Microplastics on natural beds: availability, exposure and fine-grained bed sediments promote transport

Abstract Rivers transport a variety of materials, among which, recently, plastics. The transport of plastic fragments and small pieces, referred to as microplastics (MPs), has borrowed methods and approaches from sediment transport, with due adaptations. Past studies showed that the higher variability of densities and shapes encountered in plastics can be addressed within the classical sediment transport framework, but questions remain about how to fit plastic into such framework. Sediment and plastic transport in rivers requires both the understanding of the plastic-specific transport behaviour per se , and an expansion of definitions and methodology to fit the new needs imposed by plastic presence. Here we propose a method to define the onset of transport depending on the availability of mobile material, and apply it to the case of sparse MPs on natural beds. Laboratory experiments on homogeneous (corresponding to MPs’ total bed cover) and mixed (corresponding to much reduced MPs bed cover spanning 0.05–1%) bed experiments are presented and discussed, allowing a better understanding of the onset of transport process in the case of a bed composed of different material fractions. The role of bed sediments and their induced hydrodynamics aligns with literature results, with the exception of the lightest MPs we tested, which moved on a sandy bed at conditions one order of magnitude lower than previously observed. When looking at the role of availability, we observed that the higher the availability, the lowest the bed shear stresses needed to start transport. Average values disregarding the availability variations show good agreement with published literature. The effects of availability are here investigated for the first time, and shed light on the potential underestimation of MPs fluxes by present models.