Sustainable Site Design: Making It Happen

Thomas H. Cahill, P. E. 1 Michele C. Adams, P. E. 2

1. President, Cahill Associates, Inc. 104 S. High Street, West Chester, PA. Phone: +1 610

696-4150. Fax: +1 610 696-8608. E-mail: tcahill@thcahill.com 2. Principal Engineer, Cahill Associates, Inc. 104 S. High Street, West Chester, PA.

Phone: +1 610 696-4150. Fax: +1 610 696-8608. E-mail: madams@ thcahill.com

1. INTRODUCTION The traditional approach to land development begins with the desires and goals of a client. Be they industry, land developer, homebuilder or government agency, they wish to build a structure or group of structures in a chosen location. As these objectives, needs or opportunities are described to an architect and their supporting cadre of engineers and landscape architects, they carefully craft a “program” that will create the needed structure or built system on the land. The issue of “where” is usually determined by market conditions, and when a location is selected and a suitably-sized parcel of land purchased, the methods, materials and costs drive the process, since all land development is constrained by the economic realities of the program. Unlike traditional development where the site elements of water, wastewater, and stormwater are “engineered” after the building is designed and landscape design applied as a final beautification process, Sustainable Site Design begins very early in both the site selection and site design processes. When a specific parcel is selected, consideration is given to existing natural features, including topography, vegetation, and soils, as well as sensitive elements such as floodplains, areas of shallow bedrock, and high water table, to develop a sense of “where you are.” The Site Designers work as part of the Design Team from the concept stages of the design process, formulating a strategy that will avoid excessive earthwork and sustain important natural elements such as wooded tracts and stream corridors, using the techniques of Sustainable Site Design. Thus, the initial step is to define the intrinsic values – or opportunities – of the land and identify the site constraints, and create a plan that will allow the owner to complete the desired program while sustaining most of these values. The most important aspect of Sustainable Site Design is that if the building program is carefully integrated into the opportunities and constraints of the land, it costs substantially less. By considering the site in the context of the various design elements, such as landscape design, stormwater management, water and wastewater, grading and erosion control, construction cost, as well as operation and maintenance are reduced. Thus sustainable site design does not translate into increased development cost, but just the opposite. Achieving a LEED rating requires this new approach to site design. This paper presents technologies, techniques, and an overall approach to achieve sustainability. The best efforts of Sustainable Site Design focus on the need for management of water, stormwater, and wastewater. Techniques that demonstrate true sustainability of the resource, such as stormwater

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management techniques including porous asphalt pavement that infiltrate rainfall rather than build detention basins are foremost in the methods and materials described in this paper. 1. 1. Hydrologic Cycle The potential impacts that the built environment may have on the natural cycles of water are important to consider. For example, will the new impervious surfaces reduce the soil moisture levels in nearby woods and hence affect the vegetation, or will there be adverse water quality impacts to nearby streams and water bodies? In urban or previously developed sites, is there a need for restoration as part of the design? What techniques can be used to reduce the volume and pollutant load of stormwater and wastewater? Most importantly, how will we develop and maintain a supply of potable water to serve this site? All of these issues are better understood when we evaluate the hydrologic cycle on a parcel. Figure 1 illustrates the annual cycle throughout the Piedmont Plateau of the eastern US, where annual rainfall varies from 42 inches to 48 inches a year. The numbers vary throughout the country, but the basic arithmetic remains the same. Under natural conditions prior to development, about half of that rainfall amount is returned to the atmosphere through evaporation and by the vegetation as transpiration. A tree is basically a water pump, removing the moisture from the upper soil mantle during much of the year as the rain percolates through the soil. Only 8 inches to 12 inches a year actually runs from the land surface during heavy precipitation, with most of the rainfall soaking into the ground.

Figure 1 The annual hydrologic cycle for an undisturbed acre in the Piedmont Plateau. When we develop a site, removing much of the vegetation and covering the soil with impervious surfaces of rooftop, pavements and roads, we greatly alter the hydrologic cycle. Figure 2 illustrates the effects of these surfaces on the hydrologic cycle. By constructing impervious land cover we effectively increase the immediate runoff during a storm, adding some 36 inches, or

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three feet, of additional volume of water. This runoff scours the land surface, transporting everything we drip, drop, spill or spray on the land as non-point source pollutants. These two issues, increased runoff volume (water quantity) and water quality degradation, define the stormwater impacts of land development.

Figure 2 The impact of development on the hydrologic cycle. 2. SUSTAINABLE SITE DESIGN GUIDELINES Stormwater impacts of land development are minimized when Sustainable Site Design strategies and guidelines are incorporated into the planning process. Cahill Associates (CA) has developed a set of principles for Sustainable Site Design strategies, methods and materials:

Balance the Natural Hydrologic Cycle Hold runoff volume constant. Avoid increased runoff volumes, worsening flooding downstream. Avoid increased bankfull flows, impacting streams and aquatic biota. Maintain recharge of rainfall to groundwater. Maintain infiltration for existing vegetation. Maximize preventive approaches through sound planning and design. Minimize impervious surfaces through low impact development. Protect sensitive and special value areas through conservation design. Use Best Management Practices such as Porous Pavement/Recharge Beds. Collect and re-use rainwater for plant and garden watering. Use vegetated roof systems in highly developed sites for volume control.

