A numerical model approach on floating riverine plastic debris with wind-induced re-drifting process from beaches along the Gulf of Thailand

Southeast Asian countries are widely acknowledged as major sources of plastic waste entering the ocean. The seasonality of floating marine debris abundance in the Gulf of Thailand (GoT) was investigated through visual observations in situ and a particle tracking model (PTM). The observations documented the highest concentration of floating debris (3,329 pieces) during the northeasterly monsoon (dry season), characterized by offshore-ward winds on the beaches along the eastern coast. A PTM representing riverine plastic debris was developed to reproduce these observations by incorporating ocean surface currents, horizontal diffusion, Stokes drift, windage, and beaching/re-drifting processes. Two re-drifting processes were examined in the PTM experiments: one where re-drifting occurs on a timescale assigned to each particle with an average of 200 days and another where modeled particles were re-drifting after the onset of offshore-ward winds above the beaches. The latter experiment successfully reproduced the seasonal patterns observed in reality, although re-drifting should occur after 10-60 days from the onset of offshore-ward winds, suggesting that plastic beach litter is prevented from re-drifting immediately due to various obstructions such as beach vegetation. The results indicate that the floating plastic debris observed visually in the GoT does not directly come from rivers but from the beaches where plastic debris accumulates during the onshore-ward winds. This information will support evidence-based measures, such as beach cleanup campaigns, to effectively reduce plastic debris impact in the GoT.