The Pennswood Village Stormwater System
Design and construction of a multifunctional riparian corridor for stormwater-quality management
By
Stephen J Souza,
Mark Gallagher,
Stuart D Appel
As you turn into the entrance drive to Pennswood Village, a broad meadow suddenly opens in front of you. The busy arterial road is left behind, and the noises of birds, frogs, and tall rustling grasses can be heard all around. As you drive directly over two stormwater swales—two pieces in a linked system—the croaking of frogs becomes louder, the temperature drops slightly, and the eye follows the green swaths of wetland grasses as they curve around gentle slopes (Figure 1, seen in this article's title above). If not for the young trees dotting the landscape, it would be difficult to tell whether the swales, basins, and meadow had always been there. Even if visitors to the site were unaware that the system is a highly functional regional stormwater facility (Figure 2), they doubtless would be struck by the sheer beauty of the place. Herein lies part of this project's success—it functions on a variety of levels and serves a multitude of purposes, thus increasing its value to the community.
Pennswood Village is a retirement community situated on 82 acres. The site is part of the Neshaminy Creek watershed, a sub-watershed of the Delaware River. This is a large watershed of approximately 233 square miles that encompasses portions of central and lower Bucks and Montgomery counties in Pennsylvania. Over the past few decades, it has become increasingly developed through the conversion of farm fields into residential and commercial properties (Figure 3).
In response to a major flood in June 1996, Middletown Township commissioned a regional stormwater study to identify problem areas, one of which was the area downstream of Pennswood Village. The flood caused the existing 17-foot-deep detention basin on the property to overflow, severely damaging a recently opened health center and many other properties downstream of the retirement village.
In 1999, as part of Pennswood Village's site redevelopment and expansion plans, a public-private partnership was developed with Middletown Township to address local traffic safety and regional stormwater management issues. The agreed-upon stormwater management plan was to be designed to manage the runoff from Pennswood Village and the adjoining George School properties, and much of the runoff from the highly developed outlying watershed areas.
The landscape architecture firm of Wells Appel Land Strategies was about to begin work on the village's expansion plans within the main building complex. Nancy Spears, executive director of Pennswood Village, realized that the traffic and regional stormwater challenges were land-use issues and asked the design firm to review the proposed solutions. The plans, which had been in the works for 10 years, involved rerouting the entrance road to Pennswood Village, demolishing a beautiful old stone barn that stood in the way, and installing a large regional detention basin on the property. As they walked the site together, Stuart Appel, president of the firm, explained to Nancy how the agreed-upon planning did not support the community's commitment to environmental stewardship and advised her that more intelligent options were available, which could solve the traffic and stormwater issues, preserve culturally significant structures like the barn, and provide for additional functions expressive of the community's core values.
Two aerial photographs proved especially helpful in illustrating to the client the importance of intelligent site design (Figure 3). The 1995 aerial photo made clear just how important the open field at the entrance to Pennswood was for the area's ecosystems. It was, and continues to be, the only large piece of open space connecting Core Creek State Park to the east and the Neshaminy Creek Corridor to the west. If the Pennswood field were to be used for regional stormwater management, all three areas would be protected from development indefinitely. At this point, the village decided to put off any new construction until a master plan could be developed—one which included the regional stormwater facility, identified suitable places for construction, and preserved open space. In putting its architectural plans for new buildings on hold, the community reorganized its priorities and made a serious commitment to landscape and stormwater management design (Figure 4).
This commitment was a direct result of the landscape architect's initiative in educating the Pennswood Village committee, residents, and the township about alternative approaches to stormwater management. Although the township's requirements could have been satisfied with the construction of a standard detention basin, Pennswood Village preferred a design that performed multiple functions, held cultural and material significance, and added a sense of wild beauty to the landscape. The final forms of the basins function not only to effectively treat stormwater but also to take advantage of experiential, spatial, and aesthetic opportunities.
Designing a Stormwater-Quality Management System for Pennswood Village
The idea for Pennswood's stormwater system was conceived by Wells Appel as a logical expression of the community values placed on ecology, stewardship of the land, and intergenerational education. The concept not only addressed the technical challenges but also created a much needed "sense of entrance" to the community.
As the project landscape architect, Wells Appel assembled a team of plant ecologists, wildlife biologists, environmental planners, and civil engineers. It was the genuine collaboration of this interdisciplinary team that enabled the vision to become reality.
To construct the Pennswood system, permits were required from the Pennsylvania Department of Environmental Protection (PADEP), Army Corps of Engineers (ACOE), Bucks County Soil Conservation District, and Middletown Township. Mellon Biological Services inventoried the existing conditions and natural resource features of the site. Much of the compiled data was also used as the supporting information for the PADEP and ACOE permits. Particular attention was given to the site's environmental attributes of existing wetlands and grasslands habitats and their use by various forms of wildlife. Wells Appel would later use these data in collaboration with Princeton Hydro to develop the riparian corridor concept, to determine the ideal configuration and grading of the system, and in the selection and utilization of plant materials.
