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

Abstract. Coastal cities frequently face various types of flooding triggered by heavy rainfall and storm surges, such as fluvial flooding and pluvial flooding. Currently, Currently, there is a lack of comprehensive methods to analyse the sources of severe compound flooding. This study, using the Shahe River Basin in Guangzhou, China as an example, establishes and validates a coupled 1D and 2D hydrodynamic model. Based on historical data, it constructs joint probability distributions of rainfall and tidal levels with different return periods and durations. Using the results from the coupled model under various design scenarios, it proposes an impact index to quantify the contributions of rainfall and tides to flooding. Furthermore, it quantifies the interactions between fluvial flooding and pluvial flooding. Flood-prone areas are delineated, and the causes of flooding are analyzed. The results show that when the return periods of rainfall and tide level are both 10 years, the Kendall return period for the combined event of rainfall and tide level is 36.35 years, greater than the “Or” return period (5.40 years) and less than the “And” return period (66.88 years). The impact degree index of rainfall on flooding varies between 0.5 and 1, with the minimum at 24-hour duration, indicating that the study area is primarily affected by rainfall and the influence of tide level is most significant at 24-hour duration. The pluvial flooding caused by the influence of river water level on the drainage outlet accounts for 19.08 % of the total volume at most. This shows that fluvial flooding affects the seriousness of pluvial flooding by influencing the water levels of outlets. The flood-prone area is divided into different regions based on the main natural factors (rainfall and tidal level) and social factors (pipeline network, drainage outlets, 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.