Factors Controlling Mud Floc Settling Velocity in a Highly Turbid Macrotidal Fluvial-Estuarine System

This study assesses the settling dynamics of suspended sediments along the hyper-turbid Gironde Garonne fluvial-estuarine system, with an innovative optical SCAF instrument (System of Characterization of Aggregates and Flocs). Two fields campaigns were carried out to determine the settling velocity and properties of suspended sediments during a semi-diurnal tidal cycle, as well as hydrodynamic conditions and water properties. The two sampling stations were representative of two regions: a tidal river dominated by fresh water and an estuary affected by salty or brackish waters. A high spatial variability of the settling velocity was observed along the fluvial-estuarine system and vertically along the water column. Settling velocities ranged from 0.02 to 0.4 mm/s. This study confirms that in hyper-turbid systems, the suspended sediment concentration (SSC) is predominantly driving the settling dynamics of suspended sediment. Threshold concentrations have been defined for the flocculation and hindered regimes where the settling velocity may vary by one order of magnitude. Although in natural environments it is difficult to distinguish between the effects of SSC and turbulence (as they are correlated), in the Gironde-Garonne system the turbulent shear G seems to affect the settling of suspended sediment to a lower extent. Settling velocity variations cannot be directly correlated to salinity or organic matter content. Despite differences in hydrodynamic and environmental conditions in fluvial and estuarine regions, a common prediction law has been found to estimate settling velocity of suspended sediment as a function of suspended sediment concentration. Estuaries and rivers are biotically rich environments strongly impacted by human activities. Mud trapping capacity of such systems has a major influence on water quality by reducing light availability, promoting oxygen depletion and by trapping adsorbed contaminants, bacteria and nutrients. A key dynamical parameter impacting the trapping of mud is the sediment settling velocity. Sediment settling is influenced by a wide range of environmental factors such as salinity, sediment concentration, turbulence of the flow and organic matter. This manuscript presents sediment settling data from field surveys carried out along the Garonne River-Gironde Estuary system (France), where large quantities of mud are trapped during the dry season. It highlights the driving role of sediment concentration on the settling dynamics along the entire system, despite the hydrodynamics and water properties of riverine waters differing from the estuarine waters. The turbulence of the flow appeared to be of secondary importance. An unique empirical prediction law has been established for the whole system contrary to other systems around the world. An improved understanding of sediment fluxes contributes to effective waterways management and the preservation of essential ecological environments. In such hyper-turbid system, the suspended sediment concentration predominantly drives the settling dynamics of suspended sediment The turbulent shear appeared to be a control parameter of secondary importance A common prediction law can be found for the whole system