Salinity Intrusion in the Rhine Meuse Delta: Estuarine Circulation or Tidal Dispersion?

The Rotterdam Waterway is a partially stratified estuary in the Rhine Meuse Delta, The Netherlands. Salt intrusion through this waterway regularly threatens fresh water availability and is expected to increase because of channel deepening and changing river discharge characteristic. Various mitigation measures are currently considered, based on the idea to reduce salt intrusion by enhancing vertical mixing. To develop effective measures, a proper understanding of the salt intrusion mechanisms in this system is essential.

In partially stratified systems, salt intrusion can be driven by gravitational circulation (Hansen and Rattray, 1965). The transport can be further enhanced by ebb-flood asymmetry in stratification (Simpson et al. 1990) and vertical shear (Jay & Musiak, 1994) leading to ‘tidal straining circulation’ (Burchard et al, 2011), all related to stratification. However, also tide-correlated mechanisms like tidal trapping (Okubo, 1973) and tidal pumping (Fischer, 1979) can lead to salt intrusion, both in stratified and non-stratified systems.

In this study, we determine the contribution of the various dispersion mechanisms to salt intrusion in the Rhine Meuse Delta. For this we use flow and salinity results from 3D model simulations, and decompose the salt flux at various locations along the estuary into run-off related, estuarine circulation related, time-dependent shear related, and (depth-averaged) tide related flux contributions, following Lerczak et al. (2006). We translate these into ‘Effective Dispersion Coefficients’ and study their relationship with run-off, tide and wind induced set-up. The results show that in this system – even though it is mostly stratified – tide-related fluxes can become more important than stratification-related fluxes. Their relative contribution increases with decreasing river discharge and with set-up. These findings on processes have implications for development of effective mitigation measures.