NCDOT is required by the NPDES stormwater program to mitigate stormwater runoff from its facilities. NC Senate Bill 2008-107 further mandates runoff from bridge decks be evaluated and treated using best methods. Bioretention and bioswales show promise for stormwater treatment in the linear highway environment. Two bioretention cells and a bioswale were constructed in a DOT maintained right-of-way underneath a bridge deck at the intersection of Mango Creek and I-540. One bioretention cell was sized correctly per NC DENR requirements, while the other has half the bowl storage and surface area that is required for its drainage area. The bioretention cells and bioswale were constructed using advanced practices, including internal water storage, engineered fill media, and specialized vegetation.
Undersized bioretention cells have a high potential to be used as retrofits. However, little is known about their relative performance when compared to full-sized bioretention cells. This grant may allow NC DOT and NC DENR to award undersized bioretention cells a portion of the credit that full-sized cells receive. Also, this research will evaluate bioswale hydrologic and water quality performance. No research has been completed on bioswales in North Carolina.
Monitoring of these practices began in November 2009 under NC DOT grant HWY-2009-29, but is forced to be completed by June 30, 2010. This grant proposal will allow continued monitoring of the bridge deck runoff quality, the undersized bioretention cells, and bioswale beyond FY 2010, essentially for a full year. Completing the data collection will allow the researchers to make appropriate conclusions about performance of each system.
