Measurements of the Rate of Dissipation of Turbulent Kinetic Energy in a High Reynolds Number Tidal Channel

The rate of dissipation (ε) of turbulent kinetic energy at mid-depth in a high speed tidal channel is estimated using measurements from both a standard acoustic Doppler current profiler (ADCP) and shear probes mounted on an underwater, streamlined buoy. The investigation was carried out in Grand Passage, Nova Scotia where the depth-averaged flow speed reached 2 m/s and the Reynolds number was 4 x 10⁷. The dissipation rates estimated from the ADCP data agree with the shear probe data to within a factor of two, with some of the discrepancy attributed to the 40 m separation of the instrument platforms. Both the ADCP and the shear probe measurements indicate a linear dependence of ε on the cube of the flow speed during flood tide with maximum values reaching 5 x 10^-5 W/kg. Much lower dissipation rates were observed on the ebb tide when the convergence of the flow in the narrowing channel suppresses the turbulent fluctuations. Two significant limitations of ADCPs in measurements of turbulence will be discussed: (1) the calculated dissipation rates are highly sensitive to the choice of Doppler noise level, and (2) the ADCP measurements do not capture the intermittency in the flow.