Compound floods in Hong Kong: Hazards, triggers, and socio-economic consequences

Study region: Hong Kong is a megalopolis located on the southeast coast of China. Study focus: Megalopolis located in low-lying coastal areas such as Hong Kong is especially vulnerable to compound flood risks from multiple flood drivers (e.g., heavy rainstorms and extreme sea levels). Using long-term hydrological observations and socio-economic data, this study aims to reveal the risks, causes and socio-economic impacts of compound floods from precipitation and sea level in Hong Kong. New hydrological insights for the region: We find that the most extreme sea levels are mostly accompanied by heavy precipitation and thus lead to compound floods, while the most extreme precipitation events are less likely to coincide with extreme sea levels. TCs are the major trigger of compound floods with fractional contributions ranging from 48.8% to 83.8% in the gauges. Typhoon Wanda (1962) triggered the most severe compound flood with a joint return period of 178 years which is much shorter than the return period (>10,000 years) without the consideration of the dependence between the extremes. During the past decades, Hong Kong has experienced an increase in compound flood hazards, which is mainly attributed to the mean sea level rise. Socio-economic data reveals that TCs associated with compound floods are more destructive than TCs that induced single-type floods, highlighting the urgent need to establish a multi-hazard flood risk management strategy.