Imagine driving into a parking lot on a rainy day and trying to find that spot where you can get out of your car without stepping into a puddle. Happens all the time, right?
Not always. A small, but growing number of parking lots remain puddle-free in wet weather because they have porous pavement that soaks up the rain. And it does more than keep one’s shoes dry.
Porous pavements — made of pervious asphalt or concrete, or permeable paving blocks — are being tried across the Chesapeake Bay watershed to tackle the vexing problem of urban and suburban runoff.
Rainfall and snow melt washing off roads, parking lots, sidewalks and roofs account for one-sixth of the nitrogen and phosphorus, and one-fourth of the sediment fouling the Bay and its tributaries, according to the state federal Bay Program. And that runoff is the only source of those pollutants that’s growing.
Porous pavements are considered another type of green infrastructure — along with rain gardens, green roofs, trees and bioswales — which are used to manage runoff in a way that mimics nature: letting the ground soak up the water.
But porous pavements have some handicaps that limit their usefulness in certain situations. For starters, they are costlier to install than conventional asphalt, concrete and brick. That’s partly because to be effective, such pavements need to be laid on a porous base to let the runoff soak into the ground — usually one or more layers of rocks or gravel. If installed over poorly draining soils, though, “under drains” need to be laid below the pavement to carry water away, adding to the cost.
Porous pavements also require different maintenance; an annual vacuuming is often recommended to keep dirt from clogging up their openings. Finally, they’re considered less durable than conventional pavements when subjected to heavy vehicular traffic.
Even with those limitations, porous pavements are widely used in some parts of the United States. Shoreview, MN, a suburb of St. Paul, has installed it on many of residential streets lain on sandy soil.
If the soil at the project site drains well, Shoreview public works director Mark Maloney said, “I won’t say it’s a no-brainer…but gosh, you owe it to yourself to look it into pervious pavement.”
Since starting with porous concrete in 2006, Maloney has gravitated to porous pavers: interlinking pre-cast concrete blocks as deep as 8 inches, with thin gaps filled with crushed rock.
The pre-cast concrete blocks “are made for porous streets” and hold up well in Minnesota’s extreme weather, Maloney said.
Porous pavements have been slower to catch on in the Bay watershed, for a variety of reasons, including the variety of soils and expense.
For instance, in Maryland, Prince George’s County is counting on green infrastructure to bring down the cost of complying with the Chesapeake Bay Total Maximum Daily Load, or TMDL. Simply enlarging and retrofitting the storm sewer network could cost more than $2 billion by 2025, estimated Adam Ortiz, director of the county’s Department of the Environment.
But at this point, porous pavements are not on the county’s menu for the Bay “pollution diet,” as the TMDL is commonly called.
Prince George’s has forged an unusual partnership with a private company, Corvias Solutions, delegating to it much of the decision-making about how to reduce stormwater pollution. Under the partnership agreement, the size of the private partner’s profit and local tax rates both hinge on keeping costs down. Corvias has opted against using porous pavements because they are more expensive than other types of green infrastructure.
Company spokeswoman Cindy Willhite said Corvias is working with the porous pavement industry to find ways of reducing the costs of using their products.
Localities and states across the region also have differing standards for installing and maintaining them, and some demand far more regulatory review than others.
But when space is at a premium for dealing with stormwater, or other methods prove more expensive, porous pavements can be cost- effective, advocates and experts agree.
More space for parking
Commercial developers are proving more willing to try porous pavements, especially when there is little room for other types of green infrastructure.
A new development in Baltimore County, MD, will use porous pavement on its 2,196-space-parking lot to maximize the space that can be devoted to rent-generating stores.
The Foundry Row shopping and office complex in suburban Owings Mills will have 356,000 square feet of ground-level retail, anchored by a Wegmans supermarket, plus office space. It is fronted by acres of parking, with large truck delivery lanes in back.
To maximize the space available for revenue-generating real estate and still meet stormwater control requirements set by the county and state, Baltimore-based developer Greenberg Gibbons opted for porous asphalt parking lanes.
“Foundry Row is the first place Greenberg Gibbons has used porous pavement,” the property’s development director, Gustavo Arango, said as he showed a visitor the partially completed parking lot.
At first glance, the finished portion of the lot looks like any other expanse of asphalt, though with vegetated islands between every other parking lane.
But upon closer inspection, each area between the islands has three broad strips of asphalt. The slightly shinier and bigger center strip, the driving lane, is made of conventional asphalt. On either side, the asphalt is coarser, with pores to let water through.
The developer opted for standard asphalt in the driving lanes at the recommendation of the Maryland Asphalt Association, which said it would last longer, Arango said.
Under most conditions, the development director said, the porous pavement will absorb not only the rainfall on the parking lanes but also the stormwater running off the impervious travel lanes. As insurance, the parking lot includes drains that empty into the vegetated islands, known as bioswales. And beneath the lot are under drains that will carry any remaining uncaptured runoff to the county’s stormwater system.
