Estuaries are complex systems influenced by many different parameters and processes, for example tidal currents from the ocean side and river discharge from the inland side. Accordingly, estuarine circulation is composed of many contributions, the knowledge of which is essential for a proper management of estuaries. The circulation contributions and their dependencies on the influencing parameters can be investigated by means of numerical hydrodynamic models.
The simulation results reveal that the tidal straining circulation is the dominant contribution in a large part of the parameter space. This circulation arises from tidal asymmetries of stratification and shear. In tidally energetic estuaries, stratification is generally diminished by the along-estuary flood currents and induced by the along-estuary ebb currents, but it can also be induced by across-estuary circulation. Depending on the cross-sectional shape of an estuary, the across-estuary circulation can even be much more efficient in inducing strong ebb stratification than the along-estuary currents. Thus, also the tidal straining circulation and the total, estuarine circulation depend on the shape, for example the depth-to-width ratio, of an estuary. Other influencing parameters identified to be important are the minimum-to-maximum depth ratio, the along-estuary salinity gradient and the across-estuary diffusivity.