As a stormwater manager in a state where folks joke that there are two seasons–wet and dry–Elizabeth Wong of North Port, FL, is always on the lookout for stormwater treatment methods that are cost-effective, address water quality, need little maintenance, and are even aesthetically pleasing.
Wong is turning her focus on permeable pavement and paver systems.
Permeable pavement is one of four recommended low-impact development (LID) methods promoted in an LID manual being developed by Sarasota County, FL, and the Southwest Florida Water Management District.
Florida’s Department of Environmental Protection is working on a statewide water-quality treatment rule to get everyone on the same page, says Wong. One of the key factors to encourage LID is getting credit for the use of permeable pavement, Wong points out.
“LID is going to be a very key component of meeting those new stormwater treatment rules,” she says, noting that a developer might receive credit for using permeable pavers and reducing the size of a retention pond on the same site. “A pond takes up space in the development of an expensive piece of property. If you’ve got permeable pavement, then you don’t need as big a pond. More of the runoff will go in the ground rather than be held back in a pond.”
Permeable pavement can take the form of concrete pavers, plantable and drivable grass products, bricks, and recycled tires, as well as asphalt and poured-in-place concrete. This article focuses mainly on paver-type products.
How to choose which product is most appropriate depends on the application, says Jack Miriam, the environmental manager for Sarasota County. General questions to consider include installation procedures; what type of base course is needed; and whether the native soil needs to be replaced with another type for better drainage and infiltration. The installation and the soil type can also affect load-bearing capacity, maintenance requirements, and ability to filter pollutants from stormwater runoff. In cold climates, the surface’s ability to be plowed may also be a consideration.
According to the Interlocking Concrete Pavement Institute (ICPI), permeable pavement is a best management practice (BMP) for stormwater control under the National Pollutant Discharge Elimination System (NPDES).
Additionally, permeable pavement can receive Leadership in Energy and Environmental Design (LEED) points for sustainability, a fact echoed by SF Concrete Technology, which licenses concrete producers to manufacture interlocking permeable pavers, among other products. The use of permeable pavement can help maintain the predevelopment hydrology of a watershed.
The ICPI reports that open-graded aggregate in the surface and base course helps receive, store, and infiltrate runoff into the soil underneath.
All permeable pavements have high initial surface infiltration rates, according to ICPI, and studies show that permeable pavement can significantly reduce runoff while lowering suspended solids, nutrients, and metals.
Pavers, Blocks, and PICP
Bruce Ferguson, the director of the School of Environmental Design at the University of Georgia, notes that the industry for manufacturing open-jointed paving blocks was the first to establish industry standards for permeable paving. (See a related article by Ferguson, in this issue–“Porous Pavements Q&A.”)
Ferguson says he’s disappointed that concrete blocks are not being used more for permeable paving. He chalks it up to a lack of knowledge about their use.
“They are very reliable,” he says. “This type of permeable paving, as long as you follow the rather simple points from the established standards, is the hardest one to mess up. During installation, you just need to get the right materials and assemble them in the right order, and it’s going to work.”
The ICPI points out that permeable interlocking concrete pavement (PICP), because it is factory-manufactured, has a consistency in quality, can be mechanically installed, can be installed even in freezing temperatures, and is immediately traffic-ready.
While pervious concrete and porous asphalt rely on small-sized aggregates bound with asphalt or cement to create a porous matrix to support vehicular traffic, PICP relies on solid, high-strength concrete units, surrounded by small, highly pervious, stone-filled joints to receive and infiltrate stormwater. PICP typically has an optional geotextile on the bottom and sides of open-graded base that sits atop the soil subgrade.
To accommodate additional parking, a porous lot was added to this site.
The ICPI recommends a No. 2 stone sub-base; the optimal thickness varies with the design. On top of that is a 4-inch-thick open-graded base. Ferguson notes that it’s similar to that used for any other porous pavement. “It’s an open-graded, single-sized type stone. An ASTM [American Society for Testing and Materials] No. 57 stone about an inch in diameter is most typical.”
Next comes the surface course. “With block-type pavers, there’s a setting bed for them, considered part of the surface coarse. That setting bed needs to contain smaller, single-sized aggregate. The No. 8 or 89 aggregate, which is about three-eighths of an inch in diameter, is the most typical. The setting bed is about an inch thick. It’s one of the simple but important parts to getting this done right.”
The blocks or concrete pavers, usually a minimum of three-and-one-eighths inches thick, are placed on top of the surface course.
