Holey Pavement

Is it possible for a parking lot to be “not boring?”

When the Washington Conservation District moved into a commercial business park in Oakdale in 2013, staff began looking for opportunities to reduce stormwater runoff and showcase some lesser-known conservation technologies. They settled on a retrofit design that includes a permeable overflow parking lot with a large raingarden, 5000 square feet of native prairie, native plantings along the building foundation and parking lot entrance, and a demonstration bee-lawn. Believe it or not, the permeable parking lot steals all the attention.


I was going to show you poison sumac but nature said “lol no” #urbandesign #stormwater #parking

♬ original sound – Angie Hong

Conventional pavement is impermeable (non-porous, impervious, and impenetrable), which is a fancy way of saying that water does not soak through it when it rains. As a result, developed areas generate large volumes of stormwater runoff that can cause flooding and pollution downstream. In contrast, porous pavement is holey – in a good way.   

The 15,325 square foot overflow parking area at the Washington Conservation Center features permeable pavers in the center with “grassy pave” along the sides where the cars park. The grassy pave is inlaid with a structural system that is designed to withstand the weight of cars and trucks without compacting the soil beneath. The area below the pavers and grass was over-excavated 18 inches deep and filled with gravel and sand so that it can hold 783 cubic feet of water – the equivalent of 0.6 inches of rain – until it has time to infiltrate. During larger rainstorms, the overflow moves through a buried underdrain and into a swale that wraps around the prairie to a wetland behind the building.

Please park on the grass! The grassy pave at Washington Conservation Center is designed to hold cars and also absorb rain and melting snow.

Two similar technologies – porous asphalt and porous concrete – are also designed to allow water from rain and melting snow to soak in, instead of run off. Of the four, porous asphalt is the cheapest, though also the least durable. Long term studies conducted in Rockville, Maryland and Prince William, Virginia indicate that porous pavement technologies remove 82-95% of sediment, 65% of total phosphorus, and 80-85% of total nitrogen from runoff.

The photo above shows porous concrete (left) and regular concrete (right) side by side. This was installed at Mahtomedi Public Works building in 2009.

In general, porous pavement is best suited for parking lots and driveways rather than roads. The cost is higher than for conventional pavement and requires annual maintenance with a vacuum truck to prevent the pore spaces from filling in. However, since these systems provide stormwater management and parking all in one, they are relatively cost-effective options for urban locations where space is limited.    

Of course, in Minnesota, there are always concerns about the winter. Surprisingly, however, porous pavement is actually quite well-suited for our snow, ice and freezing temperatures. During the winter months, snowmelt soaks through porous pavement instead of accumulating on the surface like with conventional pavement, so ice is less likely to form. This decreases the need for salt and reduces the risk of slip and fall injuries. The layer of gravel of sand beneath also decreases the likelihood of heaving during freeze and thaw cycles.

Porous asphalt parking lot in winter. Unlike with conventional asphalt, the snow will melt through the pavement once it warms up instead of melting and refreezing on the surface.

For parking lot owners considering a switch to porous pavement, some watershed districts offer cost-share incentive grants to help defray the extra cost. For example, the Ramsey-Washington Metro Watershed District provided $100,000 in funding for the demonstration project at Washington Conservation Center, which helped to pay for the porous pavement, raingarden, and native plantings. Staff from the Washington Conservation District can also provide advice on how to design a retrofit project and where to find materials and contractors for porous pavement projects.

To learn more, request a free site visit at www.mnwcd.org.