Sediment suspension, flocculation and settling over bio-physical cohesive substrates in saline water

Biologically-active mud-sand mixtures, commonly known as cohesive sediments, are ubiquitous in estuaries and continental shelves. Due to flocculation, cohesive sediments transport as flocs and their impacts to ecosystem and earth system are distinctly different from those of non-cohesive sediments. Typical floc samples generated from kaolinite clay, fine sand and xanthan gum (a natural proxy of extracellular polymeric substances (EPS)) were collected and analysed from a series of controlled laboratory flume experiments. Not only floc characteristics but also their interactions with flow turbulence in the boundary layer and bed deposition are presented. The results indicate that flocculation occurs pervasively and rapidly in all the cohesive experimental runs from within 0.5 hour of the start of the experiment. Pure mineral cohesion without xantham gum lead to higher suspended sediment concentration (SSCs) in the water column than that of mixed bio-physical cohesion. Bed elevations and turbulence kinetic energy (TKE) near the bed show more fluctuations in bio-physical cases with low qantities of xanthan gum. Moreover, in the low bio-physical cohesive condition, the flocs aggregation show a dominance in flocculation from 0.5 to 6 hours and then breakage increasing between 6 to 9 hours. With the addition of xanthan gum and the associated higher bio-physical cohesion, flocculation can be impeded by lower sediment entrainment and suspension from the substrates. The number of flocs, Macroflocs size, settling velocity and mass settling flux (MSF) have reductions compare with those of low bio-physical case. Finally, a conceptual schematic illustrating the processes of bio-physical cohesive sediment suspension, flocculation, settling, and deposition has been presented to emphasise the importance of taking flocculation and settling into consideration in future morphodynamics, sedimentation studies and predictive models