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 Sediment forebays, Vegetative swales, Naturalized basins, and a Meadow conversion. A coalition of nearly 100 volunteers planted over 400 trees and shrubs over October 12th and 13th, 2007. Click here for plant list.
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.
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.
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.
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.
In Fall 2010, Phase II began. Phase II of the retrofit project, funded by Growing Greener, installed two sediment forebays and stabilized eroded streambanks along the Plymouth Creek. The forebays capture stormwater as it flows into the basin, providing a settling period to separate sediment and debris before discharging water to the Plymouth Creek.
Montgomery County Conservation District coordinated a two day volunteer work event to install concrete blocks on the forebay floors, perform maintenance, and remove invasive species. The concrete block surface will aid in removal of sediment that is trapped in the forebays. The blocks are open celled to encourage infiltration and allow for vegetative growth. The volunteers installed 1300 blocks in the forebays providing a savings of over $11,000 to the grant.
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.
Total cost for project was $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).
Montgomery County Conservation District
143 Level Road
Collegeville, PA 19426