Over the past few years I've fielded a number of questions about the use of permeable pavement in Oklahoma. Permeable pavements are an alternative to conventional concrete and asphalt paving that allow for infiltration of stormwater through the pavement and into a subsurface storage area or drain. Recently, there have been some significant installations, including Chicago's U.S. Cellular Field parking lot consisting of 6 acres of interlocking pavers. Its important to look into the advantages and limitations for its use on local sites.
Initially, I began by looking into the ratings for permeable pavement provided by the EPA. EPA showed permeable pavements to have a failure rate of about 75%1,2. That was not encouraging. I looked into the studies behind these findings and found that they were from 1992. Well, a lot has happened in the LID world since 1992, so what about more recent studies?
My inquiries took me to states like North Carolina and Georgia, where water quality regulation is intense, and to the EPA's LID Development Center in Portland, Oregon. As you may know, some advantage of permeable pavement include:
* The lifespan of a northern parking lot is typically 15 years for conventional pavements due to freeze/thaw stress, which is also common in Oklahoma. Porous asphalt or concrete parking lots can have a lifespan of more than 30 years because saturation during freeze/thaw is reduced3.
* Permeable pavement transforms areas that were a source of stormwater runoff to a system that can reduce or eliminate runoff that would have been generated from traditional paving. Studies show stormwater retention rates ranging from 25-100% depending upon storm intensity. http://www.thewaterchannel.tv/index.php?option=com_hwdvideoshare&task=viewvideo&Itemid=53&video_id=349
* Permeable pavement can reduce the amount of pollutants in stormwater runoff and can address suspended solids (high removal rates), acid rain (some types effective), phosphorous (moderate removal), nitrogen (moderate removal), and metals pollution (best in vegetated systems).
So what's not to like? There are some drawbacks and limitations to use including:* Permeable pavement is not recommended, or in some states even allowed, where surface soils are silt or clay. That's most of northeastern Oklahoma. Generally, permeable pavement is only recommended in areas with sandy surface soil, like the western part of the State, or where stormwater has been pretreated.
* Even when properly sited, maintenance requirements are critical for the success of permeable pavement. Proper maintenance includes vacuum sweeping or high-pressure water washing at regular intervals and inspections after each storm. If you can't commit to maintenance, there are better options.
* Porous pavement has reduced strength compared to conventional materials and isn't appropriate for applications with high volumes, high speeds, or heavy loads.
So how might permeable pavement be used in Oklahoma? Walkways, driveways, alleyways, and overflow parking are some options. And consideration should be given to preventing sediment from reaching the pavement, especially during construction, which can trouble a pavement system before it's even in use.
Finally, we have a responsibility to put our best foot forward for LID as these topics are being introduced in local communities, to hit the ground with practices we know have positive reproducible results. Who wants to hear, "More of my tax dollars gone to waste." again? The future of LID is in our hands!
1 Schueler, T.R., P.A. Kumble and M.A. Heraty. 1992. A Current Assessment of Urban Best Management Practices. Metropolitan Washington Council of Governments, Washington, DC.
2 J. Galli. 1992. Analysis of Urban BMP Performance and Longevity In Prince George's County, Maryland. Prepared for Department of Environmental Resources, Prince George's County, MD.
3 Gunderson, J., Pervious Pavements: New Findings About Their Functionality and Performance in Cold Climates, Stormwater, September 2008.
