2026 CSDMS meeting-047
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Field evidence for mud as flocculated bed-material load: Insights into improving fluvial mud transport predictions
Justin Nghiem,
University of Minnesota Minneapolis Minnesota, United States. justinnghiem@gmail.com
Gen Li, UC Santa Barbara Santa Barbara California, United States.
Gerard Salter, California Institute of Technology Pasadena California, United States.
Kyle Wright, ETH Zürich Zürich , Switzerland.
Paola Passalacqua, ETH Zürich Zürich , Switzerland.
Michael Lamb, California Institute of Technology Pasadena California, United States.
Mud dominates the sediment load in many rivers and deltas, but modeling mud transport is difficult because cohesion impacts mud entrainment and deposition via complex mechanisms. Here we present field evidence from the Wax Lake Delta, Louisiana and a global compilation of alluvial rivers to show that in situ flocculation—the aggregation of mud grains into larger particles—provides a mechanistic framework to predict river mud transport beyond empirical equations. Fitting the Rouse-Vanoni equation to suspended sediment concentration-depth profiles shows that river mud ubiquitously has enhanced settling velocity likely due to flocculation. This finding challenges the idea that river mud is washload—fine particles with such exceedingly slow settling velocity that they are washed passively through the river, absent from the riverbed, and set by external inputs rather than local bed sediment entrainment. Instead, we hypothesize that enhanced floc settling velocity causes mud to behave as bed-material load in which active sediment exchange between the bed and water column determines sediment concentration. To test this hypothesis, we conducted fieldwork in the Wax Lake Delta. In situ turbidity sensors and concentration-depth profiles show that mud concentration varies temporally and spatially in response to shear stress variations, consistent with bed material load. Furthermore, mud is present in bed. Bed-material entrainment theory explains mud concentrations using a formulation that accounts for floc growth and densification near the bed. Our results reveal that mud fluxes can be predicted in sediment transport models using local flow conditions, bed grain size distribution, and floc settling velocity.
