The multi-decadal morphodynamic changes of the mouth bar in a mixed fluvial-tidal estuarine channel

Inner-channel mouth bars (IMB) are common and vital geomorphological structures in estuaries that can efficiently promote the progradation of a fluvial delta. However, these significant structures face serious interferences by natural and human forces. This study mainly focuses on IMB deposited in the mixed fluvial-tidal dominated South Passage (SP) of the Changjiang Estuary between 1959 and 2018 to reveal how multidecadal morphodynamic variability in a mouth bar responds to natural forcing and human interferences. The results show that the volume of the IMB in the SP increased from 1128 × 10⁶ m³ in 1959 to 1636 × 10⁶ m³ in 1989 and then decreased to 462 × 10⁶ m³ in 2018. Meanwhile, the evolution of the IMB could be divided into five stages, the growing phase (1959 to 1979), which showed a ‘V’ shape, the partial adjustment phase (1999–2003), which showed a ‘crest-dent’ shape, the stable phase, which showed a flat shape (2004–2010), the partial adjustment phase (2011–2017), when Jiangyanan Shola inserted into the SP, and finally evolved into a flat shape in 2018. Fluvial water discharge and suspended sediment discharge (SSD) did not control the variation in the mouth bar. ENSO events were normally responsible for not only the periodic variations in the landward slope and the water depth of the crest of the IMB but also occasional extreme changes in the IMB. Moreover, local sandbar insertion, sediment from the seaside induced by strong tidal power and intensified engineering projects resulted in the continual shrinkage of the mouth bar by depositing more sediment at the mouth bar. Our work implies that understanding and governing this IMB will bring additional economic benefits to this fast-developing society, but the protection of this delicate estuarine geomorphology system should still receive great attention.