Tidal Trapping and Its Effect on Salinity Dispersion in Well-Mixed Estuaries Revisited

Abstract

In well-mixed estuaries, the up-estuary salt flux is often dominated by tidal dispersion mechanisms, including tidal trapping. Tidal trapping involves volumes of water being temporarily trapped in dead zones or side channels adjacent to the main channel and released later in the tidal cycle, which causes an additional up-estuary salt flux. Tidal trapping can result from a diffusive exchange between a channel and a trap, or from filling and emptying of the trap by a tidal flow that is ahead in phase compared to the flow in the main channel (advective out-of-phase exchange). This study revisits the dispersive contribution from tidal trapping in a single dead-end side channel using an idealized numerical model. The results indicate that advective out-of-phase exchange yields the largest additional salt flux for the largest realistic velocity phase difference of 90∘. Mixing of the trapped salinity field enhances the dispersive effect for small velocity phase differences. A continuous diffusive channel-trap exchange also enhances the dispersive trap effect when the velocity phase difference is small, but can dampen it when the phase difference is large. We demonstrate that the effect of a trap is twofold: firstly, channel-trap exchange alters the salinity field and introduces an additional salt flux in the main channel over a distance equal to the tidal excursion length; secondly, the altered salinity gradients are advected in both up- and down-estuary direction, influencing the tidal salt flux over twice the excursion length.