Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Marylandby Lucas Sharkey,
William F. Hunt,
Allen P. Davis,
Document Type: Proceeding Paper
Part of: World Environmental and Water Resources Congress 2008: Ahupua'A
As an increasingly adopted stormwater 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 requirements. This study investigated hydrologic performance at six bioretention cells in Maryland and North Carolina over a 10 to 15 month period. The cells varied in media composition, media depth, and drainage configuration. Outflow from each cell was recorded using a pressure transducer and data logger as outflow passed over a v-notch or Thelmar weir. Inflow was determined by either a similar set up to inflow with weirs or flumes, or by triggering amples from a tipping bucket rain gauge from a >90% impermeable watershed. In College Park, Maryland, a 181 m² Cell (Cell CP) with a 50-80 cm media depth was monitored for 22 events. In Silver Spring, MD, a 102 m² Cell (SS) with a 90 cm media depth was monitored for 60 events. In Greensboro, NC, where soil media depth was nominally 120 cm, an estimated 65.0 cm (cell G1) and 72.1 cm (G2) of 132.1 cm precipitation evapotranspired or exfiltrated. In Louisburg, one of the two cells studied was lined with an impermeable membrane to eliminate exfiltration (cell L2) and the other was unlined to allow both exfiltration and evapotranspiration (L1). In both Louisburg cells, media depth was 70 cm. Of 58.1 cm of inflow to cell L1, 15.6 cm exfiltrated or evapotranspired; 9.3 cm of 49.9 cm evapotranspired from cell L2. More than two-thirds of water lost at cell L1 evapotranspired. Results indicate that the bioretention facilities can achieve substantial hydrologic benefits through delaying and reducing peak flows and decreasing runoff volume. A large cell surface area: drainage area ratio and media volume appear to improve the performance. Results suggest out that hydrologic performance is minimally impacted for high rainfall depth and long rainfall duration. The runoff volume reduction promotes pollutant mass removal and links bioretention water quality benefits with hydrologic performance. Findings from all six sites will help designers determine individual storm performance of bioretention as well as estimate annual water budgets for implementation in Low Impact Development scenarios.
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