Estimating and mapping compound flood potential from precipitation and storm surge along the Coastal Chinese Margin

The Coastal Chinese Margin (CCM) is highly developed but vulnerable to compound flood arising from extreme precipitation and storm surges. This paper estimates the probability and intensity of compound floods due to extreme precipitation and storm surges using daily data from 1979 to 2014 in the CCM, based on copula functions and joint probability models. The objectives of this research are to clarify the spatiotemporal evolution of the compound events, characterize their encounter risks and uncertainties, identify the underlying driving mechanisms, and ultimately optimize coastal prevention and control strategies. The results indicate that: (1) compound events demonstrate pronounced seasonality, predominantly occurring in July and August, and exhibit a distinct increasing trend in both frequency and intensity; (2) the two islands (Hainan and Taiwan) and two peninsulas (Shandong and Liaoning) experience a higher frequency of compound events. The Yangtze River Delta and the Zhejiang-Fujian coastal region are hotspots for compound events characterized by elevated precipitation and storm surge levels, especially those of high intensity; (3) the uncertainty associated with the design value is larger in regions with high values and increases with the Kendall return period. Precipitation is the primary factor contributing to the uncertainty of future compound events; (4) tropical cyclones are identified as the primary driver, accounting for more than half of the compound events south of the Yangtze River Estuary and exerting a broader influence on precipitation. El Niño events enhance the frequency of compound events, whereas the Interdecadal Pacific Oscillation (IPO) and Indian Ocean Dipole (IOD) suppress precipitation, thereby reducing the occurrence of compound events. This study provides a quantitative assessment of the risk of combined flood events in grid scale, theoretically enhances our understanding of the complex interplay between storm surge and precipitation in compound flood events, and practically aids in informing policymaking and interventions aimed at strengthening flood defenses, as well as sustainable development of the CCM.