Floc variability under changing turbulent stresses and sediment availability on a wave energetic muddy shelf

The interplay between cohesive sediment flocculation, turbulence, and sediment availability is studied using field observations collected on the muddy Atchafalaya inner shelf, Louisiana, USA. Observations were made at two sites, characterized by surficial sediments of different grain size. At the coarse-sediment site, a negative correlation between floc size and suspended sediment concentration was observed for the first time in a field experiment under energetic wave conditions. This agrees with previous laboratory studies and field observations, and suggests that strong near-bed turbulence associated with significant sediment resuspension also results in the breaking of macro-flocs (230-μm230-μm mode). At the fine-sediment site, the relative abundance of clay and fine silt available for flocculation appears to promote persistent macro-flocs of approximately 300-μm300-μm. The observations support the assumption that sediment supply and surficial sediment properties, in addition to turbulence and suspended sediment concentration, play an important role in flocculation.

► Flocculation under varying turbulence and sediment availability is observed.
► Floc size is inversely proportional to turbulence and SSC in wave-energetic conditions.
► Floc mode is at a larger size at the fine sediment site than the coarser-sediment site.
► Sediment supplies (river and seafloor) play an important role in flocculation.