Regulatory agencies have begun to respond to these issues, and the United States Environmental Protection Agency (EPA)has mandated that regional water quality control boards reduce pollutants in storm water discharge. The Stormwater Phase II rule was published on December 8, 1999, and generally requires operators of small Municipal Separate Storm Sewer Systems (MS4) in urbanized areas to develop and implement a stormwater management program which addresses six minimum control measures identified by the National Pollutant Discharge Elimination Systems (NPDES). In turn, the regional water quality control districts now require their districts and developers in those cities to implement various types of Best Management Practices (BMPs) to comply with these new standards. The EPA recognizes there are often site-specific, regional and national variables in BMPs design constraints and pollution control practices,therefore it does not preclude MS4s from using other practices to achieve the minimum requirements.

Previous approaches have focused on excess runoff retention and detention typically in local or regional detention/retention basins. However, research shows the best solution is to restore the natural drainage system through a porous pavement system. A permeable pavement surface with an underlying porous reservoir temporarily stores surface runoff before it infiltrates into the subsoil and or gets discharged at a controlled rate to an existing storm drainage system.

The most practical way to achieve a natural drainage system is to increase infiltration and groundwater discharge through a process called re-naturalization. Interlocking permeable pavers embrace this type of drainage system. The pave stones interlock, yet are designed to provide an open void between the stones creating a continuous, high-compressive strength permeable surface. The openings are filled with aggregate to allow infiltration and filtration of the surface drainage. Stormwater enters through these openings and is stored in the open-graded base below the paver surface. Once the water infiltrates into the base material, the natural processes of infiltration may occur. Because permeable pave stones are engineered to meet ASTM 936 standards offering a minimum compressive strength of 8,000 pounds per square inch, surface durability and wearability are not compromised.

Environmental Benefits of Permeable Pave Stones

Permeable pave stones provide needed harmony between economic development and ecological concerns. In comparison to traditional paving structures, permeable pave stones appear to have a higher cost. However, since the permeable pavers reduce or eliminate the need for drainage infrastructure and onsite retention systems, the overall project developmental costs can be reduced. ?EUR??,,????'??Research studies suggest a runoff coefficient using the rational method of 0.15 to 0.25 in newly constructed permeable paver pavements, comparable in most areas to a grass field,?EUR??,,????'?? explains Tom Zais, Pavestone Company. ?EUR??,,????'??They therefore provide a more efficient use of available land that would otherwise not be developed due to stormwater runoff requirements or by the limitations of how much land can be impervious. Inner city redevelopments, ?EUR??,,????'??tear down?EUR??,,????'??? projects and commercial pad sites can all benefit from a reduction in overall land and construction expense by utilizing a permeable pave stone pavement system. Also, permeable pavers reduce pollutants. Studies show that asphalt, made from the combustion of petroleum products, releases toxins into the environment as it naturally ages. Oils, grease and phenols originate from the asphalt as it undergoes normal degradation. These are washed into the drainage system. Stormwater can be contaminated by this process, but permeable pavers eliminate the contaminant leaching into the environment.?EUR??,,????'?? Zais suggests additional permeable pavement benefits include improved erosion control, minimization of downstream flooding, attenuation of runoff, superior water quality, less need for curbs and storm sewers, retention of natural drainage patterns and vegetation, improved road safety, ground water recharge (if desired) and mitigation of pollutant impact.

Design and Installation

The installation process is critical to the overall success of the infiltration process. Sediment should be prevented from entering the base and pavement surface during construction as this can create surface clogging. Steps to assure sediment is kept away include: installing silt fences, excavating in stages, routing mudding construction equipment away from the area, and constructing temporary drainage swales to divert area runoff. Until the entire contributing drainage area is completed, the pavement should not receive runoff.

The amount of soil compaction during construction can impact the pavement?EUR??,,????'???s infiltration ability. If the soil is inadvertently compacted during construction, a substantial reduction of infiltration will occur. If compacted soil is considered in the drainage calculations, compaction will be acceptable.

As with any interlocking paving stone application, edge restraints are an important component. For permeable pave stones, the concrete edge restraints should be a minimum of six inches wide and twelve inches deep.

Green Parking

Green parking is a term describing a parking lot, often for overflow and emergency routes, wherein turf or grass paving stones are utilized. With an open void in the large paving stone, natural or planned vegetation can grow. Aggregates can be used as an alternative to filling the voids when vegetation is not required for ecological, environmental or aesthetic needs. The concrete grass paving stones are manufactured with an average compressive strength of 4000psi and a water absorption maximum of seven percent. Typically, a paved area designated as green space is subject to wheeled or vehicular traffic and may become so compacted that the permeability of the soil structure is inhibited to the point that the grass is unable to survive. Green paving structures reduce the impervious pavement while improving stormwater runoff concerns.

Performance and Maintenance

?EUR??,,????'??Storing runoff in pavement is counter to conventional pavement design due to the degrading effects water has on surface materials,?EUR??,,????'?? Zais said. ?EUR??,,????'??However, experience has demonstrated that permeable pave stones can perform reliably. The most critical component of permeable pavement is the base section; to assure infiltration rates are not impacted or reduced and structural integrity is not sacrificed, special attention should be given to this component in the design and specification writing process.

?EUR??,,????'??The performance behavior of interlocking pave stones is high,?EUR??,,????'?? said Zais. ?EUR??,,????'??Under load, a concrete paving stone will transfer shear via interlock to its neighboring pave stone. The shape and laying pattern of the paver can help facilitate interlock and therefore enhance structural capacity.?EUR??,,????'??

The permeable pavement should be inspected regularly. Clogging can occur in heavy traffic areas due to contaminants such as oil, dust, and other fine road debris. Conventional street sweepers equipped with vacuums, water and brushes can generally restore any clogging that occurs. Permeable pavers are not ideal for cold climates that use sand snow removal systems due to the clogging potential.

?EUR??,,????'??With most metropolitan areas experiencing a resurgence of urban redevelopment and an aging drainage infrastructure, permeable pavement surfaces are quickly being adopted as BMPs by municipal agencies,?EUR??,,????'?? Zais said. ?EUR??,,????'??Federal and local mandates are also placing emphasis on sustainable development practices minimizing environmental impact. Permeable pavers have been extensively studied and documented to perform in a variety of conditions.