Pennswood Village, a Quaker directed retirement community in Bucks County places great value on ecological systems, stewardship of the land, and education. As part of the facility’s planned expansion, Pennswood Village entered into a public-private partnership with Middletown Township, which involved the correction of existing runoff and stormwater problems associated with land adjacent to the Pennswood site. The resulting stormwater quality management system was designed to mimic the functional properties of a riparian corridor floodplain (see Site Plan below). The design integrated a number of stormwater BMPs, including a sediment basin, vegetated swale, infiltration basin, created treatment wetland, and a small wet pond. These BMPs work in series to reduce peak flows, promote groundwater recharge, and remove pollutants through a combination of filtering, settling and biological treatment mechanisms. The final design exceeded the Township’s stormwater management requirements while providing an attractive, passive recreation space and learning environment for Pennswood Village and the community at large.
The 82-acre site is part of the Neshaminy Creek watershed that encompasses portions of Bucks and Montgomery counties. Over the past few decades, the watershed has become increasingly developed from the conversion of farm fields into residential and commercial properties.
Wells Appel, the project landscape architect, assembled a team of plant ecologists, wildlife biologists, environmental planners and civil engineers. It was the genuine collaboration of this interdisciplinary team, which enabled the vision to become a reality. Pickering, Corts and Summerson, the project engineers, conducted a comprehensive hydrologic analysis of the watershed. Princeton Hydro studied the pollutant loading characteristics of the watershed. The results of both of these analyses were used to determine which BMPs would work best with the site’s natural resource characteristics, while being capable of decreasing the pollutant load and facilitating the recharge of groundwater to the maximum extent possible.
The system consists of an integrated series of BMPs, each sized and located to address a specific stormwater management issue. The alignment and grading of the swales, basins and wetlands, combined with the careful selection of native grasses, shrubs and trees, diminishes the velocity of the runoff, biofilters and settles pollutants, and creates opportunities for groundwater recharge. The four major elements of the system and their functional attributes are as follows:
1) At the uppermost end of the system is a sedimentation basin that collects runoff directed by a series of pipes from Route 413 and the contributing watershed. This basin is a stone-lined structure whose purpose is to slow the flow of runoff, and allow larger sized particles and debris to settle out of runoff. Upon entering the basin, the velocity of the concentrated runoff is reduced by a flow dissipation structure. A weir regulates the volume and time that the collected runoff is detained in the sedimentation basin. The basin is easily accessed for routine maintenance and the periodic removal of accumulated sediments. It is also screened from direct view by a landscape buffer.
2) Runoff discharged from the sedimentation basin is directed into a grassed swale that conveys the runoff to an infiltration basin. The soils in this section of the site are highly permeable and the depth to both the seasonal high water table and bedrock layer is in excess of 6 feet. These conditions of good soil permeability and lack of a constraining horizon are conducive to the infiltration of runoff and the recharge of the shallow aquifer. The infiltration basin is sized to manage the first flush runoff volume of a storm event.
3) Flows exceeding the infiltration capacity of this basin will be discharged from the basin over a broad crested weir into a long, winding vegetated swale. On either side of the swale is a broad, flat meadow that is graded and designed to function in a manner similar to a riparian corridor or stream floodplain. It consists of a series of shallow, stepped channels, each of which accommodates and detains the runoff from increasingly larger storm events. The meadow supports a variety of vegetation including grasses, shrubs or trees having different flood and drought tolerances.
4) At the terminus of the system is a constructed treatment wetland (CTW) and small wet pond. CTWs are capable of achieving very high pollutant removal efficiencies, particularly with respect to the removal of nutrients and dissolved pollutants. Outflow from the wet pond is controlled by an outlet structure. Initially, during the early part of a storm, runoff that exceeds the capacity of the infiltration basin will flow via the swale into the wet pond. The outlet control structure on the pond will cause water to flood back into the CTW. As water is detained in the CTW and wet pond, it will back up further, eventually overflowing into the broad meadow and the created riparian corridor.
Watershed: Neshaminy Creek
Stuart D. Appel, FASLA, RLA, PP
1516 Locust Street
Philadelphia, PA 19102