Compound Flooding Potential from the Joint Occurrence of Precipitation and Storm Surge in the Qiantang Estuary, China

In coastal regions, compound fl ooding, driven by multiple fl ood hazard sources, can cause greater damage than when the fl ood drivers occur in isolation. This study focuses on compound fl ooding from extreme precipitation and storm surge in China ' s Qiantang Estuary. We quantify the potential of compound fl ooding by measuring bivariate joint statistical dependence and joint return period (JRP). We fi nd a signi fi cant positive dependence between the two fl ood drivers considered, as indicated by Kendall ' s rank correlation coef fi cients. Compound events occur frequently, with an average of 2.65 events per year from 1979 to 2018, highlighting the signi fi cant concern of compound fl ooding for this estuary. Using a copula model, we demonstrate that considering the dependence between the two fl ood drivers shortens the JRP of compound fl ooding compared to the JRP assuming total independence. For a 1-in-10-yr precipitation event and 1-in-10-yr storm surge event, the JRP is 1 in 100 years when assuming total independence. However, it decreases to 1 in 32.44 years when considering their dependence. Ignoring the dependence between fl ood drivers can lead to an increase in the JRP of compound events, resulting in an underestimation of the overall fl ood risk. Our analysis reveals a strong link between the weather patterns creating compound events and extreme storm surge only events with tropical cyclone activity. Additionally, the extreme precipitation only events were found to be connected with the frontal system of the East Asian summer monsoon. This study highlights the importance of considering the dependence between multiple fl ood drivers associated with certain types of the same weather systems when assessing the fl ood risk in coastal regions.