Transport of plastic pollution by ocean waves

Thesis

Ocean waves are one of the drivers of ocean transport of floating and submerged particles, including microplastics, oil droplets, sediment, and wreckage. This thesis examines the wave-induced transport of purely Lagrangian particles by wave packets and the effect of changing size and density of floating objects on their transport by regular waves. Particle tracking velocimetry is used to examine Lagrangian particle trajectories under deep-water wave packets in a laboratory flume. Particle motions are dominated by Stokes drift near the free surface, and the Eulerian return flow at depth. Close agreement is achieved between experimental measurements and leading-order solutions of the irrotational water wave equations. A multiple-scales solution is derived for Eulerian mean flow under wave packets that applies to all water depths. The solution is validated against experimental data, using particle tracking velocimetry corrected for background and paddle wave generation errors. It is found that the magnitude of the horizontal return flow is enhanced by divergence of the Stokes transport at wave packet scale and the confining effect of the mean setdown underneath the packet. This enhanced return flow has potentially large ramifications for the transport of particles in coastal waters. A combination of analytical, numerical and experimental approaches are used to examine the transport of inertial, spherical objects (representing large marine debris) by Stokes drift in regular waves. It is found that such objects are transported at different rates depending on their size and density, and that larger objects experience increased drift compared with Lagrangian tracers. The mechanism for increased drift comprises the variable submergence and the corresponding dynamic buoyancy force components in the direction perpendicular to the local water surface, which leads to an amplification of Stokes drift when averaged over the wave cycle. Using an expansion in wave steepness, a closed-form approximation is derived for this increased drift, which can be included in ocean-scale models of marine litter transport.

Authors: Calvert, R

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: plastic pollution; ocean waves; wave-induced transport; Eulerian return flow; Lagrangian transport; inertial floating objects; analytical perturbation expansions; wave flume experiments
• Subjects: Oceanography; Applied mathematics; Physics; Environmental engineering