Development Length and Transport Regimes of Buoyant Plastics in Open-Channel Flows

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

Recent studies suggest that the vertical distribution of positively buoyant plastic particles in turbulent open-channel flows can be described by a modified Rouse profile. However, experimental evidence also indicates that floating particles often remain confined to the air–water interface due to surface tension, with limited vertical exchange. These observations suggest a tension in the interpretation of Rouse-type profiles for positively buoyant plastics. To address this, we first derive an expression for a particle development length, defined as the streamwise distance required for particles to interact with the controlling boundary. This provides a diagnostic measure of whether particles have had sufficient opportunity to interact with the boundary, and suggests that previous experimental studies may have operated within a developing regime. Second, we develop a regime map that integrates free-surface detachment and bed-entrainment processes within the Rouse framework, providing a predictive description of when Rouse-type behaviour is physically valid for both positively and negatively buoyant plastics. Together, these results highlight the critical role of boundary interactions in plastic transport and support improved prediction of plastic pollution dynamics in riverine environments.