- Published: 14 February 2007 14 February 2007
Putting a smooth, hard surface over bare ground is always an improvement – or is it?
It’s not the benefits of parking lots, driveways, and even patios that are being questioned; it’s what they don’t do. By preventing rainwater from going directly into the ground, improved surfaces are seen by designers and planners as contributing to more urban flooding, water pollution and additional infrastructure costs.
Fortunately, there are products available to address this, from permeable pavers to joints incorporating polymeric sand to give the benefits of a sand-set job with more stability. And, there are also other, less-costly options.
The demand for conventional pavers will certainly continue. However, as more individuals and communities begin to think (and legislate) green, and developers look to better utilize their properties, it’s a sure bet that permeability will loom larger on the landscape.
THE NEED FOR CONTROL
Every structure that goes up creates an obstacle to permeability. However, Jim Schwab, a senior research associate with the Chicago-based American Planning Association (APA) says planners are now recognizing that common ways of managing runoff aren’t necessarily enough.
“Certainly, even in a fairly dense urban area, open space and parks can provide some grassy surfaces where the rain can leach through,” he says. “However, with parking lots especially, we’re developing a sense that you have to do something other than a standard concrete or asphalt surface.”
Part of the problem, he explains, is that where stormwater management isn’t sufficient, the risk of flooding increases.
“We like to say that it’s not just how you build, but where you build,” Schwab adds. “There’s definitely a link between flood potential and stormwater management. The closer you get to the flood plain with impermeable surfaces, the bigger problem you create in terms of managing potential flood problems.”
Local regulators certainly need to consider flood potential in their zoning and building regulations, and often incorporate retention or detention basins on larger projects to contain runoff on-site. However, those don’t fit well with a coming trend especially popular in residential projects called low-impact development (LID).
“That’s a buzz word we’re hearing a lot in the stormwater community these days,” says Michele Adams, principal engineer with Cahill Associates, a stormwater-consultant firm in West Chester, Pa. “These are developments with more permeable surfaces and more vegetation to manage stormwater.”
While managing stormwater onsite is important to reduce flooding, an even larger shadow is cast over the matter by the federal government’s Environmental Protection Agency (EPA) and the Clean Water Act.
Studies by the EPA have found that stormwater runoff is a major cause of water pollution by washing everything, from pesticides and chemicals to petroleum products leaking from motor vehicles, into nearby water systems.
“Their big concern is capturing what they call the first flush, or the first half-inch of rain, because that’s the most-contaminated as it comes off impermeable surfaces,” says Gale Schroeder, who handles architectural sales for the McMinnville, Ore., office of Willamette Graystone. “If it can be captured naturally through the pavers and the aggregate and then down into the soil – rather than going through a traditional storm sewer system – it’s cleansed along the way.”
The concept of permeable pavers seems, at first glance, to be an oxymoron, although no more so than permeable asphalt or permeable concrete – two other products making their bids in the green arena.
“Natural stone is such a green material,” says Miles Chaffee, owner of Santa Fe, N.M.-based Milestone Imports. “They have low toxicity, they’re non-carcinogenic, and they don’t omit VOCs (volatile organic compounds).”
Although Chaffee is an importer of an impervious surface – porphyry – he’s stressing to his customers the ability to create permeable installations utilizing a sand-set approach ... but stabilizing them with polymeric sand.
“The polymeric sand acts as a stabilizer and it allows the moisture to go through, but it also inhibits weed growth and keeps ants from coming through,” he says. “You can eliminate the use of concrete mortar, and you can recycle the sand.”
As with the use of any other permeable material, the key to taking this approach is the compacted gravel foundation, overlayed with a thin sand course, which goes underneath the pavers.
While Chaffee admits it’s a relatively new approach to increasing permeability, the major paver manufacturers offer a variety of options that incorporate some sort of pockets within their products filled with a permeable material – anything from gravel to soil.
“It’s one of the fastest-growing segments of our company in terms of commercial products,” says Ray Rodenburgh, director of marketing for Toronto-based Unilock. “What’s important about our products is they’re designed for mechanical installations, so large expanses can be paved with permeable pavers in a short amount of time.”
“It’s definitely an exploding market, and we’re not even in the embryonic stages of this yet,” echoes John Kemp, vice president of marketing for Atlanta-based Oldcastle Architectural Stone. “I can’t tell you where it’s going to end.”
