Author(s): Arefeh Shamskhany; Shooka Karimpour

Linked Author(s): Arefeh Shamskhany, Shooka Karimpour

Keywords: Microplastic pollution; Sediment transport; Entrainment and mixing; Settling velocity; Backward Facing Step

Abstract: Plastic generation has increased drastically over the last decades, leading to a proportional increase in plastic waste generation. Mismanaged plastic wastes get into the aquatic environment through different pathways, such as rivers, wastewater treatment systems, and stormwater runoff. Plastic debris is present in the aquatic environment for countless years, turning into smaller pieces through the fragmentation process. Therefore, the presence of Microplastics (MPs), plastic particles identified with sizes smaller than 5mm, grows exponentially over the years. MPs pollution is an emerging environmental threat that has detrimental impacts on the ecosystem and human health. Based on recent studies, only about 1% of aquatic MP input is found in water surface layers, and the rest are hidden in the water column and sediments. Due to variability in their origins and exposure to various weathering processes, MPs have an extensive range of densities, shapes, and sizes. In addition to the different physical properties of MPs, turbulent flow can also intensify the intricacies of MPs distribution and behavior in aquatic environments. Turbulent structures induced by different mechanisms such as temperature gradient, or a sudden bed elevation change, can become the dominant factor affecting MP transport and distribution. Due to their small size, MPs are entrained in turbulent coherent structures and move against their natural buoyance (Shamskhany and Karimpour, 2021). Thus, MPs distribution and transport in the aquatic environment is further intricated due to complexities in their properties and the turbulent structure in the ambient flow. In this study, using the numerical simulations, we aimed to formulate MPs mixing and vertical transport in a turbulent flow based on the combined effect of particles' size and density. We used microbeads of spherical shapes due to their well-established drag coefficient and settling behaviors. These microbeads are injected into turbulent flows with different turbulent intensities, induced over the Backward Facing Steps. We have established a dimensionless parameter to explain the settling behavior and entrainment of regular shaped MPs with different sizes and densities in transient flows with different turbulent intensities. Results of this study are compared with reported regimes of sediment particles mixing behavior in a turbulent flow based on particles settling velocity (Stout et al., 1995). Using sediment analogy, our results quantify the unique entrainment behaviors of MPs in a turbulent flow. References: Shamskhany, A. and Karimpour, S. (2021). The role of microplastic characteristics on vertical transport and mixing, Canadian Society of Civil Engineering Annual Conference, 26 – 29 May, Virtual. Stout, J. E., Arya, S. P., & Genikhovich, E. L. (1995). The effect of nonlinear drag on the motion and settling velocity of heavy particles. Journal of Atmospheric Sciences, 52(22), 3836-3848.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221339

Year: 2022