Numerical plastic transport modelling in fluvial systems: Review and formulation of boundary conditions

In recent years, it has become clear that plastic pollution poses a significant threat to aquatic environments and human health. Rivers act as entry points for land-based plastic waste, while a certain fraction of entrained plastics is carried into marine environments. As such, the accurate modelling of plastic transport processes in riverine systems plays a crucial role in developing adequate remediation strategies. In this paper, we review the two main multiphase flow numerical approaches used in plastic transport modelling, comprising Lagrangian Transport Models (LTMs) and Eulerian Transport Models (ETMs). Although LTMs and ETMs can be regarded as complementary and equivalent approaches, LTMs focus on the transport trajectories of individual particles, whereas ETMs represent the behaviour of particles in terms of their mass or volume concentrations. Similar results of the two approaches are expected, while our review shows that plastic transport models are yet to be improved, specifically with respect to the formulation and implementation of boundary conditions, comprising plastic interactions with the channel bed, river bank, and the free surface, as well as interactions with biota. We anticipate that an implementation of these boundary conditions will allow for a better representation of different plastic transport modes, including bed load, suspended load, and surface load. Finally, we provide suggestions for future research directions, including a novel threshold formulation for free surface detachment of plastics, and we hope that this review will inspire the plastic research community, thereby triggering new developments in the rapidly advancing field of numerical plastic transport modelling.