Impact of Tidal Wave on Salinity Variation in Convergent Estuaries

As an environmental challenge, salt intrusion in estuaries has been an urgent issue across world countries. An estuary has both riverine and marine characteristics as it is the transition between a river and a sea. The salinity of the estuary water is the result of the balance between river and tidal fluxes, and mixing in between. In this study, we proposed an unsteady-state analytical solution of salt intrusion based on one-dimensional advection-diffusion equation for salinity, with assuming the harmonic tidal wave with multi-frequency and the mixing coefficient independent of distance. The analytical model of salinity variation retains the basic physical characteristics involved and offers a simple and efficient method to study salt intrusion in estuaries. It can directly reflect the influence of the interaction between tidal wave and runoff. The result indicates that the amplitude of the salinity is proportional to the river velocity and tidal flow amplitude, and inversely proportional to the dispersion coefficient as well as the tidal angular frequency. In addition, the unsteady analytical solution demonstrates that the phase lag between tidal wave and salinity transportation is π/2, which reveals the effect of river flow on the feature of the maximum salinity appears after high water slack and the minimum salinity appears before low water slack. The deduced analytical solution is illustrated with an application to the Humen estuary of the Pearl River Delta (PRD) in China, and proves to be an efficient and accurate approach to predicate the salt intrusion in convergent estuaries. The unsteady analytical solution is tested against 14 surveys made at six study sites over a complete spring-neap tidal cycle, to validate its capability of predicating salt intrusion variation. The results show that the proposed unsteady analytical solution can be successfully used to reproduce the temporal process of salinity under interaction of runoff and tidal flow in convergent estuaries.