Graduation of Tess Wegman

Due to climate change and human interventions saltwater intrusion is becoming an increasingly relevant topic of concern worldwide. Saltwater intrusion is the flow of saline(sea)water into freshwater bodies and has adverse consequences for the environment and economic activities. The Dutch Rhine Meuse delta has an open connection with the sea by the New Waterway (NWW) and is therefore susceptible to saltwater intrusion. Periods of low river discharge are the most critical conditions for saltwater intrusion.

Generating resonant internal waves over dredged bottom topography is an idea to reduce salt intrusion length. Internal waves can induce shear instabilities or wave breaking, and hereby contribute to increased turbulent transport of mass and momentum, and hereby have the potential to decrease the salt intrusion length. The objective of this study is to investigate whether internal waves generated over undular bottom topography in the NWW can generate a significant additional amount of vertical mixing and thus reduce stratification. The hypothesis is that this additional vertical mixing breaks down stratification, with the underlying assumption that the decrease in stratification decreases horizontal salt intrusion.

In the numerical study, FinLab, a finite element numerical modelling tool which includes the nonhydrostatic processes is applied. Simulations of a 2D channel stretch with sinusoidal bottom topography and a linearly stratified fluid and a linearly varying background velocity are run.

The main conclusions are that the generation of internal waves and the generated internal wave energy as found in the modelling results are in line with the theory. The amount of kinetic energy as function of the vertical velocity is a good measure from the model results to indicate occurrence of internal waves. The first resonant mode is most energetic, however the total energy found for these waves is only a few percent of the amount of energy required to fully mix a stratified water column. For the observed trapped waves the only instabilities that could transfer internal wave energy to TKE are shear instabilities near the bed. The extent of those shear instabilities increases with increasing bed wave amplitude. Over the full simulation the net vertical buoyancy transport is of negligible magnitude. The total potential energy, does show significant relative increase between 6% and 99% compared to a similar case without bed waves. This increase in potential energy is caused by the interaction of bed friction and flow velocity, which is enhanced during the presence of internal waves.