SW6110103_Stormwater Narrative_20230127

A=COM
JSOC - Microgrid
Installation and Integration
Post -Construction Stormwater Management
Permit Package
USACE Project No.: PN97470
AECOM Project No.: 60652522
October 17, 2022
Delivering a better world

JSOC - Microgrid
Project number: 60652522
Prepared for:
U.S. Army Corps of Engineers, Wilmington District
Prepared by:
AECOM
10 South Jefferson Street
Roanoke, VA 24011
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JSOC - Microgrid
Project number: 60652522
Table of Contents
Attachment A
— Stormwater Permit Application................................................................
Attachment B
— NCDEQ Supplement and O&M Forms ....................................................
Attachment C
— Land Development Drawings..................................................................
Attachment D
— Hydrology Report....................................................................................
AttachmentE —
USGS Map..............................................................................................
Attachment F —
Erosion & Sediment Control Checklist.....................................................
Attachment G
— NRCS Websoil Survey............................................................................
Attachment H
— Geotechnical SHWT and Infiltration Report .............................................
AECOM

JSOC - Microgrid
Project number: 60652522
1. Introduction
The JSOC Microgrid project provides for the construction of a new microgrid system to
integrate new and existing diesel generators to the JSOC Compound at Fort Bragg in order
to provide emergency backup power independent of the Fort Bragg utility grid. The project
scope generally consists of the following:
• New 2 megawatt (MW) diesel generator
• Integration of four (4) existing 2 MW generators
• A new fuel supply system consisting of three (3) 50,000 gallon above ground, double
wall, protected diesel fuel tanks, pumping and polishing equipment, and underground
fuel distribution to the existing and new generators.
• Pavement improvements to include a new fuel delivery access road and contained fuel
delivery area.
• Supporting utility connections to include medium voltage and low voltage electrical
feeders, underground telecommunication and fiber optic connections, domestic water
for fire protection, and stormwater drainage infrastructure.
2. Site Description
The project site is generally divided into three project areas:
Fuel Storage Tank Yard
Building 3-1549 Vicinity
Utility routing from Building 3-1549 to Malvesti Street
The proposed Fuel Storage Tank Yard site is located approximately 90.0 feet to the east of
Building 138 and outside the JSOC compound perimeter fence. Approximately 70-percent
of the site is wooded with a mix of pines and hardwoods. A new bulk fuel oil facility will be
located at this location to include above ground double wall fuel storage tanks, fuel truck
unloading facilities, and paved access road for fuel truck delivery and fire truck access. The
existing topography at this location generally slopes from the west to the east. This site is
bounded by Building 138 to the west, Building 137 to the north, and the existing JSOC
perimeter fence to the south and east.
Proposed work in the vicinity of Building 3-1549 (within the JSOC compound) will consist of
new electrical switchgear and generator installation, with supporting utility infrastructure. A
portion of the existing parking lot to the south of Building 3-1549 will be removed for
installation of the new electrical equipment. This area is already developed with paved
access roads, parking, and numerous underground utilities. Stormwater runoff is collected
within storm drains and discharge either into an underground infiltration trench (primarily
serving Building 3-1549) or into existing detention basins, with an existing outfall to the
southwest of the parking lot.
The remaining work area is linear in nature for the installation of underground electrical
feeders and communication cable from the proposed microgrid switchgear equipment to
the existing SUS electrical switches.
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JSOC - Microgrid
Project number: 60652522
3. Site Layout
The new fuel tank storage yard will be located approximately 90.0 feet to the east of
Building 138. The fuel storage tanks, and ancillary equipmentwill be housed on a reinforced
concrete pad within a closed security fence. A new 20-foot asphalt access road will be
provided to support fuel delivery and fire truck access to the fuel tank storage yard and to
maintain access to Building 138.
A fuel truck unloading area will be provided adjacent to the fuel storage tanks to facilitate
unloading of fuel to the new fuel tanks. The unloading area will consist of rigid concrete
pavement and containment curbing to collect any potential spills by sloping the pavement
to a central grate inlet, which drains to a new containment tank. The containment tank has
sufficient volume to contain a potential spill from the largest anticipated delivery vehicle.
The new electrical switchgear and generator will be located within the existing parking lot to
the south of Building 3-1549 and will require the removal of existing parking spaces. A
maintenance roadway will be provided to allow for maintenance vehicle access to the new
equipment.
4. Stormwater Management
Storm drainage and stormwater management is designed to meet all applicable
requirements from the North Carolina Department of Environmental Quality (NCDEQ)
Stormwater Design Manual.
NCDEQ: Fort Bragg maintains a MS4 permit with NCDEQ. Consequently, this project is
subject to North Carolina Stormwater Rule 5A NCAC 02H.1017 and will require a North
Carolina Post -Construction Stormwater Permit since there is more than one acre of
disturbance. This project will modify the existing stormwater permit (SW6110103) that was
granted in connection with the C4 Building construction. The existing permit information
