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1. Cost: The material and installed cost of Pervious Pavers / Permeable Pavements (PP/PP) generally ranges from approximately 3 to 6 times the cost of properly designed and installed asphalt pavement with equal or better performing stormwater structural management measures included.
2. Infiltration: The infiltration capacity/potential of PP/PP decreases with time, even with regular maintenance. This decrease can result in infiltration capacity/potential of mature PP/PP (5 years in place with expected loadings) decreasing to 20%+\- infiltration for an average precipitation event versus 80%+ for newly installed PP/PP. The infiltration capacity/potential of bioretention, in contrast, generally increases over time.
3. Clogging: Greases, oils, and other petrochemicals as well as fine particulates (dust, clays, silts, other) and various non-petro-organics readily clog and/or seal PP/PP installations. Once these are in the PP/PP matrix, it is difficult and expensive to clear the clogging without removing the PP/PP in the clogged/sealed area. PP/PP can be exposed to such clogging agents whenever a vehicle leaks fluids or has fine soils on tires. There is a very high probability of exposure to such at many sites. Wet ponds and bioretention areas - especially, on the other hand, “self-heal” after exposure to such agents, require much more exposure for reduction in performance, and are much more readily fixed if such did become a problem.
4. Maintenance: Regular maintenance - generally weekly or more frequent vacuuming - is required for PP/PP installations. PP/PP installations have one of the highest - if not the highest - maintenance costs of any stormwater structural management measure.
5. Utilities: Even if PP/PP installations work properly, utilities under the PP/PP installations may fail. When this happens, the PP/PP installation is removed for utility repair work. A special fix of the affected area is required to both aesthetically and functionally match the removed area. This must extend beyond the immediate area to “tie-in” to the adjoining PP/PP. It is unlikely this will be done with as much care as the original installation and the patch would almost certainly be more noticeable than a conventional patch (if the PP/PP material is even still available to match the original PP/PP installation - ever tried matching bricks on an old house?).
6. Proof: Studies undertaken to date which show “positive results” have been undertaken in very favorable circumstances - intense maintenance, regular inspection, low traffic loadings, and little to no clogging agent exposure. These are not real world situations and produce misleading results. Studies that take into account real world situations are needed to provide design data on long term performance. There is little monitoring data on the existing installations to determine the performance or conduct a benefit-to-cost assessment relative to alternates in “real-world” situations.
* Also see post, "Rational C Value for Brick Pavers in a Sand Bed" (thread162-200106)
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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation
