SW6190504_Basin sizing and Drawdown Calculation_7/22/2019

Infiltration Basin Sizing Summary
Hoke County Cooperative Extension office Building
633 East Balmer Street
Infiltration sized per NC DEQ Minimum Design Criteria (MDC) Section C-1.
Infiltration Basin 1
Drainage Area: 0.91 ac
Built Upon Area (BUA): 0.58 ac
RV=0.05+0.9xIA
IA = 0.64
RV = 0.05 + 0.9 x 0.64 = 0.63
Design Volume
DV = 3630 x RD x RV x A
RD = 1.0"
RV = 0.63
A = 0.91 ac
DV = 3630 x 1.0" x 0.63 x 0.91 ac — 2,081 cu ft
Surface Area
SA = FS x (DV X 12 / K x T)
FS=2
DV = 2,081 cu ft
K = 2.0 in/hr
T = 72 hours
SA=2x(2,081 CFx 12 / 2.0 x 72) = 347 SF
(2,937 SF Provided)
Draw Down Calculations
Infiltration Basin 1
Design depth = 0.70in = 8.40 inches
Infiltration Rate = 2 in/hr
Factor of Safety = 2
8.40 inches / I in/hr = 8.4 hours
Infiltration Basin 2
Drainage Area: 1.17 ac
Built Upon Area (BUA): 0.69 ac
RV=0.05+09xIA
IA = 0.59
RV = 0.05 + 0.9 x 0.59 = 0.59

Design Volume
DV=3630xRDx RVxA
RD = 1 A"
RV = 0.58
A= 1.17 ac
DV = 3630 x 1.0" x 0.58 x 1.17 ac = 2.463 cu ft
Surface Area
SA—FSx(DV X12/KxT)
FS=2
DV = 2,463 cu ft
K = 2.0 in/hr
T = 72 hours
SA = 2 x (2,463 cf x 12 / 2.0 x 72) = 411 SF
(3,172 SF Provided)
Draw Down Calculations
Infiltration Basin 2
Design depth = 0.78in = 9.36 inches
Infiltration Rate = 2 in/hr
Factor of Safety = 2
9.36 inches / 1 in/hr = 9.36 hours