The turbulent boundary-layer flow over flat rough beds due to a wave or to combined wave-current interaction is studied using a numerical turbulence closure model of K-L type. Different cases of modelling are discussed and illustrated by numerical examples. The model was run for a wide range of parameters of wave, current, bed roughness and angle of interaction. The curves of dimensionless mean and maximum bed shear stresses obtained present the non-linearity of the wave-current interaction. The parameterization of results is realized in order to facilitate the incorporation of bed shear stresses into coastal morphodynamic models.