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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.
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