Particle Beaching: A Study on the Transport of Buoyant Microplastics in the Swash Zone

Abstract

Predicting the transport of microplastics on beaches is critical to modeling plastic pollution in coastal environments. The transport of these particles is dependent on the particle characteristics and local swash zone hydrodynamics. The swash zone is the region where broken waves travel up and down the beach’s slope. Individual swash events are capable of transporting microplastic particles, as well as sediments. To study microplastics, some previous experiments studied small particles in continuous wave forcing in wave tanks. While these studies can measure particle transport after a number of waves, they are unable to isolate the interactions between one wave and one particle. In this dissertation, we focus on simplifying the study of microplastics in the swash zone. Specifically, we consider the transport of buoyant microplastics in a single swash event, where we can isolate the swash zone’s role in transporting buoyant plastic particles onto and off of beaches. In Chapter 2, we designed an experimental wave tank that isolates small swash like flows; using this tank, we tracked disk-shaped particles which were initially placed on the beach. Our observations showed that smaller particles were carried higher up the beach during the uprush phase of a single swash event, and that they were more likely to remain on the shore after backwash. In contrast, larger particles had reduced uprush transport and were more likely to leave the beach. We explain these results by calculating the Stokes number at the instant the wave first impacts each particle. We find that this Stokes number correlates with both the maximum and final beaching positions of the particles. Lower-inertia particles generally stayed near the swash front during uprush, leading to high maximum positions and resulting in higher beaching locations. In contrast, higher-inertia particles lagged behind the swash front, leading to lower maximum and final positions and had a greater likelihood of returning to the water. These findings demonstrate the importance of particle inertia to buoyant particle transport in the swash zone. In Chapter 3, we develop a statistical model to predict particle residence times on beaches and the evolution of the distribution of particle positions across multiple waves. This model uses empirically-derived beaching probabilities from our laboratory experiments for a single swash event. Our simulations results show that buoyant particles continuously leave the beach, such that the particle count decreases exponentially over time. We use an exponential fit to particle count over time to estimate the average residence time of particles on the beach in our simulations. With our simulations, we find that residence time decreases with increasing particle size and decreasing average wave run-up length. We characterize this relation with the average particle Stokes number, where we find that the residence time of particles on the beach decreases with increasing Stokes number. We observe that particles that on the beach are generally transported to high positions where they form a wrack line. Over time, the largest run-ups will move the wrack line farther up the beach. Finally, we also consider the relative effect of tides, where we find that falling tides lead to particles having higher residence times on the beach, whereas rising tides reduce particle residence times on the beach. Overall, our simulations provide a useful tool for describing buoyant particle transport on a beach under variable wave forcing. In Chapter 4, we review and analyze the relevant forces for the incipient motion of buoyant microplastics in the swash zone. This work provides a short review on sediment incipient motion, highlighting the challenges when extending to buoyant microplastics in the swash. In our analysis, we focus on identifying a characteristic timescale of microplastic incipient motion considering the effects of lubrication and drag. We define a lubrication timescale by scaling the lubrication force with net buoyancy of the particle. The timescale due to drag is characterized by the particle relaxation time, which is the time it takes for a particle to go with a surrounding flow. We find these due timescales differ with particle and flow characteristics. Moreover, we compare both estimates with a timescale derived from experiments, and find that relaxation time scales closer with our experimental values compared to the lubrication timescale. This suggests that buoyant particles on a beach may be easily mobilized by incoming swash waves. Overall, this work highlights the need to accurately describe the incipient motion for buoyant particles, as well as presents an analysis of timescales derived from relevant forces in the system for disks on an impermeable beach. Finally, in Chapter 5, we summarize the contributions from this thesis and describe some important avenues for future work.