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Maintain Water Quality Use native species and limit/avoid future chemical site maintenance. Limit artificially landscapes areas such as maintained lawns. Avoid discharges of wastewater and stormwater to streams and lakes. Recycle wastewater nutrient by-products (nitrogen, phosphorus). Maximize use of optimal pollutant reducing BMPs for stormwater. Filter runoff from pollutant producing “hot spots.” Avoid excessive earthwork that creates erosion and sediment impacts. Sustain native vegetation and use low maintenance materials. Minimize water temperature impacts to streams and to sensitive aquatic biota. Minimize Overall Site Disturbance Minimize disturbance of the existing soil mantle and site vegetation. Tailor building to natural contours; avoid excessive earthmoving. Limit removal of existing vegetation by requiring rigorous tree replacement. Prohibit structures, all forms of disturbance in floodplains and other sensitive areas. Keep building/parking envelops as compact as possible. Use clustering, reduced setbacks, other innovative planning and design techniques. Maintain riparian buffer along streams. Maximize Water Conservation – Recycle Wastewater Use on-site water and wastewater treatment systems if feasible. Use low-flow fixtures. Capture and re-use precipitation with cisterns, rain barrels. Use dual “gray water” systems. Use alternatives to chemical disinfection (i.e., UV disinfection). Re-use treated wastewater for flushing. Use low maintenance, low energy wastewater treatment systems. Apply treated effluent to land surface to recharge groundwater.

While much more can be said to describe these guidelines, no one project can incorporate every principle; resistance to change, unsuitability, and economic constraints are all contributing factors. However, consideration of a sustainable stormwater management approach to site design is absolutely attainable. Figure 3 is a flowchart that translates Sustainable Site Design Guidelines into an implementation reality. This flowchart operationalizes a comprehensive and sustainable approach to stormwater management during site development. Perhaps the best way to illustrate this sustainable site design approach is to describe several projects completed during the past twenty years that include the construction materials and methods developed.

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Figure 3 Comprehensive stormwater management for sustainable site design. 3. IMPLEMENTATION OF GUIDELINES 3.1 Site Selection and Resource Evaluation Most development projects do not have the opportunity to select a site based on environmental considerations, but the design and construction of a Clinical Laboratory for the SmithKline Beecham Company in 1994 did allow for this process. As a design/build project, the design team was included in the initial selection, planning and evaluation, with the selected site situated in a large office complex in suburban Philadelphia. After utilizing the Sustainable Site Design guidelines presented above, the building was situated on the cleared upper portion of the parcel and the surrounding woods were substantially preserved. Figure 4 shows the aerial overlay used to identify site opportunities and constraints. Earthwork was limited to fit the building on the land surface, while satisfying use criteria of circulation and access. A small wooded stream valley was protected from disturbance by avoiding the construction of detention basins. Instead, stormwater management includes a porous pavement parking lot with subsurface infiltration

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beds. The beds provide significant recharge to the critical aquifer system, an important regional water supply source, although off-site water and wastewater systems are utilized. All landscape materials and planting systems are native species that do not require irrigation.

Figure 4 The SmithKline Clinical Laboratory site in East Norriton Township, PA. 3.2 Porous Pavement with Recharge Beds First developed in the 1970’s at the Franklin Institute in Philadelphia, Pennsylvania, porous asphalt pavement consists of standard bituminous asphalt in which the aggregate fines have been screened and reduced, allowing water to pass through the asphalt. Figure 5 illustrates the system, whereby underneath the pavement is placed a bed of uniformly graded, clean washed aggregate with a void space of 40%. Stormwater drains through the asphalt, is held in the stone bed, and infiltrates slowly into the underlying soil mantle. A layer of geotextile filter fabric separates the stone bed from the underlying soil, preventing the movement of fine particles into the bed.

Figure 5 Cross-section through porous asphalt showing subsurface infiltration bed.

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Porous pavement with recharge beds is especially well suited for parking lot areas. Several dozen large, successful porous pavement installations, including some that are now 20 years old, have been developed by Cahill Associates, mainly in Middle Atlantic States locations. These systems continue to work quite well as both parking lots and stormwater management systems. In fact, many of these systems have outperformed their conventionally paved counterparts in terms of both parking lot durability and stormwater management. 3.2.1 Installations Old and New. One of the first large-scale porous pavement/recharge bed systems that CA designed is located in a corporate office park in the suburbs of Philadelphia (East Whiteland Township, Chester County). This particular installation of about 600 parking spaces posed a challenge because of the both the sloping topography and the underlying carbonate geology that was prone to sinkhole formation. The site also is immediately adjacent to Valley Creek, designated by Pennsylvania as an Exceptional Value stream where avoiding nonpoint source pollution is of critical importance. Constructed in 1983 as part of the Shared Medical Systems (now Seimens) world headquarters, the stormwater system consists of a series of porous pavement/recharge bed parking bays terraced down the hillside connected by conventionally paved impervious roadways. Both the top and bottom of the beds are level, as seen in Figure 6, hillside notwithstanding. After 20 years, the system continues to function well and has not been repaved. This particular area is naturally prone to sinkholes due to underlying carbonate geology, but no sinkholes have occurred in the porous asphalt areas. On the other hand, sinkholes have occurred in the conventional asphalt areas, which the site manager attributes to the concentrated distribution of stormwater into previously constructed detention basins.

