Abstract

Understanding how floating particles are transported by streaming waters is crucial in predicting the transport of plastic pollution, which is dramatically abundant in rivers, lakes, and oceans. Using particle tracking velocimetry, we investigate the motion of floating particles of different shape and size on the turbulent free-surface of a field-scale meandering stream. We consider two locations with different turbulence levels where the role of surface waves on the transport is deemed negligible. Millimetre-sized spheres are used as tracers to characterise the surface flow. These are compared with centimetre-sized discs and rods, much larger than the tracers, approximating typical-sized pieces of litter found in rivers. These larger particles exhibit similar velocities as the small tracers but filter out the extreme accelerations. Consequently, their motion is more time-correlated and their spreading rate is larger. This notion is confirmed by the mean-square displacement of single particles and mean-square separation of particle pairs, which grow faster in time compared to the tracers. The rotation of the rods, affected by a range of turbulent scales, reduces the correlation time scale of their translational motion, and leads to a slower dispersion compared to the discs, despite the rods' length being larger than the discs' diameter. Taken together, these results indicate that the motion of finite-size objects floating on non-wavy turbulent water is consistent with the behaviour of inertial particles in three-dimensional turbulence. These results can be valuable when constructing predictive models of floating plastics in natural waters.