Floating ocean plastic can get a boost to its wave-induced transport because of its size

Plastic pollution and other ocean debris are a complex global environmental problem. Every year, ten million tonnes of plastic are estimated to be mismanaged, resulting in entry into the ocean, of which half will float initially. Yet, only 0.3 million tonnes of plastic can be found floating on the surface of the ocean. Where has the rest of the plastic gone?

The key mechanisms for plastic transport are currents, wind, and waves. Currents and wind transport ocean debris in a straightforward manner like the forces on a sailing boat. However, ocean waves predominantly move objects in circular-like orbits. The orbits do not quite close, resulting in a so-called Stokes drift in the direction in which the waves travel.

A joint team from the Universities of Oxford, Plymouth, Edinburgh, Auckland and TU Delft have investigated how waves transport floating ocean debris while including, for the first time, the effects of an object’s size, buoyancy, and inertia on its transport. Their results are published in The Journal of Fluid Mechanics.

Larger objects being transported faster than smaller objects was an unintuitive result.

Dr Ross Calvert from the University of Oxford’s Department of Engineering Science and his co-authors found that larger floating ocean debris can be transported at a rate faster than Stokes drift due to inertial effects.

The Stokes drift induced by waves has been shown to be important for the movement of ocean debris towards the coast, resulting in plastic beaching, which may be where some of the unaccounted-for plastic pollution is. It has also been shown to increase plastic pollution being transported to polar regions.

Very small objects will trace exactly what the water does and are thus transported with the exact Stokes drift.

Dr Calvert said: ‘Larger objects being transported faster than smaller objects was an unintuitive result. We expected inertia to reduce the speed at which floating debris was transported in waves, analogous to wind and currents. After checking our result experimentally and numerically, we then went on to discover the mechanisms by which these inertial objects moved faster than the water around them.’

After observing that larger floating plastic spheres were transported faster than smaller ones in the COAST wave flume at the University of Plymouth, the team developed a model to further investigate the result.

Through this model, which included gravity, buoyancy, drag and added-mass forces in a coordinate system that rotated and translated with the wave, they found that object size relative to the wavelength was the predominant driver for a change in transport, with a secondary effect from the density of the object.

Prof Ton van den Bremer at University of Oxford and TU Delft, who directed the research, said: ‘Although anyone walking on the beach will know waves transport floating debris towards the shore, the rate at which they do so depends on many factors that existing models, which are highly simplified, ignore. Examples of such factors are whether waves break and the size of the floating debris. This research provides a theoretical underpinning for the latter.’

This research is the start to understanding the mechanisms for an increase in wave-induced drift. Further study into the effect of object shape, including wave-flume and numerical testing of idealised and real ocean debris, are underway.

The research was supported by the Royal Academy of Engineering.

‘A mechanism for the increased wave-induced drift of floating marine litter’ is published in the Journal of Fluid Mechanics and can be read online here: https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/volume/FCDAB4A0D2744C2BD6F863700D51E986

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