Indications for the transition of Kelvin-Helmholtz instabilities into propagating internal waves in a high turbid estuary and their effect on the stratification stability

Abstract

Internal waves (IWs) are an ubiquitous phenomenon in natural stratified fluids, including oceanic and coastal waters. Turbulence caused by their breaking can trigger vertical mixing of sediments, nutrients, and other dissolved substances. A major generation mechanism of IWs in coastal waters is the interaction of currents with topographic features. Additionally, current shear can lead to the development of Kelvin-Helmholtz instabilities, independent of the estuarine morphology. These instabilities normally form stationary rolled up vortices, which can also be imaged in echograms. In this study, we present new indications that propagating IWs in the Ems estuary arise from Kelvin-Helmholtz instabilities. The Richardson number drops below 0.25 at lutocline at the beginning of IW generation. However, the subsequently appearing lutocline undulations did not roll up, like the typical Kelvin-Helmholtz billows, but evolve into propagating and growing IWs of Holmboe type. Some of these waves were even subject to wave breaking. IW breaking occurred mainly during the second half of the ebb tides and is accompanied by vertical up-mixing of fluid mud. The turbulence and vertical mixing, caused by IW breaking, strongly decreases the local stratification by up to 60%.