Fate of positively buoyant particles in a scaled swash flow

Microplastics are ubiquitous throughout the world's oceans including their coastlines. These coastlines act as both a source and a sink for ocean microplastics, a necessary boundary condition for modelling their transport. On beaches, the swash zone controls the transport of microplastics across the coastal boundary. And yet, there is still no definitive model to predict the fate of microplastics once within the swash zone. To address this problem we performed a dimensional analysis to identify the key non-dimensional parameters. Next, we designed laboratory experiments to explore this parameter space, where we tracked particles that were initially at rest on a beach within the swash zone. We observe that the smallest particles that start lowest on the beach are more likely to end up higher on the beach after a solitary swash event, whereas larger particles that start higher on the beach are more likely to beach lower or wash off entirely. To characterize these results, we calculate the Stokes number at the point of initial wave impact. We find that this Stokes number is well correlated with particle final position in our data, likely because the particle's ultimate position in the wave and its subsequent transport by the wave is set by how it is initially picked up.