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Modeling turbulent mixing and sand distribution in the bottom boundary layer

For the calculation of turbulent mixing in the bottom boundary layer, we present simple analytical tools for the mixing velocity wm and the mixing length lm. Based on observations of turbulence intensity measurements, the mixing velocity wm is represented by an exponential function decaying with z. We suggest two theoretical functions for the mixing length, a first lm1 obtained from the k-equation written as a constant modeled fluctuating kinetic energy flux and a second lm2 based on von Kármán's similarity hypothesis. These analytical tools were used in the finite-mixing-length model of Nielsen and Teakle (2004). The modeling of time-mean sediment concentration profiles C(z) over wave ripples shows that at the opposite of the second equation lm2 which increases the upward convexity of C(z), the first equation lm1 increases the upward concavity of C(z) and is able to reproduce the shape of the measured concentrations for coarse sand.