In an effort to protect and treat stormwater across the state, the North Carolina Department of Transportation (NCDOT) established the Highway Stormwater Program (HSP) in 1998. Guiding principles of HSP can be summarized in three categories: to comply with National Pollutant Discharge Elimination Standards (NPDES) stormwater permit requirements by managing and reducing stormwater pollutants from roadways and industrial areas; to design sustainable programs that can be effectively managed, implemented, and integrated into NCDOT; and to develop solutions that improve program delivery, are proactive, form partnerships, have technical merit, and are fiscally responsible.

As a more recent addition to HSP, NCDOT has developed design guidelines as prerequisit requirements for its NPDES stormwater permit, known as the NCDOT stormwater best management practices (BMP) toolbox. HSP defines the toolbox as, "guidance, criteria, and considerations for the design and application of post-construction structural best management practices."

Best Management Practices applied by NCDOT

The BMP toolbox consists of 11 strategies for treating highway stormwater and includes level spreaders, preformed scour holes, dry detention basins, swales, forebays, hazardous spill basins, infiltration basins, media filters, wet detention basins, stormwater wetlands, and filter strips. NCDOT has implemented a program to identify potential sites for retrofits based on the location of existing roadway drainage systems that discharge to sensitive or impaired watersheds. Upon selection of potential sites, NCDOT assigns design contracts to consultants for preparation of surveys, design, and construction documents.

Protecting a Community's Drinking Water

Our transportation professionals have been tasked with retrofitting existing drainage systems at several interchange locations using the BMP toolbox. These interchanges are located along I-85 in Durham and Granville Counties. Stormwater from both sites discharges into the Falls Lake watershed, which supplies drinking water for the majority of neighboring Wake County. The Redwood Road interchange will utilize bioswales to replace concrete lined ditches. The bioswales reduce velocity, attenuate runoff, trap sediments, and promote infiltration by use of grass lined ditches underlain with sand filters and under pipe systems. The NC-56 interchange will include a media filter basin, which will consist of a dry detention pond underlain with sand filters and under pipe systems. The pond is sized to retain the first inch of runoff and promote infiltration through the bottom of the pond and sand filter.

Industrywide, BMP standards are being created to improve the quality of stormwater runoff. Through the application of these specific toolbox design guidelines, NCDOT will continue to retrofit existing drainage systems and improve water quality. I hope that through our efforts, stormwater across the state will continue to improve in quality, and, therefore, improve the quality of life for Wake County and surrounding area residents.