The BMPs incorporated into the Warwick County Park Day-Use Area Improvements Project were needed in order to effectively manage the runoff contributed from construction of additional asphalt parking areas, a new pavilion and a new park office and administration building, as well as from the large drainage area surrounding the park. The BMPs are designed to divert the majority of bypass volume entering the project site around the proposed recreation areas while maintaining high water quality standards within the French Creek Watershed. French Creek is designated as a State Scenic River, and as an Exceptional Value stream by the Pennsylvania Department of Environmental Protection.
To compensate for previous development, the site was considered entirely pervious in the pre-development condition. To preserve the stream’s Exceptional Value status, the type, location and size of all stormwater BMPs were selected based on minimizing the amount of earth disturbance and tree removal, utilizing existing natural features, maintaining required environmental buffers, and maximizing water quality. Additionally, innovative stormwater management solutions were desired by the County to serve as examples for educational purposes. Considering these factors, McCormick Taylor designed the following BMPs to be incorporated into the site.
Parking Lot Infiltration Trench
A permanent infiltration trench along the new parking lot is designed to infiltrate the parking lot runoff from a 2-year storm. Overflow then percolates through the stone-lined trench and flows to an existing inlet which discharges to an existing plunge pool. The infiltration trench was not analyzed in the rate reduction calculations since it is intended for volume reduction only and not rate control. Rather than standard vertical curbs, wheel stops have been provided for parking spaces so that runoff can enter the infiltration trench.
Vegetated Swale with Check Dams
The existing natural vegetated swale located up-gradient of the site will be extended to divert water around the improved park day-use area. Multiple check dams within the vegetated swale will provide peak rate management. Discharge from the swale will enter a headwall that carries the flow under a trailhead parking area through a pipe that ultimately discharges to a new stilling basin designed to protect the outfall from erosion. Per Chester County Conservation District (CCCD) recommendations, the drainage swale has been designed to convey the 10-year storm event due to the upslope tributary area being mature forest, which will remain as a protected woodland within the County Park. Had the swale been designed to convey 2.75 cfs/acre (the alternative allowed by state regulations), it would be sized three times larger, causing more disturbance and removal of mature trees. A permanent channel lining was proposed for the drainage channel to prevent erosion and to provide stability while the vegetation becomes established. A conservation seed mix consisting of native warm season grasses and perennials was specified to provide a permanent vegetative cover within the swale.
Bioretention Garden 1 (BR1) is proposed near the new park office building and asphalt parking area to reduce peak runoff rates. A domed riser discharges overflow into a basin to protect the outfall from erosion. Infiltration facilities are not proposed in this area due to the proximity of the park building’s basement. An additional Bioretention Garden (BR2) is located between the asphalt parking lots. It has been designed to manage runoff rates and includes an underlying infiltration trench designed to infiltrate the runoff from a 2-year storm. A domed riser discharges overflow into an existing inlet and stilling basin. Both bioretention gardens will be seeded with a native mixture of grasses and herbaceous perennials which are tolerant of both periodic inundation and dry periods. A diversity of native plants has been specified to provide aesthetic and seasonal interest throughout the year.
Site-Wide Water Quality Measures
To meet the water quality standards for an Exceptional Value watershed, Pennsylvania requires that the post-construction stormwater infiltration volume equals or exceeds the pre-construction stormwater infiltration volume and that any post-construction stormwater discharge is pre-treated. Infiltration facilities have been designed to infiltrate the difference in the 2-year storm runoff volume. Also, all vegetated swales, bioretention areas and infiltration trenches have been designed with an underlying 18-inch layer of compost that filters runoff before it percolates into the ground. There is an existing non-structural BMP (filter strip) downstream of site as well as constructed wetlands and vernal pools which have been established since 2006. Therefore, it was ensured that all runoff from the proposed improvements reaches these non-structural BMPs to manage the water quality prior to entering French Creek.
During construction, erosion and sediment control is provided primarily through the installation of 12-inch diameter compost filter socks. These biodegradable socks are filled with a compost mixture and held into the earth with wooden stakes. This control method is ideal for the site’s location because unlike conventional super silt fencing, it requires minimal earth disturbance and generates nearly no waste since the socks are biodegradable and the compost is spread throughout the site.
Watershed: Schuylkill River
David T. Stauffer, RLA, ASLA
Chester County Facilities Department
14 East Biddle Street
P.O. Box 2748
West Chester, PA 19380