Settling and Rising Dynamics of River Litter

James Lofty , Daniel Valero, Mário Franca

Global assessments of river litter transport, accumulation, and export to the oceans remain constrained because the particle-scale hydrodynamic variables governing litter movement are currently unknown. We resolve this by explaining the vertical dynamics of full-scale river litter in quiescent water through multi-camera, three-dimensional trajectory reconstructions of over a thousand litter items spanning 24 River-OSPAR categories, representing ~80% of observed riverine litter. Our results show that settling and rising velocities span three orders of magnitude (−82.1 to −0.29 cm s⁻¹ and 0.90 to 143.3 cm s⁻¹, respectively), indicating large variability in transport among litter categories. Despite the wide variability of velocities, drag coefficients were relatively consistent, with a median value of 1.03 and interquartile range spanning 0.45 to 1.72. We identify four trajectory regimes – straight vertical, straight oblique, nonlinear vertical, and nonlinear oblique – establishing a regime map based on particle Reynolds number and litter anisotropy. We further propose a mechanistic explanation for the four regimes, arising from the coupled effects of wake dynamics and litter physical properties. These results provide the parameterisation required for predictive, physics-based models of river litter transport.