Numerical modeling of the beach process of marine plastics: A probabilistic and diagnostic approach with a particle tracking method

A model of the beach process of marine plastics was proposed based on the assumption of the beaching and backwashing flux balance, and its applicability was examined by means of time-invariant linear system analysis and particle tracking experiments with respect to the ratio between the residence time of plastics on a beach (τ_r) and the period of nearshore current variability (T₀). Based on the theory, the balance was expected to hold when τ_r/T₀ was much smaller than 1; however, good agreement was obtained between the theory and the particle tracking method for much larger values of τ_r/T₀. The parameters, which are diagnostically given in the model, will be prognostically decided by the coastal dynamics in the future to develop robust beach process models. Nevertheless, we believe that a diagnostic approach would be another pillar in the strategy for estimating the amounts and distributions of marine plastics in the coming years.