Stability of an Idealized Floating Carpet of Plastic Spheres in an Open Channel Flow

Abstract

Plastic debris can accumulate at hydraulic structures and waste-collection devices, leading to a so-called floating carpet formation. Understanding the accumulation of plastic debris at structures is pivotal in the prediction of increased flood risk and design of waste-collection devices. In this research, we studied the stability of plastic carpets under different flow conditions using laboratory experiments, and we developed analytical models to predict critical velocities that led to two instabilities: (1) squeezing—particles inside the carpet are pushed downward due to cumulative compressive force, and (2) erosion—particles at the upstream edge of the carpet mobilize completely. Velocities of the fully developed flow were measured under a stable carpet to estimate boundary shear stress, which was applied to calculate the compressive force of the particles. Using measured flow velocity data and particle’s properties, the critical flow velocities that led to instabilities were calculated. Overall, this research supports a better understanding of physical processes associated with plastic accumulation, supporting the development of optimized plastic removal strategies. This research investigates physical processes associated with waste accumulation or floating carpet at hydraulic structures such as flood gates, culverts, and waste-collection devices, using laboratory experiments and analytical considerations. In this study, we identified two kinds of carpet instabilities—squeezing and erosion. Two analytical formulas were derived for these two instabilities to estimate the critical flow velocity. Moreover, the amount of the collected waste can be anticipated for a given flow velocity by using these analytical formulas. Our research findings can underpin the design phase of collection devices and the choice of their deployment location.