Porous, Pervious or Permeable Asphalt

We toured the Ramsey County Watershed District office grounds recently through a Wild Ones summer tour series.

There were some very interesting sustainable landscaping and water retention/infiltration features including several raingardens (water from rooftops and curb cuts), rainbarrels, native plantings, a green roof and a permeable asphalt parking lot.

It was the first time I had seen permeable asphalt and on first glance it doesn't look too different from traditional asphalt other than the larger aggregates near the surface.

Our tour leader demonstrated the infiltration capabilities by pouring a bucket of water onto the asphalt.
The puddle from the bucket stayed about 2 feet in diameter and the water quickly infiltrated.

The asphalt make up is almost the same as traditional asphalt, the difference being that the smaller aggregates are left out of the mix leaving only the large aggregate.

Why do you want water to infiltrate rather than run off a paved surface?
During a rainstorm there are huge surges of water that flow off of paved surfaces and into storm drains where it goes untreated into our local waterways. The water surge erodes soil and picks up pollutants (fertilizer, pet waste, oil etc) and dumps these pollutants untreated into local water bodies.

The ideal situation is to stop water from leaving your property to help maintain or improve water quality in your watershed. This can be done through permeable paving (asphalt or pavers), directing water from downspouts into raingardens or rainbarrels, having native plantings where plants have deep root systems and can help with infiltration, mulching exposed soil with wood or leaf mulch and keeping your lawn mowed at a higher height. Limiting your fertilizer/pesticide usage in your yard is also very important. Another advantage of water infiltrating back into the ground is that it helps recharge groundwater.

Image Source: National Asphalt Pavement Association

This diagram shows the subgrade requirements laying permeable asphalt. These requirements vary depending on the amount of frost in your region.

One advantage in the winter is that the snow melts much more quickly on permeable asphalt and the pavement therefore stays drier preventing ice buildup.

Here is a video of water being poured onto the permeable asphalt at a different site to demonstrate how quickly it infiltrates.