Effect of Shape and Size on the Transport of Floating Particles on the Free Surface in a Natural Stream

• Published in: Water resources research Vol. 59; no. 10
• Main Authors: Sanness Salmon, Henri R., Baker, Lucia J., Kozarek, Jessica L., Coletti, Filippo
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01-10-2023
• Subjects: Approximation; Channel flow; Computer models; Diameters; Floating; free surface turbulence; Free surfaces; Lakes; Litter; Mathematical models; Meandering; Meandering streams; Movement; natural stream; Natural streams; Natural waters; Oceans; Open channel flow; Open channels; particle dispersion; Particle tracking; Particle tracking velocimetry; Plastic debris; Plastic pollution; Pollution; Pollution dispersion; Prediction models; Rivers; Rods; Seafloor spreading; Shape; Shape effects; Stream pollution; Streaming; Surface flow; Surface waves; Tracers; Translational motion; Turbulence
• DOI: 10.1029/2023WR035716
• EISSN: 1944-7973
• ISSN: 0043-1397

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 different locations, where the role of surface waves on transport is deemed negligible. Millimeter‐sized spheres are used as tracers to characterize the surface flow. These are compared with centimeter‐sized discs and rods, approximating typical‐sized pieces of floating litter. The larger particles exhibit similar mean and fluctuating velocities as the tracers but filter out the extreme turbulent accelerations. Consequently, their motion is more time‐correlated and their spreading rate is larger. This behavior is also confirmed by complementary laboratory measurements in an open channel flow. 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‐sized objects floating on the surface of weakly wavy turbulent waters is consistent with the behavior of inertial particles in three‐dimensional turbulence. These results can be valuable when constructing predictive models of floating plastics.
Plain Language Summary
Plastic debris is a rising global issue severely affecting the state of our rivers, lakes and oceans. Understanding, how pieces of litter, often floating, travel in streaming waters is crucial for predicting and ultimately limiting plastic pollution. The main goal of this research is to investigate how the shape and size of small floating objects may affect their journey on the surface of water. To this end, we use high‐speed imaging to track floating objects of different shape and size in an outdoor stream laboratory. The motion of centimeter‐sized discs and rods, approximating typical pieces of plastics found in rivers, is directly compared to the motion of millimeter‐sized spheres that follow the surface flow. We find that the larger discs and rods spread faster on the surface of water. Not only can these results be used to devise effective sequestration strategies, but they can be important for computer models that predict the abundance, and fate, of plastic litter in natural waters.
Key Points
The velocity of floating particles in turbulent streams is weakly affected by their shape and size
Larger particles disperse faster on the free surface due to their ability to filter out small‐scale turbulent fluctuations
Rods re‐orient following the mean shear of the surface flow and rotate according to the integral scales of the free surface turbulence