[M-IS02] Ocean plastics, an earth science perspective

[MIS02-12] A numerical model approach on floating marine plastic debris with wind-induced re-drifting process from beaches along the Gulf of Thailand

*Pontipa Luadnakrob^1,4, Maria Belen Alfonso², Atsuhiko Isobe², Tahira Irfan¹, Keiichi Uchida³, Hisayuki Arakawa³, Sukchai Arnupapboon⁴, Nathacha Changphetphol⁴, Suchana Chavanich⁵, Voranop Viyakarn⁵ (1.Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 2.Research Institute for Applied Mechanics, Kyushu University, 3.Tokyo University of Marine Science and Technology, 4.Southeast Asian Fisheries Development Center/ Training Department, 5.Chulalongkorn University)

Keywords:Floating marine debris, particle tracking model, Gulf of Thailand

The seasonality of floating marine debris abundance in the Gulf of Thailand was investigated through visual observations and a particle tracking model (PTM). The observations documented the highest concentration of floating debris (3,329 pieces) during the northeasterly monsoon, characterized by offshore winds on the beaches along the eastern coast. To reproduce these observations, a PTM representing riverine plastic debris was developed 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 is that re-drifting occurs on a timescale assigned to each particle with an average of 200 days, while the other is that modeled particles re-drifting after the onset of offshore-ward winds on the beaches. The latter experiment successfully reproduced the seasonal patterns observed in reality, although re-drifting should occur after 1-2 months 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 floating plastic debris of which abundance has a peak in winter does not directly come from rivers, but from beaches during the northeasterly winds. Well-planned beach cleanup campaigns are likely to be highly effective in reducing floating debris in the Gulf of Thailand.