Compound flood potential from storm surge and heavy precipitation in coastal China

Abstract. The interaction between storm surge and concurrent precipitation can cause greater flooding impacts than either in isolation. This paper investigates the potential compound effects from these two flooding drivers along the coast of China. Statistically significant dependence between them exists at the majority of locations that are analysed, but the strength of the correlation varies spatially and depending on how extreme events are defined. In general, we find higher dependence at the south-eastern tide gauges (TGs) (latitude < 30° N) compared to the northern TGs. Seasonal variations in the dependence are also evident. Overall there are more sites with significant dependence in the typhoon season, especially in the summer. Accounting for past sea level rise further increases the dependence between flooding drivers and future sea level rise will hence likely lead to an increase in the frequency of compound events. We also find notable differences in the meteorological patterns driving compound and non-compound events. Compound events at south-eastern TG sites are caused by low-pressure systems with similar characteristics across locations, including high precipitable water content (PWC) and strong winds that generate high storm surge. Based on historical disaster damages records of Hong Kong, compound flood events account for the vast majority of damages and casualties, compared to univariate flooding events, where only one flooding driver occurred. Given the large coastal population and low capacity of drainage systems in many Chinese urban areas, these findings highlight the necessity to incorporate compound flooding and its potential changes in a warming climate into risk assessments, urban planning, and the design of coastal infrastructure and flood defences.