Field measurements of turbulence at an unstable interface between current and wave bottom boundary layers

In situ particle image velocimetry measurements, at a resolution of 3.5 Kolmogorov scales, have been performed in the inner part of the coastal bottom boundary layer. The spatial details enable us to directly determine the vertical distributions of mean velocity, Reynolds shear stress, shear production and dissipation rates, energy spectra, and abundance of eddies. Focusing on cases with wave velocity of similar magnitude as the mean current, velocity profiles have logarithmic distributions in the upper half of the sample area. Below the log layer, but well above the bottom ripples, an inflection point appears, indicating a region of flow instability. Based on data interpretation, which includes variations in wave phase with height, this inflection occurs near the interface between current and thinner wave boundary layer (WBL) below it. Scaling of mean velocity profiles with shear velocity and characteristic roughness is effective only above the inflection point, while turbulence parameters scale reasonably well at all elevations. Instabilities associated with the inflection are manifested by a peak in turbulent shear production rate and a rapid increase in small-scale turbulence, as is evident from trends of the dissipation rate, energy spectra, and distribution of eddies with elevation. Therefore, the presence of a WBL generates a shear production peak and rapid increase in the dissipation rate at higher elevations than those found in rough-wall steady boundary layers. Transition between current and wave boundary layers is also characterized by broad Reynolds stress peaks and shear production exceeding the dissipation rate.