Holey Pavement

What excites you? Does it need to be a true adventure like rafting through the Grand Canyon or heading out on safari in Africa? Or can you find excitement in life’s little victories, like discovering a carton of ice cream in your freezer that you didn’t know was there? Do you ever find excitement in things that other people consider mundane?

I would like to think that my coworkers and I are relatively normal people, but then I stopped to consider a short list of things that excite us:

  1. Finding a caddisfly larva on a rock in a stream. (They’re an indication of good water quality.)
  2. Plant roots. (Did you know that some prairie plants have roots 15 feet deep?)
  3. Porous and permeable pavement. (Water can soak right through the pavement!)

So, while I realize that my internal excitement barometer might be a little bit askew, allow me to try to explain why holey pavement, while not technically exciting, is actually an important and useful technology.

A woman watches rain soak into the porous pavers at Lily Lake boat launch in Stillwater, MN.

Porous pavement comes in many different forms; pervious asphalt, concrete and pavers are the most common varieties. Above ground, these paving types look like fairly traditional sidewalks, parking lots and roads, but the secret lies beneath your feet. When it rains, the water soaks right through the pavement as if it weren’t even there. As a result there is no stormwater runoff to pollute nearby lakes and streams. Though porous pavement is still relatively uncommon, local communities in Minnesota are starting to use it more often as a way to reduce runoff from businesses and parking lots where there isn’t room for other stormwater treatment practices like raingardens and stormwater ponds.

Stormwater runoff from roads and parking lots is one of the leading causes of water pollution, which begs the question, why not use porous pavement more often? Like most good things in life, porous pavement is expensive.  Of the options, pervious asphalt is the cheapest, followed by concrete and then paver stones. The real cost lies not in the pavement itself, however, but in the subsurface layers required to ensure that water has somewhere to go once it soaks through the pavement. Preparing for porous pavement requires over-excavating and filling the area below the pavement with layers of sand and gravel that can temporarily store rainwater until it has time to soak deeper into the ground. This natural filtration process cleans the water and helps to promote groundwater recharge in addition to reducing stormwater runoff.

Through experimentation, we’ve learned that you can save money by using porous pavement in strategic locations on streets and parking lots where the runoff water goes and using traditional pavement for the rest of the surfaces. For example, the City of Chicago has begun retrofitting alleys by installing strips of pervious concrete down the centers of the alleys. There are now more than 200 “green alleys” in Chicago, with 20 to 40 new ones built each year. Two of the best places to see porous pavement in Stillwater are the Lily Lake boat launch and the Post Office parking lot across the street from Trinity Lutheran Church. Visit either one during a big rainstorm and you can watch as water rushes across the pavement until it hits the edge of the porous zone and then disappears. Now that’s what I call exciting!

Expect to see more porous pavement in Minnesota as the technology improves and cities and businesses get more comfortable with its maintenance requirements. Meanwhile, the next time you are looking for excitement on an otherwise boring day, grab a friend and try your luck at finding one of a handful of porous paver driveways in Stillwater. It’ll be cheaper than booking a safari!