Impact of Maintenance and (Im)Properly Sizing Bioretention on Hydrologic and Water Quality Performance

Abstract

Routine maintenance and proper construction oversight both need to occur during and after bioretention cell construction, in order to ensure proper functionality. Two sets of bioretention cells of varying media depths (0.6-m and 0.9-m) have been monitored for two, 12-month periods, in Nashville, NC. These bioretention cells are unique in that during the first monitoring period, the bioretention cells were (1) clogged with fine sediment from construction and (2) were severely undersized. Complete drawdown of the surface storage took 48 hours or more, as compared to the recommended 12 hours. Initially, the surface storage volumes for the 0.6-m and 0.9-m media depth cells were only 28 percent and 35 percent of the design storage volume, respectively. The design event for the bioretention cells at this site was 2.5-cm, but the system was overwhelmed frequently and overflow occurred for events as small as 0.9 cm. After one year of monitoring, the fines layer present in the top 7.6-cm was removed. Removal of this layer increased the surface storage volume of both sets of cells by 89 percent. With the increase in surface storage volume, more runoff was treated, and fewer events had overflow. The smallest event with overflow was 1.9 cm, and some events up to 2.8 cm were fully captured in the bowl. Overflow volume was reduced to approximately one-third of the volume from the first monitoring period. Moreover, removal of the fines layer increased the surface drawdown rate by up to a factor of 10. Pollutant load reductions increased for nitrogen species and total suspended solids because more runoff was treated. The results of this study highlight the reduced performance associated with improperly constructed and maintained bioretention cells. Even a small construction error in setting the base elevation for the bottom of the bowl or the emergency overflow structure can drastically reduce the bowl storage volume.