Roadway Bioretention Areas

What is a Bioretention Area

A bioretention area is a planted area that collects and cleans stormwater runoff from paved surfaces, like roads and sidewalks. The plants and soil in the area slow down the water and filter out pollution before it reaches a storm drain or creek.

Why we Install Them

When we install new sidewalks, bike paths, and widen streets, we place more impervious surfaces throughout Raleigh. More impervious surfaces can lead to more pollution. We strive to keep Raleigh green but also give the people of Raleigh a safe multi-modal transportation system. That is why we try and incorporate green stormwater infrastructure into our projects.

How They Work

Water is directed from the roadways and sidewalks into areas that have been engineered to remove pollutants from the runoff before it enters the stormwater system.

The drain from the street to the bioretention area on Sandy Forks Road

Samples of the stone, sand, and soil used in a bioretention area

Plantings in bioretention area on Milburnie Road

Cutouts in the curb, drains, or small channels direct the water into the bioretention area.
The water slowly seeps through layers of mulch, dirt, sand, and stone before entering the stormwater system.
The plantings in these areas also play a large part in removing pollutants, as plant roots help clean the water.

These systems are engineered so that water can use the roadway to move some of the water to the storm drain system so as not to overburden the bioretention area during heavy rain events. Bioretention areas stay dry when it is not raining.