SW4200702_SW Narrative_20200708

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. Due to the
large size of the parcel, the proposed impervious area does not exceed the minimum threshold of high-
density design and a low-density design approach has been utilized. Vegetated swales have been
provided to convey the stormwater in a safe, non-erosive manor.
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 will be combined into a single parcel which has been
defined as the site for stormwater purposes. Two existing swales convey existing drainage: one flowing
to the south through the crop land, and the second flowing west through the wooded area. Site soils are
generally categorized as sandy loams by the USGS soils map.
Site Stormwater Design
The 31.18-acre site contains 2.39 acres (7.67%) of existing impervious area that has been taken into
account for the stormwater design. The existing impervious are is made up of three parking areas,
walking trails, a dwelling and driveway connection, and picnic shelters. The proposed build-out includes
3.95 acres (12.67%) of impervious area which only triggers low-density design requirements per NCDEQ
guidelines.
Three vegetated swales have been utilized to handle the stormwater runoff for the project: two existing
and one proposed. The proposed swale has been designed per the NCDEQ MDC with max 5:1 side
slopes, a 1.50% bottom slope, and a non-erosive velocity during the 10-yr flow event. The existing
swales onsite also meet the above criteria.
The proposed swale located southeast of the circular parking area conveys flow from culverts at the
entry drive and a curb cut in the parking area. The swale terminates into an existing densely vegetated
swale that carries the flow offsite according to the existing drainage patterns. A third, existing swale to
the northwest of the building carries flow from the building area to the west the vegetated and wooded
area.