SW6200404_Response To Comments_20230303

C -oqwFE
CAPE FEARENGINEERING
March 3, 2022
Jim Farkas
State Stormwater Engineer
NCDEQ
512 N. Salisbury Street
Raleigh, North Carolina 27699
RE: SW6200404 Modification for SOF SSA Site
Stormwater Permit Comments
Dear Mr. Farkas,
Please find our response to the subject review comments received on February 16, 2023
below:
1. Increase the permanent pool surface elevation to 243.0' so that the pond is adequately sized per
the HRT method (and so that the bottom of the vegetated shelf is at the permanent pool surface
elevation). Increasing the permanent pool surface elevation can be accomplished by either
replacing the drawdown orifice with one at the correct elevation or by adding an upturned elbow
with the corrected invert to the drawdown orifice pipe on the inside of the outlet structure (l can
provide a picture if my explanation is unclear).
Response:
A 6" upturned elbow will be installed inside of the outlet structure at the exiting orifice stub.
Revised orifice calculations included with this package show that a 4" diameter is required to
drawdown the design volume in 3 days. As such, a cap shall be installed on the upturned elbow
with a 4" hole drilled. The top of cap shall be at an elevation of 243.0'. Installation of the elbow is
depicted on sheet C-500. Erosion control general sheet notes and sheet keynotes have been
updated/added to ensure the original drawdown orifice is installed with modifications once the
site is fully stabilized.
2. Revise the orifice diameter so that the design volume (volume stored in the wet pond above the
permanent pool and below the lowest bypass elevation (top of riser @245.5')) draws down within
2-5 days.
Response:
Revised calculations have been provided to show that a 4" orifice diameter will drawdown the
design volume in approximately 3 days. As such, the drawdown orifice will be modified as
described in the response to #1.
151 Poole Road, Suite 100 • Leland, NC 28451 • TEL: [phone] • FAX [phone]
www.copefearengineering.com

3. Provide revised plans/calculations showing that the above two items have been incorporated into
the project.
Response:
Applicable revised plans and calculations have been provided.
4. While the foreboy volume is not technically within 15%-20% of the main pool volume, we con
approve a variance for this project since the wet pond has already been graded1constructed and
no further grading1chonges to the wet pond are proposed as port of this modification (The forebay
is approximately 110 cf, or 0. 6% of the foreboy volume, shy of being within the acceptable range
per the MDC).
I trust the information provided is sufficient to address the comments listed above. Please let me know
if you have any questions, concerns or need any additional information.
Kind Regards,
Mason Manhertz, P.E.
Project Engineer
Cape Fear Engineering, Inc.
Cc: (554-47 "36")
151 Poole Road, Suite 100 • Leland, NC 2845I • TEL: [phone] • FAX [phone]
www,capefearengineering.com