Expertise improving significantly
Arango said construction permits for the parking lot, issued by the Maryland Department of the Environment and Baltimore County, came only after extensive reviews of the plans and the site, including soil tests. Both agencies say they intend to monitor the project’s handling of stormwater over time.
Greenberg Gibbons is unlikely to use porous pavements again because other stormwater control methods are less expensive, Arango said. But in this case, the developer had an essentially free source of rocky material at hand with which to make a permeable sub base for the parking lot. The site had been a former Solo Cup manufacturing plant, and the factory’s demolition left 10,000 cubic feet of concrete rubble, which could be reused, said Greenberg Gibbons president Tom Fitzpatrick.
Knowledge about and expertise with porous pavements — among vendors, contractors, engineers and others — has improved significantly in recent years, experts said. The U.S. Environmental Protection Agency once frowned on it but now considers it an appropriate stormwater control technology.
Even so, one expert cautions that localities should be wary of selecting the lowest-cost bidders for such work. Only contractors with specialized training and experience can do it in a way that holds up, captures enough stormwater and requires only routine maintenance, said Stuart Schwartz, a senior research scientist at the University of Maryland Baltimore County. His work at UMBC’s Center for Urban Environmental Research and Education has focused on methods for controlling runoff.
Soil tests are essential before deciding how or even whether to put down porous pavement, Schwartz stressed.
A high-performing porous pavement system installed in an appropriate site can reduce runoff reliably enough, Schwartz said, to count it against whatever regulatory requirements a community faces to reduce its stormwater pollution.
In Maryland, such installations can qualify for stormwater reduction credits. In Virginia, though, they can be counted towards reducing phosphorus as well as storm runoff, said Greg Hoffman, stormwater retrofit program manager at the Center for Watershed Protection, a nonprofit organization based in Ellicott City, MD.
Schwartz said state and local regulators may take different approaches because of varying soil conditions and installation requirements. But he suggested that few local governments have the resources to employ permitting staff with the requisite knowledge.
One Virginia town has opted to try porous pavement — though with sizable financial assistance from the state and a nonprofit.
2017 goal met using pavers
In Ashland, nearly 20 miles north of Richmond, contractors retrofitted about 5,000 square feet of pavement with pre-cast concrete blocks, often called permeable pavers.
Town engineer Ingrid Stenbjørn said the pavers, some dating back to 2012, do a very good job of absorbing rainfall and reducing the speed and volume of stormwater before it reaches Ashland’s storm drains.
The town has already met its 2017 obligation to reduce stormwater pollution under the Bay blueprint, thanks in part to the permeable paver system, Stenbjørn said.
But the installation was not without its challenges. Ashland lies atop clay soil that absorbs water very slowly if it at all, Stenbjørn said. As a result, the town’s pavement system is designed to slowly filter stormwater through a porous sub-base and into an underdrain. The water eventually winds up in the storm sewers.
The porous pavement system should provide an additional benefit, Stenbjørn said, by reducing the number of times the town’s sanitary sewers overflow because of stormwater leaking into them
Another benefit of the porous pavement permeable pavers has been a major reduction in stream bank erosion near outfalls for the town’s stormwater systems.
Stenbjørn said there are “lessons learned” for any future porous pavement installations in Ashland.
While the town’s street sweeper has a vacuum that does a fairly good job of cleaning the pavement, it can’t get all of the debris. A town employee must do that manually, with a hand tool, she explained.
In winter, she added, sand and salt used to treat the road become the problem.
The cost of the porous pavement permeable pavers is another concern. She said the town would not have been able to afford a 2015 project without grants and big discounts from the town’s excavation contractor and the permeable paver contractor —and more.
A $200,000 grant from the National Fish and Wildlife Foundation plus a $168,000 grant from the state Department of Environmental Quality paid for a new police department parking lot and for restoring 200 feet of a badly eroded stream.
The parking lot needed work. A new layer of conventional asphalt, which would have cost just $30,000, would have taken care of it, Stenbjørn said.
At least one resident thinks the porous pavers are an improvement. Ian Kirkland, owner of the Caboose Market & Café on Ashland’s main street, Railroad Avenue, said rain no longer puddles on the municipal public parking lot next to his business. The pavers there “do the job,” Kirkland said, as rainfall disappears instantly from the surface.
Puddles have not disappeared on the two parallel parking spaces in front of the restaurant, though, which also have permeable pavers. Kirkland said they still take time to drain.
That’s yet another lesson learned: The parallel parking spots along the main drag in the picturesque town center have proven to be less than ideal for the pavers. Installed as part of a streetscape improvement program, they “look really pretty,” the town engineer said, but pollen, dirt and trash become highly compacted in the gaps between the pavers, requiring manual labor to break it up for the street sweeper.
“I don’t know where all that trash comes from,” Stenbjørn said with a laugh.