Enhanced Macroplastic Transport and Interception in Rivers: The Role of Detached Groynes and Vegetation Patches

Small-scale hydrodynamics of plastic debris transport in freshwater environments are insufficiently understood, but it is known that groynes and vegetation play a role in debris accumulation. From the perspective of water management, pollution transport estimates and efficiency of cleaning efforts, this knowledge gap should be addressed. This research paper investigates the impact of groyne setup on macroplastic transport in rivers through observation of the uniform, floating macroplastic particles in laboratory experiments with stationary flow conditions. The effects of two factors were compared by tracking the plastic particles' movement patterns: vegetation presence and groyne configuration. Vegetation was in the form of a circular patch of wooden stems imitating common reed in the middle of the groyne area. The groynes were either in a regular setup, i.e., attached to the bank, or they were detached, allowing the flow along the bank through the groyne field. During the test with a regular setup, typical recirculation of the flow causes plastic litter to accumulate and stay in the upstream corner of the groyne field. The plant patch presence resulted in the temporal accumulation of the plastic pieces between plant stems. When the gap in the groynes was present, all the plastic pieces floating inside the groyne field made their exit from the area, and this behaviour repeated with the vegetation presence. Those results indicate that detached groynes disrupt the flow pattern, which reduces plastic retention time and enhances transport through a groyne field. Those observable changes in macroplastic transport may help in designing best practices of river cleaning activities or more ecologically friendly river structures.