Flow and turbulence in the bottom boundary layer of the coastal ocean

Lack of detailed turbulence data within the inner part of combined wave-current boundary layers is a long-standing knowledge gap in analysis of coastal bottom boundary layers (BBL). Such data are essential for validation and improvement of circulation models in addition to the study of local phenomena, e.g. sediment transport. In-situ particle image velocimetry (PIV) measurements, at an unprecedented resolution of 3.5 Kolmogorov scales, are performed in the inner part of the BBL. They enable independent estimation of vertical profiles of flow and turbulence characteristics, such as Reynolds shear stress, shear production rate of turbulent kinetic energy, and the dissipation rate, which are compared to previous studies of steady turbulent boundary layers over rough-walls.;When the wave velocity and current are similar in magnitude, the mean velocity profile has an inflection point near the interface between current and wave boundary layers, indicating a region of flow instability. Scaling of these profiles based on log layer parameters works well only above this inflection. Instabilities associated with the inflection are manifested by peaks in turbulent shear production rate and a rapid increase in small-scale turbulence, as is evident from trends of the dissipation rate. Both the shear production peak and rapid increase in the dissipation rate occur at higher elevations than values in rough-wall steady boundary layers. The transition between current and wave boundary layers is also characterized by broad Reynolds stress peaks and a shear production rate exceeding the dissipation rate.;The measurements also permit direct evaluation of assumptions used to estimate turbulence quantities from traditional oceanographic sensors, e.g., shear production - dissipation rate balance in the log layer. To evaluate the limitations of spectral curve-fitting techniques to estimate turbulent quantities, e.g., the dissipation rate, PIV data is spatially filtered in two-dimensions and its affect on energy spectra are examined. It is found that the spectral slope computed from time series data can be affected significantly by point sensor spatial resolution. Furthermore, due to the differing response of wave velocities and turbulence to spatial filtering, spectral slopes at the transition from turbulence to wave-dominated spectral ranges are modified making them inordinately steep.