Pickering, Corts & Summerson, the project's civil engineer, conducted a comprehensive hydrologic analysis of the watershed. Using the Soil Conservation Service's Technical Release 55 (TR-55), the company computed peak flows and runoff volumes under different storm intensities. These data would serve as the foundation for the stormwater management system's design. Pickering, Corts & Summerson also conducted soil testing and borings and computed the detention times and flow-attenuation capabilities of the stormwater treatment system.
Princeton Hydro conducted a screening analysis of the pollutant loading characteristics of the watershed using PADEP's Simple Method. The resulting pollutant loading data, combined with the engineer's hydrologic data, were used by Princeton Hydro to evaluate the feasibility of different best management practices (BMPs). BMPs most compatible with the site's natural resource characteristics, yet capable of decreasing the pollutant load and facilitating the recharge of groundwater to the maximum extent possible, were prioritized. Princeton Hydro developed the list of plants most appropriate for introduction in the different sections of the system and worked closely with Wells Appel in its design.
Alongside the scientific analyses, many other considerations were taken into account: pedestrian and vehicular circulation, view sheds, important focal points, existing vegetation, wildlife corridors, the future use of space, and areas for passive recreation (Figure 5). These analyses proved to be invaluable in the design of a stormwater facility serving a wide variety of functions.
Throughout the process, collaboration among the different disciplines was encouraged and highly productive, with each discipline understanding and fully considering the others' suggestions and viewpoints. The need to balance site design, landscape architecture, engineering, and stormwater management in this project was recognized by each member of the team. Wells Appel authored the concept, but the final design was the result of input from all team members. Conceptual drawings shuttled back and forth between the disciplines. For instance, Pickering, Corts & Summerson would calculate stormwater facility sizing requirements and send drawings to Wells Appel; Wells Appel would reshape the facility to make it look and function more as it would in a natural floodplain. The same level of collaboration occurred during the development of the planting plan. Princeton Hydro compiled a list of plant species appropriate for different areas of the system, and Wells Appel tailored this list and placement according to the aesthetic and spatial qualities desired.
With its team, Wells Appel finalized the design of the stormwater management system's layout, grading, planting, lighting, bridges, and passive recreational features. The landscape architects prepared a series of renderings that were used to articulate and convey the environmentally integrated design of the system to the Township Planning Board. The presentation, conducted with the entire project team, was designed to inform the directors and residents of Pennswood Village of the holistic, ecological attributes of the system and to address any concerns of the community.
The Final Design
The functional design of the Pennswood Village stormwater management system mimics that of a natural riparian stream corridor channel (Figure 6). It uses a number of BMPs linked in a specific series of passive operation. The efficiency of individual BMPs was increased by creating a routing system that integrated a series of hydrologically linked BMPs, thereby creating a pollutant removal train. Each BMP is 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 grading plan was developed from the analyses of stormwater flows and volumes but also took inspiration from the gently sloping topography of Bucks County. Likewise, the selection of hardscape and plant materials also held cultural and natural significance for the area. For instance, an old Quaker bridge found near the site provided the inspiration for the bridges in the design—and dictated the choice of the local Bucks County brownstone (Figure 7). Quaker pairings of materials were also used. One example is the entrance sign, where forged steel and wood beams are used together. Quaker ideals of functionality paired with an unadorned, elegant aesthetic were used throughout the project.
A major component of the design deals simultaneously with human experience of the project and with the goals of the stormwater system. A decision was made to realign the main entrance road to Pennswood Village so that the stormwater system and the road intersected twice (Figure 2). This design decision provides a sense of entrance into the village while calling attention to the functioning of the system. It allows the visitor to experience spatial and climatic changes resulting from the topography and planting of the system, since the road cuts across two sections of the riparian corridor. The changes in plant communities and grade make the system beautifully visible to visitors. This design feature also allows the system to meander more dramatically—thus increasing the length of stormwater flow and allowing for the creation of additional basins.
This choreography of movement through the site is even more evident in the pedestrian paths. These paths were carefully designed with shifting views and changing spatial qualities in mind. At times, views are blocked by gentle mounds and opened up again at the edge of a large basin. Parts of the path closely follow the swales and basins, while others veer into the dry meadow (Figure 8). When the trees reach maturity, they will serve to frame certain views and will define a range of enclosed and open spaces.
The plan for the original detention basin was a big "hole in the ground" with a low-flow channel, tucked into a corner of the property, away from the road. This traditional approach does not encourage human interaction and implies that stormwater management is a thing to be hidden away. The final design for Pennswood celebrates human interaction, observation, and education through its alignment with vehicular and pedestrian routes, and through its parallel use as a naturalistic entryway into the community. The inclusion of elements meant for human use and observation ensures that the stormwater system is a highly visible element in the landscape and can act as its own advocate, educating those that use the paths and roads interwoven within it.
