American Society of Civil Engineers

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

by Houng Li, (Grad. Res. Asst., Dept. of Civ. and Envir. Engrg., Univ. of Maryland, College Park, MD 20742-3021), Lucas J. Sharkey, M.ASCE, (Proj. Engr., CH2M Hill, 3201 Beechleaf Ct., Ste. 300, Raleigh, NC 27604), William F. Hunt, M.ASCE, (Asst. Prof. and Extension Specialist, Dept. of Biological and Agric. Engrg., North Carolina State Univ., Raleigh, NC 27695-7625), and Allen P. Davis, (corresponding author), F.ASCE, (Prof., Dept. of Civ. and Envir. Engrg., Univ. of Maryland, College Park, MD 20742-3021 E-mail:

Journal of Hydrologic Engineering, Vol. 14, No. 4, April 2009, pp. 407-415, (doi:

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Document type: Journal Paper
Special Issue: Impervious Surfaces in Hydrologic Modeling and Monitoring
Abstract: 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 m² cell, 50 – 80 cm media depth, monitored for 22 events, and Silver Spring (SS), a 102 m² cell, 90 cm media depth, monitored for 60 events] and North Carolina [Greensboro (G1 and G2), each approximately 317 m², 120 cm media depth, both monitored for 46 events, and Louisburg (L1=surface area of 162 m², L2=surface area of 99 m²); each had 50 – 60 cm fill depths, monitored for 31 and 33 events, respectively] over 10- to 15-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.

ASCE Subject Headings:
Best Management Practice
Nonpoint pollution
Urban areas
Stormwater management
Peak flow