Distribution of buoyant tracers in an idealized tidally averaged estuarine circulation model with application to the Delaware Bay

Abstract

Between the river and the ocean lies a highly dynamic and complex system, an estuary, that acts as a critical trap for marine debris originating from both land and fluvial sources. In this study, an idealized tidally averaged estuarine circulation model for the gravitationally driven circulation was applied to predict tracer transport and marine debris pathways using a tracer conservation equation that incorporates multiple transport processes. The modeling results reveal that tidal mixing, estuarine depth, and width determine the estuarine circulation strength and structure. Lateral surface flows converge over the deeper channel center aggregating buoyant tracers there. At the bottom of the channel center lateral currents diverge resulting in enhanced tracer concentrations of sinking particles near the estuarine flanks. An analysis of high-resolution satellite imagery of Delaware Bay suggests that buoyant material frequently aggregates near the estuarine channel in narrow bands that extend along the estuary, consistent with model results. These aggregation zones are more often observed during winter than other seasons. Finally, the cross-sectionally averaged tracer concentration of a neutrally buoyant tracer is modeled based on a balance between transport by river flow, along-estuary tidal mixing, and shear dispersion by the estuarine exchange flow. The along-channel distribution of the tracer concentration is controlled by a nondimensional number (a Peclet number) that describes the relative importance of advection by river flow and along-estuary mixing. For stronger mixing, tracer concentration gradients are more constant along the estuary. Together, these insights offer an integrated framework to better predict and manage marine debris accumulation in estuaries.