SW6210202_SW6210202_Add Info Request_20210324 - Lowe Responses_20210406

ROY COOPER
Governor
DIONNE DELLI-GATTI
Secretary
BRIAN WRENN
Director
Jerry Oberholtzer, PE
Lowe Engineers
990 Hammond Drive, Suite 900
Atlanta, GA 30328
NORTH CAROLINA
Environmental Quality
March 24, 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. A review of that
information has determined that the application is not complete. The following information is
needed to continue the stormwater review:
N
Please provide a geotechnical investigation verifying that there is sufficient separation
from the bottom of the bioretention cells to the seasonal high water table (Bioretention
MDC 1).
See "SOTF Ops Fac Geotech Report" PDF
Please provide calculations for all inlets and outlets showing that the riprap is designed to
prevent erosion from the peak flow from the 10-year storm event (General MDC 4).
See "Riprap (Letter)" PDFs
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. 4
4. 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.
The WQ design volume (which is the 95th percentile RRv for USACE) is the volume of media voids beneath the planting surface. The weir
elevation is set a the planting surface for rain events that generate runoff which will exceed the 95th percentile event hydraulic volume.
�. rroviae pars of all revisions, /_ naracopies or revisea pian sneers, ana i naracopy or orner
documents. Pdfs must be uploaded using the form at:
https://edocs.deq.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 April 23, 2021, or the
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 1", these small discrepancies will not impact the bioretention cells
ability to retain the full 1". Please see "SEPARATED SUB -BASINS COMPARISON" PDF for better clarity.
E Q North Carolina Department of Environmental Quality I Division of Energy, Mineral and Land Resources
512 North Salisbury Street 11612 Mail Service Center I Raleigh, North Carolina 27699-1612
NORhI CAROLINA -
uenaa nt of en imnmenmi awr\ r 919.707.9200

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.Anennncdenr.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,
Steven Corey Anen, PE
Environmental Engineer
Stormwater Program
cc: Fayetteville Regional Office
D�� North Carolina Department of Environmental Quality Division of Energy, Mineral and Land Resources
EQ
_ 512 North Salisbury Street 1 1612 Mail Service Center Raleigh, North Carolina 27699-1612
NORTH C oROLWA
OepartmeM Environmental UualN 0 " 919.707.9200