There are also other issues that paver manufacturers need to address as the market gains steam in the years to come.
One, says Kemp, is creating products appropriate to the residential market. A second is developing products he calls, “self-remediating,” since one critical problem with all permeable systems is the need for maintenance.
Maintenance isn’t necessarily a drawback when pavers are competing with permeable asphalt and permeable concrete, because it’s recommended that all of them be vacuumed periodically to remove debris from surface voids. However, pavers don’t always rise to the top when compared with other surfaces.
A key issue is cost. The department of general services for San Diego County in California is currently doing its own study of the three technologies. All three have so far been able to handle the heaviest rain that’s fallen on them, but the expense is also part of the equation, says Richard Watson, owner of Richard Watson & Associates Inc. in Mission Viejo, Calif.
“We had a little problem getting people to bid on the porous asphalt part of the job, but it laid down well,” says Watson, whose firm wrote the project’s grant application “The porous concrete requires a lot of hand labor, and cost more than I thought it would. The pavers were the most-expensive to put in.”
Although Watson claims the pavers are the most-attractive part of the project, which is incorporated into the general services building’s parking lot, a second phase of the study will look solely at porous asphalt by installing an additional 60,000 ft² of the product using several different combinations of asphalt and the depths of the stone reservoirs underneath.
Luckily, there’s no one best permeable answer for every project where the technology is important.
“Each system has to be designed to the project and the location,” says Cahill’s Adams. “Rainfall is different depending on where you are in the country; the soils are different, the topography is different, and the climate conditions are different.”
For instance, in applications where salt is used to melt ice, permeable pavers perform well. However, in locations where sand is a problem, they can clog up.
Particularly for large expanses, specifying the right system and then designing it properly will be the job of a landscape designer and engineer, rather than a landscape contractor.
“The designer has to design a system that can manage the stormwater so it will not only go through the permeable surface, but then has somewhere to go,” says Adams.
“The underlying system has to be a stone bed or something similar that can hold the water and allow it to soak into the underlying soil,” she explains. “There can be problems if the soils are compacted during construction, or if it’s not designed to handle flows in areas that have extreme weather events.”
Porous concrete can require a higher skill level for successful installations – but, Adams says, “The pavers install according to the manufacturer’s recommendations, and the asphalt is pretty straightforward; anyone who can install regular pavers or asphalt can install the permeable products.”
Adams adds that, in Europe, some manufacturers are now experimenting with manufacturing pavers from porous concrete.
Will the requirement to install a permeable solution to manage stormwater be coming to a commercial or residential project near you in the immediate future? As with the products themselves, there’s no single easy answer.
Unilock’s Rodenburgh says he’s aware of several New England communities now requiring permeable driveways; and, in Toronto, expansion of parking lots and driveways includes the requirement that they be permeable.
Willamette Graystone’s Schroeder says he’s also familiar with communities that require homeowners to pay a fee for stormwater leaving their properties. By installing a permeable driveway that can handle water coming off a roof, the homeowner can earn a fee reduction.
“However, I think this is being driven more by the commercial market,” Schroeder says. “There, permeable parking can serve two functions. It can serve as a detention basin and as usable parking. And, by eliminating a conventional detention basin from the property or reducing its size, you can increase your parking area.”
The Washington-based U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) program started by focusing on commercial development, but it’s expanding into other areas, and Milestone’s Chaffee believes that same process will happen with permeable pavers.
“I think the average homeowner is becoming more aware of green issues such as energy consumption and minimizing waste,” he says. “The whole idea of permeability will become more popular as people become aware of what’s washing off our road systems and into our water.”
Oldcastle’s Kemp echoes that idea. He says permeability is now an issue with commercial builders, but it will eventually be embraced by residential developers and homeowners.
“Recycling didn’t catch on for awhile, either, but today people take their trash out with the paper, plastic and glass separated,” says Kemp. “The environmental movement is gaining, and people will want to do things to give back to the earth and make things better for their children and grandchildren.”
In the meantime, consultant Watson says while San Diego County was certainly concerned about the water washing off its parking lots, its study of permeable surfaces will lead ultimately to building specifications for governments in that county.
“We just did a presentation to municipalities, the port authority and the airport,” says Watson. “We had one city that asked us to speak with them because they have a developer who wants to use porous paving, and they don’t have any specifications for it. This is one way to get them standardized.”