Influence of Breaking Bores on the Transport of Macroplastics

Each year there is an influx of several million tonnes of plastic into oceans. Transport and mobility of aquatic plastics are shaped by many factors including flow conditions. Surge waves - highly turbulent transient flows - observed in rivers and near coastal areas, can result in substantial turbulent mixing and have the potential to transport mismanaged plastic waste offshore. The transport of plastics in flow conditions induced by surge waves and the variation in the transport process owing to the changes in the hydrodynamic properties of a surge wave and macroplastic properties, have not been investigated. In this study, laboratory experiments were performed using a hydraulic flume, to produce surge wave Froude number ranging from 1.40 to 4.90. An in-house particle tracking velocimetry platform was used to capture the transport of macroplastics, which was simulated using solid macro-sized negatively buoyant acrylic, and positively buoyant high-density polyethylene and polypropylene plastic balls. The overall results highlight that the horizontal transport of all the macroplastics was governed by surge wave celerity and macroplastics initial momentum, while the vertical transport was influenced by surge wave Froude number. As macroplastics size decreased, there was an increase in horizontal and vertical transport, since it was easier to mobilize them against their buoyancy. This study illustrates that under high mixing conditions, plastics with marginal density from water, can entrain heavily with the flow, moving against their buoyancy. This research highlighted the transport and mobility of aquatic plastics to promote a healthier and cleaner aquatic environment.