Simulate the current dynamics process and predict the trajectory of plastic waste flowing out to sea from estuaries in the Gulf of Tonkin

This paper presents the results of a numerical simulation of flow dynamics in the northern Vietnam Sea. Based on these results, the trajectories of plastic waste from river mouths in the northern Vietnam Sea are predicted. The numerical simulation is based on the ROMS model for the northern Vietnam Sea and surrounding areas, in combination with data collected from a Vietnam-France international cooperation project (Study, evaluation, and prediction of the transport and dispersion of small plastic waste in the Van Uc River mouth area). The dynamic regime of the study area is considered to simulate the main wind seasons. The results show that the flow field in the Northern Vietnam Sea follows the tidal phases. In the rising tide phase, the general trend is for the flow to move from the south to the north. Conversely, in the falling tide phase, the general trend is for the flow to move from the north to the south. The flow speed in the tidal phases is largest at about 65-70 cm/s and smallest at about 5-10 cm/s. The much larger surrounding area outside the Northern Vietnam Sea has a very complex flow field, with many different flow areas and vortices. There is a belt of vortices that extends for hundreds of kilometers, with an average flow speed that is higher than the flow in the Northern Vietnam sea. Based on the flow field and the frequency of occurrence in the area near the Vietnamese coast where rivers flow out, the frequency of flow flowers are used to predict the trajectories of plastic waste from river mouths into the sea. The results of the calculations and predictions, when compared with the actual data, show good agreement.