The oscillatory nature of waves generates a thin boundary layer above the sea bed in which the fluid velocity decreases from its free stream value to zero at the bed. The wave boundary layer thickness is small compared to the current-alone boundary layer (which typically extends throughout the water depth) and, as such, is characterized by a zone of high shear velocity and associated high levels of turbulence. Strong turbulence intensities within the wave-induced boundary layer have an impact on the ambient current field, which experiences an increase in bottom resistance (the so-called ‘apparent roughness’) due to the presence of the turbulent boundary layer.

For currents on the continental shelf, it is well known that the effect of wave-enhanced bottom friction (or “apparent roughness”) is important in controlling the near-bed current speed and turbulent mixing. It can thus be reasonably expected that accounting for this apparent roughness will influence the design basis for sub-marine pipelines that cross the continental shelf. In this paper, we implement — and then demonstrate — the effect of wave-related apparent roughness on currents with the aim of providing a more accurate design basis for pipelines on the continental shelf.

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