How litter moves along a macro tidal mid-latitude coast exposed to a coastal current

A simplified particle-tracking model with an idealised coastline was used to investigate how the interaction between variable winds and water level (VaWWL) operates spatially along a coast. The model included a constant along-coast current, horizontal diffusion, onshore/offshore wind drift, beach/cliff combinations and point/distributed litter sources. The default model reproduced basic properties of observed beach litter loadings (zero net accumulation, negatively skewed loading distributions) and the observed spatial pattern along the Scottish east coast, with average loadings increasing in the coastal current direction. The VaWWL effect moved the along-coast flux of floating litter offshore as debeaching events occur during offshore winds. Varying diffusion, coastal current speed, windage, beach/cliff combinations and different foreshore boundary conditions were investigated. Reconciling model predictions with previous estimates of plastic inflow suggested sinking rates of up to 90% soon after first entry into the sea. The VaWWL effect offers a realistic boundary condition for particle-tracking models.