Developers and landowners in many communities have few options for sustainably managing stormwater runoff from parking lots, driveways, and roadways – it must be retained, treated, and infiltrated or re-used onsite. Wetlands and underground chambers may be appropriate solutions for some sites, depending on space, soil properties, and rainfall. However, an increasing number of permeable pavement technologies can offer a cost-effective design option for civil engineers and project owners to consider. Following are brief summaries of a variety of recent projects incorporating permeable pavement systems.

Thorndike Park, Arlington, Mass.
At Thorndike Park in Arlington, Mass., additional parking was needed to accommodate expanding use of the recreational park, but the client did not want to install costly best management practices (BMPs) to manage additional stormwater runoff. The project called for a cost-effective porous paving solution that required very little maintenance, was highly porous to absorb runoff from adjacent asphalt pavement, would provide a unique aesthetic, and be durable to withstand the wear associated with regular vehicular traffic and the Massachusetts climate.

The Thorndike Park Project utilized BodPave85 to reduce up-front installation and long-term maintenance costs, while creating an aesthetically pleasing multi-functional porous parking area. Porous pavement in the parking stalls absorbs runoff from the adjacent asphalt pavement (inset). Boddingtons Inc.

To eliminate runoff from the new parking area, BodPave85 was installed in the parking stall areas with drainage from adjacent drive lanes graded toward the parking spaces. BodPave85, composed of 100 percent post-consumer recycled plastic, can be filled with either grass or gravel. The client chose decorative gravel to further reduce maintenance needs. BodPave85 serves as the stabilizing structure for the open-graded aggregate, providing increased void-space in the profile and eliminating the need for vacuuming or sediment removal under normal conditions, which can be experienced with some porous paving systems. The highly porous reinforced gravel profile also provides storage capacity during rainfall events; the 4-inch clean gravel profile will accommodate an additional 0.75 inch to 1 inch of rainfall over its relative area, reducing the depth of engineered sub-base storage needed to accommodate the designed storm event.

In addition to stormwater benefits, the project required a paving grid system that could withstand long-term, heavy traffic. With its 1.35-inch integrated stabilizing ground spikes and nesting snap-fit connections, BodPave85's design eliminates the risk of lateral shear as passenger and maintenance vehicles use the site under all types of weather conditions. After its first year of use, the project is 100 percent functional, shows no signs of wear, and has required no maintenance.

Information provided by Boddingtons Inc.

Gravelpave2 provides a permeable pavement for entrance road, driveway, and parking areas at the Preserve at the Clam Farm, Charleston, S.C. The site is close to a sensitive salt marsh and runoff needed to be controlled. Invisible Structures To construct a porous pavement demonstration, a portion of a parking lot was removed and the area excavated to the plan depth to accommodate a 48-inch-thick stone reservoir. Gray and Son Inc. The highlight of a porous pavement demonstration was when water from fire hoses disappeared into the pavement. Gray and Son Inc.

The Preserve and Clam Farm, Charleston, S.C.
Designers for the Preserve and the Clam Farm in coastal Charleston, S.C., chose Gravelpave2 permeable paving system for the entrance road, driveways, and onsite parking. After completion of the three phases in the new townhome complex, the total amount of permeable Gravelpave2 will be 90,000 square feet; the first phase consists of 29,000 square feet.

This property, on Folly Beach, is surrounded by salt marsh that is designated as highly sensitive to any runoff. No consolidation of stormwater was allowed, as per Ocean and Coastal Resource Management, the agency for all coastal permitting. Gravelpave2 was approved because there is less runoff than the undeveloped land. The site designers, International Mariculture Resources, South Carolina, Inc., and Ron Felkel, RLS, of HLA Inc., selected Gravelpave2 for the natural look of the finished system.