The stormwater system itself is composed of four major elements: a sedimentation basin, a series of bioretention basins, swales, and a created treatment wetland. At the uppermost end of the system is the outfall structure and forebay (Figure 9). Here, runoff is directed by a series of pipes from Route 413 and the contributing watershed. The outfall structure is a semicircular stone-lined wall constructed with the local Bucks County brownstone. The curved form of the wall echoes the grading, and its simple but elegant construction reflects the building traditions of the region. The forebay boulders slow the storm surge, settle out gross particulate material, and contain the majority of the trash and road debris conveyed along with the runoff. A stone weir regulates the volume and time that the collected runoff is detained in a second sedimentation basin (Figure 10). The basins are easily accessed for routine maintenance and the periodic removal of accumulated sediments.
Runoff discharged from the sedimentation basin is directed into a grassed swale that conveys the runoff to an infiltration basin. At this point, the swale runs alongside the entrance road, visible intermittently through the trees. The soils that predominate in this section of the site are highly permeable. The depth to the seasonal high-water table is in excess of 6 feet, as is the depth to bedrock. 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.
Flows exceeding the infiltration capacity of this basin will be discharged over a broad crested weir into another long, winding vegetated swale, which connects additional basins and curves back and forth across the entrance road. On either side of the swale is a broad flat meadow graded and designed to function in a manner similar to a riparian corridor or stream floodplain (Figure 11). That is, it consists of a series of shallow, stepped channels, each of which accommodates and detains the runoff from increasingly larger storm events. Depending on its proximity to the grassed swale and the resulting post-construction topography, the meadow supports a variety of vegetation including grasses, shrubs, and trees having different flood and drought tolerances.
At the terminus of the system is a created treatment wetland (CTW) (Figure 12). Outflow from the wetland is controlled by an outlet structure designed to safely pass the 100-year storm. Initially, during the early part of a storm, runoff that exceeds the capacity of the wetland will flow, via the swale, into the final basin. The outlet control structure on the final basin will cause water to flood back into the CTW should the need arise. As water is detained in the CTW and final basin, it will back up further, eventually overflowing into the broad meadow and the created riparian corridor. From the meadow, it flows into another swale on the north side of the property, then into a series of three bioretention basins, and finally into Neshaminy Creek.
Drew Mason, Pennswood's landscape manager, states that stormwater reaches the final basin three to four times each year, meaning that the system overflows into Neshaminy Creek a maximum of four times annually.
A few components of the original concept plan—such as the educational amphitheater and the orchard visible in the illustrative plan (Figure 2)—were not carried through to the final design. In the place of the proposed orchard, the community planted a single specimen white oak that will eventually preside over the dry meadow. The proposed amphitheater represented the commitment to education fostered by Pennswood Village. However, even without such a formal space, Pennswood continues to host educational tours for local middle schools and high schools, as well as to give presentations to municipal officials from all over the region. Additionally, classes from the University of Pennsylvania, Temple University, and Rutgers University make annual field trips to Pennswood to learn from this multifunctional project.
Plant Selection
To achieve the desired goal of stormwater management while still creating an aesthetically pleasing setting, care was taken in the selection of plant materials used throughout the Pennswood Village system (Figure 11). The use of native plant materials fit well with the existing landscape, providing habitat for wildlife in keeping with the desires of Pennswood Village, and creating a sense of place. Additionally, the use of indigenous plants minimized problems with the acclimatization of plant material, maintenance of plantings, and the ability of the selected materials to withstand seasonal climatic fluctuations.
Plant selection was a function of grade, periodicity and duration of flooding, and the drainage characteristics of the underlying soils. However, other factors were taken into consideration. These included the velocity of runoff to which the plants would be exposed, the functional intent of the planting, their wildlife habitat attributes, and their visual and spatial characteristics.
Plant palettes for each part of the system were generally chosen according to naturally occurring plant communities. Four broad conditions for plant growth were identified on the plan: dry and wet riparian corridor conditions, and dry and wet meadow conditions. For the drier parts of the swale and basin system (the dry riparian corridor), species such as sycamore, white ash, ironwood, and hackberry were selected. For the wet riparian corridor, species included grey-stemmed dogwood, bayberry, pin oak, green ash, and river birch.
In the dry meadow, selections included drought-tolerant tall grasses such as little bluestem, Indian grass, and red top, as well as wildflowers like black-eyed Susan and common milkweed. The wet meadow plant communities included broom sedge, New York ironweed, Joe Pye weed, and boneset.
