Permeable Paving

Adapted from the Massachusetts Low Impact Development Toolkit

Description

Since impervious surfaces are the primary source of stormwater runoff, Low Impact Development strategies recommend permeable paving for parking areas and other hard surfaces. Permeable paving allows rainwater to percolate through the paving and into the ground before it runs off. This approach reduces stormwater runoff volumes and minimizes the pollutants introduced into stormwater runoff from parking areas.

All permeable paving systems consist of a durable, load bearing, pervious surface overlying a crushed stone base that stores rainwater before it infiltrates into the underlying soil. Permeable paving techniques include porous asphalt, pervious concrete, paving stones, and manufactured "grass pavers" made of concrete or plastic. Permeable paving may be used for walkways, patios, plazas, driveways, parking stalls, and overflow parking areas.

All types of permeable pavement are typically designed to handle 70-80% of all storms (the most common ones). Longer, more intense rain events are typically drained from permeable pavements, since inflow will exceed infiltration (outflow) during these storms.

Three Major Types of Permeable Paving

Porous asphalt and pervious concrete appear to be the same as traditional asphalt or concrete pavement. However, they are mixed with a very low content of fine sand, so that they have 10%-25% void space and a runoff coefficient between 0.25 and 0.35 for the range of storms that permeable pavements are designed to handle.
Permeable Interlocking Concrete Pavement (PICP, a.k.a. unit pavers) are impermeable blocks made of brick, stone, or concrete, set on a highly permeable bedding of #8 or #89 stone. The joints and opening voids within the pavers should also be filled with #8 or #89 stone to allow water to percolate downward. When built on an open-graded reservoir base course such as #57 crushed stone, the runoff coefficient range for PIPC is approximately 0.3 over the range of storms it is designed to handle. The runoff coefficient of PICP will vary depending on rainfall intensity, joint width, and materials. Some concrete paving stones have an open cell design to increase permeability.
Grass pavers (a.k.a. turf blocks) are a type of open-cell unit paver in which the cells are filled with soil and planted with turf. The pavers, made of concrete or synthetic, distribute the weight of traffic and prevent compression of the underlying soil. Runoff coefficients are similar to grass, 0.15 to 0.6.

Each of the techniques described above is constructed over a base course that doubles as a reservoir for the stormwater before it infiltrates into the subsoil. The reservoir should consist of uniformly-sized crushed stone, with a depth suffcient to store all of the rainfall from the design storm. The bottom of the stone reservoir should be completely flat so that infiltrated runoff will be able to infiltrate through the entire surface. Some designs incorporate an "overflow edge," which is a trench surrounding the edge of the pavement. The trench connects to the stone reservoir below the surface of the pavement and acts as a backup in case the surface clogs.

Applications

Grass pavers are appropriate for pedestrian-only areas and for very low-volume, low-speed areas such as overflow parking areas, residential driveways, alleys, and parking stalls. Permeable Interlocking Concrete Pavement can also be used in low to medium volume settings such as residential streets and commercial parking lots.
Permeable paving is most appropriate for sites where the underlying soils have a permeability of at least 0.3" per hour. It has also been successfully used in areas with lower infiltration rates by incorporating perforated pipes within the open-graded base reservoir to drain excess water that isn’t able to infiltrate the soil.
Permeable paving is an excellent technique for dense urban areas because it does not require any additional land. With proper design, cold climates are not a major limitation. Porous pavement has been used successfully in Norway, incorporating design features to reduce frost heave.
Permeable paving is not ideal for high traffic/high speed areas because it has lower load-bearing capacity than conventional pavement. Nor should it be used on stormwater "hotspots" with high pollutant loads because stormwater cannot be pretreated prior to infiltration. Heavy winter sanding may clog joints and void spaces.

Benefits and Effectiveness

Porous pavement provides groundwater recharge and reduces stormwater runoff volume. Depending on design, paving material, soil type, and rainfall, permeable paving can infiltrate as much as 70% to 80% of annual rainfall.
Porous pavement can reduce peak discharge rates significantly by diverting stormwater into the ground and away from the pipe-and-pond stormwater management system.
Grass pavers can improve site appearance by providing vegetation where there would otherwise be only pavement.
Porous paving increases effective developable area on a site because portions of the stormwater management system are located underneath the paved areas, and the infiltration provided by permeable paving can significantly reduce the need for large stormwater management structures on a site.

