SW6200404_Stormwater Narrative_20221123

Stormwater Management Narrative
& Calculations
For
SOF Supply Support Activity
Fort Bragg, North Carolina
Prepared for
Fort Bragg Water Management Branch
Date: September 21, 2022
Prepared by
Cape Fear Engineering, Inc.
151 Poole Road, Suite 100
Belville, NC 28451
[phone]
NC License No. C-1621

DESIGN NARRATIVE
The SOF Supply Support Activity project site is located at the northwest corner of the
intersection of Eagle Talon Drive and Tora Bora Boulevard. The GSB TEMF building is
located west of the project site. The project consists of the design and construction of a
general-purpose warehouse, a covered storage shed, access drives, utility installations, and
stormwater conveyance infrastructure.
The project site is located within a previously established stormwater permit boundary. As
such, this is a submittal to modify the existing SOF GSB TEMF PN79453 Stormwater
Permit (SW6200404). This modification will require any revisions remain in compliance
with EISA Section 438 as prescribed by the original permit.
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The site has been previously cleared and rough graded. The existing site topography where
most of the construction will occur is flat with slopes ranging from 1 to 2%. There is an
existing stormwater treatment pond directly east of the project site. On the project side of
the existing pond, elevations drop from 259 to 246 feet (pond top of bank) at an approximate
3:1 slope. A swale is located at the top of the slope to direct water to an existing catch basin.
Existing stormwater infrastructure as well as existing utilities are located north and south
of the project site. Portions of existing silt fence and a chain link fence will be removed as
required for construction. An existing Conex Storage Container will also be removed from
the project site.
There are no known contamination areas located within the project site. The presence of
unexploded ordnance or other hazardous materials are not anticipated.
The project is located in the Cape Fear River Basin and drains to Stewarts Creek (Stream
Index 18-31-24-5-4 Stream Class Q.
PROPOSED IMPROVEMENTS
Proposed improvements include a general-purpose warehouse, a covered storage shed,
access drives, utility installation, and stormwater conveyance infrastructure. Heavy -Duty
concrete pavement will be utilized for the circulation apron surrounding the buildings. The
perimeter of the site will be paved with gravel. A segmented block retaining wall will be
built between the perimeter gravel and existing stormwater pond.
DRAINAGE CONSIDERATIONS:
Proposed grading will consist of fills varying from 1 to 5 feet. Areas between the perimeter
gravel and retaining wall will require additional fill. Positive drainage away from the
building and within paved areas will be provided. Paved areas will be designed to have
slopes between 1.0% and 5.0 max%. Runoff will be collected by catch basins in paved areas
and then conveyed via stormwater pipes to the existing stormwater treatment pond.
The bulk of the impervious area for the SOF SSA site was reallocated from the future

impervious area bank previously established in the permit. However, the finalized site plan
of the SOF SSA site resulted in a net increase of 0.28 acres within the overall stormwater
pond drainage area boundary. Additionally, the drainage area of the permit boundary has
been expanded to include the ingress driveway into the project site.
Note that the originally permitted drainage area only measures 15.84 acres even though the
pond design, calculations, and permit were based on a total contributing drainage area of
16.2 acres. The expanded drainage area to include the driveway extension was an additional
0.17 acres. Therefore, the total measured drainage area equals 16.01 acres which is still less
than the originally designed and permitted 16.2 acres. As such, there was no drainage area
increase consideration in this modification.
Applicable original calculations provided by Mason & Hanger have been revised to reflect
the net impervious area increase. The revised calculations show there is still sufficient
existing pond geometry by NCDEQ Water Quality (1.0" storm) and EISA 438 standards
(1.8" storm). Furthermore, the revised composite curve number calculation to include the
SOF SSA site shows that the overall composite curve number has remained the same.
Therefore, no hydrologic and pond routing calculations required any revisions.
Any revisions to the original Mason & Hanger calculations will be crossed out and updated
values will be provided in red text. Tables 2, 3, & 4 from the original permit report are
provided below to show an overall summary of values that have remained unchanged or
have been revised.
The following items are included in this permit modification submittal:
• Stormwater Management Permit Application
• Revised Wet Pond Supplement
• Revised Water Quality Calculations
• Project Site Stormdrain Calculations
• Project Site Outlet Protection Calculations
• Project Site Stormwater & Erosion Control Plans

TABLE 2 PRE -DEVELOPED COMPOSITE RUNOFF CURVE
NUMBER CALCULATIONS AND PEAK FLOW
AREA (AC)
WEICNTED
?
DRAINAGE AREA
Q1 (CFS)
010 (CFS) Q25 (CFS)
Wet Pond
17,E
56
1.31
17.28 28.52
TABLE 3 POST -DEVELOPED COMPOSITE RUNOFF CURVE
NUMBER CALCULA T1ONS AND PEAK FLOW
AREA (AG)
WEIGHTED
DRAINAGE AREA
DRAINAGEQ1
(CFS)
Q10 (CFS) j Q25 (CFS)
Wet Pond
`16.2
191
51.48
106.52 132.28
'Total drainage area and weighted CN remains unchanged.
TABLE 4 WET POND VOLUME REQUIREMENTS
WET POND
MIN VOLUME
REQUIRED (CF)
NCDEQ
EISA
OPTION #2
REQUIRED (CF)
VOLUME
PROVIDED
CF
Volume Main Pool
-.-4r6021 I4,15e2
l4 972
118,621
Volume of Forebay
18,889 17,190
17,246
17,682
Design Volume
5- 47,045
8; 84,681
100,531
`Volume Lased on the diiierence of the pest-deveiopment runoff volume and pre -development runoff volume for the 24-hour
10-year storm event- No change in this value, as the total drainage area and past -development ON has remained unchanged.