Due to the flocculation process, suspended mud aggregates carried by rivers to the coastal ocean are thought to undergo changes in size and shape in response to environmental drivers such as turbulence, sediment concentration, organic matter (OM), and salinity. Some have assumed that salt is necessary for floc formation, and that mud, therefore, reaches the estuary unflocculated. Yet mud flocs exist in freshwater systems long before the estuarine zone, likely due to the presence of OM acting as a floc‐promoting binder. Therefore, it is important to consider how salinity affects flocculation, if at all, in the presence of OM. Here, we used experiments to examine the flocculation of a natural mud with and without OM. Results showed that the rate of floc growth and equilibrium size both increase with salinity regardless of the presence or absence of OM. However, the response of both to salinity was stronger when OM was present. In deionized water, natural sediment with OM was seen to produce large flocs. However, the size distribution of the suspension tended to be bimodal. With the addition of salt, increasing amounts of unflocculated material became bound within flocs, producing a more unimodal size distribution. Here, the enhancing effects of salt were noticeable at even 0.5 ppt, and increases in salinity past 3–5 ppt only marginally increased the floc growth rate and final size. Data from the experiment were used to develop a salinity‐dependent model to account for changes in floc growth rate and equilibrium size.
We conducted field work in South San Francisco Bay to examine cohesive sediment flocculation dynamics in a shallow, wave‐ and current‐driven estuarine environment. Drawing on data collected using a suite of acoustic and optical instrumentation over three distinct seasons, we found that the factors driving floc size variability differed substantially when comparing locally sourced sediment (i.e., through wave‐driven resuspension) to suspended sediment advected from upstream. Statistical analysis of our extensive field data revealed additional seasonal variability in these trends, with wave stress promoting floc breakup during the summer and winter months, and biological processes encouraging floc growth during the spring productive period. Combining these data with fractal dimension estimates, we found that seasonally varying floc composition can lead to differences in floc settling velocity by a factor of approximately two to five for a given floc size. Finally, by analyzing co‐located turbulence and sediment flux measurements from the bottom boundary layer, we present evidence that the relationship between floc size and the inverse turbulent Schmidt number varies with floc structure. These results can be used to inform sediment transport modeling parameterizations in estuarine environments.
more » « less- Award ID(s):
- 1736668
- NSF-PAR ID:
- 10390621
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 127
- Issue:
- 6
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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