Bedforms effect on microplastics deposits erosion

Transport processes involving both microplastics (MPs) and natural sediments are being marginally studied, for the high complexity of the system and the many factors requiring attention. Still, it is of high importance to understand the interactions between natural sediments and MPs transport, especially at the water-bed interface, a critical area for rivers’ ecology and biodiversity. To bridge this gap, we carried out flume (15-m long, 1.0-m wide and with 0.27 m water depth) experiments to study the interactions of a small bedform and a deposit of compact MPs. The compact-shaped MPs, consisting of Polyamide 6 particles with equivalent sphere diameter around 2.9 mm, were released at a low flow rate (around 20 l/s corresponding to a mean velocity of 0.1 m/s), for which deposit formed at the lee side of a 2-cm high and approximately 0.7-m long sand dune. A sudden increase of flow rate was then applied (up to 60 l/s corresponding to a mean velocity of 0.3 m/s), forcing erosion of the MPs. Measurements included velocity profiles and turbulent measurements via Acoustic Doppler instrumentation, videos and underwater photos of the small bedform. From Doppler measurements the mean flow characteristics were derived, as well as fluctuating terms of the velocity components up to 50Hz. Using Structure from Motion, a 3D model of the bedform and the MPs deposit was constructed. The erosional behaviour of deposited MPs was derived by estimating the total volume mobilised from the deposit by difference (prior and post erosion) via DEM. The MPs’ removal efficiency was then estimated, in three cases of MPs’ deposit initial volumes.