Rainfall and coastal water level relative timing as drivers of compound flooding in Haikou, China

Study Region: Urban area of Haikou, Hainan Province, China. Study focus: Compound flooding in typhoon-prone coastal cities arises from the interaction between coastal water level and rainfall, but the role of their relative timing remains underexplored. This study proposes a framework that captures the relative timing for improved compound flooding assessment. Using the D-Flow FM model, we simulate 49 time-lag scenarios to quantify how timing ((Formula presented) ) between coastal total water level and rainfall modulate inundation. For each scenario, we evaluate inundation volume and compute absolute ((Formula presented) ) and relative ((Formula presented) ) increments relative to the baseline. We delineate compound zones and decompose contributions from water level, rainfall, and their interaction. New hydrological insights for the region: The results indicate that time-lags strongly shape compound flooding and produce nonlinear responses. Inundation is consistently larger when rainfall peaks before the coastal water level peak. A window from −19 h to −4 h yields volumes exceeding the historical event, and the maximum inundation occurs at −13 h with (Formula presented) = 9.44% and ((Formula presented) = 8.53 million m³). Water level is the dominant driver, yet within compound zones, the interaction term exceeds rainfall alone. These findings highlight that taking (Formula presented) = 0 h as the worst-case can underestimate risks, emphasizing that the relative timing of rainfall and coastal water levels is a critical factor for managing compound flooding in coastal cities.