SW7100310_HISTORICAL FILE_20210628

STORMWATER DIVISION CODING SHEET
POST -CONSTRUCTION PERMITS
PERMIT NO.
SW
DOC TYPE
❑ CURRENT PERMIT
❑ APPROVED PLANS
HISTORICAL FILE
❑ COMPLIANCE EVALUATION INSPECTION
DOC DATE
YYYYMMDD

ROY COOPER
Governor
JOHN NICHOLSON
fntenm Secretary
BRIAN WRENN
Director
NORTH CAROLINA
Environmental Quality
June 28, 2021
Washington County
Attention: Curtis S. Potter, County Manager
P.O. Box 1007
Plymouth, NC 27962
Subject: Stormwater Permit Recission
Stormwater Management Permit SW7100310
Plymouth Municipal Airport T — Hanger Area Development
Washington County
Dear Mr. Potter:
The Division of Energy, Mineral, and Land Resources received your letter dated June
23, 2021 stating that you did not wish to renew stormwater permit SW7100310. It is our
understanding that this project was never built. Based on this information we aye hereby
rescinding permit SW7100310.
If you have questions, please feel free to contact me or Roger Thorpe at (252) 946-
6481.
Sincerely,
Samir Dumpor, P.E.
Regional Engineering Supervisor II
North Carolina Department of Environmental Quality I Division of Energy. Mineral and Land Resources
zf�DEQ
Washington Regional Office 1 943 Washington Square Mall I Washington, North Carolina 27889

(' c . 6 h-� l.)- ( wa A
WASHINGTON COUNTY
PLYMOUTH, NORTH CAROLINA P.O. BOX 1007
27962
June 23, 2021
Roger K. Thorpe
Environmental Engineer
Division of Energy, Mineral and Land Resources
NC Department of Environmental Quality
943 Washington Square Mall
Washington, NC 27889
Subject: Stormwater Management Permit SW7100310
Plymouth Municipal Airport
Washington County
Mr. Thorpe,
In response to your letter dated April 28, 2021, regarding the renewal of Stormwater
Management Permit SW7100310, it has been decided that Washington County will not renew
the permit. The permit was originally obtained in 2010 for the development of the Plymouth
Airport T Hanger project located at 1069 Plymouth Airport Road, Plymouth, North Carolina. The
project never took place; therefore I would like to request that the permit be rescinded.
If you have any questions, please feel free to contact me at [phone].
Sincerely,
Curtis S. Potter
County Manager
— c�'tJccaG ���aoi tuirit�� �'n�GiJer —

wa"
ROY COOPER
Governor
DIONNE ORLLI-OA'l'Ti
secretory
3RJAN WRENN
vlrm,or
Washington County
Attention: Curtis S. Potter, Manager
P.O. Box 1007
Plymouth, NC 27962
IOPIPI cA€tOLNMA
CmtrmM,"tal Quality
April 28, 2021
Subject: Stormwater Permit Renewal
Stormwater Management Permit SW7100310
Plymouth Municipal Airport
Washington County
Dear Mr. Potter:
A Division of Energy, Mineral, and Land Resources file review has determined that
Stormwater Permit SW7100310 for a stormwater treatment system consisting of a wet
detention basin to serve the Plymouth Airport T Hanger area located at 1069 Plymouth
Airport Road, Plymouth, NC expires on December 31, 2021. This is a reminder that
permit renewal applications are due 180 days prior to expiration. We do not have a
record of receiving a renewal application.
Please submit a completed permit renewal application along with a $505.00 fee for
permit renewal. Also, it was noted in the file review that a designer's certification that the
project was constructed in accordance with the approved plans as required by the
permit has not been submitted. Please include this with your application. Permit
application forms for renewal can be found on our website at:
https.//cieq. ilc.gov/about, divisions/_energy-minerai-and-land-
resourcesisiormw�aierlstormwater-program/post-coc..strucron-2. North Carolina General
Statutes and the Coastal Stormwater rules require that this property be covered under a
stormwater permit.
If you have questions, please feel free to contact me at [phone]. 1 will be glad to
discuss this by phone or meet with you. If you would like, I can e-mail you a copy of the
application forms. You can request a copy by e-mailing me at
roper thorpe(Qncdenr.gov
Sincerely,
�,/� c9M—�
Roger K. Thorpe
Environmental Engineer

