Defined as a system of concrete pavers with joints that allow for infiltration of water through the pavement, PICPs are an engineered ecological system that captures, treats, and stores stormwater. The pavement uses an open- graded base and sub-base for water infiltration and/or storage. These systems can be designed for full exfiltration of the captured water or complete storage. They can stand alone or be used in conjunction with swales, ponds, or storage tanks.
PICP systems can be designed for 100-year hydrological events, with treatment zones that can also be included to encourage the use of naturally occurring enzymes to establish a bacteria colony that will break down the first flush pollutants within the system. In contrast to impervious systems, PICPs have demonstrated ability to provide competitive capitalization costs, reduce winter maintenance costs, and improve long-term savings by providing a 50-year pavement design.
Used in conjunction with bioremediation, PICP is a BMP treatment train used effectively in the U.S. for the past 10 years and in Europe for the past 20 years to mitigate peak flows and improve water quality. A method of removing pollutants using microorganisms in soil, bioremediation can occur naturally or it can be spurred on via biostimulation, which is the addition of fertilizers to increase the bioavailability within the medium. Recently, the addition of microbe strains matched to the medium have successfully enhanced the resident microbe population’s ability to break down contaminants. Bio-swales have proven effective in this regard as well.
Designing a PICP System
PICP systems are very site-specific, engineered systems, as are all site solutions involving water, soils, and loading conditions. However, generalizations can be made for purposes of discussion. It is important to note that every PICP system requires a set of site parameters and must be designed and reviewed by a design professional—civil engineer, geotechnical engineer, or other qualified land planner/ designer. PICP systems are most effective if treated as a first design strategy by engineers or other qualified professionals.
Benefits of PICP
While the capture of stormwater at pre-development levels is a key benefit of PICP systems, they also offer a number of other advantages.
- Filtering Pollution. The absorption of the first flush of stormwater reduces the temperature of the rainwater, and the layers of aggregate act as natural filters. Also, naturally occurring bacteria in the aggregate layers and soil are able to break down some of the stormwater pollutants. Finally, underground storage negates need for retention/detention ponds.
- High Suspended Solid Removal Rate. PICP systems can provide a TSS (Total Suspended Solids) removal rate of 80 to 90 percent. Studies have shown TMDL (Total Maximum Daily Loads) may be met with PICP as well. The use of open-graded aggregates will promote infiltration rates that will exceed 150 inches an hour at initial construction. This aspect of PICP systems allows them to easily capture and treat first flush pollutants.
- System Durability. The PICP system can be designed and engineered for a 50-year pavement design. Municipalities would be well served to review plans for new streets, as well as existing streets that need replacement, and compare capitalization budgets and maintenance/ replacement costs of conventional pavement and stormwater construction with PICP systems.
- High Loading Capacity Under Pressure. PICP systems may be engineered to accept axle loads well in excess of 20,000 pounds. Often PICP systems are designed to meet hydrological requirements first and structural loading second. Subsequent evaluations for both static and dynamic loading will find an over-designed system for vehicular use due to the base and sub- base requirements of stormwater detention/retention demand. A fire truck demonstration shows the ability of the PICP system to take large amounts of water without any runoff.
- Pedestrian Friendly. Permeable paver surfaces are pedestrian friendly and will meet or exceed ADA requirements. The ADA requires that openings in horizontal pedestrian traffic areas cannot exceed 13mm (½ inch) and must not have any lippage greater than ¼ inch. Elevation changes greater than ½ inch must have a ramp. Permeable pavers with 2mm chamfers will provide a smooth surface especially for wheelchair users. Handicap delineation as well as parking lanes and other markings may be more permanently constructed with the use of different colored pavers or engraved logos.
- A Viable BMP. The federal government and many state agencies have recognized PICP as a viable BMP especially when used as an integral part of a treatment train. Various studies have been conducted in the U.S. and Canada since the early 1990s and much is known regarding water quality, volume reduction, sediment collection, and temperatures within the base and sub-base. A typical bio-aquifer storm system has surface openings somewhere between 8 and 12 percent depending on paver shape. Borgwardt determined that surface flow rates are dependent on aggregate chips used and will experience a 50 percent reduction in flow rate within the first five to eight years, due to sediment collection at the joint level. Sand should never be used as a joint filler as it yields the lowest flow rates and will clog the system, requiring remedial maintenance much sooner. Another study by Bean suggests the use of a sand filter at a discharge point would also improve water quality, while grid pavers using sand fill coincided with Borgwardt’s findings. They also determined that temperature reduction for discharge flow to streams is lowered with PICP systems as compared to discharge from asphalt surfaces and does not prove harmful to the aquatic life.
A special thanks to Oldcastle APG Masonry and Architectural Record for supplying us with this article.
