The synergistic effects of green-blue-gray infrastructure for flood risk management in plain river network areas

Climate change and urbanization have heightened flood risks by intensifying rainfall and expanding impervious surfaces, underscoring the need to integrate green, blue, and gray infrastructure (GBGI) as adaptive strategies. However, most studies examine GBGI at building or block scales, prioritizing expanding low-resilience gray infrastructure. They also lack standardized methodologies for quantifying synergistic effects, with scant mechanistic insight and rarely consider contextual drivers like rainfall intensity or spatial heterogeneity. We address these gaps at watershed scale in a plain river network region, simulating seven GBGI scenarios-green, blue, gray (depoldering), and four combinations-with an SCS-MIKE11 hydrological-hydrodynamic model for 20-, 50-, and 100-year rainfall events. Synergistic effects were assessed by the difference between combined and summed individual strategies performance via reduction rates of maximum water level (MWL), peak discharge (PD), and flooding rate (FR). Results show that (1) gray strategy through large-scale depoldering demonstrates significant flood risk management performance across all scenarios (MWL 2.34-2.46 %, PD 1.16-1.39 %, FR 4.70-16.32 %), highlighting the value of restoring hydrological connectivity; (2) blue-gray and green-blue-gray strategies generate robust positive synergies (0.98-4.92 %) concentrated upstream, whereas green-blue and green-gray mixes deliver limited or negative effects (down to -1.55 %), due to infrastructural interactions in runoff generation, storage, and drainage within polders; (3) rainfall intensity differentially influences synergistic effects, with MWLs showing minimal sensitivity (variation ≤0.24 %), PDs increasing from 0.10 % to 1.20 % with rising intensity, and FRs reaching peak synergy at 50-year return period (5.71 %). These findings advance watershed-scale GBGI synergistic effect theory and provide location- and event-specific guidance for climate-adaptive infrastructure planning.