Surface convergence zones due to Lagrangian residual flow in tidally driven estuaries

Abstract Buoyant material, such as floating debris, marine organisms, and spilled oil, is aggregated and trapped within estuaries. Traditionally, the aggregation of buoyant material is assumed to be a consequence of converging Eulerian surface currents, often associated with lateral (cross-estuary) density gradients that drive baroclinic lateral circulations. This study explores an alternative aggregation mechanism due to tidally driven Lagrangian residual circulations without Eulerian convergence zones and without lateral density variation. In a tidally driven estuary, the depth-dependent tidal phase of the lateral velocity varies across the estuary. This study demonstrates that the lateral movement of surface trapped material follows the tidal phase, resulting in a lateral Lagrangian residual circulation known as Stokes drift for small amplitude motions. For steeper bathymetry, the lateral change in tidal phase is greater and the corresponding lateral Lagrangian residual flow faster. At local depth extrema, e.g. in the thalweg, depth does not vary laterally, so that the associated tidal phase is laterally constant. Therefore, the Stokes drift is weak near depth extrema resulting in Lagrangian convergence zones where buoyant material concentrates. These ideas are evaluated employing an idealized analytic model in which the along-estuary tidal flow is driven by an imposed barotropic pressure gradient, whereas cross-estuary flow is induced by the Coriolis force. Model results highlight that convergence zones due to Lagrangian residual velocities are efficient in forming persistent aggregation regions of buoyant material along the estuary.