2008-12-05_REVISION - C1980007 (2)

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