Limitations

Permeable paving can be prone to clogging from sand and fine sediments that fill void spaces and the joints between pavers. As such, sand should not be applied on these products in the winter. Periodic maintenance is critical, and surfaces should be cleaned with a vacuum sweeper at least once per year. Additional vacuum cleaning should be conducted as needed based on site monitoring.
To avoid frost heave in cold climates, the reservoir base and soil should be designed to drain within 24 hours. In the event that the reservoir base freezes within this time period, there is suffcient void space to accommodate expanding ice without moving or heaving individual stones or surface pavers.
Permeable paving can only be used on gentle slopes (<5%).
Permeable pavement is not suited for high-traffic areas. It is generally not suitable for sites where it will be subject to heavy axle loads, although PICP can be engineered for such sites.
Caution should be used when paving in areas with permeable pavement. Snow plows can catch the edge of grass pavers and some paving stones, requiring that the plow blade be set higher or that rollers be used on the plow blade. If properly installed, snow removal from PICP should be the same as conventional pavement and should not require rollers on plow blades.

Maintenance

Post signs identifying porous pavement areas.
Minimize use of salt or sand during winter months
Keep landscaped areas well-maintained and prevent soil from being transported onto the pavement.
Clean the surface using vacuum sweeping machines.
Monitor regularly to ensure that the paving surface drains properly after storms.
Do not reseal or repave with impermeable materials.
Inspect the surface annually for deterioration.
Grass pavers may require periodic reseeding to fill in bare spots.

Design Details

For all permeable paving, base course is a reservoir layer of 1"-2" crushed stone; depth to be determined by storage required and frost penetration.
Permeable paving require a single-size grading of base material in order to provide voids for rainwater storage; choice of materials is a compromise between stiffness, permeability, and storage capacity. Use angular crushed rock material with a high surface friction to prevent traffic compaction and rutting.
The design may also include a 2" thick filter course of 0.5" crushed stone, applied over the base course.
For grass pavers, use deep-rooted grass species whose roots can penetrate the reservoir base course. Irrigation may be required but should be infrequent soakings so that the turf develops deep root systems. Grass pavers are not suitable for every day, all day parking because the grass will get insuffcient sunlight. Better for use as occasional overflow parking.
The introduction of dirt or sand onto the paving surface, whether transported by runoff from elsewhere or carried by vehicles, will contribute to premature clogging and failure of the paving. Consequently, permeable paving should be constructed as one of the last items to be built on a development site, after most heavy construction vehicles are finished and after the majority of the landscaping work is completed.

Cost

On most sites, permeable paving costs more than conventional asphalt or concrete paving techniques. In the case of porous asphalt and pervious concrete, construction costs may be 50% more than conventional asphalt and concrete. Construction costs of paving stones and grass pavers vary considerably and will depend on the application. As with any site improvement or stormwater management structure, property owners should provide a budget for maintenance of permeable paving, at an annual rate of 1%-2% of construction costs.
Permeable paving reduces the need for stormwater conveyances and treatment structures, resulting in cost savings elsewhere. Permeable paving also reduces the amount of land needed for stormwater management and may satisfy requirements for green space, allowing more development on a site.

Local Case Study

West Farms Mall – West Hartford, CT

Grass pavers were installed at the West Farms Mall off of I-84 at exit 40, to handle peak-season overflow parking associated with a mall expansion. Over four acres of reinforced turf was designed to accommodate 700 spaces of overflow parking for the peak shopping seasons. There are a few drains installed in the reinforced turf that are only used during very heavy storms. Because the reinforced turf works so well the existing storm drainage system did not have to be enlarged for the additional parking. The overflow parking area needs to be mowed on a regular basis and treated like a regular lawn. The area also needs to be plowed as any parking would be. Rollers were fit to the bottom of the snow plow so the reinforced turf would not be damaged. The manager of the Westfarms facility is satisfied with the turf.

Websites

www.unh.edu/erg/cstev/index.htm www.invisiblestructures.com/GP2/whole_lotof_turf.htm www.icpi.org
www.uni-groupusa.org/case.htm www.nemo.uconn.edu/ www.lowimpactdevelopment.org/epa03/pavespec.htm www.epa.gov/ednnrmrl/repository/abstrac2/abstra2.htm
www.forester.net/sw_0503_advances.html

This fact sheet was adapted from the Massachusetts Low Impact Development Toolkit, a production of the Metropolitan Area Planning Council, in coordination with the I-495 MetroWest Corridor Partnership, with financial support from US EPA. The Massachusetts Low Impact Development Interagency Working Group also provided valuable input and feedback on the LID Toolkit. For more information, visit www.mapc.org/lid and www.arc-of-innovation.org.