van Keulen Daan, Kranenburg Wouter M, Hoitink Antonius J F
Department of Environmental Sciences, Wageningen University, Wageningen, The Netherlands.
Deltares, Delft, The Netherlands.
Estuaries Coast. 2025;48(6):153. doi: 10.1007/s12237-025-01579-0. Epub 2025 Jul 30.
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.
在充分混合的河口，向上游的盐通量通常由潮汐扩散机制主导，包括潮汐捕获。潮汐捕获是指一定体积的水暂时被困在与主河道相邻的死水区或侧通道中，并在潮汐周期的后期释放，这会导致额外的向上游盐通量。潮汐捕获可能源于河道与捕获区之间的扩散交换，也可能源于与主河道水流相比相位超前的潮汐流对捕获区的充水和排水（平流异相交换）。本研究使用理想化数值模型重新审视了单个死端侧通道中潮汐捕获的扩散贡献。结果表明，对于90°的最大实际速度相位差，平流异相交换产生的额外盐通量最大。捕获盐度场的混合在小速度相位差时增强了扩散效应。当速度相位差小时，连续的扩散性河道-捕获区交换也会增强捕获区的扩散效应，但当相位差大时会减弱这种效应。我们证明，捕获区的作用是双重的：首先，河道-捕获区交换改变了盐度场，并在等于潮汐偏移长度的距离上在主河道中引入了额外的盐通量；其次，改变后的盐度梯度在上下游方向上平流，影响了超过两倍偏移长度的潮汐盐通量。