M1
1,41%SGcJ -7j D,�' 310
47
From: Elliott, Jason L. Ipagroup.com)
Sent: Thu�sd ay 20, 2010 7:57 PM
To: /Mymouth l
Subject: / ��lymouth Municipal Airport- T-Hangar-Area Develbpment
Dear Bill,
As a follow-up to our conversation earlier today, I would like to provide further explanation for the choice of the design,
permanent pool elevation for the wet detention pond associated with the T-Hangar Development project at the
Plymouth Municipal Airport. During our conversation, you expressed your concern about the elevation of the
permanent pool and the affect this elevation has on the health of the pond plantings on the vegetative shelf. Due to the
conditions of the site, the permanent pool elevation (30.4') was set as close as could reasonably be expected to the
seasonal high water table (" 32.0') and I feel that the alterations being made to the site will be sufficient to justify the
difference for the following reasons:
The site is restricted by the presence of an existing hangar that has a fixed floor elevation that can't be
modified. The FFE of this hangar is 33.63'. The permanent pool elevation is set at 30.4'. In order to get the
runoff from the hangar to drain to the pond, while providing adequate cover, a slope of 0.3% had to be used on
the piping from the hangar to the pond. This is, obviously, a very flat slope and I wouldn't feel comfortable
going any flatter on the slope of the pipes. Many efforts were taken to raise the permanent pool elevation as
much as possible and it is my opinion that the elevation is set as high as it can reasonably be without
compromising the drainage from the hangar area.
Alterations to the site should lower the seasonal high water table to some degree and the site has been re-
graded to provide more efficient drainage from the pond area to the main ditch, parallel to the runway. The
existing ditch was lowered approximately two feet at the point where the outlet pipe from the pond discharges.
The lowering of this ditch should have an effect on the seasonal high water table. I believe it is reasonable to
expect this lowering to also provide some lowering of the water table adjacent to the ditch. Plus, the re -grading
of the outlet ditch to the main ditch should provide adequate drainage to prevent the permanent pool elevation
from being above the design elevation except during the 2-5 day period following a storm event.
The geotechnical report that was obtained cited a seasonal high water table of -12 inches for the borings taken
at the site. The existing elevation in the pond area is - 33.0'. The seasonal high water table was determined to
be 32.0' based on the these numbers. The report also stated that the static water table elevation is -48 inches.
The permanent pool elevation is -30 inches below the existing elevation in the area of the pond. Due to the fact
that the permanent pool elevation is set between the seasonal high water table and the static water table
elevations, I think it is reasonable to expect that this would help provide the proper permanent pool elevations
throughout the entire year. This elevation should also help prevent the permanent pool elevation from
dropping too low.
I hope that these points have helped clear up your concerns about the elevation of the permanent pool. Based on my
engineering experience and what I know about the site, I feel confident that the proposed design will work despite the
site constraints. If you need more information, please let me know.
Thank you very much for your assistance.
Sincerely,
Jason Elliott. PE
LPA GROUP of North Carolina, p.a.

5000 Falls of the Neuse Road, Suite 304
Raleigh, North Carolina 27609
[phone]
[phone] (fax)
[phone] (mobile)
JLEIIiott((D,LPAGROU P.cotu
i
WARNING: All e-mail sent to or from The LPA Group corporate e-mail system is subject to archiving, monitoring and/or
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2

MJATHE LPA GROUP of North Carolina, p.a.
Transportation Consultants
5000 FALLS OF NEUSE ROAD ■ SUITE 304 In RALEIGH, NORTH CAROLINA 27609 ■ 919-954-1244 ■ FAX [phone]S
March 22, 2010
Mr. Bill Moore
NCDENR, Division of Water Quality
Washington Regional Office
943 Washington Square Mall
Washington, North Carolina 27889
Re: Plymouth Municipal Airport
T-Hangar Area Development
Dear Bill:
U-lE.'aJ.,..5 yr ZIP
-
MAR 2 4 2010
Enclosed, please find two copies of the Stormwater Management Plan for the above referenced
project for your review and approval. Also enclosed is a copy of the Wet Detention Basin
Supplement form, the original Wet Detention Basin Operation and Maintenance Agreement, and
the original, plus one copy, of the Stormwater Management Permit Application. A check, in the
amount of $505.00, has also been enclosed for the required application fee.
The T-Hangar Area Development project is being proposed to enhance the growth potential at
the Plymouth Municipal Airport. The project includes a wet detention pond that would treat the
runoff from the full build -out of the proposed project. The pond has been sized for the removal
of 90% TSS.
As part of this project, a trade-off is being requested for the treatment of some of the existing
impervious surface for a portion of the proposed impervious surface that is unable to reach the
wet detention pond. This is the trade-off that I spoke with you about back in January.
If you have any questions or need additional information, please do not hesitate to call.
Sincerely,
THE LPA GROUP of North Carolina, p.a.
/� Z";�
.11tison L. Elliott, PE
Aviation Design Engineer
Enclosures
Member of THE LPA GROUP INCORPORATED
ATIANTA a BALTIMORE a BATON ROUGE a CHARLOTTE a COLUMBIA ■ GREENSBORO a IRVINE a JACKSONVILLE a KENNESAW a KNOXVILLE a LITTLE ROCK
M<LEAN a MOBILE a NASHVILLE a OCEAN SPRINGS a ORLANDO a RALEIGH a SARASOTA a TALLAHASSEE a TAMPA a WEST PALM BEACH
N

