High‐Tide Flooding Along the China Coastline: Past and Future

Abstract

AbstractCoastal flooding has become a major issue for low‐lying coastal cities in China, and a lot of research has focused on assessing flood risk from storms and associated extreme sea levels. High‐tide flooding (HTF), however, which leads to minor inundation and occurs more frequently as sea level continues to rise has not been assessed comprehensively. Here, we analyze HTF along the China coastline using tide gauge records. We show that the frequency of HTF has increased (Xiamen doubled HTF frequency every 11.5–37.4 years, with the median being 17.5 years). As a result, the cumulative loss ratios of HTF are higher than those of major (or extreme) flooding events in several locations. To gain insights into the processes driving HTF changes, we decompose still water levels that occurred during HTF into five components, including non‐linear trend, interannual to decadal mean sea level (MSL) variability, seasonal MSL cycle, tidal anomaly, and nontidal residuals. It is evident that due to sea‐level rise (SLR) fewer components need to combine to raise the water levels above HTF thresholds. We show that the South China Sea coast already experiences HTF purely driven by high spring tides, and will also see the fastest future increase in the number of tide‐only HTF events. In general, China will experience more HTF days under all warming scenarios as SLRs. This long‐term trend will be modulated by the nodal cycle of ocean tides leading to more rapid increases in HTF in the 2030s and 2050s.