SW6110103_Stormwater Narrative_20110201

Ft. Bragg, SOF C4 Building Project
Stormwater Management Narrative
The proposed project known as Fort Bragg JSOC C4 Building consists of a new 4-
story building, associated parking and utilities, and an underground stormwater
management infiltration trench. This report presents the stormwater management
calculations for the project.
The property is located on Fort Bragg in Cumberland County, North Carolina with
access to the site from Shughart Way off of Malvesti Street. The SOF C4 Facility is
located in the Northeast section of the Joint Special Operations Command (JSOC)
compound between Shughart Way to the west and the train tracks that serve Pope Air
Force Base to the east. The undeveloped site is situated between Pope Air Force
Base Building Number 138 to the north and JSOC Building Number 3-1650 to the
south. Building 3-1650 is a newly constructed pre-engineered metal building which is
the fourth in a series of similarly constructed JSOC buildings built parallel to each other
along the east side of Shughart Way. These four buildings are served by an 82-space
asphalt parking lot located to the east of the buildings. South of these four buildings
and parking lot is a new JSOC Operational Communications Facility-JCU, (PN 66444)
currently under design. Utility systems and other site considerations for the JCU are
being coordinated with the development of the SOF C4 Facility project The scope of
work for the C4 project includes civil site work and site utilities design along with minor
demolition of the existing site and utilities to support the construction of the new SOF
C4 Facility.
The project site is located on a mostly undeveloped grassed 2.49 acre site with some
existing impervious areas and a few small areas of existing woods. The existing
portion of Shughart Way immediately within the project site will be realigned to the
west in order to provide safer transportation with an improved intersection layout. The
realigned Shughart Way will connect and use the existing drive aisle that currently
serves adjacent offsite parking. The existing portion of Shughart Way within the
project site to the north of the realignment will be demolished and removed. There is
also an existing perimeter unpaved road on the project site that will be removed. The
proposed site will disturb greater than 1.0 acre and add proposed built upon areas
greater than 24% of the proposed site area, and is therefore considered a high density
project. The state of North Carolina's Stormwater Management Phase II Post
Construction ordinance requires new high density development projects to provide
stormwater management water quality treatment for the first inch of runoff and to
provide water quantity control for the 1-year storm event.
The site has been broken down into one drainage area with all bypass stormwater
being accounted for in the calculations. Drainage Area #1 encompasses a majority of
the new impervious areas to include the new C4 Building and associated drive aisles
and parking. Drainage Area #1 also includes the associated offsite drainage area.
Water quality and quantity stormwater management are provided for Drainage Area #1
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by providing a proposed underground infiltration trench underneath the new parking
area. Stormwater pretreatment measures consisting of grass channels and water
quality Vortech chambers in concrete vaults are being provided. Water quality control
is provided in the proposed infiltration trench for the first inch of runoff and the facility
provides 85% total suspended solids removal. Water quantity control for the 1-, 2-, 10-
, 25-, 50-, and 100-year storm events is provided in the proposed infiltration trench
such that the post development runoff rates are less than predeveloped conditions for
the project. The 100-year water surface elevation is below the top elevation of the
infiltration trench. The existing off -site impervious areas that drain to the infiltration
trench have been accounted for in the proposed infiltration trench with respect to the
water quality and water quantity stormwater management requirements.
The analysis followed the SCS method and other generally accepted engineering
practices. Bentley PondPack software was used to estimate storm water discharges by
modeling the response of the project drainage basin to typical design storm events
during pre and post development. The drainage areas were delineated from field
prepared topographic survey. Pre development curve number design calculations
were based on existing field surveys. Post development curve number calculations
were based on the proposed site plan. The time of concentration was calculated
based on actual conditions using the TR-55 program. Hydrological soil groups were
obtained from USDA Soil Survey as published on the USDA NRCS soil survey website.
The soil type onsite is Wagram (WgB), of the hydrologic group "A". The geotechnical
engineer calculated the infiltration rates of the existing soils and determined that the
infiltration rate of the soils at the proposed infiltration trench bottom elevation is
expected to be 4.42" an hour; however, given the wide variations in measured
infiltration rates at varying locations and depths, a conservative infiltration rate of 0.30"
an hour has been used. Per conversations with NCDENR DWQ, this approach is
acceptable provided the drawdown calculations confirm that the infiltration device is
able to be drawn down within 120 hrs (5 days) and that the seasonal high water table
is at least 2.0-feet below the planned bottom of the infiltration trench. The bottom of
the infiltration trench is at elevation 244.00. The infiltration trench draws down in 79
hours (see attached calculations) and the seasonal high water table is over 8.0-feet
below the planned bottom of the infiltration trench as verified from the recorded
measurements at the two Ft. Bragg monitoring wells MWDAP3S and MWDAP3D in
very close proximity to the planned infiltration trench location. Refer to the attached
calculations with the included SWMU 63 Groundwater Potentiometric Map from
February 2009 which shows these monitoring wells and their recorded groundwater
elevations. The design storm events 1, 2, 10, 25, 50, and 100-year were based on the
rainfall data from NRCS for Cumberland County, NC.
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