A simple model for beaching and resuspension of plastic debris

When modelling the transport of plastics in the marine environment it is common to use a Lagrangian modelling framework. The movement of the particles is governed primarily by advective and diffusive transport, but the plastics are also subjected to a number of other physical, chemical, and biological processes that affect their fate. For transport in coastal regions, one of the more important processes is the interaction between particles and the shoreline.

Currently, there is no consensus on how to handle shoreline interactions in particle tracking models, and many resort to over-simplified descriptions such as considering a particle to be permanently beached at the position where it first hits land, or not allowing for beaching of debris at all. However, it is well-known that a lot of floating marine litter ends up on beaches, and mark-recapture studies of plastic on beaches around the world show that there can be considerable turnover in the litter on a beach. Furthermore, these studies show that both beaching and resuspension rates vary both over different beaches, and over different seasons at the same beach, indicating that these processes depend on several different factors, such as wind and wave conditions, beach morphology, and likely also the shape, size, and density of the object. Thus, in order to accurately predict the accumulation sites for floating plastic debris in coastal regions, more care should be put into modelling shoreline interactions.

Here we investigate a toy model for beaching of floating plastic debris, implemented in an idealised Lagrangian framework with analytically defined current, spatially constant wind and diffusivity, and a domain bounded on one edge by a straight, homogeneous shoreline. We implement different strategies for handling the beaching and resuspension of debris and compare the resulting distribution of particles. There is currently insufficient experimental data on the extent to which the different factors affect the beaching and resuspension processes for different kinds of plastic objects, so the purpose of this work is not to reproduce actual conditions, but rather to investigate the effect of the choice of beaching and resuspension strategies on the simulation results. We investigate e.g. a simple resuspension model where particles have an average lifetime on the beach, as well as a wave-based model where the beaching and resuspension is affected by randomly generated wave heights.