and some of the proposed modifications are summarized as follows:
Table 1. Summary of Proposed Modifications to Existing Permit
Item Existing Permit Proposed Modification
Project Limits / Limit of Disturbance
(associated with land disturbance
permit of respective project)
4.78 acres
3.58 acres
Project Area
2.49 acres
3.38 acres
Drainage Area 1
Total Area
85,000 sf (1.95 acres)
547 sf (0.01 acres)
BUA
58,400 sf (1.34 acres)
58,947 sf (1.35 acres)
Drainage Area 2 (modification)
Total Area
N/A
23,647 sf (0.54 acres)
BUA
N/A
12,890 sf (0.30 acres)
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JSOC - Microgrid
Project number: 60652522
As stated in more detail above, this project is generally comprised of three areas:
Fuel Storage Tank Yard (designated as "Site 1" elsewhere)
Building 3-1549 Vicinity (designated as "Site 2" elsewhere)
Utility routing from Building 3-1549 to Malvesti Street
Site 1 is quantified and presented as "Drainage Area 2" in the existing permit modification.
Site 2 includes new "built -upon area" (BUA, as defined by NCDEQ) that will contribute to the
existing infiltration trench drainage area, which is noted as "Drainage Area 1" in the existing
permit.
The entire project disturbs roughly 3.6 acres, largely due to utility trenching to Malvesti
Street. That utility routing pertains to the land disturbance permit but not the Post -
Construction Permit since there are no new impervious areas associated with that work.
New site development and changes to the BUA are only present in the first two areas — Site
1 and Site 2, which have a combined area of less than one acre.
SITE 1:
Only Site 1 has an increase in the BUA. A bioretention basin with internal water storage
(IWS) serves as the stormwater control measure (SCM) and is sized to treat the NCDEQ
one -inch treatment volume for the entire BUA. The bioretention surface area will be
approximately 1,260 square feet with a ponding depth of 9 inches and will provide
approximately 1,092 cubic feet of treatment volume — exceeding the Site 1 minimum
required treatment volume of 1,074 cubic feet. Runoff from the containment drainage area
is conveyed to a containment tank. The primary purpose of the tank is to capture potential
fuel spills from a tanker truck during delivery. The containment tank will also store the initial
first -flush of storm runoff until a visual inspection can be performed to ensure there is no
sheen present and that it is safe to drain the contained volume to the bioretention SCM.
The tank can store the runoff volume from a 2-inch rainfall event before overflowing to the
bioretention SCM. The containment tank storage volume is not counted as part of the
treatment volume for NCDEQ compliance but does provide some actual stormwater
benefit.
SITE 2:
Site 2 has an overall decrease in development (i.e. a net decrease in BUA). However, a
portion of Site 2 will increase the developed area contributing to an existing underground
stormwater management infiltration trench under an existing stormwater permit. Based on
as -built information, this existing SCM required 4,750 cubic feet of treatment volume and
provided 6,842 cubic feet— resulting in a treatment volume surplus. The amount of added
treatment volume required for the additional BUA in Site 2 will be less than 50 cubic feet and
can, therefore, be accommodated by the existing underground SCM.
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JSOC - Microgrid
Project number: 60652522
5. Stormwater Conveyance
New storm drainage ditches and piping are designed with capacity for the 10-year design
storm. Regarding existing drainage capacity, the fuel tank storage yard (Site 1) and the
parking lot area to the south (Site 2), currently drain eastward to an existing ditch along the
railroad tracks to an existing 30" reinforced concrete pipe (RCP) culvert, which crosses the
railroad approximately 500 feet south of the site. The existing drainage area that
contributes to this culvert crossing is approximately 45 acres. Construction of the fuel tank
storage yard area at Site 1 will increase impervious areas. The effect of this increase will be
reduced by the new bioretention SCM at Site 1 and by reductions in pavement that are
associated with the Site 2 work to the south. The reductions in imperviousness are
accomplished by replacing existing pavement with #57 gravel fill. Existing stormwater
management systems will not be modified.
Site development associated with this project is less than 1 acre and represents about 2
percent of the total drainage area that contributes to the existing ditch and pipe culvert
crossing. The post -development peak runoff from Site 1 is controlled and sufficiently
reduced using a bioretention SCM so that the increase in 10-year runoff to the existing
ditch is minimized. The adequacy of the existing culvert for the 10-year design storm and
possible correction of any inadequacy is outside the scope of this project. However, due to
the proportional area of this project with respect to the whole, this project will not result in
significant changes to the watershed and will, therefore, not adversely affect the existing
culvert.
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JSOC - Microgrid
Project number: 60652522
Attachment A — Stormwater Permit Application
AECOM

JSOC - Microgrid
Project number: 60652522
Attachment B — NCDEQ Supplement and O&M Forms
AECOM

JSOC - Microgrid
Project number: 60652522
Attachment C — Land Development Drawings
AECOM

JSOC - Microgrid
Project number: 60652522
Attachment D — Hydrology Report
AECOM

JSOC - Microgrid
Project number: 60652522
Attachment E — USGS Map
AECOM

JSOC - Microgrid
Project number: 60652522
Attachment F — Erosion & Sediment Control Checklist
AECOM

JSOC - Microgrid
Project number: 60652522
Attachment G — NRCS Websoil Survey
AECOM

JSOC - Microgrid
Project number: 60652522
Attachment H — Geotechnical SHWT and Infiltration Report
AECOM

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