Improving predictions of microplastic incipient motion through shields parameter modifications

Understanding microplastic transport mechanisms in the aquatic environment is necessary for improving load estimates to oceans as well as developing more targeted and effective management strategies. In recent years, the importance of microplastic accumulation in and interactions with sediments in riverine systems has been increasingly realized through results from field, lab, and modeling studies. Incipient motion is a relevant and understudied component of these processes. Several lab studies have demonstrated that microplastics do not exhibit the same incipient motion behaviors as sediments and may move under lower flow conditions than predicted by the traditional Shields parameter. Here, we supplement these previous studies with flume incipient motion experiments on an expanded range of plastic sizes, classes, and polymer types to better understand what drives these observed differences with sediment grains. We use this expanded dataset to evaluate several proposed modified Shields parameters. We additionally propose a new modification that accounts for the impacts of fluid drag and size on transport. Results have implications for future model development as well as guiding the design of field experiments.--Author's abstract