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<br /> <br />Calculation of runoff volumes using Rational Method (for TR-115) <br /> Accumulated Curve S=(1000/ Accumulated Accumulated No. of acres in Accumulated For <br /> precipitation, Number, CN) - 10 runoff, Q, in runoff w/o contributing acre-feet of <br /> comparison: <br /> P, in inches CN inches = [(P- initial drainage area runoff for Accumulated <br /> (acreage- (0.2XS)]^/[P+( abstractions, drainage = <br />acre-feet of <br />Pond weighted 0.8)(S) Q', in inches = (Q/12)Xacres runoff for <br /> back- PA 2/P drainage per <br /> calculation TR-115 <br /> submittal <br /> 10-yr, 24-hr. <br />MB-1 1.9 71 4.08 0.227 1.900 91.2 1.72 1.705 <br />MB-21R 1.9 82 2.20 0.584 1.900 100.6 4.89 4.918 <br />MB-3 1.9 83 2.05 0.628 1.900 8.3 0.43 0.443 <br />MB-5 1.9 66 5.15 0.126 1.900 12.4 0.13 0.126 <br />SG-1 1.9 61 6.39 0.055 1.900 2717.4 12.46 12.522 <br />RPE 1.9 71 4.08 0.227 1.900 61.7 1.17 1.2 <br /> 25-yr, 24-hr. <br />MB-1 2.3 72 3.89 0.428 2.300 91.2 3.25 2.713 <br />MB-2R 2.3 82 2.20 0.854 2.300 100.6 7.16 7.106 <br />MB-3 2.3 83 2.05 0.907 2.300 8.3 0.62 0.647 <br />MB-5 2.3 66 5.15 0.251 2.300 12.4 0.26 0.251 <br />SGA 2.3 61 6.39 0.141 2.300 2717.4 31.86 29.33 <br />RPE 2.3 71 4.08 0.395 2.300 61.7 2.03 1.988 <br />Prepared by T. Kaldenbach, 12/5/08 <br />Initial abstraction(0.2 X <br />S) <br />0.82 inches <br />0.78 inches <br />Note: The SCS CN method assumes that the initial <br />abstraction (losses before runoff begins) = 0.2 S, with S <br />calculated as (1000/CN) - 10. If P<0.2 S, then no runoff <br />occurs because all precipitation is lost to initial abstraction <br />(i.e. evaporation, plant consumption, storage in depressions, <br />and infiltration)