Tide-driven controls on maximum near-bed floc size in a tidal estuary

• Reynolds shear stress found to be the primary factor in floc size variations.
• Effect of eddy size on maximal near-bed floc size was clearly shown.
• Due to river morphology different Reynolds flux variations with SSC was found.

The evolution of the near-bed maximal floc size and of the response of a population of flocs to the fluctuations of the turbulence field, the suspended sediment concentration (SSC) and salinity was observed in mid reaches of a shallow tidal channel through a four-day campaign during the flow reversal period. Flocculation process was found to be controlled predominantly by the turbulent motion and the SSC. Maximum floc size was observed through the first half of flooding tide which is normally associated with the resupension of the loosely settled sediments (SSC > 4 FTU) and the presence of relatively intense shear stresses (<10 cm2/s2). Turbulent shear over lengths governed by Kolmogorov length scale λk escalated the maximum floc size when λ k< 500 μm. Salinity did not appear to necessarily enhance flocculation. Using an Acoustic Doppler Velocimeter (ADV) to simultaneously measure the turbulent diffusivities and SSC at 12 points along the estuary provided non-intrusive approach to measure the settling velocity of cohesive sediments in the field. Two distinct settling behaviors were distinguished and were mainly attributed to the channel morphology and its influence on inducing upward velocity fluctuations.