City's first porous lot

The city's first porous concrete parking lot is being poured this week behind Colorado College's new Cornerstone Arts Center. The technology, if more widely adopted, could have significant environmental benefits for a city that has been criticized by its neighbors over stormwater runoff into Fountain Creek and the Arkansas River.

Unlike traditional parking lots, rain and snowmelt will not run off the 7,500-square-foot parking lot and into the city's stormwater system - or find its way untreated into area creeks.

Instead, the water will permeate the concrete, filter through a special matting that traps dirt, then seep into an 8-inch layer of rock beneath the lot. From there, the stormwater will flow into 16 gravel-filled cardboard tubes sunk 5 to 8 feet deep that will carry the water far underground.

Stan Rovira, project manager for the liberal arts school, said contaminants such as oil and gas will be diluted and filtered as the runoff percolates down to the sand and fine gravel that underlies the campus, negating the need for the city to treat the runoff and then send it down Fountain Creek.

The technology is not new. It has been used successfully in areas blessed with temperate climates, such as California and Florida. It's a bit riskier in areas with harsher winters, because it requires careful engineering to keep water from pooling beneath the pavement, where it can freeze and crack pavement.

Lisa Ross, senior engineer with Colorado Springs' city engineering department, said there are three or four trial sites using the technology in Denver.

And Chicago, with a winter far nastier than that in the Pikes Peak region, is now requiring pervious pavement on all alleys because it too has a stormwater runoff problem, said Doug Groninger, whose company, Groninger Concrete, is doing CC's lot.

Rovira and Groninger are a bit coy about the parking lot cost but said it will be more expensive than a traditional impervious lot of the same size. They said most of that is attributable to the preparatory work needed to make sure the water drains properly.

Groninger, whose employees underwent training to do the work, said special equipment also is needed. The lot also needs to be pressure washed or vacuumed occasionally to keep grit from clogging up voids in the concrete.

The concrete is made from pea gravel of the same size so water will flow through the mix of sand, rock and Portland cement. It costs about the same as regular concrete, although it requires more careful mixing to assure there are voids in which water can pass through.

But there are potentially significant upsides.

The technology can mean less grading and traditional drainage work and, on undeveloped land, does not require that land be set aside for settling ponds.

Ross said the city's stormwater enterprise, which began assessing fees on impervious surfaces on property throughout the city in 2007, will give credit for what she calls porous pavement.

She said the porous concrete being poured at CC this week is a first.

"We're really pleased they're doing it," she said.

Rovira said the potential savings on stormwater fees was not a "make it or break it" consideration and was not a driving factor in the college's decision to try the technology.

The liberal arts school has been a leader in environmentally sensitive construction techniques - including in the nearby arts center - and the parking lot is just an extension of that effort to build green, said Rovira.

Personally, Rovira said, "I've been wanting to try this for years. It's a little risky but this lot is small enough that it's a perfect place for me to play."

An adjacent 10,000-square- foot lot is being paved with regular asphalt and will serve as a benchmark for how well the pervious lot sheds water and holds up to freeze-thaw cycles. Rovira said if the technology proves itself in this climate, the college could use it on sidewalks, wheelchair-accessible ramps and other parking lots.

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Contact the writer: 636-0197 or bill.mckeown@gazette.com