The flux Richardson number R_f, also called the mixing efficiency of stratified turbulence, is an important property of vertical mixing in oceans, lakes and the atmosphere. Measuring R_f in the field is usually difficult, thus parameterization of R_f based on readily observed turbulent properties is essential for quantifying ocean mixing. Estimates of R_f 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 R_f in terms of the buoyancy Reynolds number (Re_b), the gradient Richardson number (Ri), and turbulent Froude number (Fr_t). Neither the Re_b-based or Ri-based scheme is able to describe the observed variations in R_f but the Fr_t-based parameterization works well. This finding supports the further use of the Fr_t-based parameterization in turbulent oceanic environments.