Seawater intrusion related to estuarine circulation strongly decreases with increasing tidal velocity, but increases with increasing river discharge (increase relative to tidal discharge amplitude). In the case of strong stratification and resulting suppression of turbulent mixing, the strength of the estuarine circulation depends primarily on the tidal velocity. The strength of the estuarine circulation increases with increasing river discharge whereas the seawater intrusion length decreases. In the case of weak stratification and strong vertical mixing the river discharge opposes the intrusion of seawater. It shows the important role of density stratification for the suppression of turbulent vertical mixing. However, insight in the seawater intrusion process and some rough estimates of the contribution of estuarine circulation can already be obtained from the classical analytical approach. Nowadays, the intrusion if seawater in an estuary can be simulated in numerical models that incorporate detailed descriptions of turbulent exchange processes and complex bathymetries, see for example Scott (1994), Jay and Musiak (1996) and Burchard et al. The role of tidal motion is reduced to the generation of turbulence at the channel bed, which produces turbulent eddies that exchange of mass and momentum across the water column. This theory considers a stationary salinity field in a frame that moves cyclically up and down the estuary while vertical mixing processes are represented by a tide-averaged and depth-averaged diffusivity. In the classical theory of estuarine circulation, advection of seawater up and down the estuary by the tidal motion and associated mixing processes are not considered explicitly.
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Many other authors have subsequently contributed to further verification and refinement. The classical theory of estuarine circulation was first developed by Hansen and Rattray in 1965, after earlier investigations by D. In that case a more or less stationary salt wedge forms along the bottom this case is discussed in the article Salt wedge estuaries. In estuaries with little tide there is hardly any mixing of seawater with river water. The different processes that drive estuarine circulation are discussed in this article. Most of the mixing takes place vertically in the water column due to turbulence generated by tidal flow along the estuarine bottom. Mixing is the result of turbulence, which in estuaries is mainly caused by strong tidal flows. The circulation is kept going by mixing of seawater with outflowing river water.
It is driven by the longitudinal density gradient along the estuary: higher-density seawater water in the downstream part of the estuary 'diving' under lower-density mixed water in the inner estuary. This circulation is called estuarine circulation or gravitational circulation. In this way, a circulation of seawater is created, which is directed inland along the bottom and seaward along the surface, see figure 1. This allows 'new' seawater to enter the estuary via the lower water layers.
Part of the intruding seawater will mix with river water, whereby seawater is returned to the sea by estuarine outflow in the upper water layer. In estuaries where seawater and river water meet, seawater will tend to intrude beneath the river outflow. The density difference is of the order of 2.5%.
Seawater has a higher density than river water, due to its high salinity (see the article Seawater density). Figure 1: Schematic representation of the intrusion of seawater (dark blue) in a longitudinal section of a prismatic estuary.
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