Figure 6 Porous pavement parking bays are “benched” down a hillside. Other early ‘80’s sites, such as the SmithKline Beecham (now Quest) Laboratory in Montgomery County, Pa and the Chester County Work Release Center in Chester County, Pa also used the system of “terracing” the porous paved recharge beds down the hillside to overcome the issues of slope. At the DuPont Barley Mills Office complex in Delaware, the porous pavement was constructed specifically to avoid the construction of a detention basin which would have

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destroyed the last, wooded portion of the site. More recently (1997), the porous parking lots at the Penn State Berks Campus were constructed to avoid destroying a wooded campus hillside. The Penn State Berks lots, also on carbonate bedrock, replaced an existing detention basin and have not experienced the sinkhole problems that another campus detention basin has suffered. The Morris Arboretum of the University of Pennsylvania, located in Chestnut Hill, utilized standard impervious asphalt in the main driveway, with porous asphalt parking spaces. Figure 7 below shows the parking lot during Hurricane Floyd, where the stormwater is flowing over the impervious central aisle and through the pervious parking bays.

Figure 7 The Morris Arboretum in Chestnut Hill, PA during Hurricane Floyd.

Other installations have included offices, libraries, schools, municipal buildings, shopping centers, country clubs, manufacturing plants, and even a McDonald’s in Atlantic City, New Jersey. Over 100 systems have been constructed in eight states during the past twenty years, and all continue to remain pervious and function properly. 3.3 Recharge Gardens and Other Vegetative Infiltration Measures Stormwater runoff volume reduction can be achieved by a variety of methods that integrate stormwater storage and infiltration with vegetative elements. These various techniques are similar to stormwater storage/infiltration beds under porous pavement in that they are incorporated into typical site elements such as landscape beds and open grassed areas. The use of vegetated systems allows for additional environmental benefits by reducing runoff volume through evaporation and plant transpiration as well as infiltration. These systems integrate attractively into the site landscaping and work well in commercial, institutional, and residential areas. Recharge gardens are depressed vegetated areas in which a small shallow infiltration bed is installed and backfilled with a modified soil used for a planting medium (Figure 8). The stone bed is constructed similarly to other infiltration beds and is comprised of clean, washed uniformly graded aggregate with a 40% void space, and is wrapped with non-woven geotextile to prevent soil migration into the bed. The bed is typically shallow in recharge gardens and does not usually exceed 12” in depth. Also, the bed must be excavated carefully to avoid bed bottom compaction. The depressed area is graded with gradual side slopes and an overflow is installed so that no more than 6” – 8” of surface ponding is allowed. Native vegetation should be chosen

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that can withstand both wet and dry periods as the system is not designed to function as a wetland, but rather to allow runoff to fill up and soak into the soil where it will both infiltrate and be used by the vegetation.

Figure 8 The recharge garden at Pennsylvania State University/Centre County Visitor Center receives rooftop runoff.

Vegetated infiltration beds can also be used in grassed areas, such as under recreation playfields or in other open space environments. In large evenly graded areas, more storage can be provided in deeper beds that are backfilled with soil and planted with turf. These areas receive runoff from rooftops or other adjacent impervious areas. This same concept can be incorporated into smaller, linear infiltration trenches. Infiltration trenches work well in tight areas and along sloping areas where they can be built in series along the contours and down the hillside. An excellent example of a site design that integrates a variety of stormwater management techniques, especially vegetative systems, is the Pennsylvania State University and Centre County Visitor Center in State College, Pennsylvania. The stormwater management features on site were designed to capture, store, and infiltrate or evaporate/transpire the net increase in the volume of runoff for all storms of the two-year frequency or less (Figure 9). The largest portion of runoff from the site is captured and infiltrated under porous pavement parking lots that overflow to recharge gardens located throughout the site. Runoff from the rooftop is conveyed to a large recharge garden and an underground infiltration trench that was built along the contour in an open grassed area. Impervious areas were further reduced on site by utilizing porous concrete for new sidewalks. The BMPs were successfully built despite the limitations of a shallow soil mantle underlain by carbonate geology.

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Figure 9 Pennsylvania State University - Centre County Visitor Center utilizes five

different stormwater management BMPs 5. CONCLUSION As the concept of Sustainable Site Design evolves and the recommended concepts are implemented, we expect many new (or perhaps old) technologies to evolve to better meet the goals stated. Most importantly, we must encourage the process of creative design and allow the evolution of design and construction methods and materials that are truly sustainable. Learning to live within the tolerance limits of our environment is fundamental to our future on the land.

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