  Over the last decade, fluvial plastics have been identified as major threat
to aquatic environments and human health. In order to develop adequate
mitigation strategies for plastic pollution, a fundamental process
understanding of riverine plastic transport is of significant importance. In
this context, the implementation of research findings into numerical simulation
environments is anticipated to enhance modelling capabilities and to support a
rigorous decision making. Recent experimental research has focused on the
incipient motion of plastic particles, as well as on the effects of surface
tension on plastic concentration profiles. While these investigations have
advanced the state-of-the-art knowledge, current literature still displays a
lack of basic insights into layer-specific plastic transport physics. In this
study, first principles are applied to advance knowledge on free-surface
detachment and bed entrainment of fluvial plastics. A novel relationship for
the critical surface detachment velocity is derived, followed by the
development of a framework that allows to relate plastic Shields parameters to
those of natural sediments. Overall, it is anticipated that these developments
will trigger new research within the plastics community, and it is hoped that
present findings will be implemented into Lagrangian particle tracking
software.
