A mechanism for the increased wave-induced drift of floating marine litter

Ross Calvert, M.L. McAllister, C. Whittaker, Alison Raby, Alistair Borthwick, Ton Van Den Bremer

Research output: Contribution to journalArticlepeer-review

Abstract

Periodic water waves generate Stokes drift as manifest from the orbits of Lagrangian particles not fully closing. Stokes drift can contribute to the transport of floating marine litter, including plastic. Previously, marine litter objects have been considered to be perfect Lagrangian tracers, travelling with the Stokes drift of the waves. However, floating marine litter objects have large ranges of sizes and densities, which potentially result in different rates of transport by waves due to the non-Lagrangian behaviour of the objects. Through a combination of theory and experiments for idealised spherical objects in deep-water waves, we show that different objects are transported at different rates depending on their size and density, and that larger buoyant objects can have increased drift compared with Lagrangian tracers. We show that the mechanism for the increased drift observed in our experiments comprises the variable submergence and the corresponding dynamic buoyancy force components in a direction perpendicular to the local water surface. This leads to an amplification of the drift of these objects compared to the Stokes drift when averaged over the wave cycle. Using an expansion in wave steepness, we derive a closed-form approximation for this increased drift, which can be included in ocean-scale models of marine litter transport.
Original languageEnglish
Article numberA73
Number of pages35
JournalJournal of Fluid Mechanics
Volume915
Early online date18 Mar 2021
DOIs
Publication statusE-pub ahead of print - 18 Mar 2021

Keywords

  • Waves/Free-surface Flows
  • Surface gravity waves
  • Geophysical and Geological Flows
  • Ocean processes
  • Coastal engineering

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