A Laboratory Study of the Effects of Size, Density, and Shape on the Wave-Induced Transport of Floating Marine Litter

Floating marine litter is transported by several mechanisms, including surface waves. In studies of marine litter transport, the wave-induced drift is set to be equal to the Stokes drift, corresponding to the Lagrangian-mean wave-induced drift of an infinitesimally small tracer. Large-scale experiments are used to show how the wave-induced drift of objects of finite size depends on their size, density, and shape. We observe increases in drift of 95% compared to Stokes drift for discs with diameters of 13% of the wavelength, up to 23% for spheres with diameters of 3% of the wavelength, whereas drift is reduced for objects that become submerged such as nets. We investigate what these findings may imply for the transport of plastic pollution in realistic wave conditions and we predict an increase in wave-induced drift for (very) large plastic pollution objects. The different extrapolation techniques we explore to make this prediction exhibit a large range of uncertainty. In transport models for floating marine litter, surface wave effects are often included by simply superimposing their Stokes drift (the small net drift induced by waves) upon wind-driven flows and currents. However, size and shape can be important factors determining a floating marine litter object's wave-induced drift, which generally increases with object size. Large-scale experiments are used to show how the wave-induced drift of floating marine litter objects of finite size depends on their size, density and shape. We observe increases in drift of 95% compared to Stokes drift for discs with diameters of 13% of the wavelength, up to 23% for spheres with diameters of 3% of the wavelength, whereas drift is reduced for objects that become submerged such as nets. We investigate how these findings may affect the transport of plastic pollution in realistic wave conditions, where we believe there will be an increase in wave-induced transport for (very) large plastic pollution objects, although further research is needed to reduce uncertainty. For large objects, size and shape can determine a floating marine litter object's wave-induced drift, which generally increases with size Increases in drift of 95% compared to Stokes drift have been observed in experiments for discs with diameters of 13% of the wavelength Plastic from the ocean exhibited an increase in drift with size unless objects become submerged, such as nets which had decreased drift