The variety of plants used throughout the project is another manifestation of the site's multiple functions. The plant material used aids in the treatment, filtration, and detention of stormwater and at the same time provides habitat for wildlife and creates beautiful and varied spaces for human enjoyment.
Community Reception
Five years after the system was constructed, Nancy Spears, Pennswood's director, admitted that only in its fourth summer could the community appreciate the full beauty of the design and the result of their labor, time, and money. The grasses had grown in, the perennials and grasses had established and spread, the trees had stabilized, and the system was functioning perfectly. She and Drew Mason, the community's landscape manager, felt that patience was essential throughout the process, but stressed that the result was well worth the wait. The village has a saying that embodies this commitment to cross-generational stewardship: "We are the harvest of someone else's hope; we sit under trees we did not plant; we plant trees under which we will never sit." This attitude provides a wonderful foundation on which the stormwater system can grow and mature.
As for the current functioning of the system, no flooding has occurred downstream since the system was installed, in spite of very heavy rains that caused flooding in other parts of the township. Spears states that the gradual flow of stormwater from one basin into the next is a fascinating and powerful occurrence to witness. The rising and settling of the water and its movement from one basin to another is very poetic for her and for others in the community. She says that in a storm event, it is extremely clear how the system works and that observers can see the elegant manner in which the water is controlled and directed.
The residents at the village and people from the surrounding neighborhoods frequently use the paths that circulate through the swales and meadows for jogging, walking, and bird watching. Many of the residents at the village are avid bird watchers and have noted a great increase in the number of bird species present from one year to the next. One longtime resident noted that she saw Baltimore orioles, redwing blackbirds, and other species she hadn't seen on the property in 20 years. A number of hares, frogs, and a fox have also been spotted on the site.
The stormwater system and the network of paths would not have been so successful if not for the efforts of Mason and his team. They understand how the system works and appreciate the importance of maintaining grades and plant communities. Mason states that when the system was installed, there was no maintenance manual for this type of project, and they had to gradually learn how to best care for it. After installation, he and Appel walked the grounds several times throughout the years to see what needed attention and to devise appropriate maintenance procedures.
Mason and his team conduct several measures throughout the year to ensure that the system is functioning properly and to encourage the continued growth of the native plant communities. They clean out the forebay and sedimentation basin about once a year and have noted that no debris from the basin migrates into any other part of the system—it stays in the basin until it is manually removed. They also keep invasive species such as phragmites, mustard grass, and crown vetch under control through a combination of careful spot spraying and manual cutting.
Mason and his team mow the system in two segments: The lower elevations are cut in the early winter when they are relatively dry, and the upper elevations in early spring. This approach allows migratory birds to find cover on the site when they need it, while also keeping machinery away from wet soils so as not to create tracks.
Mason frequently walks the site, keeping an eye out for erosion channels and volunteer plants, and simply observing the system at work. He notes that water travels slowly through the system, allowing infiltration to occur in the basins. He and his team are essential to the continued success and functioning of the system. While he admits that the system is far from high-maintenance, he stresses the importance of regular procedures and observations, and above all, a thorough understanding of how the system works.
Conclusion
The project's successful result is a highly functional stormwater management system that exceeds the township's stormwater management requirements while providing an attractive environment for Pennswood Village and the community at large. As proposed, this system attenuates peak flows, promotes groundwater recharge, and passively removes pollutants through a combination of filtering, settling, and biological treatment mechanisms. It also provides opportunities for recreation, education, and the appreciation of nature. In taking on a variety of functions, the project works to ensure its own continued success within the community. By providing opportunities for passive recreation, the project acts as an educational tool, spreading the message of alternative stormwater management techniques to a wider audience.
The Pennswood Village stormwater management design has won several awards, including a Design Merit Award from the American Society of Landscape Architects in 2003 and the Pennsylvania Governor's Award for Environmental Excellence in Design in 2003. It has also been recognized for engineering excellence by the Pennsylvania House of Representatives, the Pennsylvania Senate, and the American Council of Engineering Companies.
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Project team: Wells Appel Land Strategies; Stuart Appel, principal-in-charge; Evan Stone, project manager; Devinder Soin, landscape designer; Deborah Lermit, landscape designer; Ngim Chea, landscape designer, Princeton Hydro; Dr. Stephen Souza, hydrologist; Mark Gallagher, plant ecologist and wildlife biologist, Pickering, Corts & Summerson; Wayne Johnson, civil engineer, Mellon Biological; Rick Mellon, wetlands scientist; General Contractor, S&H Landscaping.
The authors wish to acknowledge editorial assistance on this article from Rosa Mannion of the University of Pennsylvania Department of Landscape Architecture.
Author's Bio: Stephen J. Souza, Ph.D., is with Princeton Hydro LLC in Ringoes, NJ .
Author's Bio: Mark Gallagher is with Princeton Hydro LLC in Ringoes, NJ .
