- 1Deltares, Delft, Netherlands (wouter.kranenburg@deltares.nl)
- 2Department of Environmental Sciences, Wageningen University, Wageningen, The Netherlands
- 3Department of Hydraulic Engineering, Delft University of Technology, Delft, The Netherlands
A good understanding of dispersion mechanisms in estuaries is essential to understand the transport of physical and biogeochemical constituents in these systems. In well-mixed estuaries, up-estuary transport of salt is often dominated by tidal dispersion mechanisms. One such a mechanism is tidal trapping, where volumes of water are temporarily stored in dead zones, side channels or harbor basins adjacent to the main channel and released later on in the tidal cycle.
In this study, we analyze the dynamics and quantify the dispersive contribution of tidal trapping using an idealized numerical model. We take into account that this trapping can be the result of a diffusive exchange between the channel and trap, but also from the filling and emptying of the trap by a tidal flow, which is leading in phase compared to the tidal flow in the main channel. We systematically compare the dispersion effect for both types of channel-trap exchange, for combinations thereof and for the case where the water in the trap is mixed before returning to the main channel.
The results show that the largest trapping induced salt flux is obtained with advective out-of-phase exchange for the largest realistic tidal flow velocity phase difference of 90 degrees. This result is different from literature and we explain why. For small velocity phase differences, mixing of the trapped salinity field before release enhances the dispersive effect. A continuous diffusive channel-trap exchange on top of the advective exchange enhances the dispersive effect of the trap 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 excursion length; secondly, the altered salinity gradients are advected in both up- and down-estuary direction, influencing the tidal salt flux over a distance twice that of the tidal excursion length.
These insights in salt dispersion contribute to the understanding of transport in estuaries.
How to cite: Kranenburg, W., van Keulen, D., and Hoitink, T.: Tidal trapping and its effect on salinity dispersion in well-mixed estuaries, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20272, https://doi.org/10.5194/egusphere-egu25-20272, 2025.
