2010-03-10_PERMIT FILE - M2008012

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NuvEMCO LAST CHANCE MINE STORM WATER MANAGEMENT PLAN W = minimum rock mass (size or weight) V = velocity to which bank is exposed SG = specific gravity of rock r = 70 (for randomly place rubble, a constant) a = outside slope face angle with horizontal, DEGREES Assuming the stream section will be a constructed trapezoidal channel with 3H:1 V side slopes, a = 18.43 °. To compute the specific gravity of the riprap, the stone is assumed to be sandstone with a weight of 145 pounds /cubic foot. Therefore: SG = 145/62.4 = 2.3 Inputting the above into the equation: W = 0.2 pounds (See Appendix H) Based on this, no riprap is required in the channel downstream of Sediment Pond #1. If the channel shows signs of erosion, riprap will be installed. Maximum Runoff Volume The maximum runoff volume will be the 100 -year 24 -hour storm plus the 10 -year 24- hour storm as outlined in the State of Colorado Division of Reclamation, Mining and Safety's April 10, 2009 letter. From the NOAA Atlas 2, Volume the 10 -year 24 -hour storm will generate 1.95 inches of precipitation. The 100 -year 24 -hour will generate 3.00 inches. It is assumed that none of the precipitation that falls on the area will infiltrate and all of the precipitation will become runoff. This is a worse case. 100 -year 24 -hour precipitation event 3.00 inches = 0.25 feet Net area = 46,996 sq ft 46,996 sq ft x 0.25 ft = 11,749 cu ft = 87,888 gallons 10 -year 24 -hour precipitation event 1.95 inches = 0.163 feet 46,996 sq ft x 0.163 ft = 7,660 cu ft = 57,301 gallons The total volume from the two events equals 19,409 cu ft or 145,189 gallons. By trial and error a pond, Sediment Pond #1, 54 ft wide by 54' long with 2:1 interior slopes 5 feet deep will be required without any freeboard to contain the runoff from the 10 -yr 24 -hr storm with a 1 -foot deep spillway or outlet into the Swale that runs northeast. Other shapes that fit the area may be constructed if they have an equivalent volume. March 2010