ABCdrawdowncalcs.pdf

URGE 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