Exploring the driving factors of compound flood severity in coastal cities: a comprehensive analytical approach

Abstract. Coastal cities face severe compound flooding, including both fluvial flooding and pluvial flooding. Currently, there is a lack of comprehensive methods to analyze the driving factors of compound flooding. This study establishes a coupled one-dimensional and two-dimensional hydrodynamic model. Based on historical data, the model constructs joint probability distributions of rainfall and tide levels with different return periods and durations. Using the results from the coupled model under various design scenarios, the study proposes an impact index to quantify the relative contributions of rainfall and tide level to flooding. Additionally, the model quantifies the interactions between fluvial flooding and pluvial flooding. Taking the Shahe River basin in Guangzhou, China, as a case study, the results show that the combination of the copula function and the Kendall return period method is effective for designing hydrological variable combinations. The impact degree index of rainfall on flooding varies between 0.5 and 1, with the minimum at 24 h duration, indicating that the compound flooding is primarily affected by rainfall, and the influence of tide level is most significant at 24 h duration. The pluvial flooding caused by the influence of river water level on the drainage outfalls accounts for up to 19.08 % of the total volume. This shows that fluvial flooding affects the seriousness of compound flooding by influencing the water levels of outfalls. The flood-prone area is divided into different regions based on the main natural factors (rainfall and tide level) and social factors (pipeline network, drainage outfalls, and riverbank defenses) to help decision-makers identify the causes of flooding in each drainage unit and better formulate targeted disaster-reduction strategies to improve flood control capabilities.