Investigating the impact of river flow and tidal level variations on river delta salinization through idealized numerical modelling

The world’s river deltas are increasingly vulnerable due to pressures from human activities and environmental change. In deltaic regions, the distribution of salinity controls the resourcing of fresh water for agriculture, aquaculture and human consumption; it also regulates the functioning of critical natural habitats. Excessive salinity can harm the sustainability of these delicate ecosystems and compromise the various anthropogenic activities taking place there. Currently, many deltas face the consequences of increased salinity due to sea level rise. Salt intrusion (i.e., upstream intrusion of the saltwater zone) is further exacerbated by shortages to freshwater availability due to changes in the hydrological cycle or upstream river diversions. Despite numerous insightful studies, there are still significant uncertainties on salinity’s spatio-temporal patterns response to changes in river flow and tidal range. In this study, an effort is done to fill this gap through idealized three-dimensional numerical modelling of a typical delta configuration. A series of simulations is carried out considering seasonal freshwater flow and tidal level variations. Model results demonstrated the existence of simple correlations and relationships describing the salinity field in a delta. In particular, salinity and river discharge are exponentially correlated through an equation that shows similarities to solutions of the one-dimensional advection-diffusion equation. The use of stream labelling methods (e.g., Strahler-Horton, width function) disclosed another correlation with salinity increasing as the channels’ order decreases.  In addition, small increases of the tidal amplitude in river-dominated or low tidal regime cases were found to have positive effects against salinization because of tide-induced mixing that can increase freshwater areas and volumes. Finally, results from simulations of different flow regimes indicated that it is possible to mitigate deltas’ salinization by water regulations and better use of existing resources instead of resorting to expensive and harmful for the environment technical solutions.