Salt Intrusion in Estuaries

Abstract

In a tidal river, salt water from the sea tends to penetrate due to its slightly greater density. The form in which this happens depends on the tidal influence. If there is hardly any tide, the salt water will penetrate underneath the fresh river water as a “salt wedge”. Far more common is the situation where a strong tidal action takes care of a mixing process, such that the water in a particular cross-section has an almost uniform salt concentration; however, this concentration varies in longitudinal direction. This is called the well-mixed case. The salt concentration is now governed by the convection-diffusion equation (11.4) with two complications:

• (i) the hydrodynamic variables A and Q are now functions of space and time, to be computed as discussed in Chapter 16;

• (ii) the diffusion coefficient is influenced by the longitudinal gradient of salt concentration. This is a rather complicated matter (cf. Thatcher and Harleman 1972),that is not discussed here.