Does urban green infrastructure lead to equity issues for flood vulnerable areas? A case study in an urbanized polder area

Vegetation can help cities to mitigate and adapt to flood risks as urban green infrastructures (UGI). However, the spatial equity outcomes of UGI in flood regulation among vulnerable areas are not well understood. Taking an urbanized polder in Kunshan, China, our study estimates flood regulation efficiency (FRE) by process-based model MIKE SHE/11, and flood vulnerability index (FVI) from physical and social characteristics, to detect spatial variations in actual flood regulation benefits among different levels of flood vulnerability. The findings revealed that, UGI achieved greater FRE under longer rainfall intervals than shorter intervals, due to high evapotranspiration and infiltration. Among benefiting areas, public service and campus areas obtained the highest FRE whereas self-built housing areas had the lowest. Statistically, FRE was negatively associated with FVI, and showed a strong bias to low flood vulnerability groups (Gini = 0.64). This inequity outcome was derived from social (Gini = 0.56) other than physical vulnerability (Gini = 0.32), owing to particularly lower FRE in self-built housing areas. Therefore, policymakers could pursue more equitable outcomes of UGI by optimizing flood regulation efficiency for low socioeconomic groups. This study is expected to support the equitable and resilient design and practice of UGI for flood risk management.