Flushing and Trapping of Buoyant Particles in a Gravitationally Driven Wide Estuary

Plastics and other buoyant particles, such as oil, plankton, and seafoam, are omnipresent in estuaries, which provide a critical interface to the open ocean. Here, we explore the convergence and along-channel transport of buoyant particles using an idealized model of the gravitationally driven estuarine circulation with a deeper center channel and shallower flanks. First, the model predicts persistent surface convergence zones, aggregating buoyant material. Second, reduced vertical mixing results in accelerated surface outflow consistent with particle transport out of the bay during neap tides. We refer to this consistent outflow as flushing. Third, increased vertical mixing from neap to spring tides changes the lateral structure of the surface flows with weak up-estuary along-channel flows in the region of particle convergence during spring tides, resulting in particle trapping. We show the applicability of idealized model theory to the Delaware Bay through the use of a realistic hydrodynamic model, high-resolution satellite images, GPS-tracked drifters, and CTD cross-sections. Our results are consistent with the theory and indicate that buoyant surface-trapped particles in Delaware Bay quickly converge to the channel center of the estuary and are consequently either trapped in the estuary during spring tide or flushed to the continental shelf during neap tide. Our results contribute to the growing understanding of particle movement and retention within an estuarine system.