North Sea Bottom Steady Boundary Layer Measurements

The steady‐flow boundary layer was studied with measurements of tidal current and wave velocity made at 0.10, 0.25, 0.50, 1.00, 3.00, and 5.00 m above the sea floor at two locations in the North Sea in 70‐m and 102‐m water depths. The data cover a range of combined current and wave conditions. The logarithmic boundary‐layer flow model was successfully fitted to the current measurements using the roughness $z_0$ and friction velocity $u^{*}$ as fitting parameters. The measurements and analysis results demonstrate the existence of large boundary‐layer current gradients in typical offshore locations and large values of $z_0$ and $u^{*}\text{.}$ They also show the dependence of these parameters on the ratio of wave flow to the current flow component as well as the bottom roughness. The measurements further demonstrate that the boundary‐layer profile can vary greatly for the same general location due to movable bottom effects and general ocean turbulence. Theoretical model predictions for the wave‐current interaction effect and the movable bottom effects showed limited success. The findings here can be used to estimate hydrodynamic forces on marine pipelines.