Channel-harbour exchange and its influence on salinity dispersion in a partially stratified branch of the Rhine-Meuse estuary

Salt intrusion is known to be influenced by harbours and side channels. While the contribution of these features to tidal dispersion is well established in well-mixed estuaries, the governing processes in partially stratified system have remained understudied. We investigate the channel-harbour exchange in the New Meuse, a partially stratified branch of the Rhine-Meuse estuary.

The harbour basins subject to study are located just upstream of the junction with the Old Meuse and the New Waterway, in a region characterized by large gradients in the salinity range over short distances. During a field campaign, four shipboard surveys were conducted to study the channel-harbour exchange at two harbour basins under spring and neap tide conditions.

Decomposition of the instantaneous salt flux, aimed to unravel the exchange between the channel and the harbours, revealed large differences in the contribution of a continuous density-driven exchange. These differences were confirmed by numerical modelling of the systems. The reduced vertical exchange is attributed to a weaker salinity gradient in the main channel in front of the harbour entrance, which limits the pressure gradient between the harbour and the channel. Stark differences in the salinity range were found to be predominantly the result of interactions between the branches.

The numerical model was subsequently used to set up a balance to quantify the up-estuary salt flux resulting from the channel-harbour exchange (tidal trapping) for the different harbours in the New Meuse. This analysis showed that harbours, where the salinity range in front of the harbour was weak, contribute significantly less to the up-estuary salt flux, primarily due to the reduced vertical exchange. Additionally, the contribution of tidal filling and emptying of the harbour basins, which typically drive the dispersive effect of traps in well-mixed systems, was found to contribute negatively to the up-estuary salt flux. The negative contribution of tidal filling and emptying is enhanced by atypical tidal salinity variations in the main channel, due to interaction between the branches.

This leads to the surprising conclusion that some of the largest harbours, where the density-driven exchange between the channel and harbour was observed to be weak, contribute the least to the up-estuary salt flux.