Blue-Green Infrastructure for Climate-Resilient Urban Redevelopment: Heat-Mitigation and Runoff-Retention Co-Benefits in a Dense Subtropical Old Urban District

Dense older urban districts face combined heat and stormwater pressures, yet the thermal and hydrological functions of blue-green infrastructure (BGI) are frequently assessed separately. This study integrates ENVI-met microclimate simulations with a planning-level runoff-retention capacity assessment to compare green roofs, permeable roads, water bodies, and integrated portfolios in Central District, Taichung, Taiwan. Its contribution is a cross-typology hydrothermal comparison under common assumptions that distinguishes among absolute benefits, area-normalized efficiencies, and spatial trade-offs while incorporating implementation constraints. Full permeable-road implementation reduced the domain-averaged, pedestrian-level air temperature by 0.61 °C and physiological equivalent temperature (PET) by 1.37 °C. Water bodies provided the highest equivalent unit-area retention depth (81 cm) but could increase localized radiant exposure when they replaced shaded vegetation. Green roofs offered distributed retention despite limited cooling at 1.5 m. A feasible portfolio combining 50% roof greening, selective permeable-road retrofits, and water-body allocation reduced air temperature by 0.29 °C and PET by 0.46 °C, with a nominal retention capacity of 47,953 m3.