Mixing contributions from resonant trapped internal waves generated by bottom topography in an estuary

The Rotterdam Waterway is part of the Rhine-Meuse estuary, which is characterized a salt wedge estuary. Therefore, it is persistently strongly stratified. Field observations in the Rotterdam Waterway, described in earlier literature, reveal internal waves (IWs) generated by resonance over undular bottom topography. IWs are widely found in estuarine and coastal regions, and can contribute to mixing in stratified bodies of water. In this study we explore the generation of IWs over a series of sinusoidal bed forms and their potential of mixing.

An idealised 2D stretch of an estuary, containing sinusoidal bottom topography, is modelled in the non-hydrostatic finite element numerical model FINLAB. The effects of varying wavelength and wave height of the undular topographic features on internal wave generation and vertical mixing are evaluated.

From the model results we find that the generation of the resonant internal wave modes are in correspondence with an analytical analysis based on linear theory. Our results show that in the case of bed form induced internal waves, vertical mixing in the short 2D stretch increases, compared to a flat bed. This is predominantly caused by an increase in bottom friction. This suggests that the trapped internal waves only give a relatively small contribution to this increase in vertical mixing in the area of generation.

Further investigations are required to quantify the contribution from internal waves to vertical mixing, once the waves start to propagate through the domain. Furthermore, the model results will be compared to recent observations of internal waves in the Rotterdam Waterway. Internal wave characteristics and the generation mechanism will be compared to the model results.