SWA000115_Calculations_20211007

OUTLET DEVICE#1

The main pool surface area of WetPond #1was designed using MDC 1. (b): The SA/DA
(table 1) and Average Depth Method.

POST-DEVELOPMENT IMPERVIOUS CALCULATION

PROPOSED IMPERVIOUS AREAS TO BMP:
IMP. AREA = 3.05 AC.
TOTAL DRAINAGEAREATO BMP = 6.69 AC.
PERCENT IMPERVIOUS = 3.05 AC./6.69AC. = 45.6%
(Use 46% Impervious and Interpolate between 40% and 50% impervious to size
permanent pool for Wet Pond.)
SURFACE AREA REQUIRED
ESTIMATED IMPERVIOUS SURFACE PERCENTAGE FOR TOTAL DRAINAGE
AREA TO THE STORAGE DEVICE IS 46%.
FOR A 3-FOOT-DEEPPERMANT POOL THE SURFACE AREA REQUIRED
WOULD BE 1.68% OF THE TOTAL DRAINAGE AREA TO THE DEVICE.

IF:
D.A. TOTAL = 6.69 ACRE
THEN:
1.68% OF 6.69 ACRES = 4,896 SQUARE FEET
SURFACE AREA PROVIDED FOR PERMANENT POOL = 8170.69 SQUARE FEET

VOLUME REQUIRED
VOLUME CALCULATED BY SIMPLE METHOD DESCRIBED BY THE NCDWQ
BMP DESIGN MANUAL.
Rv = 0.05 + 0.9 * Ia
Ia = 3.05 ac imp / 6.69ac d.a. = 0.456

Rv = 0.05 + 0.9 * 0.456= 0.460

3
V = 3630 * 1” * 0.46 * 6.69 = 11,171 Ft
3
VOLUME REQUIRED = 11,171 Ft

3
VOLUME PROVIDED = 24,518 Ft
STAGE 3.0’ ABOVE WATER SURFACE
DRAW DOWN TIME TO RELEASE OVER THE COURSE OF 3 DAYS

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