In situ floc size and turbulent shear stress were measured together with suspended sediment concentration to investigate the floc properties under changing hydrodynamic forcing over the intertidal mudflat. A tripod system was established in the field for a period of approximately one month, including ~ 6 days of stormy conditions in the middle of the investigation period. Mean floc size exhibited strong temporal variations within a tidal cycle, and inverse relationship was found between mean floc size and shear stress. Suspended sediment concentration (SSC) can modulate the flocculation dynamics when shear stress decreases down to enhancing flocculation. Asymmetrical behaviors of floc sizes between flood and ebb phases were identified, with overall larger floc sizes in flood than in ebb tide under the same shear stresses. Floc structure showed different properties under calm and stormy conditions, and the variable fractal dimension and variable primary particle size were more convincing in simulating the variation of floc effective density with mean floc size during the storm period, which was inferred to be related to the resuspension of bed sediment as well as organic matter. A total of 110 mm bed erosion was measured during the storm, and erosion events occurred only around low water, due to the high current-wave combined bed shear stress and off-shore current. After the storm, ~ 40% of the erosion recovered within one week, and the fast settling of large flocs around high water plays significant role in the deposition process, leading to ~ 60% of the recovery.

In order to improve our understandings of temporal and vertical variations of sediment flocculation dynamics within the turbidity maxima (TM) of the highly turbid Yangtze Estuary (YE), we deployed LISST-100C, a laser instrument for in-situ monitor of the sizes and concentrations of flocculated particles in a wet season. Field data in terms of vertical profiles of flow velocity, suspended sediment concentration (SSC), salinity, flocculated particle size distribution and volume concentration were obtained, based on field works conducted at consecutive spring, moderate, and neap tides. Data analyses show that the mean floc diameters (D_M) were in the range of 14–95 μm, and flocculation exhibited strong temporal and vertical variations within a tidal cycle and between spring-neap cycles. Larger D_M were observed during high and low slack waters, and the averaged floc size at neap tide was found 57% larger than at spring tide. Effective density of flocs decreased with the increase of floc size, and fractal dimension of flocs in the YE was mainly between 1.5 and 2.1. We also estimated the settling velocity of flocs by 0.04–0.6 mm s⁻¹ and the largest settling velocity occurred also at slack waters. Moreover, it is found that turbulence plays a dominant role in the flocculation process. Floc size decreases significantly when the shear rate parameter G is > 2-3 s⁻¹, suggesting the turbulence breaking force. Combined effects of fine sediment flocculation, enhanced settling process, and high sediment concentration resulted in a large settling flux around high water, which can in part explain the severe siltation in the TM of the YE, thus shedding lights on the navigation channel management.