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NUVEMCo LAST CHANCE MINE <br />STORM WATER MANAGEMENT PLAN <br />this calculation is included in Appendix G. The resulting velocity across the outlet is <br />0.45 feet per second (fps). <br />Riprap Design <br />Based on the outlet velocity of less than 1 foot/second, no riprap is required on the <br />channel downstream of Sediment Pond #2. The slope just down stream of the outlet will <br />be re- sloped to 0.10 ft/ft to result in a velocity of 1.87 feet per second (ft/s). The velocity <br />in the existing Swale is approximately 2.47 ft/s. No riprap is proposed. If the channel <br />shows signs of erosion, riprap will be installed. (Appendix I) <br />Maximum Runoff Volume <br />The maximum runoff volume will be the 100 -year 24 -hour storm plus the 10 -year 24- <br />hour storm as outline in the State of Colorado Division of Reclamation, Mining and <br />Safety's April 10, 2009 letter. From the NOAA Atlas 2, Volume the 10 -year 24 -hour <br />storm will generate 1.95 inches of precipitation. The 100 -year 24 -hour will generate 3.00 <br />inches. <br />It is assumed that none of the precipitation that falls on the area will infiltrate and all of <br />the precipitation will become runoff. This is a worse case. <br />100 -year 24 -hour precipitation event <br />3.00 inches = 0.25 feet <br />Net area = 8,020 sq ft <br />8,020 sq ft x 0.25 ft = 2,005 cu ft = 14,998 gallons <br />10 -year 24 -hour precipitation event <br />1.95 inches = 0.163 feet <br />8,020 sq ft x 0.163 ft = 1,307 cu ft = 9,777 gallons <br />The total volume from the two events equals 3,312 cu ft or 24,775 gallons. <br />By trial and error a pond, Sediment Pond #2, 30 ft wide by 25' long with 2:1 interior <br />slopes 6 feet deep will be required without any freeboard to contain the runoff from the <br />10 -yr 24 -hr storm with a 1 -foot deep spillway or outlet into the Swale that runs northeast. <br />Other shapes that fit the area may be constructed if they have an equivalent volume. <br />SEDIMENT YIELD <br />The sediment yields for the small areas were not calculated. In addition to the detention <br />ponds, the rough rock on the detention pond outlets will aid in sediment deposition. If <br />the detention ponds begin to fill, the sediments will be removed and disposed on site at an <br />approved location. <br />March 2010 7 <br />