Flocculation regulates suspended sediment transport in estuarine fronts

This study examines suspended sediment transport affected by flocculation settling in a highly stratified tidal estuary. In situ observation recorded two estuarine front passages during strong‐ebb and flood‐slack tides, respectively. The strong‐ebb front enhanced turbulence, increasing sediment concentrations (~ 5 ppm), macrofloc size (~ 300 μm) and settling efficiency. The surface plume combined with middle layer together thickened from 1.5 to 2.5 m, facilitating particles mixing. The plankton and organic debris were transported into bottom water, forming fluffy, porous macroflocs. In contrast, the flood‐slack front developed stable middle barrier layer (~ 2 m) that restricted mixing and organic input, leading to denser, compact flocs (~ 150 μm) in bottom waters. Floc size distributions shifted from microfloc‐dominated to macrofloc‐dominated during strong‐ebb front passing, then returned to smaller, denser flocs. Dissolved‐oxygen changes due to sediment oxygen depletion varying. These findings help to improve predictive models for sediment transport and hypoxia dynamics in estuarine systems.