Sediment resuspension, flocculation, and settling in a macrotidal estuary

Estuarine boundary layer and water column in situ measurements of hydrodynamics, sediment resuspension, and sediment particle size distribution are presented for a macrotidal environment in SE China. Vertical and tidal variability of sediment size and its relationship with turbulence and hydrodynamic forcing are examined using time series from two week long experiments after they are phase-averaged to reconstruct typical neap and spring tidal cycles. In situ particle size distributions obtained using laser diffraction show clear evidence of flocculation processes that change dynamically during the tidal cycle. Mean particle size of particles in suspension is found to be one order of magnitude larger than the primary size of the sediment. The coarser particles in suspension were present in the upper water column, whereas the finer particles were confined predominantly within the bottom boundary layer. Correlation analysis indicated that aggregate size appears to be controlled by turbulence more than any other parameters with floc size being inversely related to turbulence dissipation, while settling velocity of aggregates being proportional (on a log scale) to turbulence dissipation. Simple statistic and dynamic models incorporating the turbulence parameter are adopted and compared with previously developed models; the comparative study using our data sets shows that the dynamic model of Winterwerp (1998) as modified by Law et al. (2013) to include advection qualitative captures both the tidal and vertical variability of aggregate size.