It’s well established that rivers, particularly large, lowland rivers, transport huge volumes of mud, that is ultimately deposited and builds floodplains, deltas, and offshore environments. However, despite being critical for our understanding of the construction and evolution of alluvial and coastal landscapes, the controls on the flocculation, transport and sedimentation of mud transported in rivers remain relatively unconstrained. Few field studies have explored the physical and chemical properties of flocs in freshwater environments, or have examined how flocs responds to variations in
water chemistry, turbulent mixing, and sediment concentration which are thought to be the primary controls on floc size, and thus settling velocity. The spatially dynamic physical and chemical nature of the alluvial to marine transition zone over which a river flows into the ocean provides an ideal
opportunity to explore the controls on the size and settling rate of mud flocs carried in suspension.
We, a team consisting of researchers from Rice University and Virginia Tech, conducted a field investigation in the Mississippi River Delta in the South Pass and Southwest Pass channels to investigate the controls on the size and sedimentation of flocculated, suspended sediment across the fluvial-marine transition. Along a longitudinal profile of the river, we move from a purely fluvial, freshwater setting out into a fully saline ocean setting, taking in-situ measurements of floc size, flow velocity, and suspended sediment concentration. Preliminary results indicate that the onset of flocculation occurs in freshwater, and that size of flocs is relatively unaffected by sharp changes in
water chemistry due to intrusions of saline, marine water up the river channel. As such, the sedimentation of flocculated mud in rivers appears to be primarily driven by the reduction in fluid shear stress due to marine water intrusions, rather than by changes in floc size and settling velocity due to shifts in water salinity as previously hypothesized. This finding is both supportive of, and supported by, laboratory studies which demonstrate that the flocculation of fine-grained, cohesive sediment occurs in freshwater, even without additions of salt and/or organic material beyond what is typically dissolved in tap water, as well as provides empirical evidence to support prior studies which have inferred that fine-grained, cohesive sediment is transported in rivers as flocs from compilations of suspended sediment data across a range of rivers.
Using a floc camera, we can actually see up close what the suspended sediment looks like and measure the size and concentration of mud flocs being transported by the flow.
PREPRINT AVAILABLE UPON REQUEST
Kieran Dunne 