Flocculation dynamics of suspended particulate matter under various tidal conditions

Understanding the flocculation dynamics of suspended particulate matter is essential for a comprehensive understanding of sediment transport in estuarine and coastal ecosystems. Field observations were conducted during both spring and neap tidal cycles at two contrasting sites, the highly turbulent, low-salinity Xuliujing site of the Yangtze River estuary, and the weakly turbulent, high-salinity Belgian coastal station MOW1. The two sites exhibited different flocculation dynamics and floc size distributions (FSDs). At Xuliujing, strong river discharge and pronounced ebb dominance intensified turbulent shear, making the fragmentation of flocs the governing process. This resulted in multimodal FSDs with high proportions of microflocs (∼13 μm) and macroflocs (∼55 μm). In contrast, at MOW1, high salinity and relatively weak, symmetric tidal currents favored salt-enhanced aggregation, resulting in larger, more stable flocs and more uniform FSDs. These contrasts indicate that in freshwater environments, turbulence controls both aggregation and breakup, whereas in saline waters, salinity governs aggregation and turbulence primarily limits the maximum size of flocs. Furthermore, a one-dimensional vertical hydrodynamic model coupled with a population balance flocculation model demonstrated satisfactory accuracy in simulating current velocities, suspended particulate matter concentrations, and FSDs at both sites, showing its capability to capture flocculation dynamics under different environmental conditions.