The flux Richardson number Rf, also called the mixing efficiency of stratified turbulence, is an important property of vertical mixing in oceans, lakes and the atmosphere. Measuring Rf in the field is usually difficult, thus parameterization of Rf based on readily observed turbulent properties is essential for quantifying ocean mixing. Estimates of Rf in an estuarine, sediment-stratified turbulent flow are obtained from measurements of covariance-derived turbulent buoyancy fluxes (B) and spectrally fitting values of the dissipation rate of turbulent kinetic energy (ε). We test scalings for Rf in terms of the buoyancy Reynolds number (Reb), the gradient Richardson number (Ri), and turbulent Froude number (Frt). Neither the Reb-based or Ri-based scheme is able to describe the observed variations in Rf but the Frt-based parameterization works well. This finding supports the further use of the Frt-based parameterization in turbulent oceanic environments.