Field Investigation of Cohesive Sediment Flocculation and Sedimentation Across the Fluvial to Marine Transition

Abstract:

Despite being critical for our understanding of the construction and evolution of fluvial 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 fluvial 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.

A field investigation is being conducted in the Mississippi River Delta in the South Pass channel 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 flow velocity 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 focculation 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.