A comparison of velocity profiles in unidirectional currents and the wave boundary layer: Implications for sediment entrainment

Although current velocity profiles are commonly used to predict sediment entrainment under unidirectional currents, it has not been possible to use this method for waves because the nature of velocity profiles within the wave boundary layer was unknown. Based upon theoretical wave modelling that allows the thickness of the boundary layer and the boundary velocity to be determined in any water depth, combined with an empirical method to determine the critical boundary velocity, the different forms of the law of the wall for smooth, transitional and rough boundary conditions are evaluated for the wave boundary layer. The results show that velocity profiles in the wave boundary layer are of the same form as in unidirectional currents, but that the critical shear velocity differs from that of the latter. For smooth boundaries, the shear velocity necessary to entrain grains of a specific size and density is more than that required under unidirectional currents, but the inverse is true for fully rough conditions. This is attributed to additional forces under waves related to the balance between the static pressure and static pressure gradient.