Abstract Submission: Plastic particles of multiple sizes spanning several orders of magnitude are found in waterbodies worldwide. Field and experimental work shows that plastic particles may impact the behavior of sediment transport and bedform morphologies. What is the magnitude of this impact? How much plastic is needed before the change is meaningful? What are its effects? To begin answering these questions, we conducted experiments in a fully recirculating flume and monitored the behavior of the channel bed as increasing amounts of plastic were added to identify tipping points in sediment transport and bedform morphology behavior. In one set of experiments, Mardi Gras beads were added in increments from 0.2% cover to 20% cover where cover is defined as the area that the beads would occupy if placed over the experimental reach in a single layer. As mass ratios, these values represent 5.9 x 10-5 to 5.9 x10-3 (total mass of plastic / total mass of sand). Dune depletion begins as early as 2.5% cover where sand bursts triggered by the entrainment of lower density plastic particles locally create a shift from bedload to suspended load transport. A marked change occurs at 12.5% cover and at 20% cover we had to stop the experiment because the erosion and scour of bedforms induced by the plastic particles was large enough to uncover the bed of the flume. Our experiments highlight the need to incorporate the role of plastic particles in sediment transport equations. Plastics are not a passive component of river systems; they interact with the riverbed and modify its natural behavior and response to flow conditions.