STORMWATER MANAGEMENT PLAN
FOR
T-HANGAR AREA DEVELOPMENT
AT
PLYMOUTH MUNICIPAL AIRPORT
PROJECT DESCRIPTION AND PURPOSE
This project is being proposed to provide new aircraft taxilanes and t-hangars at the Plymouth
Municipal Airport. In addition to the taxilanes around the hangars, the project will add paved
access to the t-hangar area. Currently, this area is only served by a turf taxilane. The project will
include the construction of a wet detention pond to treat runoff from the t-hangar area. The
proposed development in the t-hangar area will allow the Airport to grow by attracting new
based aircraft and increasing revenue.
L The T-Hangar Area Development project will be split into two phases of development. The first
phase will include the paved, taxilane access to the T-Hangar Area, a new 8-unit t-hangar
building, paved access to both sides of the new t-hangar building, as well as the existing hangars,
and a paved access road from Plymouth Airport Road to the T-Hangar area. The initial
construction project, as part of Phase 1, will be to construct the taxilanes, the wet detention pond,
a portion of the paved access road and to prepare the site for the construction of the 8-unit t-
hangar building. The construction of the t-hangars will soon follow the construction of the
taxilanes.
Phase two will include the construction of an additional 8-unit building and a new 14-unit t-
hangar building. This phase will also include the construction of taxilanes to access both sides of
the future hangars, as well as a possible widening of the access road. The new wet detention
pond has been sized to accommodate all phases of development and will be constructed, to treat
the ultimate buildout.
The development in this area will impact 0.09 acres of wetlands. A Section 404 Permit from the
Corps, as well as a 401 Water Quality Certification from DWQ are also being applied for to
address these impacts.
EXISTING CONDITIONS
This project lies within the Roanoke River Basin. The site is located northeast of the terminal
area. The site, as well as the longitudinal slopes of the existing ditches, is extremely flat. Runoff
from this area collects in a system of ditches that connects to the large ditch, running parallel to
Runway 3-21. This ditch connects, downstream, to Conaby Creek, as it leaves the property.
Conaby Creek is considered a class C; Sw waterway.
The soils in the area are generally sandy clays and clayey sands with a sandy loatm topsoil at the
surface. Hand auger borings were taken, closer to the main ditch, which runs parallel to the
runway. The approximate static groundwater level at the boring locations was found to be 48
T-Hangar Area Development
Plymouth Municipal Airport
March, 2010

inches below the ground surface. The seasonal high water level at these locations was estimated
to be about 12 inches below the ground surface.
PROPOSED DEVELOPMENT
The current phase of development for the T-Hangar Area Development will include 47,648 sq. ft
of new impervious surface area. The project will also be removing 11,729 sq. ft of impervious
area, resulting in a net increase of 35,919 sq. ft of impervious area. Phase 2 of the development
for the site will add an additional 54,931 sq. ft of impervious area. In total, the full development
of this area will result in the addition of 90,850 sq. It of new impervious surface.
The site restraints, created by the flat nature of the site and the elevation of the existing hangars,
eliminates the likelihood of getting the runoff from the entire site to the wet detention pond. In
order to make the project possible, a trade-off is being requested from DWQ to treat some of the
existing impervious area that is not currently being treated, in exchange for an approximately
equal amount of new impervious area to bypass the wet detention pond.
The proposed design would allow 15,836 sq. ft of new impervious area to bypass the pond (See
Figure DA-3). Currently the site includes 28,083 sq. ft of impervious area, none of which is
currently being treated. As part of the proposed design, 14,432 sq. ft of the existing impervious
area will either be directed to the pond or removed. Although it is a little shy of the amount of
new impervious area bypassing the pond, we hope that this, along with other proposed site
improvements, will be considered a fair trade-off.
Along with the proposed trade-off, the site conditions will also be improved. The outlet ditch,
from the pond, will be regarded and lowered. This ditch will be regarded to include a five foot
bottom width and 5:1 side slopes.
STORMWATER TREATMENT
The new wet detention basin has been sized to treat the runoff from future development for the
site. The wet detention basin was design for 90% TSS removal due to the absence of a
vegetative filter strip at the pond's outlet. The pond is sized to detain a treatment volume of
20,465 cu. ft and release it over a period of 2-5 days. In determining the amount of treatment
volume necessary, enough volume was included to keep the runoff from the proposed
development less than the runoff from the existing conditions, using a Iyr, 24 hr storm.
The geotechnical report obtained for this project estimates a seasonal high water level of
approximately 12 inches below the ground surface for the loc here the hand auger
borings were taken. The static water level was determined to b 48 inche below ground level—
These borings were approximately halfway between the wet deten ion no
and the main ditch
that runs parallel to the runway. Using this information, a depth of 12 inches below the ground
surface would indicate that the seasonal high water level in the area of the wet detention pond
would be at an elevation approximately 32.0 feet.
T-Hangar Area Development
Plymouth Municipal Airport
March, 2010

