Spatiotemporal evolution and return period analysis of compound floods along the Huangpu River in Shanghai under typhoon influence

To reveal the risk evolution characteristics of compound floods in typical riverine areas of mega coastal cities, this study utilized observed rainfall, tidal level, and typhoon data from three representative stations along the Huangpu River in Shanghai from 2002 to 2024. The peaks-over-threshold method was employed to identify extreme and compound flood events. Copula-based joint probability models were constructed to systematically analyze the spatiotemporal patterns, spatial heterogeneity, and the response characteristics of compound floods to typhoons. The results show that: ①Compound flood events are predominantly concentrated from July to September, exhibiting a spatial differentiation pattern of "rainfall-dominated type in the upper reaches and rainfall-tide compound type in the middle and lower reaches." In recent years, both the frequency and intensity of compound floods have increased significantly, with the most pronounced risk escalation in the upper reaches; ②More than one-third of compound flood events are associated with typhoons, which significantly alter the joint risk structure by intensifying extreme tidal levels or rainfall, with tidal effects dominating in the lower reaches and rainfall effects dominating in the upper reaches; ③The return periods of compound floods show significant spatial differences across river segments, with typhoon-induced storm surge and typhoon rainfall dominating the return periods in the lower and upper reaches, respectively. These findings provide important scientific references for the risk identification and prevention of compound floods in mega coastal cities.