When Luray, Virginia, Parks and Recreation Director Pat O’Brien began planning to build a paved parking lot for the new greenway along Hawksbill Creek, water quality issues were a key consideration. The town had just established a riparian buffer along the creekside trail using Conservation Reserve Enhancement Program (CREP) funds.

“Cows used to be going straight into the creek and doing their business. So the CREP riparian buffer fences the cattle away, and then we put in alternative water sources,” O’Brien says. “It also filters any runoff or fertilizer that would have been going directly in the creek.”

The riparian buffer paid off with vastly improved water quality, turning a cow-trampled creek into a Class A trout stream. Not to mention creating an instantly popular greenway trail.

“I had a lot of folks when we were doing the first section who said it was the craziest thing that Luray has ever done,” chuckles O’Brien. “But it’s truly far exceeded our expectations in terms of usage; there’s folks out there day and night on the trail.”

So the question became how to improve parking along the trail without adversely affecting those water quality gains. The solution turned out to be permeable paving.

Using stimulus funds via the Virginia Department of Forestry’s Urban and Community Forestry partnership program, the town installed a parking lot composed of two different types of permeable pavers over a bed of compacted gravel. Much of the lot consists of two different color permeable pavers, which are separated by small tabs and filled in with small grit gravel that allows the water to drain. A piping system under the last pad of the parking lot drains water into a rain garden. Another section of parking area is made of Turfstone, a honeycomb system of pavers that allows grass to grow through the middle of each block.

“It is more expensive that your traditional paving, but it was always the plan along our award-winning Luray-Hawksbill Greenway,” O’Brien says. “The environmental benefits are well worth the little bit larger investment than your traditional, old-school paving materials.”

Comparing apples to asphalt
But do permeable paving options really cost that much more? Matt Kilmartin, a planner with the Tualatin Hills Park and Recreation District (THPRD) in Beaverton, Oregon, recently researched the costs and benefits of pervious paving for both new and retrofit construction.

“What we've found is that the overall project and lifetime costs tend to balance out or even warrant higher up-front costs of permeable pavement systems in terms of permitting, utility/infrastructure, and maintenance/replacement costs, and a better public image, plus the added benefit of reduced total footprint--i.e., avoiding constructing water quality or quantity facilities adjacent to paved areas means more land is available for other park, trail, and open space amenities,” Kilmartin says.

Principal engineer John Howorth of 3J Consulting, Inc. in Beaverton, Oregon, notes that pervious paving may even save money in the short and long term. His company recently designed and managed the installation of a pervious concrete parking lot retrofit at THPRD’s Aloha Swim Center.

“In order to accurately compare pervious with conventional impervious paving of a parking lot, for instance, you must also add into the equation the cost of the associated storm water facility, both conveyance systems and any water quality retention system you may be required to construct,” Howorth says. “Another element that may also be a factor is the cost of the land that would otherwise be used for a retention pond.  And finally, the life cycle cost of pervious vs. conventional paving that includes all the maintenance costs should also be considered.”

Howorth cites the case of a townhouse development where pervious paving was a less-expensive option than constructing a stormwater pond, especially when that land could then be used for an additional unit of townhouses. Even replacement of an existing failing conventional parking lot at a school with one made of pervious materials helped avoid costly stormwater system upgrades.

Site seeking
Kilmartin notes that not every site is suitable for permeable paving. In his area of the Willamette Valley, the sub-soils are primarily silty-clay which typically doesn't provide good infiltration rates. And forests can be shady and wet, and produce a lot of debris that can interfere with drainage if not cleared regularly. Other unsuitable places include sites with high exposure to chemicals or known pollutants, sites with high groundwater or expansive soils, and highways where there is a high percentage of truck traffic or heavy loadings, according to Howorth. However, there is some disagreement in the field about whether heavy loadings are really an issue. 

Should you choose to go with pervious paving, Pat O’Brien in Luray advises, “Make sure you go with a quality paver and make very sure you go with a knowledgeable installer with experience or expertise in this type of material.”

“Permeable paving requires an expert who understands permeable paving, including the proper mix designs if using asphalt or concrete, section design, site design, and preparation of quality specifications with an emphasis on quality control,” Howorth says. “With proper site design and routing of the storm water from the surrounding areas away from the pervious pavement and the underlying infiltration area, a porous pavement can actually handle quite a large storm event even with minimal soil infiltration rates, much less than a half-inch per hour. “

Make a clean sweep
The number one maintenance requirement for all pervious paving is regular vacuuming. O’Brien says that he has not yet had to vacuum the greenway parking lot yet but will soon need to vacuum out an older paving project where cigarette butts have accumulated between the paving stones. Refreshing the fine grit gravel between the paving stones is another common task when the gravel settles or is lost to vacuuming.

“It settles about a half-inch to three-quarters, and then all you do is go in there and take a few wheelbarrows of this gravel and just sweep it in, done,” O’Brien says.

“On most of our permeable parking lots, we provide weekly to monthly sweeping or blowing, bi-annual regenerative air vacuuming, and annual flushing,” Kilmartin says. “We have a cooperative public agreement with other local government entities to share the use of their RAC vac-trucks, so we didn't have to purchase one ourselves.”

Howorth notes that standard pavement also requires some regular maintenance, such as annual crack sealing, which isn’t needed for pervious pavements. In the winter, O’Brien says that he has plowed the pervious paved areas but salt is not needed because the water drains so well. THPRD now has all three main types of pervious paving, so Kilmartin can provide some perspective on the advantages and disadvantages of each type, including maintenance concerns.

“The jury is still out on which type of permeable pavement is our preference,” Kilmartin says. “We like the affordability of permeable asphalt, but it doesn't last as long as concrete and requires more maintenance. We like the aesthetic, durability, and function of permeable pavers, but they're more expensive and require regular replacement of the fine aggregates between the pavers resulting from vacuuming loss and silt contamination. However, they're much easier to remove and replace when sub-surface utility repairs are needed. Overall though, our general preference has become permeable concrete due to recent advances in concrete mix production and the overall best combination of affordability, durability, longevity, and function.”