Gravelpave2 consists of cylinders connected by a grid with an attached geotextile fabric. The structure is anchored to a base course with ring-shank spikes. The system was specified with a 10-inch base: 5 inches of compacted, washed No. 57 stone over a non-woven fabric to keep stones from migrating into the soil, on top of which was placed 2-1/2 inches of washed No. 89 stone mixed with 2-1/2 inches of coarse, compactable, well-draining sand, compacted to between 92 percent and 95 percent modified Proctor. The top surface was finished with 1-1/4 inches of sharp, washed No. 89 granite – 1/4 inch above the cylinders. All landscaping material was delivered on a fully loaded tractor trailer and installed after the Gravelpave2 was placed and finished. All the concrete aprons to the garages also were installed with Gravelpave2 in place; loads were limited to 8 cubic yards per ready-mix truck, not unlike many completed subdivisions.

The Gravelpave2 has held up extremely well under the heavy traffic of installation and subsequent traffic from the residence. Although Gravelpave2 is designed for unlimited traffic volume, speeds should be kept to slower than 20 mph.

General maintenance includes evenly distributing migrated aggregate back over the surface. Gravel in the higher traffic areas is sometimes carried away in tire treads and deposited in other locations. Brooming is required at least once per year. Keeping the structure filled with gravel reduces UV radiation damage and prolongs the life of the system – an expected lifespan of 25 years. Gravelpave2 is inert to most landscaping and horticultural chemicals.

Information provided by Invisible Structures

Porous pavement demonstration, Timonium, Md.
Porous asphalt pavements are gaining popularity rapidly as developers look for cost-effective ways to meet new stormwater regulations. While many states consider these pavements a BMP, designers, developers, and contractors may have limited familiarity with these pavements. To help bridge this knowledge gap, a number of contractors around the country have stepped up building demonstration projects to showcase these pavements. Gray and Son Inc. and Maryland Paving Inc. recently constructed a porous asphalt pavement at their office in Timonium, Md., for this purpose. Geo-Technology Associates in Abingdon, Md., designed the pavement.

The porous pavement structure included the following (from the surface down):

• 4 inches porous asphalt;
• 2 inches stabilizing course (3/8-inch aggregate);
• 48 inches stone reservoir (AASHTO No. 2 stone); and
• clay subgrade.

To construct the porous pavement, a portion of a parking lot was removed and the area excavated to the plan depth. A perforated pipe with valve (located in an existing drop inlet) was installed near the bottom of the stone bed. Another perforated pipe was installed near the top of the stone bed as an overflow. Both pipes discharge to an existing inlet near the corner of the porous pavement. The stone reservoir uses an AASHTO No. 2 stone that was dumped from the side of the pavement, leveled using a grader, and compacted with a static steel-wheel roller. Two inches of 3/8-inch aggregate was placed on top of the stone reservoir and again compacted with a static steel-wheel roller. Finally, the porous asphalt mix was placed using a material transfer vehicle and track paver. Compaction was achieved with two to four passes of a static steel-wheel roller. Jeff Graf, vice president of operations for Maryland Paving, said, "A couple of trial mix designs were required for the porous asphalt to get the void structure they needed."

The GEOBLOCK porous pavement system supports a grassed, overflow parking area for a church in Knoxville, Tenn., and negated the need for additional stormwater retention. Presto Geosystems

A demonstration of the porous pavement was held two weeks after construction. Displays showing the different materials used in construction, a video of the construction processes, diagrams, technical materials, and presentations by national and local experts were used to educate participants including the Maryland State Highway Administration, city and county agencies, consultants, and developers. The highlight of the demonstration was when a fire hose was turned onto the pavement and participants stared in amazement as the water disappeared into the pavement.

Information provided by the National Asphalt Pavement Association

Cedar Springs Presbyterian Church, Knoxville, Tenn.
The Cedar Springs Presbyterian Church low-impact parking improvements project began as an addition to meet the church's parking needs and to provide a new site entrance to improve access and traffic circulation within the church campus. The desire to develop their property as good stewards and include green or sustainable design practices led to the decision to use pervious parking for this 169-space, 60,000-square-foot project.

Some impervious parking was required to bring the church into compliance with the city of Knoxville's parking requirements. However, additional parking could be considered as "overflow" and pervious pavement systems could be used as an alternative parking surface. A vegetated parking lot, typically recommended for occasional or infrequent use traffic, is ideal for the church parking needs. The GEOBLOCK porous pavement system was selected after evaluating pervious pavement options.

