SW6140202_COMPLIANCE_20140311

STORMWATER DIVISION CODING SHEET
POST -CONSTRUCTION PERMITS
PERMIT NO.
SW_�
DOC TYPE
❑ CURRENT PERMIT
❑ APPROVED PLANS
❑ HISTORICAL FILE
COMPLIANCE EVALUATION INSPECTION
DOC DATE
z
YYYYMMDD

Wet Pond Calculations
Project: Bedford — Section 1
Latitude: 35.0347
Long: 79.1270
Permitting Authority: State
Regulatory Credits
85% TSS
25%Total Nitrogen
40% Total Phosphorus
General Requirements
Yes Sizing shall take into account all runoff at ultimate build out, including off -site drainage
Yes Vegetated slopes shall be no steeper than 3:1.
Yes BMP shall be located in a recorded drainage easement with access to a public ROW.
Yes Per supplement form the wet pond is designed for 90%TSS removal. Basin discharge shall be evenly
distributed across a minimum 30 feet long vegetative filter strip unless it is designed to remove 90%TSS.
Yes The design storage shall be above the permanent pool.
Yes Discharge rate of the treatment volume shall completely draw down between 2 and 5 days.
Yes The average depth of the permanent pool shall be a minimum of 3 feet. (4 feet provided)
Yes Permanent pool surface area shall be determined using Tables 10-1, 10-2, 10-3, and 10-4.
Yes The flow within the pond shall not short circuit the pond.
Yes BMP shall be designed with a forebay.
Yes BMP shall not be located to produce adverse impacts on water levels in adjacent wetlands.
Yes A minimum 10-foot wide vegetated shelf shall be installed around the perimeter. The inside edge of the
shelf shall be 6" below the permanent pool elevation; the outside edge of the shelf shall be 6" above the
permanent pool elevation.
Yes The forebay volume should be about 20% of the total permanent pool volume, leaving about 80%of the
design volume in the main pool.
Yes Freeboard shall be a minimum of 1 foot above the maximum stage of the basin.
SHWT: The permanent pool elevation shall be within 6 inches (plus or minus) of the SHWT elevation.
SHWT: 193.56
Permanent Pool Elevation: 193.5
MAR 1 1 2014

Wafer Quality Treatment Volume
Volume = 3630 * Rd * Rv * Ac
Where Ac=[sf of Drainage Area]/43,560 sf/ac
Rd = Design Storm
I = Impervious Portion / Drainage Area
Rv = 0.05 + 0.9 * I
Ac= 92.3
1= 46.15/92.3 = 0.50
Rd = 1.0 Inches
Rv= 0.05 + 0.9 (0.50) = 0.50
Volume = 3630 * 1.0 * 0.50 * 92.3 Acres = 167,524 cf
Volume Provided: 198,300 cf
Q: Discharge Rate (cfs) for 2 to 5 days
Q = CdA [2 g HO] A 0.5
For a 6.0 inch diameter orifice
r = 3 inches/12 inches/ft = 0.25 ft
A=3.14r2=3.14(0.25)2=0.19625sf
Cd = 0.6
g = 32.2
Ho = (Temporary Pool Elevation -Permanent Pool Elevation) / 3 = (195.0-193.5)/3 = 0.5
Q = 0.6 (0.19625) [(64.4) (0.5)] A 0.5 = 0.668 cfs
Note: Q must be between Discharge Rate for 2 days and 5 days
Discharge Rate (2-5 days) = Water Quality Treatment Volume/(seconds/dav)/(number of days)
Q-Discharge Rate (2 days) = 167,524 cf / (86,200 seconds per day) / 2 = 0.97 cfs'
Q-Discharge Rate (5 days) = 167,524 cf / (86,200 seconds per day) / 5 = 0.388 cfs
Note: 0.97 cfs > 0.668 cfs > 0.388 cfs
Surface Area (SA) provided:
SA = DA/100 * Ratio
Average Depth: 4.0
BUA1 = 50 %
DA = Drainage Area = 4,020,588 sf
Ratio= 2.5 (from Table 10-3 based on BUA, average depth, TSS and removal )
SA = DA/100 * Ratio = 4,020,588/100 * 2.5 = 100,512.5 sf
SA provided: 121,191
' Rough approximation that does not consider head