall intensity of 2.29 inches per hour. Applying <br />the rational method produced a peak runoff of 9.13 cfs. Basin M -2 will sheetflow across the <br />property and off the site to the east. During mining conditions, runoff to design point 2 will <br />be reduced from historical conditions from 21.50 cfs to 9.13 cfs. The drainage area <br />contributing to flow leaving the site reduces from the historical condition of 23.43 acres to a <br />mining condition of 8.44 acres. <br />Basin M -3 has an area of 67.22 acres and is completely captured within the gravel mine. <br />Water collected in the mine will be collected into a drainage swale system and conveyed to <br />the Cache la Poudre River at one of two discharge points. Please refer to the Mining <br />Drainage Plan and calculations presented in Appendix E. <br />7.0 RECLAIMED DRAINAGE CHARACTERISTICS <br />Site reclamation either will not change or will significantly reduce the drainage area and <br />historical runoff leaving the site from each historical basin. Upon completion of the gravel <br />mining, the site will be reclaimed as an open water pond and upland areas as shown in the <br />Reclamation Drainage Plan presented in Appendix F. <br />Design Point 1 <br />The reclaimed site will have three drainage basins. Basin R -1 contains approximately 31.31 <br />acres and will follow historic drainage patterns and sheetflow off the property and into the <br />Cache la Poudre River. Basin R -1 contains undeveloped, agricultural land and therefore has <br />an imperviousness of 2 percent. Based on the runoff equation in the USDCM, the runoff <br />coefficient was calculated to be 0.456 for the 100 -year storm. A time of concentration of 44 <br />minutes would result in a 100 -year rainfall intensity of 3.46 inches per hour. Applying the <br />rational method produced a peak runoff of 49.40 cfs. Basin R -1 returns to the Cache la <br />Poudre River as overland flow at DP 1. Reclaimed conditions will reduce the runoff to design <br />point 1 from historical conditions from 65.79 cfs to 49.40 cfs. The drainage area contributing <br />to flow leaving the site reduces from the historical condition of 81.38 acres to a reclaimed <br />condition of 31.31 acres. <br />- 4 - <br />Final Drainage Report Iverson Mine <br />P:\23511\133-23511-10002 \Deliverables \Drainage Report \Drainage Report.doc <br />February 2011 <br />