Salt transport regimes caused by tidal and subtidal processes in narrow
Tidally averaged transport of salt in estuaries is controlled by various
subtidal and tidal processes. In this study we show the relative
importance of various subtidal and tidal transport processes in a
width-averaged sense. This is done for a large range of forcing and
geometric parameters, which describe well-mixed to salt wedge estuaries.
To this end we develop a width-averaged process-based model aimed at
conducting and analysing a large number of experiments
(~40,000). We find that the salt transport is dominated
by one of seven salt transport balances, or regimes. Four of these
regimes are dominated by subtidal processes, while the other three are
dominated by tidal processes. Which regime occurs in a part of an
estuary depends on four dimensionless parameters, representing local
geometry and forcing conditions. One of the regimes features salt import
by correlations between the depth-averaged tidal velocity and salinity.
While this mechanism was previously only associated with along-channel
geometric variations, we find it can also be a dominant mechanism in a
significant part of the parameter space due to river-induced tidal
asymmetry, independent of river geometry. We apply our classification to
a case study of part of the Dutch Rhine delta and compare to
decomposition results of a fully realistic three-dimensional model. We
find the estuary features two regimes, with import dominated by subtidal
shear transport in the seaward part of the estuary and by depth-averaged
tidal correlations in the landward part of the estuary.