Effect of variable winds on current structure and Reynolds stresses in a tidal flow: analysis of experimental data in the eastern English Channel

[摘要] Wind and wave effects on tidal current structure and turbulence throughoutthe water column are examined using an upward-looking acoustic Dopplercurrent profiler (ADCP). The instrument has been deployed on the seafloor of18-m mean depth, off the north-eastern French coast in the eastern EnglishChannel, over 12 tidal cycles, and covered the period of the transition frommean spring to neap tide, and forcing regimes varied from calm to moderatestorm conditions. During storms, we observed gusty winds with magnitudesreaching 15 m s⁻¹ and wave heights reaching up to 1.3 m. Analysis ofvelocity spectra revealed a noticeable contribution of wind-induced waves tospectral structure of velocity fluctuations within the subsurface layer.Near the surface, stormy winds and waves produced a significantintensification of velocity fluctuations, particularly when the sustainedwind blew against the ebb tide flow. As during wavy periods, thevariance-derived Reynolds stress estimates might include a wave-inducedcontamination, we applied the Variance Fit method to obtain unbiasedstresses and other turbulent quantities. Over calm periods, the turbulentquantities usually decreased with height above the seabed. The stresses werefound to vary regularly with the predominantly semidiurnal tidal flow. Thealong-shore stress being generally greater during the flood flow(~2.7 Pa) than during the ebb flow (~−0.6 Pa).The turbulent kinetic energy production rate, P, and eddy viscosity,A_z, followed a nearly regular cycle with close to a quarter-diurnalperiod. As for the stresses, near the seabed, we found the maximum values ofestimated quantities of P and A_z to be 0.1 Wm⁻³ and 0.5 m² s⁻¹, respectively, during the flood flow. Over the stormperiods, we found the highest unbiased stress values (~−2.6 Pa) during ebb when tidal currents were opposite to the southwesterly windswhile, during the flood, the surface stresses slightly exceeded thoseestimated for a calm period. A comparison of obtained results gives a goodagreement with those of other researchers working on direct measurements ofturbulence in tidal flows.

[学科分类] 海洋学与技术