Tidally induced turbulence in the Bermuda underwater cave-system

This study presents results from field measurements of turbulence made in Bermuda's underwater cave-system. To the best of our knowledge, this is the first time that turbulence velocity measurements have been taken in an underwater cave-system. Water currents in caves are unaffected by surface waves and thus provide a unique opportunity to obtain clear signals of tidally induced turbulence. An acoustic Doppler velocimeter and acoustic Doppler current profiler were deployed in several cave locations during a period of six days. Power spectral density (PSD) of velocity fluctuations was estimated using the multitaper power spectral method. Turbulence kinetic energy dissipation rates, ε, were calculated based on the PSD and were found to exhibit a clear -5/3 slope within the inertial subrange. Measurement periods covered full diurnal cycles and estimates of ε showed a strong correlation with the tide phase with values up to 10^-3 W/kg during peak ebb and flood (horizontal velocities up to 0.35 m/s). Furthermore, ε was found to closely follow the wall boundary layer parametrization, ε = u_*³/(ᴋz), where u_* is the friction velocity, ᴋ is von Karman’s constant, and z is the height above the bed.