Numerical investigation of river discharge and tidal variations impact on the salinity in deltaic systems

The present project is an effort to identify key parameters and controlling factors of salinity variations in deltaic systems. The salinity’s driving mechanisms and response to hydrodynamic changes are under investigation. This is demanded because of the risks that these ecosystems are subject to by increases of salinity due to sea level rise and human activities. Numerical modelling is implemented to study the salinity distribution in idealized deltas configuration under various hydrodynamic conditions. In particular, variations of the river discharge and tidal levels are considered. The results indicate that salinity undergoes significant seasonal changes due to dynamic flow variability. Sustained drought periods could result in critical conditions with salt intrusion in the delta’s trunk (fluvial) channel. On the contrary, high river discharges during wet periods could mix completely the vertical column with freshwater for at least some distance from the river mouth. Tides could contribute further in mixing improving the conditions in the delta by decreasing the salinity in the downstream delta areas. Hence, many tidally influenced deltas could contain higher freshwater volumes than river dominated systems. Nevertheless, tidal level increases could limit significantly the deltas freshwater availability in mesotidal or macrotidal regimes. Results analysis and implementation of simple and innovative techniques uncovered the existence of certain correlations that could be useful as prognostic tools of salinity. Channels classification by their class or number of links provides a correlation that shows the salinity to increase with the decrease of either the channels order or delta’s cross-section width. In addition, radial and depth averages of salinity are negatively and exponentially correlated with the river discharge. The extracted exponential equation summarizes successfully the complex 3D delta dynamics into a 1D analytical solution. The equation resembles theoretical solutions of the 1D advection-diffusion equations under certain theoretical assumptions of which many are satisfied in the present idealized models setup. The above correlations incur modifications when external (e.g. hydrodynamic forcing) or intrinsic (e.g. bathymetry) parameters cause changes to the spatial salinity distribution. Bathymetric effects affect the salinity distribution in both horizontal and vertical directions especially at low flow periods when the bottom friction becomes stronger. Increases of the river discharge and/or tidal level override these effects. This study aspires to provide knowledge that could be useful for the development of technical or non-technical solutions for the aversion of salinization issues in deltas. With the goal of contributing to the efforts for the development of environmentally friendlier solutions, the last part of this work investigates the effect of different shape but equal volume hydrographs on the salinity. The results show that it is possible to alleviate the consequences of increased salinity by better management of existing water resources instead of seeking for additional ones