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HomeMy WebLinkAboutABCdrawdowncalcs.pdfURGE M. ROSE, P.E. P.O. Box 53441 Fayetteville, NC 28305 910-977-5822 10-12-17 ABCSTORAGE 3768 Dunn Road, Eastover Drawdown calculations Per formula on page 16-7 in BMP manual, dewatering time T (in days) is determined using the formula A = V/2KT A,= larger of bottom surface area or one-half the wetted wall area Bottom surface area (elevation 127.0) is 2,918 sf. Stormwater surface elevation at 10 -year storm is 129.17 (see routing report — post routed 10 -year storm) with 3:1 wall slopes within the basin. Wetted wall length between elevation 127.0 and elevation 129.17 = 6.86' Length of 129.17 contour= 567.81' Wetted wall area at elevation 129.17 = (567.81)(6.86) = 3,895.18 sf '/z wetted wall area = 0.5(3895.18) = 1947.59 sf A = 2,918 sf V = volume of water requiring infiltration = volume at elev 129.17 = 9,801 cf (see post -routed 10 -year storm results on the routing report) K = hydraulic conductivity of the soil = 16.76 inches/hr ('/2 of the infiltration rate = 8.3 8 inches/hr). Solve for T T = V/2KA = 9801/2(8.38)(2918) = 0.20 days = 4.8 hours