2009-05-08_PERMIT FILE - M1978222UG

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Nuvemco Monogram Mine SWMP Assuming the stream section will be a constructed trapezoidal channel with 311: IV side slopes, a = 30°. 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 = 0.039 The minimum weight is 2.4 pounds or the minimum size is 0.465 ft. Using Hydraulic Design Chart 712-1 (Copy attached) for a design velocity of 2.82 fps the design velocity is less than shown on the Chart Sheet 1 of 2. Using the minimum design velocity on the Chart of 7.5 fps and a unit weight of for sandstone of 145 pounds/cubic foot, the D50 is 0.51 feet which is slightly larger than 6 inches. Therefore, use 6 inch rough stone for the riprap. HEC 11 recommends that the riprap be applied a minimum of 1 times the channel width upstream and 1.5 times the channel width downstream. It also recommends a riprap depth of 2 times the maximum stone diameter or in this case 2 x 6 inches = 12 inches or 1 foot. A geotextile material will be placed under the riprap and be installed in accordance with the selected manufacturer's recommendations and specifications. The selected manufacturer will be consulted for the appropriate geotextile for this location. The riprap will be collected on site. If the riprap shows signs of degradation, it will be replaced as required. SEDIMENT YIELD The sediment yield for the small area was not calculated. In addition to the detention pond, the riprap in the detention pond outlet and channel will aid in sediment deposition. If the detention pond begins to fill, the sediment will be removed and disposed on site at an approved location. MAXIMUM RUNOFF VOLUME The maximum runoff volume will be the 100-year 24-hour storm plus the 10-year 24- hour storm as outline 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