Observations of Flocs in an Estuary and Implications for Computation of Settling Velocity.

The settling velocity (ws) in estuarine environments can impact whether a region is eroding or accreting sediment on the bed, yet determining this rate can be an indirect process requiring a number of assumptions. Accurate determination of ws is especially needed for numerical models to reproduce observed sediment concentrations at the appropriate timescale. We collected information on suspended sediment flocculation at a channel site (13 m deep) and a shallows site (4 m deep) within South San Francisco Estuary, alongside timeseries of flow, wave statistics, turbulent shear, and bottle samples analyzed for both ws and particle size. Using the measurements of floc size and settling velocity, we performed a sensitivity analysis on the unknown parameters in the general explicit formula for settling velocity. The collected particle size distribution data show that multiple classes of flocs are present; these are characterized as flocculi, microflocs, and macroflocs. We show that ws of flocculi is closest to ws for the full distribution. The determined parameter values lead to near‐bed mass‐weighted settling velocities (standard deviation) of 1.18 (0.55) and 0.22 (0.15) mm/s at the channel and shallows sites, respectively. Modeling efforts can use this work to help select an appropriate sediment model and parameter values. Plain Language Summary: Tides and waves pick up mud on the bottom of estuaries and transport it between the river and ocean or between the shallows and the deep channel. One important control on this transport of mud is how fast it settles out of the water column. Larger things typically settle faster, and as small particles stick together into larger aggregates, or "flocs", the flocs settle faster than the original small particles. We collected data in the channel and shallows of San Francisco Estuary, and compute settling velocity using a range of tuning parameters. We use the observations to narrow the values of these parameters, and we show how the choice of some parameter values can be more important than others in determining settling velocity. In locations where sediment flocculates, this work will help modelers select appropriate models and parameter values. Key Points: The parameters needed to compute settling velocity are constrained using settling column dataMultiple floc classes are common, suggesting a multiple class model is necessaryThe characteristic size of particles in San Francisco Estuary is smaller than the size commonly observed with camera setups [ABSTRACT FROM AUTHOR]