Turbulent features of nearshore wave-current flow

Abstract

Waves and currents influence nearly all nearshore physical processes. Their complex interaction gives birth to complex turbulence features that are far from being completely understood. In this regard, previous studies mainly focused on mean flow or inferred turbulent features from averaged velocities, seldom examining turbulent fluctuations. Moreover, the dynamics of wave-current flow have mostly been replicated in experimental channel setups, i.e., overlooking the natural occurrence of waves and longshore currents intersecting at a near-orthogonal angle. In the present work, the hydrodynamics of near-orthogonal wave-current interaction are investigated through a physical model study. Experiments were carried out in a laboratory basin in the presence of fixed sand and gravel beds, where current-only, wave-only, and combined flow tests were performed. Flow velocities were measured by means of acoustic Doppler velocimeters, through which time-averaged, phase-averaged, and turbulent velocities were obtained. Results revealed two main features of the wave-current flow. First, we observed that the superposition of waves does not necessarily induce an increase in the current bed shear stresses. Indeed, depending on bed roughness, current freestream velocity and wave orbital velocity, enhancements or reductions of the current bed shear were observed. Moreover, application of quadrant analysis revealed a periodic evolution of the current turbulent bursts. Specifically, the number of current turbulent ejections and sweeps is reduced or increased as the wave phase progresses from antinodes to nodes and from nodes to antinodes, respectively.