Investigation of flocculation dynamics under changing hydrodynamic forcing on an intertidal mudflat

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.
Marine Geology
Chao Guo, Qing He, Bram C. van Prooijen, Leicheng Guo, Andrew J. Manning, Sarah Bass