Rayne of the Valley always keeps an eye out for new technological developments, even in areas that don’t appear to be related to water treatment. Permeable pavement looks to be one of the most promising construction technologies in years, since it has the potential to dramatically change the way cities manage urban runoff and groundwater replenishment, enabling cities to both prevent flooding and save water for public use—water that would otherwise be diverted to the ocean.

(above) A demonstration of how permeable pavement moves water off its surface
(above) A demonstration of how permeable pavement moves water off its surface

The Problem With Traditional Pavement

The main downside to traditional pavement materials like asphalt or concrete lies in their impermeability. In other words, they are not able to allow water to travel down from the surface into the earth (a process known as infiltration), which is where it will normally go when humans have not modified the environment, soaking into the topsoil and simultaneously hydrating plants and refilling groundwater supplies. This is a key part of what we know as the Water Cycle. All this water that is no longer being absorbed into the ground travels downward along the surface of the land (thanks to gravity) in a search for a place to settle. After all, water looks for the lowest point of any container.  With traditional pavement, surface runoff is amplified, causing flood diversion issues in any large enough town or city. With nowhere to go vertically, precipitation adds up and must be diverted using specially built channels. Another problem is that all this runoff dissolves the chemicals, contaminants, and pathogens that build up on pavement when the weather is dry. It’s a big problem, to say the least.

The Permeable Solution

Permeable pavement materials come in handy since they’re porous, which means they are far more effective in dealing with runoff than regular paving. According to a University of Maryland fact sheet, permeable pavement in combination with swale (a natural or man-made shallow channel in the earth designed to hold or divert water) generates 10% runoff from the water it comes in contact with, as opposed to asphalt without an associated swale, which generates more than 50% runoff. Their draining action prevents contaminants from building up in large concentrations, allowing these pollutants to seep through and settle in smaller amounts on a larger area of topsoil, enabling the land to filter out the bad stuff before it reaches the groundwater. Certain types of permeables are also able to capture heavy metals and prevent them from reaching even the topsoil, acting as a safety net to protect the environment. Further benefits include the ability of micro-organisms to grow in the open spaces inside the pavement, many of which are able to digest and break down pollutants like motor oil. And they may even solve the age-old problem of trees splitting pavement, since permeables give trees the space they need to extend their underground root systems. In short, the benefits are obvious and dramatic.

Permeables Aren’t Perfect

There are drawbacks to permeable pavement, of course. Cost estimates of installing permeables are often double or triple those of traditional technologies. Maintenance is also more complicated than with asphalt or concrete, since sediment and pebbles can fall into the pores of permeables over time, blocking the draining action. This is typically addressed with several passes of a specialized vacuum truck, which pulls the loose material out and restores the pavement’s effectiveness. There are other issues for permeables as well, such as weight limits for vehicles traveling over it, and possible color loss over time.

The Final Word

The average person doesn’t lose any sleep thinking about how to build a better parking lot, but in order to ensure the continued growth of our towns and cities, and the water that supplies that them, we need to continue looking all around our built environment and observe how they interact with the larger world. Permeable paving, despite its disadvantages, is an excellent example of modern, outside-the-box thinking that enables us to redefine the interaction of water treatment and our daily lives, since scrutinizing established practices enables us to discover new and better ways of doing things. And, sometimes, the next big idea could be lying right under our noses.