Estuarine Floc Mass Distributions from Aggregation/Disaggregation and Bed Sediment Exchange

McAnally, W. H., Mehta, A. J., Manning, A. J., & Forlini, C. (2025). Estuarine Floc Mass Distributions from Aggregation/Disaggregation and Bed Sediment Exchange. Journal of Marine Science and Engineering. 13(3), 615.

Estuarine benthos, among other lifeforms of interest to water quality, can be sensitive to size-distributed cohesive floc properties. In such a context, tide-dependent suspended floc mass distributions in the Tamar Estuary, UK are revisited. At the field site close to maximum turbidity, time-series of water level, current velocity, salinity, and suspended sediment concentration (SSC) were recorded in 1998 over several tidal cycles. Concurrently, at selected times and elevations, floc mass distributions were deduced from in-situ observations of SSC, floc settling velocities and diameters. A previously developed time-dependent model, revised to account for both multiclass floc aggregation/disaggregation and bed sediment exchange by erosion and deposition, is applied to simulate floc mass distributions during ebb/flood cycles on June 24 and August 5. Although the model does not account for the density effects of salinity intrusion or sediment advection, comparisons between the simulated and observed floc mass distributions show generally reasonable agreement on both days. It is argued that this agreement is due to the primary role of floc dynamics and secondary contribution of bed sediment exchange in governing floc properties. The effects of salinity and advection can be incorporated by coupling the model physics with a suitable multi-dimensional hydrodynamic code.