Towards guidelines for effective plastic removal from rivers : phase 1

Rivers and estuaries transport plastic particles towards the sea. Within the river the concentration of different types of plastic particles and the flow velocity varies both spatially and temporally. It is the product of the local concentration and the local flow velocity, which is the plastic flux, that determines how effective a system to collect plastic particles is at a location. The objective of phase 1 was to increase the understanding of the transport of plastic particles in rivers by model case studies and by reviewing literature. Typical characteristics of macroplastic items and microplastic particles are detailed, and a selection has been made which types of plastic particles are most relevant in a river. The rising (or falling) velocity was determined from observed rising velocities for the macroplastic items and for microplastics from a recently published paper. These characteristics per type were used for a pilot modelling exercise on the plastic transport in the Rhine-Meuse delta, including the Nieuwe Maas. The two software modules that were used showed reasonable results, although a verification with observations was not possible. Trends observed in observations by Allseas in the Nieuwe Maas and trends from the pilot modelling exercise showed similarities. From the similarities and results of software verifications, we conclude that the two software modules are suitable to model the transport of plastic particles, although some limitations apply. Results of the modelling and the literature survey gave rise to draft guidelines where in a river the plastic flux is relatively high. Optimal locations for plastic collection in rivers are within the reach where both ebb and flood flow occur, since absolute flow velocities are on average relatively high in this zone and thus the flux of plastics is high as well. The optimal location to collect falling macroplastic items (e,g. PET drink bottles) is somewhere in the deeper part of the cross section of the river, whereas the optimal location to collect rising plastic particles (e.g. bottle caps) is where the flow is highest. Wind can play a prominent role in the distribution of plastic particles that float or that have the tendency to rise. The flux of these plastic particles may be much higher near the downwind river bank.