On the effect of tidal variations of turbulent mixing on flow and salt transport in estuaries

The intrusion of sea water into estuaries creates a complex flow that results from the density difference between fresh river run-off and salty sea water. In combination with other processes, such as tides, these complex flows are of interest because they affect the transport of e.g. salt, sediments and nutrients in water, which determine the future shape and ecology of the estuary. This research focusses on mechanisms that result in exchange flows and transport of salt. The model study has led to new insights into the way in which tides and baroclinic pressure gradients contribute to the straining circulation, i.e. the exchange flow that is induced by the interaction of temporal variations of turbulence and velocity. The straining circulation is traditionally associated with the tidal flow. To make a distinction between this traditional view and the new findings, we will call this the {\em tidal straining circulation}. It is shown that the tidal straining circulation explains only a small part of the total straining circulation in a parameter space typical for well-mixed and partially stratified estuaries. This research identifies a new and more important contribution to the straining circulation. This is caused by interactions between the gravitational circulation and temporal variations of turbulent mixing, which we will call the {\em gravitational straining circulation}. The gravitational straining circulation increases non-linearly with increasing temporal variations of turbulence. Large tidal variations of turbulent mixing are typically found in well-mixed and partially stratified estuaries. Such temporal variations of turbulence can be caused by strain-induced periodic stratification (SIPS), asymmetric mixing or symmetric variations of mixing, such as the variation of turbulence with the tide. The dominant contribution of gravitational straining circulation to the total straining circulation explains why the straining circulation is larger than the gravitational circulation in partially stratified estuaries and why both the gravitational circulation and the straining circulation have the same dependency on the along-channel salinity gradient. It also explains why the straining circulation is much smaller in strongly stratified estuaries, where the tidal variations of turbulence are not as large as in partially stratified estuaries. The direction and magnitude of the gravitational straining circulation is additionally shown to be independent of the timing of temporal variations of turbulent mixing. The magnitude and direction of the tidal straining circulation depend not only on the timing of temporal variations of mixing, but are shown to also depend on the bed roughness and the rate of mixing. This implies that the tidal straining circulation can act in the opposite direction as is expected from current theory in certain model parametrisations. Concerning the transport of salt, it is shown that temporal variations of turbulent mixing are able to create a strong salt transport. A potentially large salt transport is caused by temporal correlations of the salinity and the velocity. This salt transport contribution is shown to be highly sensitive to the phase of the salinity, which is strongly dependent on the phase of turbulent mixing. Accurate modelling of turbulence is therefore essential to obtaining accurate results for the salinity.

Subject

estuary
exchange flow
salt intrusion
perturbation model
turbulence modelling
estuarine circulation
straining circulation

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Embargo date

2015-11-14

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Student theses

Document type

master thesis

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