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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.
1)
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.
2)
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.
3)
Bioretention Gardens
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.
4)
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.
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