Turbulence intensity in the wave boundary layer and bottom friction under (mainly) flat bed conditions

Abstract

Variations with wave energy of near‐bed turbulence and the wave friction factor are investigated in the near‐shore zone for bed states spanning low‐steepness sand ripples and flat bed, and for wave energies extending well into the sheet flow regime. The measurements were made using a 1.7‐MHz pulse‐coherent Doppler profiler in ca. 3‐m mean water depth. Near‐bed turbulence intensities, phase‐averaged over the highest‐1/3 waves, peak at phases between 10° and 55° after the wave crest, this phase decreasing with increasing wave Reynolds number. Wave friction factors computed from near‐bed vertical turbulence intensity fall within the range predicted by existing semi‐empirical formulae, and exhibit broadly similar trends. At the higher end of the observed wave energy range (i.e., in the sheet flow regime), however, the measured friction factors increase with sea‐and‐swell energy faster than the predictions. This anomalous increase is correlated with infragravity wave energy and with mean cross‐shore current speed, but not with other forcing parameters including mean long‐shore current speed, wave skewness, wave asymmetry and wave breaking frequency. It is argued that the anomaly is partly due to additional near‐bed turbulence associated with infragravity waves, and therefore that these data are not inconsistent with Wilson's (1989) parameterization for bottom roughness in oscillatory sheet flow. Peak near‐bed turbulence intensities are independent of wave Reynolds number for Re ≲ 1.2 × 106, but proportional to Re for Re ≳ 1.2 × 106, this abrupt change possibly indicating a critical dependence on Re of turbulence production in the WBL over flat or nearly flat mobile beds.