Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland

As an increasingly adopted storm water best management practice to remedy hydrologic impairment from urban imperviousness, bioretention facilities need rigorous field performance research and monitoring to confirm performance and improve design and maintenance recommendations. This study investigated hydrologic performance at six bioretention cells in Maryland [College Park (CP), a $181{\mathrm{m}}^2$ cell, $50–80\mathrm{cm}$ media depth, monitored for 22 events, and Silver Spring (SS), a $102{\mathrm{m}}^2$ cell, $90\mathrm{cm}$ media depth, monitored for 60 events] and North Carolina [Greensboro (G1 and G2), each approximately $317{\mathrm{m}}^2$ , $120\mathrm{cm}$ media depth, both monitored for 46 events, and Louisburg ( $\mathrm{L}1=\text{surface}$ area of $162{\mathrm{m}}^2$ , $\mathrm{L}2=\text{surface}$ area of $99{\mathrm{m}}^2$ ); each had $50–60\mathrm{cm}$ fill depths, monitored for 31 and 33 events, respectively] over $10–15\text{month}$ periods. Outflow from each cell was recorded and inflow was either recorded or calculated from rainfall data. In Louisburg, L2 was lined with an impermeable membrane to eliminate exfiltration while L1 was unlined to allow both exfiltration and evapotranspiration. Results indicate that bioretention facilities can achieve substantial hydrologic benefits through delaying and reducing peak flows and decreasing runoff volume. A large cell media volume: drainage area ratio, and adjustments to the drainage configuration appear to improve the performance. Media layer depth may be the primary design parameter controlling hydrologic performance. Performance diminishes as rainfall depths increase and rainfall durations become longer. Annual water budget analysis suggests that approximately 20–50% of runoff entering the bioretention cells was lost to exfiltration and evapotranspiration.