Plastic Transport in Coastal Seas: Lagrangian Tracking of Microplastics in the Nearshore and Evaluation of Beaching.

Huge advances have been made over the last decade in the modeling of plastic transport in our global seas. The main interaction between plastic and humans and marine life takes place in the coastal environment. This is exacerbated by the fact that plastic concentrations in the coastal environment can be high due to plastic sources like rivers or polluted beaches (1). It is thus crucial to be able to model and predict the fate of microplastics in the near-shore region. However, this is highly challenging since the horizontal and vertical transport of microplasticts is influenced by a large number of strongly interacting processes. These include wind, waves, currents, bathymetry variations and coastline shape, wave breaking and sheet-flow processes in the swash zone (2).

In this work, the multi-layer non-hydrostatic free-surface model SWASH is used together with Lagrangian tracking software Parcels in order to predict the transport of weakly inertial particles in the nearshore. The top panel of Fig. 1 shows the modeling framework. We have successfully incorporated the capability to simulate inertial and buoyant particles and have validated this implementation with an analytical solution for the settling of particles under linear waves (3). The results are shown in the middle panels of Fig.1.

Flume-like (2DV) flow simulations allow us to study in detail the cross-shore vertical and horizontal processes that govern the motion of plastic (bottom panels Fig. 1). The full 3D simulations give us insight into the behaviour of plastic in flows where alongshore components are important. Current work focuses on how particles with different properties behave in breaking waves as well as the larger scale behaviour of particles in bar/rip systems.

This work will firstly contribute to the understanding of the behaviour of plastic particles in the coastal region. However, if we zoom out and consider the global problem of ocean plastic pollution, it provides a versatile modeling framework for the boundaries of the ocean. The modeling scale of this work addresses the gap between the global scale transport estimates on one hand, and local beach observations of plastic on the other hand (4,5).

DOI:

(1) 10.1038/ncomms15611

(2) 10.1029/2006RG000215

(3) 10.1175/2008JPO3793.1

(4) 10.3389/fmars.2017.00030

(5) 10.1016/j.marpolbul.2017.05.012