In August at the StormCon conference, we continued with what has become a regular feature over the last few years, the discussion panel on BMPs. This year’s topic was “BMPs and LID and the Highway Environment,” and one of the main topics of interest–both in the panel discussion and elsewhere during the conference–was porous or permeable pavement.
It is well accepted that permeable pavement has great water-quality and quantity benefits when used in parking lots or alleyways–areas that receive relatively little traffic. Used in these settings, porous pavements have been shown to hold up well even in cold climates, where they’re subjected to freeze/thaw cycles and punishing snow plows.
Widely believed to be less feasible, though, is using porous pavement for actual roadways, which have heavy traffic in both senses of the word–lots of vehicles, some of them multi-ton fire engines and trash trucks and 18-wheelers. Yet permeable pavement is making inroads here, too, several panel members pointed out. John Gulliver of the University of Minnesota and the St. Anthony Falls Laboratory mentioned that a portion of I-94 in Minnesota, as well as a mile-long section of a side street, have a porous topcoat or overlay. Michael Barrett of the Center for Research in Water Resources at the University of Texas, Austin, described an approximately 2-inch-thick, 20% porosity overlay used over conventional asphalt that allows water to seep to the edge of the roadway. Monitoring of runoff from highways with porous asphalt overlay has shown a reduction in some pollutants, including total suspended solids and metals.
John Sansalone of the University of Florida, Gainesville–who assembled and led the panel–said that in Italy, where he teaches for part of the year, both overlays and full-depth permeable pavement are being used. These permeable surfaces were adopted mainly for other purposes: partly to reduce noise and partly to eliminate splash or spray behind vehicles in wet weather, thereby improving safety. However, he noted that the drainage and water-quality benefits are beginning to be recognized as well.
What are some of the considerations in using permeable pavement on highways? As Masoud Kayhanian of the Center for Environmental and Water Resources Engineering at the University of California, Davis, noted, the strength and load-bearing capacity of full-depth permeable pavements would need to be carefully studied before their effect on hydrology is considered. Barrett also pointed out that, even though it drains quickly, porous pavement freezes faster because it has greater air space, so there is concern under certain conditions about the formation of black ice; a liquid deicer might be recommended.
Despite these concerns, this looks to be one area where one solution can help meet the goals of highway engineers and stormwater professionals. I’m eager to hear your thoughts on this; please drop me a note at [email protected]. Research into types and feasibility of porous pavements is ongoing throughout the US and in other countries, and we will continue to follow developments in stormwater.
