What is “porous pavement” and how does that help soils capture and clean water?

Porous pavement can look just like regular pavement, but it has an important environmental benefit. It allows water to trickle down through the usually impermeable street or sidewalk. This means the water can be naturally filtered by the soil, and does not have to be transported to storm drains and eventually main waterways. There are a many benefits to using porous pavement, and the technology is getting better all the time, so it is likely to pop in in your local city if it hasn’t already.

In Skokie, IL, this parking lot of porous pavers allows rainfall and snowmelt to soak into the soil. Credit: SV Fisk

With more people living in cities, more impermeable surfaces are being made. Parking lots, sidewalks, and even rooftops don’t absorb water like a grassy field. As cities continue to expand, there is a ‘concrete jungle’ effect. There is so much impermeable pavement everywhere that when there are heavy rains, the storm drains become overwhelmed. This can sometimes cause damage to stormwater and septic systems or unsafe road conditions.

All of these man-made storm-drains route water to rivers and oceans. And, it’s often not clean water. Water that washes over city surfaces picks up contaminants from cars and trucks that could be harmful to local animals literally ‘downstream’ of the city. They also pick up excess salts and other chemicals.

For example, a study in the Baltimore area showed how the storm flows into rivers from an agricultural area and highly urbanized area were chemically different from each other. This study found a pattern of more carbon and nitrogen in the water coming from areas with an increasing percentage of impervious surface coverage, like cities.¹ This is no doubt a pollution-problem that plagues every large city.

Porous concrete looks like “regular” concrete, but allows for water to seep through. Photo © City of Portland,
courtesy Bureau of Environmental Services.

One solution to this problem is to reconnect precipitation to the soil. Porous pavement allows water to filter through to the soil, using it as the natural and robust filter that it is!

In 2002 the city of Portland, Oregon, started installing a range of porous pavements. They installed porous concrete, porous asphalt, and porous pavers to see how they looked and behaved over time. Both the porous concrete and asphalt look identical to the current materials. However, their internal structure has added void space between particles – compared to conventional concrete or asphalt. You can see in this video just how quickly porous pavement allows water to move through compared to conventional materials.

This construction required some extra muscle to pour the mixture, because it’s typically thicker than conventional material. The installation also uses a base gravel-rock layer beneath the porous material. This allows water to move freely into the soil beneath.

Porous pavers also reconnect precipitation with soil and groundwater – saving the stormwater system. Photo © City of Portland,
courtesy Bureau of Environmental Services.

Porous pavers look identical to regular pavers. But, they have small gaps between each paver that allows water to seep into the rock and soil below. These pavers also use a gravel-rock layer beneath like porous concrete and asphalt. They also use some geotextile fabric between layers to minimize plant seeds getting into the lower layers. Since porous pavers are individually installed, they can be easily removed or replaced if any become damaged or need repairs.

Porous pavement is a simple way to let water access the soil in areas that usually don’t let water infiltrate. Building parking lots or streets with the ability for water to pass through it helps minimize the cost needed for storm-drain infrastructure. They also decrease the amount of contaminants getting into surrounding river or streams, and help to replenish the aquifers and other groundwater sources beneath our feet!

Answered by Adrian Gallo, Oregon State University

To watch a video showing how porous pavement works done by the City of Portland, OR, visit here:  https://www.portlandoregon.gov/bes/article/535567

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1. Kaushal, S., P. Hroffman, L. Band, M. Elliott, C. Shields, C. Kendall. 2011. Tracking nonpoint source nitrogen pollution in human-impacted watersheds. Environmental Science & Technology. 45:8225-8232. https://pubs.acs.org/doi/abs/10.1021/es200779e