SW6210202_Add Info Request_20210428

ROY COOPER
Governor
DIONNE DELLFGATTI
Secretary
BRIAN WRENN
Director
Jerry Oberholtzer, PE
Lowe Engineers
990 Hammond Drive, Suite 900
Atlanta, GA 30328
NORTH CAROLINA
Environmental Quality
April 28, 2021
Subject: Request for Additional Information
Stormwater Permit No. SW6210202
Fort Bragg Special Operations Facility Addition — PN88658
Cumberland County
Dear Mr. Oberholtzer:
The Division of Energy, Mineral and Land Resources (DEMLR), received a Stormwater
Management Permit Application for the subject project on February 10, 2021 and more
information on April 6, 2021. A review of that information has determined that the application is
not complete. The following information is needed to continue the stormwater review:
REPEAT COMMENT: When calculating the water quality volume using the NRCS
method, the runoff volumes from impervious surfaces should be calculated separately from
the runoff volumes from pervious surfaces. Using a weighted curve number greatly
underestimates the runoff from the drainage area.
ENGINEER'S RESPONSE: According to Hydraflow, using a composite CN results in higher
flowrates when compared to separating sub -basins into pervious and impervious areas then
adding those areas together. Hydraulic volumes for the "separated" sub -basins are slightly
higher for the 1-, 2-, 10-, and 25 year events, but are slightly lower for the 50- and 100 year
events. Because all bioretention cells were designed to retain 1.8" (95th percentile) rather
than the NCDEQ-required I ", these small discrepancies will not impact the bioretention cells
ability to retain the full I ". Please see "SEPARATED SUB -BASINS COMPARISON" PDF for
better clarity.
FURTHER EXPLANATION: The runoff volume from the 1" storm shall be calculated
using one of the accepted methods in Part B of the Stormwater Design Manual. Neither
the flow rates or the volumes from larger storms are considered for the post -construction
water quality requirements. The volume for the 1" storm must be calculated and used to
ensure that the volume can be stored while meeting all of the MDC. Please see the
comment below for further explanation.
North Carolina Department of Environmental Quality I Division of Energy, Mineral and Land Resources
512 North Salisbury Street 1 1612 Mail Service Center I Raleigh, North Carolina 27699-1612
NORTH CAROLINA
uepaa nt of enmmnmmi Q.1; 919.707.9200

2. REPEAT COMMENT: The weir elevations from bioretention cells 3 and 5 are set at the
same elevation as the planting surface. The outlets from all the bioretention cells must be set
at an elevation greater than the ponding volume from the water quality design volume.
ENGINEER'S RESPONSE: The WQ design volume (which is the 95th percentile RRv for
USAGE) is the volume of media voids beneath the planting surface. The weir elevation is set
at the planting surface for rain events that generate runoff which will exceed the 95th
percentile event hydraulic volume.
FURTHER EXPLANATION: The porosity of the media shall not be included when
calculating the required storage volume. The entire runoff volume from the 1" event must be
stored above the bioretention media at a depth no greater than 12". Please calculate the
appropriate runoff volume and revise the outlet so that this volume can be stored between the
bioretention media surface and the outlet.
3. Provide pdfs of all revisions, 2 hardcopies of revised plan sheets, and 1 hardcopy of other
documents. Pdfs must be uploaded using the form at:
hgps:Hedocs.deg.nc.gov/Forms/SW_Project Submittal
Please note that this request for additional information is in response to a preliminary review.
The requested information should be received by this Office prior to May 28, 2021, or the
application will be returned as incomplete. The return of a project will necessitate resubmittal
of all required items, including the application fee. Please reference the State assigned project
number SW6210202 on all correspondence.
If you need additional time to submit the information, please submit your request for a time
extension to the Division at the contact below. The request must indicate the date by which you
expect to submit the required information. The Division is allowed 90 days from the receipt of
a completed application to issue the permit.
The construction of any impervious surfaces, other than a construction entrance under an
approved Sedimentation Erosion Control Plan, is a violation of NCGS 143-215.1 and is subject
to enforcement action pursuant to NCGS 143-215.6A.
If you have any questions concerning this matter please feel free to contact me at
Corey.Anen(cncdenr.gov or [phone]. Please note that due to the ongoing Covid-19
pandemic, I will be teleworking and will be most easily reached by email.
Sincerely,
.4�
Steven Corey Anen, PE
Environmental Engineer
Stormwater Program
cc: Fayetteville Regional Office
D E Q�� North Carolina Department of Environmental Quality I Division of Energy, Mineral and Land Resources
512 North Salisbury Street 1 1612 Mail Service Center I Raleigh, North Carolina 27699-1612
NORTH CAROLINA -
D.panm.mof Envlloamaotal Duality 919.707.9200