On the suspended microplastic dispersion in turbulent shallow flows under progressive gravity waves*

When wind waves approach the coast over a relatively shallow water, the wave-induced orbital motion promotes the development of a turbulent boundary layer at the bottom, enhancing the transport and dispersion of suspended microplastics. Furthermore, the strong wave-bed interaction results in a net non-periodic shoreward Longuett-Higgins current close to the bottom superimposed to the wave orbital motion [1]. Via direct numerical simulation, we analyse the effect of the wave orbital motion, the steady current and the near-wall turbulence on the dispersion and vertical distribution of microplastics both inside and outside the bottom boundary layer. For performing the simulations, we utilise a boundary-fitted hybrid pseudo-spectral/finite-difference code which implicitly solves the Navier-Stokes equations together with the dynamic and kinematic boundary conditions at the free-surface. The motion of the suspended microplastics is treated by a Lagrangian particle tracking algorithm in the framework of the point-particle approach. The aim of this work is to understand the physical mechanisms behind the microplastic transport processes under shallow progressive waves. This has important implication in parameterising the mechanistic behaviour of suspended microplastics and assessing existing predictive microplastic transport models.

[1] Longuet-Higgins, M. S. (1953). Mass transport in water waves. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 245(903), 535-581.