Variability of turbulent quantities in the tidal bottom boundary layer : Case study in the eastern English Channel
Résumé
Tidal current structure and turbulent quantities within a tidal bottom boundary layer (BBL) have been examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument was deployed on the seafloor, off the north-eastern French coast in the eastern English Channel over 12 tidal cycles and covered the period of the transition from mean spring to neap tide. Forcing regimes varied from calm to moderate storm conditions during the deployment. For the study of turbulent quantities in the BBL, we have chosen a calm period, when an effect of surface waves on the velocity structure was negligible. Stresses were found to vary regularly with the predominantly semidiurnal tidal flow, with the along-shore stress being generally greater during the flood flow (∼3.0 Pa) than during the ebb flow (∼−1.5 Pa). The turbulent kinetic energy (TKE) production rate, P, TKE density, Q, and its dissipation rate, ε, followed a nearly regular cycle with close to quarter-diurnal period. Near the seabed, peak values of P, Q and ε were found to be 0.5 W m−3, 0.5 m2 s−2 and, 0.04 W m−3, respectively, during the flood while, during the ebb, these quantities reached lesser values: 0.1 W m−3, 0.1 m2 s−2 and 0.03 W m−3, respectively. Near the bottom, eddy viscosity, Az, peak ranged from about 0.1 m2 s−1 during the flood to 0.03 m2 s−1 during the ebb flow. Away from the bottom, Az increased to reach a maximum near the mid-depth. Time-depth variation of the P/ε ratio indicated that the turbulence in the BBL, most of the time, was at a non-equilibrium state (P/ε≠1). The largest deviation from the equilibrium occurred during the flood, when P/ε exceeded about one decade near the bottom. During the ebb, P/ε was close to the equilibrium state, slightly decreasing with height above the bottom. Results are found to be in a good agreement with those of the other researches working on direct measurements of turbulence in tidal flows.