Turbulence characteristics of a tidally driven bottom boundary layer of the coastal ocean

Seven sets of 2D Particle Image Velocimetry (PIV) data obtained in the bottom boundary layer of the coastal ocean along the South Carolina and Georgia coast (at the SABSOON site) are examined, covering the accelerating and decelerating phase of a single tidal cycle at several heights above the seabed. Additional data sets from a previous deployment are also included in the analysis. The mean velocity profiles are logarithmic, and the vertical distribution of Reynolds stresses normalized by the square of the free stream velocity collapse well for data obtained at the same elevation but at different phases of the tidal cycle. The magnitudes of ⟨u'u'⟩, ⟨ w'w'⟩ and -⟨u'w'⟩ decrease with height, above bottom in the 25-160 cm elevation range and are consistent with the magnitudes and trends observed in laboratory turbulent boundary layers. If a constant stress layer exists, it is located below 25 cm elevation for which we have no data.; Two methods for estimating dissipation rate are compared. The first, a direct estimate, is based on the measured in-plane instantaneous velocity gradients. The second extends the resolved part of the dissipation spectrum using the universal dissipation spectrum available in Gargett et al. (1984). Being undervalued, the direct, estimates are 2 to 2.5 times smaller than the spectrum based estimates. Taylor microscale Reynolds numbers for the present analysis range from 27 to 665. Anisotropy is present at all resolved scales. At the transition between inertial and dissipation range the longitudinal spectra exhibit a flatter than -5/3 slope and form spectral bumps. Second order statistics of the velocity gradients show evolution towards isotropy with increasing Reynolds number. Dissipation exceeds production at all measurement heights, but the difference varies with elevation. Close to the bottom, the production is 50 to 90% of the dissipation, but it decreases to 10-20% for elevations greater than 80 cm.; In Part II we discuss methods for conditionally sampling Reynolds stresses based on the wave phase. Since wave contamination prevents direct calculation of stresses, PIV data is used for estimating the stresses from the second order structure functions of the spatial velocity distributions. Hilbert transforms of pressure signal and spatially averaged velocity are used for determining the wave phase. For most cases, a phase mismatch occurs when the wave amplitude is small or when the turbulence is high. The data are then sub-sampled, keeping only points for which the phase difference is less than 35°. Such sub-sampling has little impact on the Reynolds stresses. Conditional sampling shows that all the Reynolds stresses vary with wave phase, but the variations of the shear stress are particularly high. Except for a consistent minimum in ⟨ u'u'⟩ at the phase of maximum wave induced velocity, there are significant differences between trends of data recorded at different elevations and/or times.