  Microplastic (MP) pollution is a well document threat to our aquatic and
terrestrial ecosystems, however, the mechanisms by which MPs are transported in
river flows are still unknown. The transport of MPs and natural sediment in
aquatic flows could be somewhat comparable, as particles are similar in size.
However, it is unknown how the lower density of MPs and their different
material properties impact their transport dynamics. To answer this, novel
laboratory experiments on bed load saltation dynamics in an open-channel flow,
using high-speed camera imaging and the detection of 11,035 individual
saltation events were used to identify the similarities and differences between
spherical MPs and spherical natural sediments transport. The tested MPs varied
in terms of size and material properties (density and elasticity). Our analysis
shows that the Rouse number accurately describes saltation length, height,
transport velocity and collision angles equally well for both MPs and natural
sediments. Through statistical inference, the distribution functions of
saltation trajectory characteristics for MPs were analogous to natural sediment
with only one sediment experiment (1.4 % of cases) differing from all other
plastic experiments. Similarly, only nine experiments (9.3 % of cases) showed
that collision angles for MPs differed from those of natural sediment
experiments. Differences observed in terms of restitution become negligible in
overall transport dynamics as turbulence overcomes the kinetic energy lost at
particle-bed impact, which keeps particle motion independent from impact.
Overall, spherical MP particles behave similarly to spherical natural sediments
in aquatic environments under the examined experimental conditions. This is
significant because there is an established body of knowledge for sediment
transport that can serve as a foundation for the study of MP transport.
