Rethinking Vertical Transport of Buoyant Plastics in Open Channels

Charuni Wickramarachchige, Felipe Condo-Colcha, Robert K. Niven, Matthias Kramer

Recent studies have demonstrated that the vertical distribution of positively buoyant plastic particles in turbulent open channel flows can be described by a modified Rouse profile. However, implicit observations in the literature also suggest that floating particles remain confined to the air–water interface due to surface tension forces. To shed more light on this apparent contradiction, we analytically and numerically develop an expression for the minimum development length, the streamwise distance required to achieve converged concentration profiles, revealing that previous studies may have not reached this critical distance. Building on this, we develop a regime map that integrates free surface detachment and bed entrainment processes with the Rouse profile, providing a comprehensive framework to predict the transport modes of both positively and negatively buoyant plastics in open channel flows. We anticipate that this framework will enhance the understanding and prediction of plastic pollution dynamics in riverine environments, ultimately supporting more effective monitoring and mitigation strategies.