Effects of Trees Planted on a Viaduct on Thermal Comfort in Street Canyons during Heatwaves.

The decision to demolish outdated viaducts or transform them into green spaces is an increasing focus in urban planning. This study investigates the environmental impact of repurposing an elevated road into a linear park, with a particular emphasis on its effects on airflow and thermal comfort in street canyons during heatwaves. Using a computational fluid dynamics (CFD) model incorporating parameterization schemes for tree drag, shading, and evapotranspiration, various scenarios were evaluated, including tree presence, height, placement, and solar altitude. The elevated road alone reduced wind speeds and increased surface temperatures. However, trees planted on the structure provided notable cooling effects, enhancing thermal comfort by lowering wind speeds and air temperatures. Tall trees (over 6 meters) reduced average temperatures by up to 1.7°C on the viaduct and 0.6°C in pedestrian zones. While shrubs under 1 meter effectively cooled surfaces, they lacked adequate shading, whereas trees taller than 4 meters improved the Universal Thermal Climate Index (UTCI) by 2–8°C. Centrally placed trees on the elevated road achieved the most significant UTCI improvements, whereas side plantings enhanced thermal conditions beneath the structure. The findings underscore the importance of tree evapotranspiration, height, and strategic placement in optimizing thermal comfort, offering valuable insights for sustainable urban planning and the design of green infrastructure on repurposed viaducts.