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

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Design Point 2 Basin R -2 contains approximately 23.42 acres and will follow historic drainage patterns and sheetflow off the property to the east and into wetlands adjacent to the Cache la Poudre River. Basin R -2 contains undeveloped, agricultural land and therefore has an imperviousness of 2 percent. The runoff coefficient was calculated to be 0.409 for the 100 - year storm. A time of concentration of 83 minutes would results in a 100 -year rainfall intensity of 2.24 inches per hour. Applying the rational method produced a peak runoff of 21.49 cfs. Reclaimed conditions for this basin will return to historic conditions and will not increase the runoff, or area, flowing to design point 2. Basin R -3 is 50.08 acres and will consist entirely of the reclamation pond. The proposed pond has a surface area of 36.8 acres and an available storage volume for storm water capture of 115 acre -feet above the normal pool. The unlined pond is assumed to have a minimum of approximately 3 feet of freeboard above natural high water level before overtopping could occur. The surface hydrology associated with the reclaimed condition will not adversely impact the adjacent properties. Final grading will result in contours that either match or are below pre - mining contours. Surface runoff from the majority of the site will flow directly into the unlined pond and upland areas. 8.0 SAFETY HAZARDS The drainage structures proposed for this project have minimal associated safety hazards. Safety concerns for the mining operation have been addressed in the permit applications to the State. 9.0 CONCLUSION This report was prepared in compliance with the Weld County Code and the Weld County Storm Drainage Criteria Addendum to the Urban Storm Drainage Criteria Manuals Volumes 1, 2 and 3. The proposed Iverson Mine development will reduce the runoff from the site due to the capture of runoff within the mining area. Upon completion of the mining project, the screening/overburden stockpile berms will be removed and used for reclamation of the unlined open water pond and upland areas. For existing conditions, the sum of all drainage basin areas is 104.81 acres. Direct precipitation on the reclaimed unlined pond and the sum of the basin area that would be captured by the unlined pond is 50.08 acres. The reclaimed site will reduce the contributing drainage area to approximately 48 percent of the historic area without increasing the imperviousness of the contributing drainage area. - 5 - Final Drainage Report Iverson Mine P:\23511\133-23511-10002 \Deliverables \Drainage Report \Drainage Report.doc February 2011