SW4210601_Response To Comments_20230917

KENNERLY
ENGINEERING&DESIGN,INC.
September 17, 2023
Jim Farkas
Environmental Engineer
Dept. of Environmental Quality
Division of Energy, Mineral &
Land Resources Stormwater Program
512 N. Salisbury Street
1612 Mail Service Center
Raleigh, NC 27699-1612
RE: Comments and Responses - Stormwater Permit
Stormwater Permit No. SW 4210601 Request for Additional Information
Wynnfall Subdivision — Phase One, Davidson County
Mr. Farkas,
Attached are Kennerly Engineering & Design, Inc. (KED) responses to emailed comments from
your review on June 06th, 2023.
The comments and responses are in the order of the letter received.
Should you have any questions, please do not hesitate to contact us at 336.775.2118.
Sincerely,
Timothy S. Ken erly, PE
Vice President/ Senior Engineer
Phone:[phone]
12133 N.HWY 150,Ste.F Winston-Salem,NC 27127 Fax:[phone]
www.kennerlyengineering.com

2 I P a g e WYNNFALL SUBDIVISION PHASE ONE STORMWATER PERMIT MOD
PERMIT#SW4210601 -REQUEST FOR ADDITIONAL INFORMATION
ADDRESS: HINKLE LANE, LEXINGTON, NC/DAVIDSON COUNTY
Review Comments
1. " Prior Comment 2c(Original Comments 6a, 6b, &6c)—The velocity shown in the calculations for
Swale 15 (4.27 fps) exceeds the V allowable shown in the swale table in the Supplement-EZ Form
(4.0 fps)."
"NOTE: The Vactual for Swale 15 is shown as 4.0 in the swale table, however this does not
correspond to the calculations. Please revise as needed."
#1:Response: The velocity entered in the Supplement EZ form is 4.27 but the spreadsheet
which is locked by NCDEMLR rounds the velocity down to 4.0. I can not
change that input so that it does not round down the velocity.
2. "Prior Comment 4bi—The maximum velocity in the vegetated conveyances (Line 5 of the Low-
Density Page of the Supplement-EZ Form) should be based on the maximum value in the Vactual
column of the swale table (the revised calculations now show a maximum Vactual of 4.93 fps).
Please revise as needed."
#2:Response: Have adjusted the V actual to the max. velocity of 5.74 FPS for the existing
drainage conveyance Ditch 22.
3. "Please specify the rainfall intensity value used to determine the flowrates for the swales as they do
not appear to be consistent. For example,for Swale 3,the rainfall intensity value appears to be
about 7.2 in/hr(back calculated from Q=2.35 cfs, C=0.48, &A=0.68 ac) and for Swale 19,the rainfall
intensity value appears to be about 6.05 in/hr(back calculated from Q=5.92 cfs, C=0.55, &A=1.78
ac)."
"NOTE:If using times of concentration other than 5 minutes,please include the time of
concentration calculations in the calculation booklet. Please revise as needed."
#3:Response: Have corrected the BUA for Swale 3 and Swale 19. The rainfall intensity for
all swales is 6.87 in/hr.
4. "The swale detail for the "V" ditch on plan sheet C-702 shows both side slopes of the ditch as having
4:1 side slopes, however the calculations indicate that one side slope is 4:1 and the other side slope
is 3:1. Please revise as needed.
#4:Response: Have revised the detail for the "V"Ditch to have 4:1 slope on the roadside
and a 3:1 slope on the backslope.
5. "The swale detail for the trapezoidal ditch on plan sheet C-702 shows the bottom width of the ditch
as 8 ft, however the calculations indicate that the bottom width of Swale 21 is 4 ft(the bottom
width for Swale 22 is 8 ft). Please revise as needed."
#5:Response: Have revised calculations and details for Swale 21 is a 4 ft wide bottom.
Existing Swale 22 was calculated using an 8 ft wide bottom.
K E N N E R LY 12133 N.NC FIVJY 150,Ste.F Phone:[phone]Fax:[phone]
ENGINEERING a DESIGN.INC. Winston-Salem,NC 27127 www.kennerlyengineering.com

3 I P a g e WYNNFALL SUBDIVISION PHASE ONE STORMWATER PERMIT MOD
PERMIT#SW4210601 -REQUEST FOR ADDITIONAL INFORMATION
ADDRESS: HINKLE LANE, LEXINGTON, NC/DAVIDSON COUNTY
6. "Please include the design information for Swale 22 on the table on plan sheet C-702. Please revise
as needed."
#6:Response: Have included the requested information on the table on C-702.
7. "Please clarify why the Manning's n value used for Swale 22 is 0.045 while the Manning's n value for
the other swales (some with comparable designs and flow depths) use 0.033. Please revise as
needed."
#7:Response: have revised swale 22 manning's N value to 0.033. The drainage draw is
unmaintained and irregular in shape, information obtained from county GIS TOPO for width of
channel was used for the calculations.
8. "Please clarify why the Vallowable value used for Swales 21 & 22 is 5.5 fps while the Vallowable
value for the other swales is 4.0 fps. Please provide a source for the Vallowable values used and
revise as needed."
"NOTE:Maximum allowable swale velocity values can be found in Part C-11 of the Manual(or from
another reputable source)."
#8:Response: The V allowable for Swales 21 &22 are obtained from Table 8.05a
Maximum Allowable Design Velocities for vegetated Channels Erosion Resistant Plastic Soils
for Tall Fescue which is 5.5 ft/s. The V allowable for all of the swales have been adjusted per
Table 8.05a. I have included the table with the calculations. The V actual for swale 21 is 4.52
ft/s and the V actual for swale 22 is 5.74 ft/s. Swale 22 is an existing natural channel that will
not be maintained and is not being graded during the construction and is not experiencing
any erosion.
"Please provide the following:
- lx hard copy&lx electronic copy of the revised calculations.
- lx hard copy&lx electronic copy of the revised Supplement-EZ Form.
- 2x hard copies&lx electronic copy of the revised plan sheet(s).
- lx hard copy&lx electronic copy of any other items needed to address these comments.
KE N N E RLY 12133 N.NC HWY 150,Ste.F Phone:[phone]Fax:[phone]
ENGINEERING a DESIGN.INC. Winston-Salem,NC 27127 www.kennerlyengineering.com