Disentangling the Synergistic Mechanisms of Urban Waterlogging Risks in High-Density Coastal Cities: A Quantitative Contribution Analysis

High-density coastal cities face escalating risks from compound flooding driven by drainage limitations and natural hydrometeorological extremes. However, underexplored non-linear interactions between these factors hinder effective mitigation. Focusing on Shalang District, Zhongshan City, China, this study proposes a quantitative framework to disentangle the contribution rates and interaction effects of critical factors: insufficient drainage capacity, low-lying terrain, and river backwater effects. A coupled 1D-2D model integrating the Storm Water Management Model (SWMM) and the Integrated Terrestrial Fluxes Model (ITF-FLOOD) was constructed to simulate inundation scenarios under varying rainfall return periods. An improved contribution rate calculation method based on factorial scenario analysis is adopted to quantify the synergistic and antagonistic mechanisms driving waterlogging severity. Results reveal that while insufficient drainage capacity is the dominant factor (contributing 30% − 109% to overflow volume), its influence is non-linearly modulated by natural boundary conditions. A critical antagonistic effect between drainage capacity and low-lying terrain is identified, suggesting that in deep depressions, simply upgrading pipe networks yields diminishing returns. Conversely, the river backwater effect exhibits a dynamic amplification mechanism: its contribution rate significantly escalates with the rainfall return period (surging from 16% to 65% at critical nodes), acting as a major constraint on drainage efficiency. The study concludes that single-measure retrofitting is insufficient for resilient flood management. Instead, a synergistic strategy integrating drainage network expansion with active river level regulation is essential to break the hydraulic lock in coastal urban areas. These findings provide a theoretical basis for prioritizing retrofit investments in complex urban water systems.