Author's Bio: Stuart D. Appel, RLA, is with Wells Appel Land Strategies in Philadelphia, PA.
January-February 2006
The Pennswood Village Stormwater System
Design and construction of a multifunctional riparian corridor for stormwater-quality management
By
Stephen J Souza,
Mark Gallagher,
Stuart D Appel
As you turn into the entrance drive to Pennswood Village, a broad meadow suddenly opens in front of you. The busy arterial road is left behind, and the noises of birds, frogs, and tall rustling grasses can be heard all around. As you drive directly over two stormwater swales—two pieces in a linked system—the croaking of frogs becomes louder, the temperature drops slightly, and the eye follows the green swaths of wetland grasses as they curve around gentle slopes (Figure 1, seen in this article's title above). If not for the young trees dotting the landscape, it would be difficult to tell whether the swales, basins, and meadow had always been there. Even if visitors to the site were unaware that the system is a highly functional regional stormwater facility (Figure 2), they doubtless would be struck by the sheer beauty of the place. Herein lies part of this project's success—it functions on a variety of levels and serves a multitude of purposes, thus increasing its value to the community.
Pennswood Village is a retirement community situated on 82 acres. The site is part of the Neshaminy Creek watershed, a sub-watershed of the Delaware River. This is a large watershed of approximately 233 square miles that encompasses portions of central and lower Bucks and Montgomery counties in Pennsylvania. Over the past few decades, it has become increasingly developed through the conversion of farm fields into residential and commercial properties (Figure 3).
In response to a major flood in June 1996, Middletown Township commissioned a regional stormwater study to identify problem areas, one of which was the area downstream of Pennswood Village. The flood caused the existing 17-foot-deep detention basin on the property to overflow, severely damaging a recently opened health center and many other properties downstream of the retirement village.
In 1999, as part of Pennswood Village's site redevelopment and expansion plans, a public-private partnership was developed with Middletown Township to address local traffic safety and regional stormwater management issues. The agreed-upon stormwater management plan was to be designed to manage the runoff from Pennswood Village and the adjoining George School properties, and much of the runoff from the highly developed outlying watershed areas.
The landscape architecture firm of Wells Appel Land Strategies was about to begin work on the village's expansion plans within the main building complex. Nancy Spears, executive director of Pennswood Village, realized that the traffic and regional stormwater challenges were land-use issues and asked the design firm to review the proposed solutions. The plans, which had been in the works for 10 years, involved rerouting the entrance road to Pennswood Village, demolishing a beautiful old stone barn that stood in the way, and installing a large regional detention basin on the property. As they walked the site together, Stuart Appel, president of the firm, explained to Nancy how the agreed-upon planning did not support the community's commitment to environmental stewardship and advised her that more intelligent options were available, which could solve the traffic and stormwater issues, preserve culturally significant structures like the barn, and provide for additional functions expressive of the community's core values.
Two aerial photographs proved especially helpful in illustrating to the client the importance of intelligent site design (Figure 3). The 1995 aerial photo made clear just how important the open field at the entrance to Pennswood was for the area's ecosystems. It was, and continues to be, the only large piece of open space connecting Core Creek State Park to the east and the Neshaminy Creek Corridor to the west. If the Pennswood field were to be used for regional stormwater management, all three areas would be protected from development indefinitely. At this point, the village decided to put off any new construction until a master plan could be developed—one which included the regional stormwater facility, identified suitable places for construction, and preserved open space. In putting its architectural plans for new buildings on hold, the community reorganized its priorities and made a serious commitment to landscape and stormwater management design (Figure 4).
This commitment was a direct result of the landscape architect's initiative in educating the Pennswood Village committee, residents, and the township about alternative approaches to stormwater management. Although the township's requirements could have been satisfied with the construction of a standard detention basin, Pennswood Village preferred a design that performed multiple functions, held cultural and material significance, and added a sense of wild beauty to the landscape. The final forms of the basins function not only to effectively treat stormwater but also to take advantage of experiential, spatial, and aesthetic opportunities.
Designing a Stormwater-Quality Management System for Pennswood Village
The idea for Pennswood's stormwater system was conceived by Wells Appel as a logical expression of the community values placed on ecology, stewardship of the land, and intergenerational education. The concept not only addressed the technical challenges but also created a much needed "sense of entrance" to the community.
As the project landscape architect, Wells Appel assembled a team of plant ecologists, wildlife biologists, environmental planners, and civil engineers. It was the genuine collaboration of this interdisciplinary team that enabled the vision to become reality.
