Shallow water tidal currents in close proximity to the seafloor and boundary-induced turbulence

Abstract

Velocity measurements with vertical resolution 0.02 m were conducted in the lowest 0.5 m of the water column using acoustic Doppler current profiler (ADCP) at a test site in the western part of the East China Sea. The friction velocity u* and the turbulent kinetic energy dissipation rate ε wl(ζ) profiles were calculated using log-layer fits; ζ is the height above the bottom. During a semidiurnal tidal cycle, u* was found to vary in the range (1-7) × 10 -3 m/s. The law-of-the-wall dissipation profiles ε wlζ were consistent with the dissipation profiles ε mc(ζ) evaluated using independent micro structure measurements of small-scale shear, except in the presence of westward currents. It was hypothesized that an isolated bathymetric rise (25 m height at a 50-m seafloor) located to the east of the measurement site is responsible for the latter. Calculation of the depth integrated internal tide generating body force in the region showed that the flanks of the rise are hotspots of internal wave energy that may locally produce a significant turbulent zone while emitting tidal and shorter nonlinear internal waves. This distant topographic source of turbulence may enhance the microstructure-based dissipation levels ε mc(ζ) in the bottom boundary layer (BBL) beyond the dissipation ε wl(ζ) associated with purely locally generated turbulence by skin currents. Semi-empirical estimates for dissipation at a distance from the bathymetric rise agree well with the BBL values of ε mc measured 15 km upslope