“The blocks come in irregular shapes or with spacers so they have open joints between adjacent blocks–that’s where the permeability comes from,” says Ferguson. “That space gets filled with the same type of aggregate that is used for the setting bed.”
Ferguson says it’s not necessary to replace native or clay-like soil.
“If you are going to do that, you have to ask how far are you going to keep going before you think you’ve got infiltration into the soil? There’s infiltration even into clay soil in the long run,” he says.
With clay, “you cannot put a 100-year storm on top of it and have it disappear in 24 hours,” he continues. “That’s too much water too fast. But most water that comes in the course of a year is in calm storms. If you let that water sit there for a period of time, then it infiltrates slowly, which is very good news in terms of water quality.
“In the event that it’s not fast enough for a design storm, when people want to make sure they know where the overflow is going, they insert a perforated pipe at some level in the base course,” he adds. “That drainage pipe takes out the excess water during a short period of time.”
Maintenance Questions
Permeable paver maintenance may require surface vacuuming, says Ferguson.
“Whether sediment of that kind is going to occur depends on what kind of situation you are in. The most common type of sediment would be sand, which is applied in northern municipalities for traction in the winter. Then you need to vacuum at least once a year in the spring after a snow melt to get out what was applied over the winter.”
The ICPI points out that even after vacuuming–which is effective because aggregate in drainage openings traps most sediments at the surface–the aggregate can be removed and replenished if it’s deeply clogged, if the base is damaged, or to install in-service utility lines. The repair can be done to match the surrounding surface.
Ferguson says permeable pavers are easily plowed. “The paving blocks are constructed to an ASTM standard, with the strength being 8,000 psi. Anybody familiar with concrete knows that is a very high number. The blocks are extremely durable to physical stresses.”
The ICPI adds that PICP is resistant to freeze-thaw cycles and deicing salt; water in the base does not freeze and heave, and complete saturation when frozen will not cause damage. It drains after snow melts and accepts normal snow-plowing equipment, although sanding is prohibited. Fewer deicing materials are needed than with regular asphalt.
Ferguson says he’s seen installations in Ontario, Canada, in which a steel plow has been grinding away on top of the pavers, “and the blocks and the plow, both of them, have seemed to survive. At least the blocks did. The blocks are not broken or dislodged.”
A feature of many paver blocks is a bevel around the top quarter-inch or so of the edge. “The original purpose of that in the design of these blocks is to prevent chipping,” says Ferguson. “But it also ensures the plow will ride up on top of every single block as it’s going along. It does not catch on these things as you drive into them. They can really take physical abuse.”
Pavers and Pollutants
The ICPI reports that PICP has a high initial surface infiltration and can handle most design storms. Runoff storage capacity depends on base reservoir design and soil subgrade infiltration rate. The organization says interlocking concrete pavement also reduces total suspended solids, nutrients, and metals.
Pavers can also help reduce the urban heat island effect and can achieve good solar reflectance with selected aggregate colors and cements, according to ICPI. Manufactured concrete units can accommodate cement substitutes, such as flyash, slag, and silica fume, and pavers can be crushed and recycled.
Research about stormwater pollutants and permeable paving show promise, says Ferguson.
“Coventry University in England has done some of the best research on the oil that drains down slowly from automobiles. There are biodegrading, naturally occurring microorganisms [in the pavement] to the extent that the oil never makes it to the bottom of the typical base course,” says Ferguson. “It goes back into the atmosphere as water vapor and carbon dioxide and very little else. It ceases to exist as a water-quality pollutant. That’s the best thing you could ask for.”
A Range of Choices
There are new materials on the horizon to add to the array of choices in permeable pavement. Ferguson points out that in the past few years, the brick industry has entered the permeable pavement business for the first time. He cites two companies that have made inroads into the market: the Belden Brick Company and the Pine Hall Brick Company.
“One reason why some designers go to a block-type pavement is for the appearance factor–it’s something that fits a historic environment or an urban situation,” points out Ferguson. “If they say they want a brick, they can literally get a clay brick. They don’t need to have imitations done in concrete. We have more choice now than before.”
Additionally, the concrete block industry continues to roll out more models in a variety of appearances, Ferguson says.
“You can get a relatively informal irregular look, a geometric look, or one that looks like a four-by-eight brick,” he says.
As opposed to other types of permeable pavement, PICP features a wide color range with various shapes and textures that blend in with the surrounding architecture and landscape, the ICPI points out. The organization states that initial costs are “competitive,” with lifecycle costs possibly lower than other choices in some markets.
