Evolution of tidal flats influenced by tidal bores and runoff in the Qiantang Estuary, China

Tidal flats provide crucial ecosystem services and coastal protection. Comprehending the evolution of tidal flats, influenced by tidal bores and runoff, is key to protecting these environments in the Qiantang Estuary′s tidal bore zones. Based on 151 observations of the Jianshan mudflat terrain, taken approximately every two days, the research explored the mudflat′s evolution. Utilizing water level, suspended sediment concentration, river discharge, and remote sensing data, the study further investigated the driving mechanisms behind this evolution. The results are as follows : ① The Jianshan mudflat maintained a dynamic equilibrium. In the observation area, the dry-season deposition rate was 0.01 m/d, with the slope stabilizing at 1‰. During the flood season, the mudflat bank retreat rate reached 3 m/d, causing a retreat width of 113 m. After the flood season, the mudflat recovered at a rate of 0.05 m/d. ② During the flood season, the intensity of tidal bores slightly increased. Flood events lead to a bifurcated river regime, directing the main sediment transport of tidal bores through the southern channel, thereby reducing sediment delivery to the tidal flats in the northern channel. ③ Qiantang River runoff was the key factor controlling seasonal erosion and deposition changes. The evolution rate of the tidal flats showed a multivariate linear relationship with river runoff, initial elevation, and tidal range. ④ Floods altered the river regime, reducing sediment supply to the tidal flats. Runoff and ebbing tides combined to cause bank erosion and retreat. However, when water erosion decreased and sediment transport by tidal bores and rising tides became dominant, the tidal flat rapidly recovered.