To construct the Pennswood system, permits were required from the Pennsylvania Department of Environmental Protection (PADEP), Army Corps of Engineers (ACOE), Bucks County Soil Conservation District, and Middletown Township. Mellon Biological Services inventoried the existing conditions and natural resource features of the site. Much of the compiled data was also used as the supporting information for the PADEP and ACOE permits. Particular attention was given to the site's environmental attributes of existing wetlands and grasslands habitats and their use by various forms of wildlife. Wells Appel would later use these data in collaboration with Princeton Hydro to develop the riparian corridor concept, to determine the ideal configuration and grading of the system, and in the selection and utilization of plant materials.
Pickering, Corts & Summerson, the project's civil engineer, conducted a comprehensive hydrologic analysis of the watershed. Using the Soil Conservation Service's Technical Release 55 (TR-55), the company computed peak flows and runoff volumes under different storm intensities. These data would serve as the foundation for the stormwater management system's design. Pickering, Corts & Summerson also conducted soil testing and borings and computed the detention times and flow-attenuation capabilities of the stormwater treatment system.
Princeton Hydro conducted a screening analysis of the pollutant loading characteristics of the watershed using PADEP's Simple Method. The resulting pollutant loading data, combined with the engineer's hydrologic data, were used by Princeton Hydro to evaluate the feasibility of different best management practices (BMPs). BMPs most compatible with the site's natural resource characteristics, yet capable of decreasing the pollutant load and facilitating the recharge of groundwater to the maximum extent possible, were prioritized. Princeton Hydro developed the list of plants most appropriate for introduction in the different sections of the system and worked closely with Wells Appel in its design.
Alongside the scientific analyses, many other considerations were taken into account: pedestrian and vehicular circulation, view sheds, important focal points, existing vegetation, wildlife corridors, the future use of space, and areas for passive recreation (Figure 5). These analyses proved to be invaluable in the design of a stormwater facility serving a wide variety of functions.
Throughout the process, collaboration among the different disciplines was encouraged and highly productive, with each discipline understanding and fully considering the others' suggestions and viewpoints. The need to balance site design, landscape architecture, engineering, and stormwater management in this project was recognized by each member of the team. Wells Appel authored the concept, but the final design was the result of input from all team members. Conceptual drawings shuttled back and forth between the disciplines. For instance, Pickering, Corts & Summerson would calculate stormwater facility sizing requirements and send drawings to Wells Appel; Wells Appel would reshape the facility to make it look and function more as it would in a natural floodplain. The same level of collaboration occurred during the development of the planting plan. Princeton Hydro compiled a list of plant species appropriate for different areas of the system, and Wells Appel tailored this list and placement according to the aesthetic and spatial qualities desired.
With its team, Wells Appel finalized the design of the stormwater management system's layout, grading, planting, lighting, bridges, and passive recreational features. The landscape architects prepared a series of renderings that were used to articulate and convey the environmentally integrated design of the system to the Township Planning Board. The presentation, conducted with the entire project team, was designed to inform the directors and residents of Pennswood Village of the holistic, ecological attributes of the system and to address any concerns of the community.
The Final Design
The functional design of the Pennswood Village stormwater management system mimics that of a natural riparian stream corridor channel (Figure 6). It uses a number of BMPs linked in a specific series of passive operation. The efficiency of individual BMPs was increased by creating a routing system that integrated a series of hydrologically linked BMPs, thereby creating a pollutant removal train. Each BMP is 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 grading plan was developed from the analyses of stormwater flows and volumes but also took inspiration from the gently sloping topography of Bucks County. Likewise, the selection of hardscape and plant materials also held cultural and natural significance for the area. For instance, an old Quaker bridge found near the site provided the inspiration for the bridges in the design—and dictated the choice of the local Bucks County brownstone (Figure 7). Quaker pairings of materials were also used. One example is the entrance sign, where forged steel and wood beams are used together. Quaker ideals of functionality paired with an unadorned, elegant aesthetic were used throughout the project.
A major component of the design deals simultaneously with human experience of the project and with the goals of the stormwater system. A decision was made to realign the main entrance road to Pennswood Village so that the stormwater system and the road intersected twice (Figure 2). This design decision provides a sense of entrance into the village while calling attention to the functioning of the system. It allows the visitor to experience spatial and climatic changes resulting from the topography and planting of the system, since the road cuts across two sections of the riparian corridor. The changes in plant communities and grade make the system beautifully visible to visitors. This design feature also allows the system to meander more dramatically—thus increasing the length of stormwater flow and allowing for the creation of additional basins.
This choreography of movement through the site is even more evident in the pedestrian paths. These paths were carefully designed with shifting views and changing spatial qualities in mind. At times, views are blocked by gentle mounds and opened up again at the edge of a large basin. Parts of the path closely follow the swales and basins, while others veer into the dry meadow (Figure 8). When the trees reach maturity, they will serve to frame certain views and will define a range of enclosed and open spaces.
