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Abstract This study presents field observations of fluid mud and the flow instabilities that result from the interaction between mud-induced density stratification and current shear. Data collected by shipborne and bottom-mounted instruments in a hyperturbid estuarine tidal channel reveal the details of turbulent sheared layers in the fluid mud that persist throughout the tidal cycle. Shear instabilities form during periods of intense shear and strong mud-induced stratification, particularly with gradient Richardson number smaller than or fluctuating around the critical value of 0.25. Turbulent mixing plays a significant role in the vertical entrainment of fine sediment over the tidal cycle. The vertical extent of the billows identified seen in the acoustic images is the basis for two useful parameterizations. First, the aspect ratio (billow height/wavelength) is indicative of the initial Richardson number that characterizes the shear flow from which the billows grew. Second, we describe a scaling for the turbulent dissipation rate ε that holds for both observed and simulated Kelvin–Helmholtz billows. Estimates for the present observations imply, however, that billows growing on a lutocline obey an altered scaling whose origin remains to be explained. 

Abstract The flux Richardson numberR_f, also called the mixing efficiency of stratified turbulence, is important in determining geophysical flow phenomena such as ocean circulation and air‐sea transports. MeasuringR_fin the field is usually difficult, thus parameterization ofR_fbased on readily observed properties is essential. Here, estimates ofR_fin a strongly turbulent, sediment‐stratified estuarine flow are obtained from measurements of covariance‐derived turbulent buoyancy fluxes (B) and spectrally fitted values of the dissipation rate of turbulent kinetic energy (ε). We test scalings forR_fin terms of the buoyancy Reynolds number (Re_b), the gradient Richardson number (Ri), and turbulent Froude number (Fr_t). Neither theRe_b‐based nor theRi‐based scheme is able to describe the observed variations inR_f, but theFr_t‐based parameterization works well. These findings support further use of theFr_t‐ based parameterization in turbulent oceanic and estuarine environments.