SW4200702_Stormwater Narrative_20210810

Lewisville Community Center Stormwater Narrative
Summary
The proposed Lewisville Community Center project consists of a 11,719 SF community building with
associated parking, walkways, and utilities. The site has been laid out in a manor to help preserve
several existing trees that will serve as a centerpiece to the parking area and entry drive. A biocell has
been utilized to treat the increase of impervious area from the project. While quantity detention is not a
requirement, staging has been incorporated into the biocell in order to help limit outflows to the
neighboring property.
Site Characteristics
The existing site consists of dense wooded area to the west and crop land to the east and south. A
Town -owned park lies to the north to which pedestrian and automobile connections have been
provided. The park and community center parcels have been combined into a single parcel. For
stormwater purposes, the limits of construction have been used to examine existing and proposed
impervious areas. Some existing impervious exists within the limits and has been grandfathered into the
stormwater design. Site soils are generally categorized as sandy loams by the USGS soils map.
Site Stormwater Design
The 4.76-acre limits area contains 0.23 acres (4.80%). The existing impervious area is made up of an
existing dwelling and associated paved driveway access. The proposed build -out includes 1.90 acres
(39.92%) of impervious area.
A biocell has been utilized to handle the stormwater runoff for the project. The biocell has been
designed per the NCDEQ MDC to include internal water storage as required. Per site boring explorations
completed by Stewart Engineering, the seasonal high water table was not evident in the biocell area
down to a depth of over six feet beneath the biocell bottom.
Runoff will be conveyed to the biocell via storm drainage piping from the building and parking areas.
Water will enter the biocell through a riprap forebay at the furthest point from the outlet structure as
possible to increase retention time. A ponding depth of 12" is proposed in the biocell with an additional
12" of attenuation. Water will drawdown to the underdrain system below and collect in a single pipe to
be carried to the outlet. Larger storms will overflow through the provided weirs and grate in the riser
structure.
Site Stormwater Modification
The original, approved plans under permit SW4200702 have been slightly modified in this edition to
include a paved fire lane in lieu of grass pavers. This provided an increase in BUA and therefore
triggered a permit modification. The biocell geometry did not need to change as a result of this
additional BUA. The original biocell was oversized and was able to handle the increase. All applicable
calculations and documentation have been provided to support the new numbers.