Pennswood Village Regional Stormwater Management System

Challenge

Pennswood Village is located in a rapidly suburbanizing area between New York City and Philadelphia, where development has increased stormwater problems. In 1996, heavy rains caused Pennswood’s 17-ft deep detention basin to overflow, flooding properties downstream. In 1999, Middletown Township asked Pennswood to help by incorporating regional stormwater management on their site. The design had to manage not only the site’s runoff, but that of the adjacent George School properties and much of the runoff from the highly developed outlying watershed area. Additionally, due to the democratic values of the Quaker client, all decisions had to be made by consensus, with all 300 members of the Quaker community agreeing to the design. This lengthened the process to two years.

Solution

The landscape architect and client wished to use a biological/ecological approach to stormwater that was more in line with the client’s land stewardship values than a standard engineering approach. Though special waivers were required, the system was designed to work with rather than against the land, using 13 acres of wet meadows and bioswales to manage large volumes of stormwater. Existing wetlands and grasslands were retained and became the basis for a riparian corridor concept developed in conjunction with wetland ecologists, which determined the ideal configuration and grading of the linked wetlands. Designers enthusiastically accepted the democratic consensus process and invested more time up front to explain and educate the client about this design approach.

• 13 acres of wet meadows and bioswales manage large volumes of stormwater from on and off site, mimicking the function of a riparian corridor floodplain.
• The stormwater management system begins with a semicircular outfall structure constructed from local Bucks County brownstone, reflecting the building traditions of the region. A series of pipes direct runoff here from Route 413 and the contributing watershed.
• The stormwater empties into a stone-lined sedimentation basin with a weir of boulders, which slows the water, settles out gross particulate matter, and contains the majority of the trash and road debris conveyed along with the runoff. The basin is easily accessed for maintenance, but screened from direct view by a landscape buffer.
• From the sedimentation basin runoff flows into a grass swale that runs alongside the entrance road and is intermittently visible through the trees.
• The swale directs the water into an infiltration basin with highly permeable soils with depths of at least 6 ft to both the seasonal high water table and bedrock layer. The infiltration basin is sized to manage the first flush runoff volume of a storm event. Runoff beyond the infiltration basin’s capacity flows over a broad weir into a long, winding vegetated swale.
• The vegetated swale consists of a series of shallow, stepped channels, each designed to detain runoff from increasingly larger storm events. On either side of the swale is a broad, flat meadow that is graded to function like a riparian corridor or floodplain. The meadow includes native grasses, shrubs, and trees with varying flood and drought tolerances.
• The system terminates in a constructed treatment wetland (CTW) and small wet pond. Water flows from the swale into the wet pond where outflow is controlled by an outlet structure designed to safely pass the 100-year storm. When the need arises, the outlet control structure allows water to flood back into the CTW. Excess water will back up further, eventually overflowing into the broad meadow and the created riparian corridor.
• ADA- accessible paths wind through the swales, wetlands, and meadows, providing Pennswood residents with an inspiring setting for exercise, photography, and nature observation.
• The road was relocated to accommodate the stormwater system, which now serves as an aesthetic entry feature welcoming residents and guests to the site.

• The cost of maintaining the 20-acre wetland meadow landscape is approximately $7,000 per year, substantially less than the $54,000 annual cost of maintaining an equivalent area of traditional lawn with ornamental plantings.

• Continuity of management is a key success factor in maintaining landscapes in accordance with the original design goals and intent. Pennswood has had the same landscape manager since its inception. Since this type of continuity may not always be possible, the designer‘s scope of work should include a document that clearly outlines design intentions and long-term management policy and goals in layman’s terms. This document could require annual reports from the site manager to the designer.
• Designers should establish a plan to manage invasive species over time with site managers. On an annual or biannual basis, the landscape architects and ecologists should walk the site with the landscape manager to inspect, identify threats, and discuss steps already taken or yet to be taken. They should use their combined expertise to understand what type and how many (if any) invasive species will be tolerated.

Project Team

Landscape Architect/Land Planner: Wells Appel Land Strategies
Civil Engineer: Pickering, Corts & Summerson, Inc
Hydrologist and Ecologist: Princeton Hydro
Wetlands Specialist: Mellon Biological Services
Arborist: Morris Arboretum Consultants

Role of the Landscape Architect

Wells Appel assembled a team of plant ecologists, wildlife biologists, environmental planners and civil engineers. The firm developed the riparian corridor concept and determined the ideal configuration of and grading of the system, as well as the selection and utilization of plant materials. Wells Appel provided the master plan for the entire site and oversaw the project through construction.