TABLE OF CONTENTS In well-mixed estuaries, salinity penetrates through the process of mixing while the river discharge flushes it back towards the sea. This struggle for dominance can have two winners: the mixing, and in that case we observe an increasing salinity over time; or the river flow, and then we see the estuary water become fresher. When the two mechanisms tie, we have a steady-state situation where the salinity remains constant over time. What remains is a longitudinal gradient of the salinity, gradually diminishing in upstream direction from sea salinity at the mouth to fresh water at the toe of the salt intrusion curve.
There are several mixing mechanisms that vary in importance depending on the shape of the estuary, the location, the level of stratification, the density, and the strength of the tide. One can distinguish different types of mixing, such as mixing by turbulence, mixing by tidal shear, mixing by residual currents, mixing by trapping, and density-driven mixing. These mechanisms will be described in the following sections. All these mixing mechanisms together drive longitudinal dispersion of salinity, which can be decomposed into many smaller constituting fluxes. There are different methods for flux decomposition, but we shall see that this approach does not really lead to practical results. In order to obtain a predictive model, we require a predictive equation for the effective longitudinal dispersion. This one-dimensional (1D) predictive equation will be derived and illustrated by empirical data. A general equation that integrates all mixing processes will be presented.