Important selection criteria included the following:

• grass parking was a relatively new, sustainable parking surface that could support the traffic loading;
• the vegetated surface negated the need for pre- and post-improvement stormwater retention; and
• the vegetated surface allowed for both maximum use of the church property by minimizing the area required for retention and correction of existing drainage issues.

The church envisioned a park-like setting with maximum green space, extensive landscaping, and a minimal stormwater retention footprint. To irrigate these features properly, an irrigation system fed from an onsite well was designed, which eliminated the need to use processed potable water for irrigation needs, reducing overall water consumption costs for the church campus.

Sustainable design considerations and environmental impact include the following:

• the porous surface provides infiltration of rainfall with minimal amount of runoff;
• the grass surface reduces heat island effects associated with asphalt.

The porous pavement design allowed for the required 80-percent total suspended solids removal without use of additional mechanical water quality devices or water quality detention ponds, thereby maximizing the use of the parking footprint on the church property.

The site is located within the lower portion of its watershed and is relatively level. The low gradient presented challenges in the design of the drainage facilities. Other challenges included the presence of soft soils and topsoil, which required extensive undercut and replacement.

Months after completion, the vegetated parking lot is functioning as planned and is heavily used for services and special events. The city of Knoxville has used this project as an example of successful urban low-impact development and the church leadership is very satisfied with the results of the project. Maintenance of the grass pavement is similar to maintaining any grass surface.

Information provided by Presto Geosystems; summarized from a project brief written by Brad Salsbury, P.E., Cannon & Cannon

Company parking lot, Carlsbad, Calif.
In an effort to prevent polluted stormwater runoff from entering the nearby Buena Vista lagoon, Soil Retention decided to retrofit its existing asphalt parking lot with a permeable, flexible, and plantable pavement called Drivable Grass at the low point of the parking lot. The site is located on the Southern California coastline in the city of Carlsbad, roughly 2,000 feet from the Pacific Ocean and 1,000 feet from the Buena Vista Lagoon, which is a 223-acre wetland home to 103 bird species, 18 mammal species, and frequented by fishermen.

Drivable Grass, installed at the low point of a parking lot, treats the first flush of stormwater runoff with biofiltration. One year after installation, the grass is growing vibrantly and there are no signs of rutting or product degradation. Soil Retention Systems

The main intent of the installation is to treat the first flush of stormwater runoff from the parking lot with biofiltration. Existing soils have low permeability, so only partial exfiltration can occur. A secondary consideration was to detain and store water during rainfall events that can be re-used for landscape irrigation. The Drivable Grass installation had the following performance criteria: durable enough to withstand vehicle traffic on a daily basis, able to be planted for biolfiltration of harmful pollutants, economical to install and maintain, and aesthetic.

Installation and preparation of the site's subgrade involved removal of the existing asphalt and soil to a depth of 2 feet using a backhoe. Next, a geotextile was laid to separate the subgrade from the structural base and storage section (18-inch-thick crushed rock with a void ratio of approximately 35 percent). Another geotextile was used to separate an additional 4-inch-thick sand bedding for turf grass roots to grow into. The Drivable Grass was then installed in 2 foot by 2 foot mats and infilled with grass seed and sand.

One year later, the grass is growing vibrantly and there are no signs of rutting or product degradation despite continuous traffic five days a week. The current system has the capacity to capture and retain an estimated 317 cubic feet of stormwater runoff for an area composed of three parking stalls and a small driveway corner of Drivable Grass. To comply with National Pollutant Discharge Elimination System (NPDES) requirements for stormwater treatment facilities in San Diego County, a volume-based facility must "infiltrate, filter, or treat the volume of runoff produced from a 24-hour, 85th percentile storm event." In Carlsbad, a 24-hour storm event would produce 0.6 inches of rain or roughly 322 cubic feet of water in the subject 20-car parking area and drive isle. Currently, there are no regulations for existing development in the city of Carlsbad. However, the state of Maryland is currently implementing similar installations/regulations for improving stormwater quality.

Aside from typical lawn care, there is no maintenance.

Information provided by Soil Retention Systems