Harbor‐Induced Tidal Salinity Dispersion in Partially Stratified Estuaries

Abstract The contribution of tidal trapping to salt dispersion has been well described for well‐mixed estuaries, in terms of barotropic filling and emptying of the traps. How traps contribute to salt dispersion in deeper, partially stratified systems remains underexplored. We investigate the dispersive effect of temporary storage of saltwater in harbors adjacent to a partially stratified estuary using field observations and numerical modeling. Our results show that instantaneous channel–harbor salt exchange is dominated by density‐driven exchange flows arising from baroclinic pressure gradients between the channel and the harbors. This pressure gradient, and consequently the exchange flow, reverses during the tide due to tidal variations in main‐channel salinity. Quantification of the trapping‐induced additional salt transport from individual basins reveals substantial differences in contributions of individual basins. These differences are linked to a region in the main channel where the tidal salinity range has a minimum, thus limiting the set‐up of baroclinic pressure gradients, reducing exchange flow strength and tidal trapping. Analysis of the density‐driven exchange reveals that it scales with the tidal salinity range raised to the power 3/2. Using this relationship, we derive an expression for the dispersion coefficient associated with density‐driven tidal trapping. This formulation indicates that the resulting dispersion is governed by the main‐channel tidal excursion length and the propagation speed of the density current within the trap, and that the dispersion coefficient scales with the square root of the along‐channel salinity gradient, in contrast to tidal trapping driven by basin filling and emptying, which is independent of this gradient.