In North Port, Wong is testing strips of Flexi-Pave, a permeable product made of recycled tires from K.B. Industries. “It seems to be holding up pretty good,” she says of the Flexi-Pave. “It is very permeable.”
As a stormwater manager, Wong is focused on how permeable pavement addresses pollutants. In choosing permeable pavement, she says her primary concerns are capital costs and operation and maintenance costs.
She says that in a city like North Port, a product has to be not only functional but also aesthetically pleasing.
Just as important is its water-quality-treatment aspects.
“If you don’t get the water back in the ground, you’re going to have to have a larger pond, which is more costly,” says Wong. “Then you have all of your wells drying up and people complaining. It’s all interlinked.”
Other permeable options that can support vehicle traffic include Drivable Grass from Soil Retention, Grasspave2 and Gravelpave2 from Invisible Structures, and Turf Cell from ACF Environmental. Turf Cell uses interlocking blocks made from recycled plastic that can be filled with soil and either seeded or sodded. Wong likes it for its openness and strength for traffic loading; it may be used in a grass parking lot the city is designing.
“You have a nice soil bed, you put this plastic frame down, and then you throw in dirt and plant grass on top,” she says. “There are lots of openings so the grass roots can intertwine with each other and be stronger. It looks green on top because of the grass. It’s a very environmentally friendly concept without having the problems of rutting in the grass.”
Wong says a similar product on the market is the Geoblock Porous Pavement System. The high-strength interlocking units offer turf protection and load support in trafficked areas.
Looking at Options in Florida
Stormwater managers are seeking tools to help decide what is most appropriate for their municipality’s needs.
Miriam is a fan of a set of spreadsheet tools presented by the Water Environmental Research Foundation, User’s Guide to the BMP and LID Whole Life Cost Models: Version 2.0. The models help identify and combine capital costs and ongoing maintenance expenditures for estimation of whole life costs for stormwater management models, including permeable
pavement.
Miriam says it’s also important that permitting is considered as Sarasota County works with the local water district to develop its own LID guidelines. “We want to end up with something a developer can count on getting a permit for from both the county and the district,” he says.
The University of Central Florida’s Stormwater Management Academy has done extensive testing of permeable paving using its rain machine.
“They did clogging and cleaning experiments on pavers, permeable concrete and asphalt, and Flexi-Pave,” says Miriam. “We have test results in Florida. Dr. Martin Wanielista did a study for the Florida Department of Environmental Protection and the Florida Department of Transportation, looking at pervious paving installations, testing them to see if they were still working.
“Some were as old as 20 years. It does seem to work. Even if it’s not maintained at all, 20 years later, it was still working. Out of that study came recommendations for maintenance, such as vacuuming once a year.”
The Stormwater Management Academy also has a pervious pavement design aid available online at www.stormwater.ucf.edu/default.asp.
Sarasota County’s LID guidelines are preliminary, but they are based upon research that draws upon the expertise of Wanielista and Manoj Chopra of the Stormwater Management Academy, the Natural Resources Conservation Service, and the St. Johns River Water Management District in Florida.
The guidelines recommend that pervious systems be considered as part of a water-quality treatment train. It also recommends that credit be given based on both the available storage volume of the system, and the ability to readily recover the storage volume.
Improper design or construction can result in premature failure of the pavement surface. To optimize surface infiltration rates, fine particles and material must be vacuumed from the surface of pervious pavements to prevent clogging. Pressure washing is not recommended, as it has a tendency to force clogging material deeper into the pervious pavement system where it is more difficult to extract and thus permanently reduces infiltration rates, the guidelines state.
They also point out that individuals with physical disabilities may have difficulties walking across certain pervious pavement surfaces, and that void spaces filled with filler material can cause the pavement surface to be uneven and difficult for those using crutches, walkers, or high-heeled shoes.
The guidelines note that pervious pavement design has two components: structural–the ability to support traffic loading–and hydraulic. In hydraulic design, computations must be made for precondition runoff volume and post-condition design storm runoff volume.
Miriam says he believes the LID manual being developed in his region is likely to be adopted by areas outside the water district.
“People outside the state are looking to see if permeable pavement works here,” he says. “We have every reason to expect that it will work and work well, given the right design.”
Miriam says no particular pervious pavement type is a standout among others, but rather that the choice is application-specific. Having developed the guidelines, Sarasota County is now inviting product representatives to “brown-bag seminars” to talk in more detail about their systems.
