Green Stormwater Infrastructure Retrofits for Urban Areas

by Debbie Stringer, Carolina Stalite Company

City of Raleigh bioretention device for stormwater under construction

When the City of Raleigh and the North Carolina DOT developed a plan to calm traffic and increase pedestrian safety on Milburnie Road, they also included vegetated bump-outs and a bioretention device for stormwater.

Urban development in Raleigh is booming which brings more impervious surfaces and stormwater runoff. Rainfall events are increasing in frequency and duration and that strains outdated stormwater infrastructure especially in older parts of the city. Retaining rainfall on site to slow it down or allow it to infiltrate is a best management practice.

Bioretention bump-out retrofits in urban areas must have curb appeal and move water. Rotary kiln expanded aggregate sand media was selected for this project to do just that. In North Carolina, this alternative to typical ASTMC33 sand has been utilized in bioretention for 15 years and each project is still performing as intended. The NCDOT specifies expanded aggregate sand for projects where hydraulic conductivity and structural integrity are needed. University research demonstrated that expanded aggregate sand also excels at fixing pollutants like TSS and Phosphorus. Expanded aggregate porosity promotes infiltration and reaeration for plant roots, which are important for healthy vegetation. Research also demonstrates that properly aerated root zones support microbial activity contributing to breaking down pollutants.

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