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NUVEMCO LAST CHANCE MINE <br />STORM WATER MANAGEMENT PLAN <br />SEDIMENT POND #2 <br />Time of Concentration <br />In order to use this information it is necessary to calculate the Time of Concentration, or <br />duration for the study area. The time of concentration used to determine the average <br />intensity of rainfall, is equivalent to the amount of time needed for runoff to travel from <br />the most remote point of the basin to the design point. <br />For the study area, this distance is only approximately 65' using the previously described <br />AutoCAD drawing, Figure 1. There are various methods available to compute the time <br />of concentration. Various publications recommend minimum times from 5 minutes in <br />urban areas to 10 minutes in rural areas. Using the Kirpich formula and the sdsu t, <br />calculator, the time of concentration for the small study area is less than 1 minute. See <br />Appendix E. For this study a time of concentration of 5 minutes will 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 the attached printout. <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 = 0.18 acres <br />Q = 0.90 cfs (See attached printout from sdsu.edu.in Appendix E.) <br />Pond Outlet <br />The peak discharge computed above of 0.90 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 />Inputting the above in the Haestad Methods, Inc computer program FlowMaster and <br />solving for the depth of water, it was determined that the depth would be 0.26 feet. As <br />the outlet silts in, the coefficient will be reduced and the depth will be less. Output for <br />March 2010 6 <br />