Hybrid Nature-Based and Grey Infrastructure for Urban Flood Resilience Under Intensifying Rainfall: A Socio-Ecological Assessment of Gangnam, Seoul
Abstract
As climate change intensifies urban precipitation extremes, high-density cities face escalating pluvial flood risks that grey infrastructure alone cannot manage. Impervious surfaces displace the hydrological functions provided by green and blue spaces, compounding flood exposure with urbanization. Nature-based solutions (NbS) offer a pathway toward restoring these functions while delivering biodiversity, heat mitigation, and water quality co-benefits, yet their integration with engineering systems remains underexplored in compact Asian megacity contexts. This study evaluates the feasibility of NbS implementation through an integrated socio-ecological assessment of the Gangnam District, Seoul, a high-density area rendered severely flood-vulnerable by rapid urbanization, aging drainage infrastructure, and intensifying rainfall. We combine GIS-based spatial analysis of 58,901 hexagonal cells, machine-learning modeling of flood susceptibility (Random Forest, AUC = 0.87), hydrodynamic simulation across four return periods, and a socio-ecological assessment (institutional interviews: n = 9; community survey: n = 50). Flooded areas are located nine times closer to roads (14.7 m vs. 130.6 m), contain 2.5 times more buildings per cell, and exhibit urban flood rates 12.3 times higher than non-urban areas. Preliminary hydrodynamic simulations suggest that hybrid NbS-grey infrastructure strategies could achieve a 28-36% reduction in flooded areas, with NbS being most effective for frequent storms (10-30-year return periods) and the Deep Tunnel Sewer System (DTSS), providing complementary capacity for extremes. The socio-ecological assessment reveals that institutional and financial factors, rather than public awareness, present the primary barriers to NbS deployment and that community preferences align with the highest-priority NbS types identified through spatial analysis. Findings support a spatially targeted NbS pathway for Gangnam, advancing evidence on hybrid green-grey strategies for flood resilience in high-density megacity contexts.