Observations of fine sediment flocculation in the turbidity maximum of the Changjiang Estuary

Better understanding of sediment flocculation will better help to predict fine sediment transport. Multiple advanced instruments were employed to get the field observations in the turbidity maximum of the Changjiang Estuary. Floc pictures were taken by a scanning electron microscope in the laboratory. The sediment flocculation dynamics and the floc characteristics are investigated in this paper. The results show that the morphology and size of flocs show obvious tidal variations. There are diverse flocs in each tidal cycle, mainly including loose, porous and dense flocs, which are mainly comprised of silt and clay. They have indistinct or distinct shapes, rough or less rough surfaces, incompact or compact structures. The floc size is highly related to the tidal processes. The average floc size (94.8 μm) during water slacks is larger than that (24.1 μm) during maximum floods and ebbs, owing to the lower flow velocity during water slacks. The average floc size (105.6 μm) during high water slacks is larger than that (83.9 μm) during low water slacks due to the higher salinity during high water slacks. The average floc size (74.7 μm) during neap tide is larger than that (44.3 μm) during spring tide, which can be attributed to the lower flow velocity, the higher salinity and the finer primary particle during neap tides. Additionally, the floc size increases gradually from the surface to the bottom. The turbulent shear stress is the controlling factor for sediment flocculation. The maximum flocs can be formed under the condition of mid-low flow velocity and the optimal flow velocities for the maximum flocs during neap and spring tides are 0.41 and 0.56 m/s, respectively. The floc size decreases significantly with the increase of turbulent shear when the turbulent shear rate parameters during neap and spring tides are higher than about 3 and 4 s−1, respectively. The conclusions can help to extend our understanding of flocculation dynamics and help to predict the fine sediment transport.