Patterns and controlling factors of seasonal erosion and accretion of estuarine tidal flat in the Minjiang River estuary

Tidal flats are fine-grained sediment deposits formed under tidal action, widely distributed worldwide, and play a crucial role in carbon sinks and coastal protection. With the reduction of sediment flux from river to ocean, patterns and factors of erosion-accretion in estuarine tidal flats have been changed, which directly affects the function of tidal flats. It is urgent to study the short-term erosion-accretion process of estuarine tidal flats to provide scientific supports to understand functions of tidal flats. The tidal flat of Langqi Island at the Minjiang River estuary in Fujian Province was scrutinized in terms of the sediment dynamic parameters (including: inundation height, tidal current velocity, wave height, suspended sediment concentration, and grain size distribution) and on-site observations in different seasons. Results show that, the surficial sediments of the tidal flat are mainly composed of fine particles of silt and clay in summer, and coarse particles of sand and silt in winter. The marsh areas covered with Spartina alterniflora in the upper intertidal zone were characterized by mainly continuous accretion, while the middle-lower intertidal zone exhibited periodic erosion and accretion. The general pattern of erosion-accretion in the tidal flats was characterized by accretion in summer and erosion in winter, and the variations in erosion and accretion in the middle intertidal zone were greater than those in the lower intertidal zone. The tidal current velocity, wave height, and suspended sediment concentration in winter were all greater than those in summer. In winter, the net flux of suspended sediment near bottom was mainly transported seaward within a tidal cycle, while in summer, it was mainly transported landward. Therefore, the seasonal variations of erosion-accretion pattern of the tidal flats in Minjiang River estuary were mainly controlled by hydrodynamic processes. In winter, strong wave action caused significant resuspension of surficial sediment and seaward transportation by tidal currents, leading to erosion of the tidal flat; in summer, the wave action was weak, and the suspended sediment settled due to low bottom shear stress and was transported landward by tidal current, leading to accretion in the intertidal zone.