Bioretention Outflow: Does it Mimic Non-Urban Watershed Shallow Interflow?

Bioretention, a key structural practice of Low Impact Development (LID), has been proven to decrease peak flow rates and volumes, promote infiltration and evapotranspiration and improve water quality. Exactly how well bioretention mimics pre-development (or "natural") hydrology is an important question that continues to be researched. Do bioretention outflow rates mirror shallow groundwater inter-event stream recharge flow associated with natural watersheds? Three small, undeveloped watersheds, located in the piedmont of central North Carolina, were chosen to represent natural hydrology. These watersheds ranged from 50 to 78ha and were comprised primarily of forest and pastureland. Each drained to a small stream, where flow rate was monitored for an extended period of time. Data collected from the natural watersheds was compared to outflow rates from four bioretention cells. The cells selected are located within the piedmont region and drain predominantly urban watersheds ranging from 0.2 to 0.9ha in size. Flow rates and cumulative volumes were determined for each site at the following intervals after stormflow/outflow began: 3, 6, 12, 18, 24, 30, 36, 42 and 48 hours. After normalizing the flow rates and volumes by watershed size, data were combined to form two data sets: bioretention outflow and stream inter-event flow. Nonparametric statistical analyses were performed on the datasets using the Wilcoxon signed rank test. Results indicate that there is no statistical difference between flow rates in streams draining undeveloped watersheds and bioretention outflow rates for the first 24 hours following the commencement of flow. Similarly, there is no statistical difference between the cumulative volumes released by the two systems during the 48 hours following the start of flow. These results indicate that bioretention cells behave comparably to natural, undeveloped conditions with respect to both flow rates and flow volumes and that bioretention outflows somewhat mirrow post storm event shallow groundwater recharge.