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During
the
construction of Metroplex Shopping Center, a stormwater management basin capable of retaining 1.6
million cubic feet was built along Plymouth Creek, a tributary of the
Schuylkill River, at the request of local townships and the
DEP. Runoff from a six mile radius of mostly impervious surfaces drains
into this 8 acre basin, which is adjacent to Chemical Rd. A 1,560 foot
arched culvert also was constructed on the site. These original stormwater management techniques addressed the downstream flooding of
Conshohocken, but still lacked best management practices (BMPs) to
control sedimentation and erosion, improve water quality, and promote
biodiversity. Before this retrofit project was initiated, the basin was
composed of mud and turf.
Priorities for improving
the stormwater management facilities were based on the consensus of
project partners, the Goldenberg Group, the Department of Environmental
Protection (DEP), and the Montgomery County Conservation District (MCCD).
The objective of this retrofit project was to introduce BMPs to the site
to better manage the high water levels common in the basin, while
encouraging cleaner water and native species.
Overall, retrofits include
1) Sediment forebays; 2) Vegetative swales; 3) Naturalized
basins; 4) Meadow conversion; and 5) Future phase II retrofits. A coalition of
nearly 100 volunteers planted over 400 trees and shrubs over October
12th and 13th, 2007. Click
here for plant list. This weekend marked the first phase out of the
project’s two phases, the second of which is planned for 2009.
1)
Sediment Forebays
Sediment forebays were
constructed near the inlet of a retention basin, facilitating the
removal of sediment and debris. These first-phase forebays range from
1,290 square feet to 2,660 square feet. The sediment forebays were lined
with ECS-2 erosion control blanket.
2)
Vegetative Swales
A vegetated swale was
also constructed to extend an eroded flow path. A trench was dug out at
the location of a water channel formed out of mud. Relatively shallow
and flat, the trench was recessed less than a foot with a gradual
inclination. A ECS-2 erosion control blanket 15 feet wide by 115 feet
long was placed over the trench, and was stapled into the mud on either
side of the trench. 750 Herbaceous plugs were planted on-site, some of
which were planted with bulb planter tools into the erosion blanket.
These plugs were native varietals including Cardinal Flower, Blueflag
Iris, Switchgrass, Bluejoint Grass, and New England Aster. As the
plants’ roots expand, they should further secure the erosion blanket in
the event of disruptive waterflows. These native plants provide a more
effective swale liner than turf since they slow the flow of water and
promote the absorption of pollutants and water, while regulating water
temperature.
3)
Naturalized Basins
Naturalized basins were
also constructed using shrubs, trees, and the native perennial plugs.
These plants have the tenacity to withstand both wet and dry conditions.
A constructed naturalized basin has many benefits: the velocity of
stormwater run-off is slowed, more water than traditional sod is
absorbed, the temperature of stormwater is reduced before it flows into
natural waterways, and native plants neutralize pollutants like oil,
fertilizers, anti-freeze, and other residues. Wildlife will find new
habitat and food within the verdant basin plantings and hummingbirds and
butterflies will be attracted to the vibrant flowers the plants produce.
Aquatic life will re-emerge.
Naturalized basins
require initial funding for construction and maintenance, but once the
plantings are stable, maintenance fees drop below pre-construction
maintenance costs. In contrast to the regular mowing a sod basin
requires, a naturalized basin needs to be mowed no more than once per
year.
4)
Meadow Conversion
A 37,900 square foot
meadow conversion has been effectuated by the management group, the
Goldenberg Group. The meadow conversion consisted of the removal of
existing vegetation (turf grass), amending soil with a compost mix, and
seeding and matting.
5) Future Phase II Retrofits
Future phase II retrofits
are contingent upon Growing Greener 2008 grants and include the plans
for a 5,240 square foot vegetative forebay and live stakes for channel protection.
Live stakes will be installed to help reinforce plantings and to
aid in erosion control. Willow species cuttings measuring a half meter
to a meter long will be placed into the ground and will grow into stable
bushy plants
A
full case study summary and video are available on the StormwaterPA
website.
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County:
Montgomery
Watershed: Schuylkill River
Metroplex Shopping Center is located at the intersection of Gallagher and
Chemical Roads in Plymouth Township. The site of a former toxic waste
dump, the land has been recycled into this development containing 780,000 square feet of retail space. Metroplex Shopping Center sits at the crossroads of three major
highways: Interstate 476, Germantown Pike, and the Pennsylvania
Turnpike.
Plymouth
Creek, a tributary of the Schuylkill River, runs through the Metroplex
development.
View all
TVSSI BMPs in a larger map
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Total cost for project is
$59,020
$34,000 in grant funding was provided
by the
TreeVitalize Watershed Program, a program sponsored by the
Department of
Conservation and Natural Resources (DCNR). This grant was used for
shrubs and trees ($19,000), herbaceous plugs ($1,500), the erosion
control blanket ($2,000), and some landscaping fees ($10,000).
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