Investigation of the variability of floc sizes on the Louisiana Shelf using acoustic estimates of cohesive sediment properties

• Sediment concentration and floc size are estimated from acoustic backscatter.
• Floc size increases toward the bottom except within the bottom few-cm.
• Flocculation is promoted by low concentrations and low turbulence.
• When turbulence is strong, floc size is negatively correlated to SSC and shear rate.

Vertical profiles of suspended sediment concentration and floc size estimated from acoustic backscatter are used together with turbulent shear estimates to investigate cohesive sediment flocculation under different flow conditions. Concentration measurements by optical backscatter sensors at two levels are used to calibrate the acoustic backscatter intensity. A strong correlation is observed between suspended sediment concentration and turbulence intensity. Mean size of flocs increases toward the bottom except within the bottom few centimeters, where floc breakage is observed due to strong turbulence. Simultaneous shear rate and floc size profiles show that low shear promotes flocculation at low concentrations. Increasing turbulence intensity increases the amount of sediment in the water column, but decreases the floc size, indicating floc breakage. Thus, these effects are seen at cm-scale vertical resolution in the field for the first time. The results support previously published numerical, experimental and lower resolution field studies.