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NuvEMco LAST CHANCE MINE <br />STORM WATER MANAGEMENT PLAN <br />minutes in urban areas to 10 minutes in rural areas. Using the Kirpich formula and the <br />sdsu t calculator, the time of concentration for the small study area is only 3.59 minutes. <br />See Appendix D for a printout. For this study a time of concentration of 5 minutes will <br />be used. <br />Using a time of concentration of 5 minutes, the rainfall intensity for a 100 -yr 24 -hr storm <br />is 5.22 inches/hour. (See Appendix D.) <br />Area - See the above section for discussion and calculations. <br />Q - Using all of the above and inputting it into the Rational formula Q = CiA where <br />Q = peak discharge, cfs <br />C = Rational Method runoff coefficient =0.95 <br />i = rainfall intensity, inches /hour = 5.22 inches/hour <br />A = Area, acres = 1.08 acres <br />Q = 5.40 cfs (See attached printout from sdsu.edu.in Appendix D) <br />Pond Outlet <br />The peak discharge computed above of 5.40 cfs will be used to size the detention pond <br />outlet as shown on Figure 1. The following assumptions were made in sizing the outlet: <br />• The outlet is trapezoidal with <br />• A bottom width of 8.00 feet; <br />• The side slopes are 3H:1V <br />• Slope = 0.001 ft/ft <br />The Mannings Coefficient is 0.040 which is for rough rock. This will result in the <br />greatest depth over the outlet. <br />Using the above in the Haestad Methods, Inc computer program FlowMaster and solving <br />for the depth of water, it was determined that the depth would be 0.68 feet. As the outlet <br />silts in, the coefficient will be reduced and the depth will be less. Output for this <br />calculation is included in Appendix F. The resulting velocity across the outlet is 0.79 <br />feet per second (fps). <br />Riprap Design <br />The next step was to size the riprap for the channel downstream of Sediment Pond #1 <br />where it enters the existing natural channel. The slope of the channel was calculated <br />using the contours shown on Figure 1. The average slope of the channel was calculated <br />to be 0.020 ft/ft <br />The riprap will be designed for the first channel section from Elevation 6277 to 6276 <br />which is the first section from the pond outlet. The riprap will use stone from on -site. <br />Using the equation: W = 0.0002 V SG /(SG -2) sin -3 (r -a) where: <br />March 2010 4 <br />