Numerical evaluation of the effects of underdrained bio-infiltration media on runoff thermal mitigation

Increased impervious surfaces due to urbanization lead to higher stormwater runoff volumes and degraded surface water quality. In addition to carrying pollutants, urban runoff often exhibits elevated temperatures due to heat absorption by impervious materials, contributing to thermal pollution in receiving waters. Even small temperature increases can disrupt aquatic ecosystems, reduce dissolved oxygen, and threaten temperature-sensitive species such as trout. Green infrastructure, or stormwater control measures (SCMs), such as bio-infiltration basins, have been increasingly used to control runoff and mitigate temperature. However, due to the potential conflict between runoff reduction and thermal mitigation, there is no consistent findings regarding to the optimal bio-infiltration basin designs. This study utilized a numerical model, validated with field data, to simulate the hydraulics and the heat transport in a bio-infiltration and further determined the effect of basin media thickness and media type on reducing thermal loads under different storm events and runoff temperature conditions. The bio-infiltration studied reduced peak runoff temperature by 2.55 to 3.42 °C, varying with the influent runoff temperature. It mitigated short-term spikes in flow temperature, reducing the potential for acute thermal stress. This study also found that media thickness, compared to media type, had a greater impact on the bio-infiltration basin's runoff reduction and thermal reduction performance. More sandy Mix D resulted in slightly greater peak temperature reduction than Mix A and C, with their mean temperature reduction being unobservable. These findings highlight the effectiveness of bio-infiltration basins in mitigating thermal load from stormwater runoff and inform further works on optimizing infiltration basin designs or the adoption of active controls to optimize system operations and enhance thermal reduction performance.