| Summary: | <p>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.</p>
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