Saving the Paving: New Bay-wide Permeable Pavement Specification Released
The promise of permeable pavement has always been powerful – producing a parking lot that prevents surface runoff. Alas, the promise has seldom been realized, and permeable pavers are rarely used today. In my view, however, permeable pavement is now poised for a resurgence given the new shift to runoff reduction, as well as major advances in paver research, testing and installation. Everyone has their own personal history when it comes to permeable pavement, so I should disclose mine.
One of the first jobs in my career was to monitor the pollutant removal performance of a porous asphalt parking lot in Rockville, MD in the early 1980s. Back then, the late Rod Lafever of the city of Rockville was a real pioneer in advancing the use of porous pavement. By the end of the 1980s, however, porous pavement went into a steep decline. Numerous reports, some authored by me, revealed that many permeable pavement projects had clogged or otherwise failed, including a number of cases where they were actually re-surfaced with conventional pavement (sadly, this happened at the Rockville monitoring test site).
Some of my ambivalence about permeable pavement reflected the industrial rivalry among the three principal purveyors of permeable pavement products: porous concrete, porous asphalt and interlocking concrete pavers. By continually smacking down the properties of their competitor’s products, the industry has created bad perceptions about permeable pavers in general.
Over the years, though, the prophets of permeable pavement, such as Tom Cahill and Bruce Ferguson have kept the faith, and more research and experience from industry and academia has boosted my confidence in pavers again. What reasons might prompt a known paver-denier such as me to change my mind?
- To start with, paver installation has been much improved due to better construction techniques and more industry certified installers. Also, there are a lot more informed “green” owners these days, which is a critical prerequisite to maintain paver performance. This was not the case back in the 1980s, when few parking lot owners knew or cared about the unique maintenance responsibilities of permeable pavers. In recent years, however, the green building movement has created a new generation of property managers that is really conscientious about the environmental performance of their buildings.
- The other key driver is that designers are increasingly dealing with individual components of impervious cover (driveways, sidewalks, parking lots and residential streets) to meet their future runoff reduction requirements. Thus, pavers can be a partial solution for small sites in waterfront and ultra urban areas where space is at a premium.
- Another reason is that permeable pavement has been reconstituted as a runoff reduction practice as opposed to an infiltration practice. Many past failures were caused by forcing infiltration into marginal soils, or construction erosion that diminished infiltration capacity. By contrast, many of the new designs involve underdrains so that runoff reduction is achieved by extended filtration rather than infiltration alone.
- The last reason is economic. While permeable pavement does cost more than traditional pavement, it can sharply reduce the cost of other parking lot stormwater infrastructure (such as underground detention, large storm drain pipes, inlets and curb and gutters). Also, permeable pavement is very cost effective when compared against other LID applied in tight urban settings (e.g., green roofs, street bioretention and foundation planters).
The current version of the CSN permeable paver design specification is now available. Also, a slideshow describing key design issues for permeable pavers is available. As always, feedback and comment is invited from the Network over the next six weeks, particularly from folks that have had good/bad experiences with pavers in the past.
Some of the specific new wrinkles in the new specification include:
- The spec provides guidance on all three types of permeable pavers (porous concrete, porous asphalt and interlocking concrete pavers) and compares their properties so designers can choose the best paver product for their unique site conditions. The basic idea is not to pick industry winners and losers, but to help reduce the confusion among designers.
- Like other practices, the paver design spec is based on the scale or size by which it is applied. Micro-scale pavers are used on applications less than 1000 square feet, whereas small-scale pavers serve up to a quarter acre, and large scale pavers are used for parking lots more than 10,000 square feet in area. The engineering and testing requirements become more sophisticated as the scale of the paving application increases.
- Thanks to Kelly Collins of the Center for Watershed Protection, the new spec contains a series of equations to enable designers to figure out to size the underground reservoir whether it has an underdrain or not. The equations can also be used to size the stone reservoir for channel protection and flood control storms, and standard design assumptions are included.
- Because the new spec focuses on how to design permeable pavers with underdrains, it has fewer and less stringent feasibility constraints. Also, the spec quietly debunks the notion that permeable pavers need underground injection permits and do not comply with the American with Disabilities Act.
- Another unresolved issue is whether the sub-grade should be protected by filter fabric or filter layer, which is still a matter of debate among industry professionals and practitioners alike. Both options are presented, but I would love to hear from folks about their experience.
- Perhaps the most notable aspect of the new spec is the extensive write-up on the proper construction sequence to install each types of permeable pavement. The spec not only includes an annual inspection checklist to trigger various maintenance tasks, but also emphasizes the need to write maintenance agreements to identify the conventional parking lot maintenance tasks that must be avoided (e.g., sanding, re-sealing, re-surfacing, power-washing)
- Other areas where feedback is solicited is how to couple permeable pavement with bioretention and other practices in the same parking lot, and whether the uniform material specifications presented in the spec need to be modified to deal with individual state differences in how they classify grades of stone and rock.
In closing, special thanks are extended to two people who helped write and/or review this spec —Kelly Collins of the Center for Watershed Protection and Dave Smith at ICPI.