Impact of Wind on Tide-Induced Advective Salt Transport in A Well-Mixed Estuary

The tide-induced net advective salt flux in well-mixed estuaries consists of five terms according to the method from Kjerfve. The term resulted from the vertical variation in salinity can be negligible in well-mixed estuaries with four tide-induced salt flux terms, known as F₁-F₄. To explore the effects of wind on these salt fluxes, the current-salinity analytical model combined with the perturbation analysis is extended by including wind. Analytical expressions for the four salt fluxes are derived separately in the present model. Under the assumption that only the M₂ tidal component is accounted for and the salt flux generated by diffusion is not studied, the tide-induced net advective salt flux Q_sx is in the seaward direction without the wind effect. By applying the Western Scheldt estuary case, the wind influence on the tidal advection salt flux (TASF) distribution in the F₄ term was investigated. The phase difference between zero-order velocity and first-order salinity (Δϕ) at the surface layer of the estuary is larger than 90° and smaller than 90° at the bottom layer, which leads to landward TASF in the surface layer and seaward TASF in the bottom layer. The distribution of Δϕ is not uniform in the horizontal direction with wind included, which differs from the result without wind. In the case of seaward wind with the speed of 18 m/s, the decrease in the zeroth-order velocity phase (ϕ_u) at the surface layer is larger than that of the first-order salinity phase (ϕ_s) downstream, which leads to an abnormal seaward TASF in this region. Owing to the surface stress caused by wind, the Stokes compensation flow in the middle and lower reaches increases/decreases with the increase of the landward/seaward wind, while the upstream situation is opposite. Thus, the first-order velocity in the middle and lower reaches increases/decreases with the increase of the landward/seaward wind, while the upstream situation is also opposite. The first-order salinity also increases/decreases with the increase of landward/seaward wind, while the upstream salinity tends to zero. Therefore, the tide-induced net advective salt flux Q_sx increases/decreases with the increase of the landward/seaward wind, which is contrary to the usual recognition.