Salt Transport Regimes Caused by Tidal and Subtidal Processes in Narrow Estuaries

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 analyzing a large number of experiments (similar to 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. Plain Language Summary The distribution of salt water in an estuary is controlled by the balance of various processes related to tidal flows and subtidal processes. We investigate which processes are most important in determining this salt distribution for a wide range of parameter configurations including depth, river discharge, and tidal flow, thereby representing many types of estuaries found around the World. To this end, we develop a width-averaged model that is specifically suitable for investigating a large number of configurations (similar to 40,000). We find that different processes are important for different parameter configurations, resulting in seven possible balances of processes, or regimes. The regime found in each part of an estuary is determined by four dimensionless parameters. We apply our classification to part of the Dutch Rhine delta and show that this estuary features two regimes depending on the location along the estuary.