SW5210202_Supplement Form_20210224

Permit No.______________________
(to be provided by DWQ)
STORMWATER MANAGEMENT PERMIT APPLICATION FORM
401 CERTIFICATION APPLICATION FORM
WET DETENTION BASIN SUPPLEMENT
This form must be filled out, printed and submitted.
The Required Items Checklist (Part III) must be printed, filled out and submitted along with all of the required information.
I. PROJECT INFORMATION
Franklin County E911
Project name
Blake Hall, PE
Contact person
919-866-4933
Phone number
2/12/2021
Date
1.44 acres
Drainage area number
II. DESIGN INFORMATION
Site Characteristics
2
Drainage area1
ft
2
Impervious area, post-development1
ft
% impervious42.36%
Design rainfall depth1.0in
Storage Volume: Non-SA Waters
3
Minimum volume required2,254
ft
3
Volume provided9,348
ft
OK, volume provided is equal to or in excess of volume required.
Storage Volume: SA Waters
3
1.5" runoff volume
ft
3
Pre-development 1-yr, 24-hr runoff
ft
3
Post-development 1-yr, 24-hr runoff
ft
3
Minimum volume required
ft
3
Volume provided
ft
Peak Flow Calculations
Is the pre/post control of the 1yr 24hr storm peak flow required?y(Y or N)
1-yr, 24-hr rainfall depth2.9in
Rational C, pre-development(unitless)
Rational C, post-development(unitless)
Rainfall intensity: 1-yr, 24-hr storm0.12in/hrOK
3
Pre-development 1-yr, 24-hr peak flow16.90
ft/sec
3
Post-development 1-yr, 24-hr peak flow15.23
ft/sec
3
Pre/Post 1-yr, 24-hr peak flow control-1.67
ft/sec
Elevations
Temporary pool elevation251.00fmsl
Permanent pool elevation249.00fmsl
SHWT elevation (approx. at the perm. pool elevation)fmsl
Top of 10ft vegetated shelf elevation249.50fmsl
Bottom of 10ft vegetated shelf elevation248.50fmsl
Sediment cleanout, top elevation (bottom of pond)245.00fmsl
Sediment cleanout, bottom elevation244.00fmsl
Sediment storage provided1.00ft
Is there additional volume stored above the state-required temp. pool?n(Y or N)
Elevation of the top of the additional volume251.0fmslOK
Form SW401-Wet Detention Basin-Rev.9-4/18/12Parts I. & II. Design Summary, Page 1 of 2

Permit No.______________________
(to be provided by DWQ)
II. DESIGN INFORMATION
Surface Areas
2
Area, temporary pool5,508ft
2
ft
Area REQUIRED, permanent pool0
SA/DA ratio1.57(unitless)
2
ft
Area PROVIDED, permanent pool, A
3,611OK
perm_pool
2
ft
Area, bottom of 10ft vegetated shelf, A
2,951
bot_shelf
2
ft
Area, sediment cleanout, top elevation (bottom of pond), A
858
bot_pond
Volumes
3
Volume, temporary pool9,348OK
ft
3
Volume, permanent pool, V
6,389
ft
perm_pool
3
Volume, forebay (sum of forebays if more than one forebay)1,007
ft
Forebay % of permanent pool volume15.8%%Insufficient forebay volume.
SA/DA Table Data
Design TSS removal85%
Coastal SA/DA Table Used?n
(Y or N)
Mountain/Piedmont SA/DA Table Used?y
(Y or N)
SA/DA ratio1.57(unitless)
Average depth (used in SA/DA table):
Calculation option 1 used? (See Figure 10-2b)n
(Y or N)
3
ft
Volume, permanent pool, V
6,389
perm_pool
2
ft
Area provided, permanent pool, A
3,611
perm_pool
Average depth calculatedftNeed 3 ft min.
Average depth used in SA/DA, d, (Round to nearest 0.5ft)
ft
av
Calculation option 2 used? (See Figure 10-2b)y
(Y or N)
2
ft
Area provided, permanent pool, A
3,611
perm_pool
2
ft
Area, bottom of 10ft vegetated shelf, A
2,951
bot_shelf
2
Area, sediment cleanout, top elevation (bottom of pond), A
858
ft
bot_pond
"Depth" (distance b/w bottom of 10ft shelf and top of sediment)3.50ft
Average depth calculated3.01ftOK
Average depth used in SA/DA, d, (Round to down to nearest 0.5ft)
3.0ftOK
av
Drawdown Calculations
Drawdown through orifice?y(Y or N)
Diameter of orifice (if circular)1.00in
2
Area of orifice (if-non-circular)
in
Coefficient of discharge (C)
0.60(unitless)
D
Driving head (H)
0.52ft
o
Drawdown through weir?n(Y or N)
Weir type(unitless)
Coefficient of discharge (C)
(unitless)
w
Length of weir (L)ft
Driving head (H)ft
3
Pre-development 1-yr, 24-hr peak flow16.90
ft/sec
3
Post-development 1-yr, 24-hr peak flow15.23
ft/sec
3
Storage volume discharge rate (through discharge orifice or weir)0.08
ft/sec
OK, draws down in 2-5 days.
Storage volume drawdown time2.40days
Additional Information
Vegetated side slopes6:1OK
Vegetated shelf slope6:1Insufficient shelf slope.
Vegetated shelf width6.0ftInsufficient shelf length.
Length of flowpath to width ratio2:1Insufficient flow path to width ratio. Must not short-circuit pond.
Length to width ratio1.8:1OK
Trash rack for overflow & orifice?y(Y or N)OK
Freeboard provided1.0ftOK
Vegetated filter provided?y(Y or N)OK
Recorded drainage easement provided?y(Y or N)OK
Capures all runoff at ultimate build-out?y(Y or N)OK
gravity w/ valve
Drain mechanism for maintenance or emergencies is:
Form SW401-Wet Detention Basin-Rev.9-4/18/12Parts I. & II. Design Summary, Page 2 of 2