A numerical model of the combined wave and current bottom boundary layer

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Abstract
A numerical model of oscillatory rough‐turbulent boundary layer flow, featuring closure via the turbulent energy equation, is used to examine the principal features of wave‐current interaction above the seabed. Results are presented from case studies carried out with water depth of 10 m, bed roughness of 0.5 cm and wave period of 8 s. The steady surface current in the absence of waves is nominally 100 cm/s, and waves having near‐bottom velocity amplitudes of 50, 100, and 150 cm/s are superimposed on this current at angles ϕ = 0, π/4, and π/2. Velocity, turbulent energy, shear stress, and eddy viscosity distributions are compared for the steady current alone, for waves alone, and for waves superimposed on the current, and the interaction in the wave‐current boundary layer is examined. Cycle‐averaged vertical profiles of horizontal velocity illustrate the extent to which the mean flow is retarded by wave‐current interaction. For the general case in which the waves are superimposed at an arbitrary angle ϕ, the mean current veers to form an angle with the wave direction greater than that of the initial steady current. Finally, the enhancement of the bed shear stress and the increase in oscillatory boundary layer thickness associated with wave‐current interaction are both quantified.