SW4240501_Design Calculations_20240819 (5)

Permit No.
(to be provided by DWQ)
AVA �pF IVA7-Fj�P
NCDENR
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
Project name Reserve at Old Hollow
Contact person Robin Yaun
Phone number 336-669-7651
Date 8/16/2024
Drainage area number 1
II. DESIGN INFORMATION
Site Characteristics
Drainage area 180,307 ft2
Impervious area,post-development 93,320 ft2
%impervious 51.76%
Design rainfall depth 1.0 in
Storage Volume: Non-SA Waters
Minimum volume required 7,740 ft3
Volume provided 8,111 ft3
OK,volume provided is equal to or in excess of volume required.
Storage Volume: SA Waters
1.5"runoff volume ft3
Pre-development 1-yr,24-hr runoff ft3
Post-development 1-yr,24-hr runoff ft3
Minimum volume required ft3
Volume provided ft3
Peak Flow Calculations
Is the pre/post control of the 1 yr 24hr storm peak flow required? Y (Y or N)
1-yr,24-hr rainfall depth 2.8 in
Rational C,pre-development 0.40(unitless)
Rational C,post-development 0.70(unitless)
Rainfall intensity:1-yr,24-hr storm 0.12 in/hr OK
Pre-development 1-yr,24-hr peak flow 0.20 ft3/sec
Post-development 1-yr,24-hr peak flow 0.09 ft3/sec
Pre/Post 1-yr,24-hr peak flow control -0.11 ft3/sec
Elevations
Temporary pool elevation 963.10 fmsl
Permanent pool elevation 962.00 fmsl
SHWT elevation(approx.at the perm.pool elevation) N/A fmsl
Top of 10ft vegetated shelf elevation 962.00 fmsl
Bottom of 10ft vegetated shelf elevation 961.00 fmsl Data not needed for calculation option#1,but OK if provided.
Sediment cleanout,top elevation(bottom of pond) 956.00 fmsl
Sediment cleanout,bottom elevation 955.00 fmsl Data not needed for calculation option#1,but OK if provided.
Sediment storage provided 1.00 ft
Is there additional volume stored above the state-required temp.pool? Y (Y or N)
Elevation of the top of the additional volume 963.1 fmsl OK
Form SW401-Wet Detention Basin-Rev.9-4/18/12 Parts I.&II.Design Summary,Page 1 of 2

Permit No.
(to be provided by DWQ)
II. DESIGN INFORMATION Surface Areas
Area,temporary pool 7,904 ft2
Area REQUIRED,permanent pool 2,903 ft2
SA/DA ratio 1.61 (unitless)
Area PROVIDED,permanent pool,Ape��,00i 5,386 ft2 OK
Area,bottom of 10ft vegetated shelf,Abot shelf 3,908 ft2
Area,sediment cleanout,top elevation(bottom of pond),Ahouond 1,542 ft2
Volumes
Volume,temporary pool 8,111 ft3 OK
Volume,permanent pool,Vperm_pool 17,757 ft3
Volume,forebay(sum of forebays if more than one forebay) 2,725 ft3
Forebay%of permanent pool volume 15.3%% Insufficient forebay volume.
SA/DA Table Data
Design TSS removal 85%
Coastal SA/DA Table Used? N (Y or N)
Mountain/Piedmont SA/DA Table Used? Y (Y or N)
SA/DA ratio 1.61 (unitless)
Average depth(used in SA/DA table):
Calculation option 1 used?(See Figure 10-2b) Y (Y or N)
Volume,permanent pool,Vperm_pool 17,757 ft3
Area provided,permanent pool,Aperm_pool 5,386 ft2
Average depth calculated 3.30 ft OK
Average depth used in SA/DA,dan,(Round to nearest 0.5ft) 3.0 ft Insufficient.Check calculation.
Calculation option 2 used?(See Figure 10-2b) N (Y or N)
Area provided,permanent pool,Apem,_pool 5,386 ft2
Area,bottom of 10ft vegetated shelf,Atwt shelf 3,908 ft2
Area,sediment cleanout,top elevation(bottom of pond),Abot_pond 1,542 ft2
"Depth"(distance b/w bottom of 10ft shelf and top of sediment) 5.00 ft
Average depth calculated ft
Average depth used in SA/DA,dan,(Round to down to nearest 0.5ft) ft
Drawdown Calculations
Drawdown through orifice? Y (Y or N)
Diameter of orifice(if circular) 1.25 in
Area of orifice(if-non-circular) in2
Coefficient of discharge(CD) 0.60(unitless)
Driving head(Ho) 0.37 ft
Drawdown through weir? (Y or N)
Weir type (unitless)
Coefficient of discharge(Ca,) (unitless)
Length of weir(L) ft
Driving head(H) ft
Pre-development 1-yr,24-hr peak flow 0.20 ft3/sec
Post-development 1-yr,24-hr peak flow 0.09 ft3/sec
Storage volume discharge rate(through discharge orifice or weir) 0.02 ft3/sec
Storage volume drawdown time 3.60 days OK,draws down in 2-5 days.
Additional Information
Vegetated side slopes 3:1 OK
Vegetated shelf slope 6:1 Insufficient shelf slope.
Vegetated shelf width 6.0 ft Insufficient shelf length.
Length of flowpath to width ratio 3:1 OK
Length to width ratio 2.0:1 OK
Trash rack for overflow&orifice? Y (Y or N) OK
Freeboard provided 1.0 ft OK
Vegetated filter provided? N (Y or N) Insufficient. Vegetated filter required.
Recorded drainage easement provided? Y (Y or N) OK
Capures all runoff at ultimate build-out? Y (Y or N) OK
Drain mechanism for maintenance or emergencies is: 6"Dewatering Valve
Form SW401-Wet Detention Basin-Rev.9-4/18/12 Parts I.&II.Design Summary,Page 2 of 2