Generation and evolution of high-frequency internal waves in the Ems estuary, Germany

The breaking of internal waves (IWs) is one of the most important factors for the vertical mixing of suspended sediments and nutrients in estuaries. Information on the generation and evolution of IWs in such environments is therefore required to better understand their physics and related processes of sediment transport. New data on highly resolved IWs recorded in the turbidity maximum zone of the Ems estuary (Germany) have provided new insights into the dynamics of estuarine IWs.To investigate the generation and evolution of IWs as well as their contribution to vertical mixing, a variety of hydro-acoustic devices was deployed in combination with a vertical sampling of suspended particular matter. Wave parameters such as significant wave height, wave frequency, wave length and steepness were computed from these data.The hydro-acoustic data reveal the formation of a prominent lutocline during slack water, at which IWs begin to be generated with the onset of tidal forcing. The two water bodies, which are characterised by markedly different suspended sediment concentrations, show clear differences in flow behaviour. As a consequence of current shear along the lutocline, Kelvin–Helmholtz instabilities are generated, which then produce IWs.The IWs break when high shear stresses between the two layers are coupled with great wave steepness, and the breaking causes vertical mixing of the sediment. Most IW breaking events occur during the decelerating ebb phase and thereby promote downstream sediment transport.

► Internal waves are generated by shear instabilities along the lutocline.
► Internal waves are generated during the onset of ebb current.
► Internal wave breaking occurs mainly during ebb tide.
► The fluid mud and the water layer show different responses to tidal forcing.
► Three different hydro-acoustic devices revealed the same phenomena.