Field Assessment of the Hydrologic Mitigation Performance of Three Aging Bioretention Cells

Abstract

Increasing imperviousness has driven regulation and design philosophies to offset consequent increases in runoff volumes and peak flows. Previous research has shown bioretention to reduce runoff volumes and peak flows. Since most research has focused on newly constructed systems, the long-term performance of bioretention has been questioned. Because bioretention is a biologically based practice, changes over time could impact hydrologic performance. This research examined and compared the hydrologic mitigation performance of three bioretention cells (BRCs) in central North Carolina with postconstruction ages ranging from 8 to 17 years old. Observed runoff volumes were significantly reduced at each of the three cells by 90%, 81%, and 64%. The volume discharge ratio for each cell was at or below low impact development (LID) target thresholds (0.33) for 63%, 67%, and 48% of observed storm events. Similar to volume reduction, all three BRCs significantly reduced peak flows. Peak discharge ratios at each site were less than the LID target threshold (0.33) for over 75% of observed storm events, and the interquartile range of peak discharge ratios was less than the LID target threshold for all observed storm events <25.4 mm. All three BRCs struggled to mitigate volumes and peak flows for large storm events (>50 mm). As the frequency and magnitude of larger events increases, guidance recommending additional surface storage should be considered. When compared to the hydrologic performance of “young” BRCs (less than 3 years old), “old” BRCs (at least 3 years old) perform at least as well with respect to peak flow mitigation while appearing to reduce runoff volumes better than newly constructed BRCs. That the three BRCs presented herein ranged from 8 to 17 years old during their respective monitoring periods while significantly reducing peak flows and runoff volumes (while meeting LID target thresholds) supports the prediction of long-term hydrologic mitigation of bioretention.