Drag reduction and shear stress saturation caused by tide-induced high suspended sediment concentration in the bottom boundary layer

Having been discovered in both the atmosphere-ocean and ocean-seabed boundary for decades, drag reduction in the bottom boundary layer (BBL) has aroused widely attention in both academic and engineering area and plays an essential role in the physical processes such as turbulent mixing and transport of mass, momentum and heat. Yet, no consistence has been reached regarding to its mechanism owing to the complexities and difficulties involved to test the proposed theoretical hypothesis practically. An explicit relationship has been put forward in this study that the bottom drag coefficient C_d increases with current velocity in the BBL until a tipping point, over which C_d decreases inversely with the increasing velocity and the associated incrementing suspended sediment concentration (SSC). The in situ observations over an intertidal flat approved such a relationship, particularly indicating that drag reduction occurred with turbulent diffusivity tending towards a constant as the current velocity reached 0.4–0.5 m s^{− 1} with a high SSC of 600–800 mg L^{− 1} under normal weather conditions with mild winds. This suggested that turbulence was damped as a result of the high SSC in the BBL, which serves as an isolated layer to damp/block the drag/turbulence. In contrast, the turbulence or mixing being induced by tough waves under windy conditions turned out to be so intense as to penetrate the whole water column to reach the seabed. More supports of the formation of the high-SSC-resultant drag-reduction layer came from the observations that drag reductions occurred commonly in a series of environments with various sediment dynamic conditions at varied critical current velocities, though. The causal linkage of “high velocity – high SSC – weakened turbulence on the bottom – drag reduction” provides a promising and tangible start point, from which the mechanism of drag reduction and thus turbulence weakening might be revealed hopefully.