Wave, current and bottom topographical interactions in the coastal ocean bottom boundary layer

Abstract

A submersible PIV system has been deployed to characterize mean flow and turbulence in the inner part of the coastal ocean bottom boundary layer (BBL), and their interaction with the bottom roughness. Five large datasets, each consisting of ~20,000 velocity distributions, are chosen for analysis to represent flow conditions spanning a range of relative wave-current magnitudes, and ripple height and orientations. In each case, vertical distributions of mean velocity, Reynolds stresses, production and dissipation rates and spatial energy spectra have been determined. In cases where the amplitude of wave induced motions are similar or greater in magnitude to the mean current, there is an inflection point in the mean velocity profile, presumably at the interface of the thin wave boundary layer (WBL) and the much wider mean current boundary layer. Furthermore, the so-called “hydrodynamic roughness” felt by the current boundary layer, as determined from the mean profiles, is substantially larger than that determined from the measured roughness characteristics. When the current is more than 70% larger than the wave amplitude, the inflection disappears, and the hydrodynamic roughness falls below the measured values. As expected, velocity profiles vary from point to point within one roughness height away from the bottom, but collapse at higher elevations. The Reynolds shear stress profiles peak at the lower portion of the log layer, and decrease with increasing elevation further above. The Reynolds stress profiles do not collapse when scaled using either the wall unit or the roughness height as length scales. The shear production rate profiles appear to form three distinct groups based on relative wave-current magnitude and roughness orientation with the mean flow, peaking either at the top of the roughness elements or just above the inflection point. In most cases, production and dissipation converge within the log layer, but trends differ below it. Spatial energy spectra exhibit bumps at wavenumbers corresponding to 1-3 roughness heights, which are indicative of excess small scale energy produced by interaction of large scale turbulence with the roughness. With decreasing elevations, spectra become increasingly anisotropic, and an inertial range develops only at low wavenumbers at high elevations.