A Mechanistic Model for Mud Flocculation in Freshwater Rivers

The transport and deposition of mud in rivers are key processes in fluvial geomorphology and biogeochemical cycles. Recent work indicates that flocculation might regulate fluvial mud transport by increasing mud settling velocities, but we lack a calibrated mechanistic model for flocculation in freshwater rivers. Here, we developed and calibrated a semi-empirical model for floc diameter and settling velocity in rivers. We compiled a global data set of river suspended sediment concentration-depth profiles and inverted them for in situ settling velocity using the Rouse-Vanoni equation. On average, clay and silt (diameters <39 mu m) are flocculated with settling velocity of 1.8 mm s(-1) and floc diameter of 130 mu m. Among model variables, Kolmogorov microscale has the strongest positive correlation with floc diameter, supporting the idea that turbulent shear limits floc size. Sediment Al/Si (a mineralogy proxy) has the strongest negative correlation with floc diameter and settling velocity, indicating the importance of clay abundance and composition for flocculation. Floc settling velocity increases with greater mud and organic matter concentrations, consistent with flocculation driven by particle collisions and binding by organic matter which is often concentrated in mud. Relative charge density (a salinity proxy) correlates with smaller floc settling velocities, a finding that might reflect the primary particle size distribution and physical hosting of organic matter. The calibrated model explains river floc settling velocity data within a factor of about two. Results highlight that flocculation can impact the fate of mud and particulate organic carbon, holding implications for global biogeochemical cycles.