The elevation of the proposed permanent pool is set at 30.4 feet. Many efforts were taken to
raise this elevation, but due to the limitations presented by the existing hangar, raising the
permanent pool did not seem to be a possibility. However, in re -grading the outlet ditch from the
pond, the bottom of the ditch was substantially lowered. The existing bottom elevation of the
ditch, at the pipe outlet, is right at 32.0 feet. The proposed elevation of the bottom of the ditch is
29.8. This ditch is graded to drain to the main ditch, parallel to the runway. The proposed ditch
lowering should offset any issues that may have been a concern due to the elevation of the
pond's permanent pool elevation. This lowering should help reduce the height of the seasonal
high water table in the adjacent area. In addition, the re -graded ditch should allow any additional
water to flow to the main ditch. With the proposed improvements, the temporary pool should be
able to be released over the designed 2-5 days without a problem.
STORMWATER QUALITY ANALYSIS
The entire drainage area that drains to the pond (Drainage Area 1) was analyzed using XP-
SWMM software. The area was modeled using the SCS Method to calculate runoff. The area
was modeled using both the proposed design for the current phase of development, as well as the
proposed future buildout.
The drainage for the site was designed to accommodate runoff from a 10-yr storm event. The
pond was also checked using the 50-yr storm event to ensure that the overflow weir was enough
to keep it from overtopping under these conditions. Using the XP-SWMM software, the pond
elevation rose to 31.77 feet during the 10-yr storm, for the ultimate development. The 50-yr
storm rose to an elevation of 32.08 feet. The top of the pond is 32.9 feet, with the emergency
overflow weir being set at an elevation of 31.9 feet.
The runoff to Culvert 2 (Drainage Area 2) was,also analyzed for the 10 yr and 50 yr storms.
This area was analyzed using Hydraflow Express software and the Rational Method.
CONSTRUCTION SEQUENCING / EROSION CONTROL
Erosion control features, along with accompanying details will be added to the construction
plans. These plans will be submitted to obtain the required Erosion and Sediment Control
permit. The contractor will be required to follow these plans and detailed instructions at all
times during the construction of the project. The Contractor will be required to take appropriate
erosion and sediment control measures during construction and to conform to all local and state
laws pertaining to the environment. The Contractor will be responsible for the installation and
maintenance of all erosion control features throughout construction as will be noted in the plans.
The proposed wet detention pond will be used as a sediment basin during the construction of this
project. All accumulated sediment must be removed upon completion of the project. Drop inlet
protection and pipe inlet protection will be used for all pipes during construction, as well.
It is anticipated that the Contractor will proceed with construction as follows:
• Install construction entrance pad.
T-Hangar Area Development
Plymouth Municipal Airport
March, 2010

• Install silt fence, rip rap, and all erosion control measures, to be included on the grading
plans, as soon as construction allows.
• Strip topsoil and stockpile onsite.
• Begin construction operations, being sure that all erosion control measures are installed
as soon as possible, as construction proceeds.
• Cover all slopes with Type I temporary erosion control matting. Seed and mulch within
21 days of construction.
• Once permanent vegetation has been established, Contractor shall remove the temporary
erosion control devices and reseed as necessary.
• Accumulated sediment shall be removed from the forebay and wet detention pond at the
completion of construction.
Maintenance of the all BMP's will remain the responsibility of the Contractor until completion
of the project and permanent vegetation has been established. The Plymouth Municipal Airport
will be responsible for maintaining permanent vegetation once the Contractor has removed all
temporary erosion control measures and the project is completed and accepted.
T-Flangar Area Development
Plymouth Municipal Airport
March, 2010