SW4200901_Project Optic Stormwater Narrative_20200914

Project Optic Stormwater Narrative
Summary
Project Optic consists of a 254,095 SF manufacturing and distribution building with associated parking,
truck courts and docks, walkways, and underground utilities. The site is situated in the southern corner
of the NC-65 and US-220/1-73 interchange and is laid out similarly to most other manufacturing and
distribution complexes. Truck traffic will utilize a designated entrance to access the building's loading
docks while associate traffic will enter through a separate entry to get to the main parking area. The
west side of the project will be mass graded to accommodate future expansion of the facility.
Site Characteristics
The existing site consists of densely wooded and grassed areas. Surrounding parcels are mostly
agricultural with some residential. The site drains to the south to Troublesome Creek, a WS-III/NSW
water body that ultimately flows to the Cape Fear River. The site soils mostly consist of sandy clay loams
with hydrologic soil class B. Public utilities will be extended to the project during the construction phase.
Site Stormwater Design
The 43.64-acre site contains a marginal amount of existing impervious area that has not been
grandfathered into the stormwater design. The project includes 10.75 acres (24.63%) of impervious area
which triggers high -density design requirements per NCDEQ guidelines. Because of the high -density
designation, a wet pond is proposed to treat the stormwater runoff. The minimum area and depth of
the main pool and forebays have been calculated per the NCDEQ Minimum Design Criteria. Rockingham
County does not require quantity detention and therefore a pre- and post -development flow
comparison is not required; however, the wet pond is oversized in an effort to reduce flow impacts.
Stormwater will be captured onsite by drop inlets and piped to the wet pond to the south. A series of
swales and culverts have been utilized on the inside of the project in order to reduce the required
stormwater infrastructure. All piping has been sized for the 10-yr 5-minute intensity. The wet pond
routing was examined using the 1-, 2-, 10-, and 100-year 24-hour events to check stage elevations and
freeboard.
The wet pond consists of a 29,941 sf main pool and two forebays totaling 4,977 sf, or 16.6% of the main
pool volume. Both the main pool and forebays are 6' deep and a 6' wide vegetated shelf has been
included at the top of the main pool. A 6'x6' concrete riser has been utilized as the primary overflow
outlet, and a 30' wide riprap spillway is provided for events over the 100-year storm. A 4" diameter
water quality orifice has been included on the outlet structure and draw down time has been kept
below 5 days for the temporary pool. The outlet pipe exits the pond to the south and discharges to a
riprap pad adjacent to existing Troublesome Creek.