“We have a lot of county projects where we’re trying to stimulate the economy, so we’re trying to build parks and fire stations, roads, and environmental projects,” he says. “We’re trying to get an accelerated course in what’s out there in the way of products and their strengths and weaknesses.”
Miriam says interlocking pavers seem to be able to handle heavier loads and turning movements common in certain applications.
The guidelines themselves provide recommendations for the type of traffic that such systems can accommodate. “Pervious pavements are used for low-traffic loading and low-turning areas, such as parking lots; residential street parking; cart, bicycle and pedestrian paths; driveways; and emergency vehicle access lanes,” they state. And, while pervious pavement can support light traffic loads, some such as pervious concrete are vulnerable to shear stress, which might cause structural failure. “Caution should be used when designing pervious pavement in areas subjected to high volumes of vehicular traffic, frequent braking, or frequent turning,” the guidelines state. “To address this concern, pervious pavements can be incorporated with impervious areas to provide a more durable surface in certain areas while infiltrating runoff in other areas.”
Miriam says, “It seems as though pervious concrete and asphalt are subject to undue wear or degradation by turning movements–by people turning into a parking place or a drive- through. So, while pervious concrete or asphalt might be quite acceptable once the car has turned into the parking place, you probably are going to want to use either impermeable concrete, asphalt, or something like interlocking pavers that will handle those turning movements and heavier traffic.
“One of the common concerns we’ve heard is how will these products be able to handle fire trucks and emergency vehicles,” he adds. “Whatever we install, we want to make sure we’ve provided for access for emergency vehicles and typical maintenance like garbage trucks.”
When considering the cost of permeable pavement, there are many considerations beyond installation costs, Miriam points out.
“There are capital costs, regular maintenance costs, and corrective maintenance costs when something has gone wrong and you have to fix it,” he says. “A complete cost analysis includes cost avoidance. For instance, if we use other impervious materials and therefore convey pollutants downstream into a stream or bay, what is the cost going to be to then remove those nutrients if we have an impaired water body?”
Cost avoidance entails avoiding the necessity of building a stormwater treatment pond or system downstream or in the future, Miriam says.
“Usually we don’t have enough vision to be able to include all of those other costs that are very indirect in a way, but it’s common knowledge that as we’ve made our urban areas very impervious, we’ve had greater volumes of runoff and therefore greater pollutant loads going into our water bodies,” he points out.
“That’s certainly been one of the contributing factors to impairments or degradation of water quality, so it stands to reason that we would try to understand the cost savings we get by installing pervious paving and having to do a separate water quality treatment system somewhere downstream,” continues Miriam.
Studies have shown that failed permeable pavement systems are often due to improper installation. The ICPI is dealing with that by offering certification in installing interlocking concrete permeable pavers. Having a certified installer is one of the requirements in Sarasota County, Miriam says.
“It’s not just the surface that we see, but it’s everything underneath it that’s important,” says Miriam. “Permeable paving installation is very important, and that’s part of where we get the storage or the pollutant reduction that happens in those different layers.”
He adds, “We’re trying to build more walkable communities and to have a much more appealing environment in our urban areas. More and more, we’re emphasizing the aesthetics and other aspects of low-impact development, such as trees and swales that are attractively planted. They might be adjacent to a permeable sidewalk that would be attractive as well. The two work together well as a treatment train.”
He acknowledges that he’s heard concerns about the longevity of permeable systems, including what tree roots might do to them. “Most of the opinions I’ve heard say that in building permeable surfaces, there’s going to be more oxygen and water available, and tree roots aren’t likely to push up looking for oxygen and water as they would do under an impermeable surface. If necessary, we can always use root barriers.”
Another concern is permeable pavement installed adjacent to a road section with a lime sub-base that might be eroded. “In that case, you might have either a subsurface curb or some kind of a liner that might help protect them,” says Miriam.
Ultimately, when it comes to permeable pavement, Miriam says he and others in stormwater management are still new to the concept.
“We’re glad to be moving along with our LID manual,” he says. “We think it’s really going to improve our environment and our economy in the long run and create a more aesthetically pleasing environment.
“It will certainly help us achieve our pollutant-load-reduction goals without building those downstream stormwater treatment systems, so, in the long run, we’re going to find that it’s very cost-effective even though we might have a little higher capital costs upfront. But we’re not going to be building so many stormwater treatment systems in the future.”
