Choosing a hot top for a greener parking lot
Porous asphalt reduces water runoff
EXETER, N.H. - Paying a premium to line a parking lot with pavement full of cavities may sound strange for a dental office, but here it makes great sense, economically and environmentally.
Porous pavement is just that: asphalt with holes in it that allow water to seep through to the ground below. That simple difference was enough to prompt the Core General Dentistry and Oral Surgery building on Hampton Road to recently spend 30 percent more than on conventional asphalt to pave its new parking lot.
"It's just unbelievable," said Michelle Proulx, the office manager at the complex. "When it rains the water seeps down through the pavement and you never see a puddle. You never see anything. The technology is amazing."
The technology is also quite simple, according to Daniel Balfour, senior project manager with Jones & Beach Engineers Inc., the Stratham-based civil engineering and survey firm that worked on the Core project. The holes in po rous pavement that allow the rainwater to pass through come from the larger-than-usual gravel mixed into the asphalt, Balfour said. This gravelly mixture is then rolled out onto an extra-deep bed of sand and stone that absorbs and filters rainwater in much the same way soil in a field or forest does.
That's a crucial difference for many community planners who often restrict development on open land based on how much of an area is covered by impervious surfaces such as buildings and pavement. For Core, using the porous pavement meant it could build 20 percent more office space and know it was helping the environment at the same time, said Robert Corson, staff architect with Core Physicians LLC, which owns the dental practice.
Still, the decision to go with the porous pavement was not straightforward, Corson said. There are disadvantages. Because it is porous, the pavement is more brittle than conventional asphalt, so snow plows must be more careful in winter. The holes on the asphalt also tend to trap dirt and must be cleaned out occasionally with a huge vacuum or the pavement won't remain porous, Corson said.
On the plus side, snow does not build up so much on the surface, making for better winter traction and 75 percent less salt required for de-icing.
"We see it as part of our role as community leaders to look into these technologies to initiate some positive changes within the community," said Corson. "People often don't try anything new until somebody else has tried it."
Porous pavement is catching on in Exeter and in surrounding communities, according to Balfour. St. Michael's Roman Catholic Church in Exeter is using porous pavement at its new location on Newfield Road. That installation, and others in Amherst, Greenland, and Londonderry are larger and thus actually cheaper because they don't need the extensive storm-water control systems increasingly being mandated by local planning agencies in low-lying areas such as Exeter, where road-runoff is a problem, Balfour said.
"It's almost a no-brainer now to skip the drainage stuff and go with the porous pavement because it gets cheaper and cheaper" in bigger projects, he said.
Soon those clients may have no choice, Balfour said. The US Environmental Protection Agency is cracking down on storm-water runoff and forcing towns like Exeter, that are near sensitive rivers and ponds, to clean up their acts.
Storm water carries chemicals such as phosphates and chloride into rivers and ponds, and as the country has become more developed, road-runoff problems have sparked growing federal concern over pollution.
The EPA issues permits allowing towns to discharge storm water into rivers and streams and is becoming more adamant about reductions in those chemical-laden discharges with each new permit it issues, Balfour said.
"If they are highly requesting these reductions now that usually means they will make it a requirement real soon," he said.
In New England, chloride from road salt is particularly problematic and porous pavement can help there, too, according to Robert Roseen, director of the University of New Hampshire's Stormwater Center, the environmental research group that helped tailor this technology for New England applications. However, it's no good for repaving existing roads without the extra-deep bed of sand and gravel underneath.
That was discovered when porous pavement was used on sections of Interstate 93 where the experimental asphalt can now be seen shearing off, Roseen said. However, if those communities where road salt contamination in local waters is a problem start mandating the use of porous pavement on new street and parking lot construction, they can start addressing EPA concerns, Roseen said.
"It's exactly these [EPA] regulations that are pushing the use of these porous pavement technologies," he said. "This stuff works really, really well." 