Microplastic dynamics within turbulence for improved modelling and monitoring strategies

Microplastics are an unavoidable part of riverine systems, altering the natural composition of rivers and the associated processes. Within a riverine system, microplastics can be present throughout the water column or within bedload, implying different transport, deposition, and erosion mechanisms. Some recent models have been developed to predict and understand the depth distribution concentration of microplastics and macroplastics, primarily relying on the Rouse model for sediments. However, the great variety in microplastics shape and size accompanied by the dynamic nature of riverine systems (e.g., variety of flow conditions, sediment composition and bedforms) question how realistic and reliable models for plastic distribution along the water column and within sediments are. Present models are often analogous to suspended sediment models and assume diffusivity to be equal to turbulent viscosity, they often exclude the bedload, neglect the effect of turbulence and bed morphology, and come short in explaining the behaviour of mixtures of microplastics and sediments. Understanding these aspects is crucial to improve present models and to aid mitigation efforts and to optimize collection systems and policy. This project targets employing both physical and numerical modelling techniques to further develop depth concentration models of microplastics. We aim to further examine and quantify the influence of turbulence on microplastics transport and concentration distribution by establishing preliminary estimates for the eddy viscosity and diffusivity of microplastics and further examine a wider range of mixtures of microplastics and sediments with different shapes and sizes.