Temperature Dynamics of a Rock Swale and Bioretention Green Stormwater Infrastructure System Treating Runoff from an Elevated Highway Interchange

A growing number of urban surface waters are impaired for high temperatures due to the heat exchange that occurs between the warm urban land surface and stormwater runoff before it enters downstream waterbodies. Green stormwater infrastructure (GSI) has the potential to reduce runoff temperatures; however, it is unclear how specific treatment approaches, such as rock swales, or sequencing of GSI in series can most effectively mitigate temperature in stormwater runoff. The objective of this study is to capture the temperature dynamics of a GSI treatment train consisting of rock swales leading to bioretention that captures runoff from an elevated highway interchange in Milwaukee, WI. Temperature was observed continuously during 17 runoff events at nine locations, including the primary influent from overpass downspouts, rock swale effluent, bioretention media, and bioretention underdrain effluent. Results indicate that rather than mitigate temperature, rock swales warmed runoff to a greater degree than that of highway overpass decks. However, the sequencing of bioretention practices at the end of the GSI system resulted in temperature reductions of 0.9°C–2.4°C on average to levels below thresholds for downstream trout species. Overall, this study demonstrates the importance of effective sequencing of GSI practices for effective temperature mitigation, which is a leading water body impairment in many states in the United States.