The plan for the original detention basin was a big "hole in the ground" with a low-flow channel, tucked into a corner of the property, away from the road. This traditional approach does not encourage human interaction and implies that stormwater management is a thing to be hidden away. The final design for Pennswood celebrates human interaction, observation, and education through its alignment with vehicular and pedestrian routes, and through its parallel use as a naturalistic entryway into the community. The inclusion of elements meant for human use and observation ensures that the stormwater system is a highly visible element in the landscape and can act as its own advocate, educating those that use the paths and roads interwoven within it.
The stormwater system itself is composed of four major elements: a sedimentation basin, a series of bioretention basins, swales, and a created treatment wetland. At the uppermost end of the system is the outfall structure and forebay (Figure 9). Here, runoff is directed by a series of pipes from Route 413 and the contributing watershed. The outfall structure is a semicircular stone-lined wall constructed with the local Bucks County brownstone. The curved form of the wall echoes the grading, and its simple but elegant construction reflects the building traditions of the region. The forebay boulders slow the storm surge, settle out gross particulate material, and contain the majority of the trash and road debris conveyed along with the runoff. A stone weir regulates the volume and time that the collected runoff is detained in a second sedimentation basin (Figure 10). The basins are easily accessed for routine maintenance and the periodic removal of accumulated sediments.
Runoff discharged from the sedimentation basin is directed into a grassed swale that conveys the runoff to an infiltration basin. At this point, the swale runs alongside the entrance road, visible intermittently through the trees. The soils that predominate in this section of the site are highly permeable. The depth to the seasonal high-water table is in excess of 6 feet, as is the depth to bedrock. 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.
Flows exceeding the infiltration capacity of this basin will be discharged over a broad crested weir into another long, winding vegetated swale, which connects additional basins and curves back and forth across the entrance road. On either side of the swale is a broad flat meadow graded and designed to function in a manner similar to a riparian corridor or stream floodplain (Figure 11). That is, it consists of a series of shallow, stepped channels, each of which accommodates and detains the runoff from increasingly larger storm events. Depending on its proximity to the grassed swale and the resulting post-construction topography, the meadow supports a variety of vegetation including grasses, shrubs, and trees having different flood and drought tolerances.
At the terminus of the system is a created treatment wetland (CTW) (Figure 12). Outflow from the wetland is controlled by an outlet structure designed to safely pass the 100-year storm. Initially, during the early part of a storm, runoff that exceeds the capacity of the wetland will flow, via the swale, into the final basin. The outlet control structure on the final basin will cause water to flood back into the CTW should the need arise. As water is detained in the CTW and final basin, it will back up further, eventually overflowing into the broad meadow and the created riparian corridor. From the meadow, it flows into another swale on the north side of the property, then into a series of three bioretention basins, and finally into Neshaminy Creek.
Drew Mason, Pennswood's landscape manager, states that stormwater reaches the final basin three to four times each year, meaning that the system overflows into Neshaminy Creek a maximum of four times annually.
A few components of the original concept plan—such as the educational amphitheater and the orchard visible in the illustrative plan (Figure 2)—were not carried through to the final design. In the place of the proposed orchard, the community planted a single specimen white oak that will eventually preside over the dry meadow. The proposed amphitheater represented the commitment to education fostered by Pennswood Village. However, even without such a formal space, Pennswood continues to host educational tours for local middle schools and high schools, as well as to give presentations to municipal officials from all over the region. Additionally, classes from the University of Pennsylvania, Temple University, and Rutgers University make annual field trips to Pennswood to learn from this multifunctional project.
Plant Selection
To achieve the desired goal of stormwater management while still creating an aesthetically pleasing setting, care was taken in the selection of plant materials used throughout the Pennswood Village system (Figure 11). The use of native plant materials fit well with the existing landscape, providing habitat for wildlife in keeping with the desires of Pennswood Village, and creating a sense of place. Additionally, the use of indigenous plants minimized problems with the acclimatization of plant material, maintenance of plantings, and the ability of the selected materials to withstand seasonal climatic fluctuations.
Plant selection was a function of grade, periodicity and duration of flooding, and the drainage characteristics of the underlying soils. However, other factors were taken into consideration. These included the velocity of runoff to which the plants would be exposed, the functional intent of the planting, their wildlife habitat attributes, and their visual and spatial characteristics.
Plant palettes for each part of the system were generally chosen according to naturally occurring plant communities. Four broad conditions for plant growth were identified on the plan: dry and wet riparian corridor conditions, and dry and wet meadow conditions. For the drier parts of the swale and basin system (the dry riparian corridor), species such as sycamore, white ash, ironwood, and hackberry were selected. For the wet riparian corridor, species included grey-stemmed dogwood, bayberry, pin oak, green ash, and river birch.
