Porous pavement gets another tryout in Maryland

State, city experimenting with methods for curbing storm-water runoff

July 19, 2014|By Timothy B. Wheeler, The Baltimore Sun

The concrete oozed rather than poured out of the mixer truck, almost as if reluctant to cover the ground — partly because it won't, entirely.

Laborers shoveled pebbly gobs around to form a new sidewalk at a park-and-ride lot in Waysons Corner, one of two where the State Highway Administration is laying "pervious" concrete this summer as a test of its environmental friendliness.

Porous paving surfaces have been around for decades, but they're expensive and often didn't work well. Interest in such surfaces among governments and developers is on the rebound, though, in response to new state regulations aimed at curbing stormwater pollution from pavement smothering the Chesapeake Bay watershed.

"There has been a resurgence in permeable pavement in the state and across the bay watershed in recent years," said Tom Schueler, head of the Chesapeake Stormwater Network, an organization that trains engineers and others how to deal with runoff.

Stormwater runoff is the fastest-growing type of pollution in the region, according to the U.S. Environmental Protection Agency, accounting for one-sixth of all the nitrogen and phosphorus and one-fourth of the sediment fouling the Chesapeake.

Rooftops, roads and other pavement are the conduits, as rainfall washes off their hard surfaces, eroding stream banks and carrying accumulated dirt, oil and other pollutants. The amount of ground covered by pavement and buildings has grown nearly twice as fast as the population, the EPA figures.

Maryland and the other states in the Chesapeake's watershed are under pressure from the EPA to do more to curb polluted runoff.

The pervious concrete being poured in Waysons Corner is a lightly cemented batch of pea gravel with air pockets between the stones intended to let rainfall soak through into the ground rather than run off into storm drains and streams.

Water-absorbing pavement can help mitigate the impact of new development, and it's one of the few ways of curbing runoff in inner-city neighborhoods, where there's little or no open ground.

The use of permeable pavement and pavers has spread gradually in commercial and residential developments, starting in the South, according to Colin Lobo, senior vice president for the National Ready Mixed Concrete Association.

"A lot of universities adopted it, doing permeable parking lots and walkways," said Kelly Lindow, owner of CityScape Engineering in Baltimore. It fit with the institutions' adoption of green building practices, she said, and helped manage runoff when trying to squeeze more buildings and vehicles onto campus.

Municipalities in the Chicago area and nationwide also have embraced permeable pavement.

Its growing popularity represents a turnaround from 15 years ago, when the EPA warned that porous pavement had a high rate of failure, with the pores tending to clog up. Federal officials blamed the problems on improper installation and maintenance. Research since then has shown how to reduce the failure rate.

Cost remains an issue. Standard asphalt or concrete can be installed for $2 to $5 per square foot, on average, while pervious mixes can cost two to four times as much. Paving blocks may run as high as $15 or even $20 per square foot if they have to be laid by hand, experts say.

Much of the extra cost comes from the need to provide a porous base to capture runoff. Often, that means putting down a 6- to 8-inch layer of gravel and maybe tilling or replacing poorly drained, compacted soils. It also requires more and different maintenance than standard pavement, including vacuuming on an annual basis, or more often.

Permeable pavement is more competitive in new development, where the ground has to be prepared for paving anyway, said Greg Hoffman, a water resources engineer with the Center for Watershed Protection, a research and training nonprofit in Ellicott City. But where space is at a premium, he added, permeable pavement soaks up runoff while still allowing parking or traffic on top of it.

Mindful of the learning curve, higher costs and potential snags, the State Highway Administration is taking its first steps to see how pervious concrete will work.

Tim Smith, the agency's materials technology director, said it's being tried out in a limited way. With funds limited, the agency is only looking for now at trying it in new projects, like the park-and-ride lot in Waysons Corner and another one off Interstate 83 in Baltimore County, for a combined cost of $1.7 million. Pervious pavement also is being put down on a hiker-biker path near the C&O Canal in Allegany County, a $350,000 item in a larger resurfacing and drainage upgrade project there.

Stuart Schwartz, a scientist with the University of Maryland, Baltimore County whose research has focused on how to control runoff, said he's glad to see the state transportation agencies take even those tentative steps.

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