Published Saturday, November 10, 2012
By SARAH FAY CAMPBELL
To the casual observer, the newly-paved area in front of the History Center train depot in downtown Newnan just looks like a pretty parking lot.
But there’s more to it than meets the eye — those good looks have a purpose.
The parking lot at the East Broad Street depot is “pervious.” That is, rainwater trickles down through it instead of running off it. The parking lot design eliminates the need for a detention pond or other “stormwater management” structure, and it cleans and filters the water before it sinks into the ground or, on rare occasions, flows into a storm drain.
A demonstration of the parking lot’s remarkable ability to absorb water was held Friday.
Crews from the Newnan Fire Department sprayed hundreds of gallons of water onto the parking lot over the span of about half an hour, and a crowd watched the water immediately soak in.
“It’s a different way of looking at stormwater management,” said Brent Davis of Belgard Hardscapes, the company that provides the permeable paving system. Belgard is made by Oldcastle.
When the city constructed the new parking lot, there were three goals in mind, said Rob Hill, engineering inspector with the city of Newnan for engineering, stormwater management and erosion control. The paving system covers all three: water quality, detention, and good looks.
“This does everything we’re looking for — plus it looks so much better,” Hill said.
The parking lot can handle a large amount of water. “People don’t expect it to work” as well as it does, Hill said.
In fact, a standard lot, such as the one in Newnan, can provide the stormwater management for an asphalt parking lot or other impervious surface five times its size.
Even after the water was sprayed onto the lot Friday, not a drop of it made its way to the storm drain.
The secret is not in the concrete “bricks” themselves. For the most part, they’re regular, high pressure concrete pavers.
Instead, its the bed of gravel, or “aggregate,” on which they sit, and which fills in the voids between the pavers.
The paving system typically consists of the pavers, atop a 2-inch thick bed of small gravel. That sits on a 4-inch bed of medium gravel, and a 12-inch bed of coarser gravel. The “aggregate” bed is designed for each particular situation, and can be made thicker if needed in order to allow it to serve an even larger asphalt or concrete lot.
This kind of parking lot is more expensive than a traditional asphalt parking lot, but once all the costs are taken into account, it is roughly the same price, and can even be cheaper as time goes by.
The biggest benefit is not having to have a detention pond. The ponds are expensive to build and maintain, but even more importantly, they take up land that can serve a better purpose. If a pond had needed to be built at the downtown Newnan parking lot, it would have reduced the number of parking spaces and been an eyesore.
“When it’s centrally located, you don’t want to waste space with a detention pond,” said Mike Leedy of Abby Group, who did the actual installation.
The paver and aggregate combination is also very durable — and lasts longer than asphalt. Whereas asphalt has to be repaved after a number of years, the pervious lot only needs regular sweeping. After many years the top layer of fine gravel can get clogged up and may need to be replaced. But only the areas that need it have to be fixed — a few pavers can be removed, the top later of aggregate cleaned out and replaced, and the pavers can be put back down.
Seeping through so many layers of stone provides significant water filtering — something a detention pond can’t do. Eighty-nine percent of dissolved solids and other sediment is captured in the first inch-and-a-half of aggregate, said Davis said. And naturally occurring enzymes in the aggregate break down oils and grease, he said.