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

To better understand how plastic is moved by the ocean, we need to measure more accurately the key mechanisms for plastic movement: currents, wind, and waves. Currents and wind transport ocean debris in a straightforward manner similar to the force on a sailing boat.

However, ocean waves predominately move objects in circular-like orbits. These orbits do not quite close, resulting in so-called Stokes drift in the travelling direction of the waves.

A joint team from Oxford, Edinburgh, and Plymouth universities has now investigated how waves transport floating ocean debris while including, for the first time, the effect of an object's size, buoyancy, and inertia on its transport.

The team’s results were published this week in The Journal of Fluid Mechanics.

Beyond Stokes drift

The Stokes drift induced by waves has been shown to be important for the movement of ocean debris towards the coast. This potentially results in plastic ‘beaching’ along the coastline, which may account for the location of some of the plastic pollution. This drift effect 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 therefore transported with the exact Stokes drift solution. However, the team found that larger floating ocean debris can be transported at a rate faster than Stokes drift due to inertial effects.

The study’s lead author Dr Ross Calvert (University of Oxford) explained: “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 it.”

Sophisticated modelling

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

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

This 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, is underway.

Professor Ton van den Bremer (TU Delft and University of Oxford) 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.”

Professor Alistair Borthwick (School of Engineering, University of Edinburgh) added, “Microplastics contaminate the oceans and are detrimental to marine life and coastal habitats, as well as littering the shoreline. Our research, led by Ton van den Bremer and Ross Calvert, has found that the drift of microplastic particles floating in waves depends very much on their size, with larger particles receiving an additional boost causing them to travel longer distances faster.”

Team

The team working on this research were Professor Ton van den Bremer, formerly Chancellor’s Fellow and now Visiting Professor at the School of Engineering, University of Edinburgh, and Associate Professor at the University of Oxford and TU Delft; Dr Ross Calvert and Dr Mark McAllister who both completed their PhDs at Edinburgh’s School of Engineering before joining Professor van den Bremer’s research group as postdoctoral research associates at Oxford; Alistair Borthwick, Emeritus Professor at the School of Engineering, University of Edinburgh; and Alison Raby, Professor in Environmental Fluid Mechanics at the University of Plymouth.

The research was supported by the Royal Academy of Engineering.

Find out more

• Read the study: A mechanism for the increased wave-induced drift of floating marine litter, Journal of Fluid Mechanics, 25 May 2021