In the dry meadow, selections included drought-tolerant tall grasses such as little bluestem, Indian grass, and red top, as well as wildflowers like black-eyed Susan and common milkweed. The wet meadow plant communities included broom sedge, New York ironweed, Joe Pye weed, and boneset.
The variety of plants used throughout the project is another manifestation of the site's multiple functions. The plant material used aids in the treatment, filtration, and detention of stormwater and at the same time provides habitat for wildlife and creates beautiful and varied spaces for human enjoyment.
Community Reception
Five years after the system was constructed, Nancy Spears, Pennswood's director, admitted that only in its fourth summer could the community appreciate the full beauty of the design and the result of their labor, time, and money. The grasses had grown in, the perennials and grasses had established and spread, the trees had stabilized, and the system was functioning perfectly. She and Drew Mason, the community's landscape manager, felt that patience was essential throughout the process, but stressed that the result was well worth the wait. The village has a saying that embodies this commitment to cross-generational stewardship: "We are the harvest of someone else's hope; we sit under trees we did not plant; we plant trees under which we will never sit." This attitude provides a wonderful foundation on which the stormwater system can grow and mature.
As for the current functioning of the system, no flooding has occurred downstream since the system was installed, in spite of very heavy rains that caused flooding in other parts of the township. Spears states that the gradual flow of stormwater from one basin into the next is a fascinating and powerful occurrence to witness. The rising and settling of the water and its movement from one basin to another is very poetic for her and for others in the community. She says that in a storm event, it is extremely clear how the system works and that observers can see the elegant manner in which the water is controlled and directed.
The residents at the village and people from the surrounding neighborhoods frequently use the paths that circulate through the swales and meadows for jogging, walking, and bird watching. Many of the residents at the village are avid bird watchers and have noted a great increase in the number of bird species present from one year to the next. One longtime resident noted that she saw Baltimore orioles, redwing blackbirds, and other species she hadn't seen on the property in 20 years. A number of hares, frogs, and a fox have also been spotted on the site.
The stormwater system and the network of paths would not have been so successful if not for the efforts of Mason and his team. They understand how the system works and appreciate the importance of maintaining grades and plant communities. Mason states that when the system was installed, there was no maintenance manual for this type of project, and they had to gradually learn how to best care for it. After installation, he and Appel walked the grounds several times throughout the years to see what needed attention and to devise appropriate maintenance procedures.
Mason and his team conduct several measures throughout the year to ensure that the system is functioning properly and to encourage the continued growth of the native plant communities. They clean out the forebay and sedimentation basin about once a year and have noted that no debris from the basin migrates into any other part of the system—it stays in the basin until it is manually removed. They also keep invasive species such as phragmites, mustard grass, and crown vetch under control through a combination of careful spot spraying and manual cutting.
Mason and his team mow the system in two segments: The lower elevations are cut in the early winter when they are relatively dry, and the upper elevations in early spring. This approach allows migratory birds to find cover on the site when they need it, while also keeping machinery away from wet soils so as not to create tracks.
Mason frequently walks the site, keeping an eye out for erosion channels and volunteer plants, and simply observing the system at work. He notes that water travels slowly through the system, allowing infiltration to occur in the basins. He and his team are essential to the continued success and functioning of the system. While he admits that the system is far from high-maintenance, he stresses the importance of regular procedures and observations, and above all, a thorough understanding of how the system works.
Conclusion
The project's successful result is a highly functional stormwater management system that exceeds the township's stormwater management requirements while providing an attractive environment for Pennswood Village and the community at large. As proposed, this system attenuates peak flows, promotes groundwater recharge, and passively removes pollutants through a combination of filtering, settling, and biological treatment mechanisms. It also provides opportunities for recreation, education, and the appreciation of nature. In taking on a variety of functions, the project works to ensure its own continued success within the community. By providing opportunities for passive recreation, the project acts as an educational tool, spreading the message of alternative stormwater management techniques to a wider audience.
The Pennswood Village stormwater management design has won several awards, including a Design Merit Award from the American Society of Landscape Architects in 2003 and the Pennsylvania Governor's Award for Environmental Excellence in Design in 2003. It has also been recognized for engineering excellence by the Pennsylvania House of Representatives, the Pennsylvania Senate, and the American Council of Engineering Companies.
Project team: Wells Appel Land Strategies; Stuart Appel, principal-in-charge; Evan Stone, project manager; Devinder Soin, landscape designer; Deborah Lermit, landscape designer; Ngim Chea, landscape designer, Princeton Hydro; Dr. Stephen Souza, hydrologist; Mark Gallagher, plant ecologist and wildlife biologist, Pickering, Corts & Summerson; Wayne Johnson, civil engineer, Mellon Biological; Rick Mellon, wetlands scientist; General Contractor, S&H Landscaping.
The authors wish to acknowledge editorial assistance on this article from Rosa Mannion of the University of Pennsylvania Department of Landscape Architecture.