Field performance of bioretention: Hydrology impacts

Flows into and out of two bioretention facilities constructed on the University of Maryland campus were monitored for nearly 2 years, covering 49 runoff events. The two parallel cells capture and treat stormwater runoff from a 0.24 ha section of an asphalt surface parking lot. The primary objective of this work was to quantify the reduction of hydrologic volume and flow peaks and delay in peak timing via bioretention. Overall, results indicate that bioretention can be effective for minimizing hydrologic impacts of development on surrounding water resources. Eighteen percent of the monitored events were small enough so that the bioretention media captured the entire inflow volume and no outflow was observed. Underdrain flow continued for many hours at very low flow rates. Mean peak reductions of 49 and 58% were noted for the two cells. Flow peaks were significantly delayed as well, usually by a factor of 2 or more. Using simple parameters to compare volume, peak flow, and peak delay to values expected for undeveloped lands, it was found that probabilities for bioretention Cell A to meet or exceed volume, peak flow, and peak delay hydrologic performance criteria were 55, 30, and 38%, respectively. The probabilities were 62, 42, and 31%, respectively, for Cell B.