Have you ever felt like the terminology of a subject strands you in an unknown land?  Take asphalt -- if you’re just learning about it, terminology can be a barrier.  Those in the industry don’t think twice about terms like “hot-mix” or “sealcoating,” but for the layman, these can be confusing.

Pervious and Permeable and Porous, Oh My! 

Some terms that often get mixed up are pervious, permeable, and porous.  These sound similar, like they all mean the same thing.  In fact, if you look up “pervious,” “permeable” is part of the definition!  Both mean a substance that allows liquids or gases to pass through (as opposed to being a solid barrier).  These terms are also used synonymously in asphalt. Pervious or permeable pavements have connected spaces that allow water to percolate through the surface. 

“Porous” has a similar, but slightly different, meaning.  Holes or spaces in the surface allow water in, but these are not necessarily connected.  While fluids could get in, they have nowhere to go from there and can't easily percolate down to the water table.  This distinction, however, is not generally recognized in the industry. 

With each year, smart business owners look for ways to maximize their return on their investment.  One thing to consider is getting a “green” reputation (and getting ahead of environmental regulations).  And one way to decrease a negative environmental impact is to use pervious pavements for your parking lots and thoroughfares.

What are pervious pavements?

It can be somewhat difficult outside the industry to understand the differences between pervious, porous, and permeable pavement and asphalts.  The terms are often used interchangeably.  This is because the ultimate goals achieved are similar – water, instead of simply running off, can percolate through.  Allowing it to do so helps decrease flooding, better manage storm water because less of it has to rush into gutters and sewers, helps keep water tables filled, and even helps the environment by removing toxic substances via the percolation process.  While they all accomplish the same goals, there are some differences between them.

Pervious pavement is generally considered the umbrella term for all the types of paving that accomplish these goals.  Within that, you have porous or permeable pavements, such as porous asphalt pavement, which allow the water to percolate through the entire surface area.  You also have permeable pavers, which are really impermeable except that they allow water to percolate through wherever they’re joined to one another.  So, in this case, the paving stone itself is not porous, but using paving stones still allows water to percolate through a lot more than non-pervious pavements because of how many paving stones are used and thus how many joins there are throughout the surface.  Paving stones are often more decorative and more expensive.  However, regardless of which type of pervious pavement is used, the benefits to the environment are similar.

The Milwaukee Metropolitan Sewerage District (MMSD) is continuing its work on keeping Milwaukee “green.” In July 2012, an article in the Milwaukee Journal Sentinel revealed that MMSD began required rooftop plants and the installation of other “green infrastructures” to help collect and absorb storm water. MMSD announced in a recent article that permeable pavement will be used along portions of three Milwaukee county roads.

One goal of this work is to improve the water quality in nearby wetlands and along rivers. “Reducing the volume of soil and sand eroding from the shoulder and capturing storm water laden with pollutants washed off the pavement will improve water quality.”