IECA 2022 Technical Paper of the Year: Short-circuiting Assessment of a Roadside Bioretention Facility in Atlanta, GA

Short-circuiting Assessment of a Roadside Bioretention Facility in Atlanta, GA

Short-circuiting of stormwater ponds occurs when water flows through the cell unevenly, allowing some of the influent to take preferential pathways to the outlet and bypass the treatment process. The performance of bioretention projects is directly affected by this phenomenon, being mainly dependent on the flow patterns and detention time inside the unit. With the goal of evaluating the hydraulic performance of a bioretention cell located in Atlanta, GA, this study constructed a physical hydraulic model in a laboratory setting and conducted analysis of residence time distribution functions originated from tracer experiments. Tracer methods are commonly used to investigate hydraulic performance. It consists of an instantaneous injection of a known solution concentration at the inlet of the pond and the measuring of the outlet concentration with time. This study used fluorescent dye as a tracer solution and focused on the quantitative short-circuiting aspect of the flow condition. In order to simplify the analysis, the hydraulic index (S-value) was used as a measure of short-circuiting for different simulated rainfall events. The results show that all rainfall events simulated had low S-value (i.e., high short-circuiting effects). The cases with a lower simulated flowrate showed a higher S-value (i.e., lower short-circuiting effects) compared to bigger simulated storms. Design recommendations related to short-circuiting generally give emphasis to the shape and location of the inlet and outlet of the pond. The studied bioretention has a shape where the in- and outlet are placed closed to each other in relation to the length and volume of the pond, representing, this way, the effect when the tracer dye travels quickly from the inlet to the outlet. This presentation will provide information on the conducted hydraulic experiments and discuss potential mitigation solutions to improve hydraulic performance concerning short-circuiting.