Turbulence and drag in a high Reynolds number tidal passage targetted for in-stream tidal power

Results are presented from an investigation of turbulence and bottom drag carried out in Grand Passage, lower Bay of Fundy. Flow measurements were made using a broadband 600 kHz acoustic Doppler current profiler (ADCP) sampling at nearly 2 Hz, and two single-point sensors: an acoustic Doppler velocimeter (ADV) sampling at 1 Hz, and a time-of-flight velocity sensor (MAVS) sampling at 12 Hz. All instruments were bottommounted. The maximum depth-averaged tidal current speed was 1.6 m/s. The local bathymetry was characterized by 20 m mean water depth and ca. 0.5 m high, 8 m wavelength dunes. The ADCP was deployed to one side of the dune field; the singlepoint sensor platform was within the dune field. Due to high water clarity, the ADV correlations at moderate to high flow speeds were very low, precluding estimation of turbulence quantities. In contrast, the time-of-flight velocity data are noise free - the measurement does not require the presence of sound scatterers - and the spectra exhibit a well-defined inertial subrange. Turbulent Reynolds stress estimates from the time-offlight sensor data yield friction velocities (u_*) and bottom drag coefficients (C_d) comparable to those determined from the ADCP profiles via the law-of-the-wall. The ratio of RMS vertical velocity variance to friction velocity in the time-of-flight data is close to 1.2, consistent with results obtained within the constant stress layer in the atmospheric boundary layer and in rough boundary laboratory experiments. Turbulence quantities are estimated from the (de-noised) ADCP velocity spectra via the variance method, modified here for application to a sloping seabed by taking advantage of the orientation relative to the local isobaths of the orthogonal acoustic beam pair planes. Noise levels were determined from the spectra ensemble-averaged in equal mean flow speed intervals, and are very close to the manufacturer´s quoted value. When the constant stress layer was sufficie- tly thick - i.e. during ebb tide - the RMS turbulence intensities in the lower ADCP range bins are entirely consistent with the anisotropic relationships between Cartesian RMS turbulent velocity components and u_* obtained in rough boundary wind tunnel experiments [1]; the ADCP Reynolds stresses agree with the Law-of-the-Wall shear stress estimates; and vertical profiles of Reynolds stress are very different from the linear decrease with height expected for (lower Reynolds number) straight and narrow open channel flows. During flood tide, the boundary layer was much thinner, and the lowermost ADCP bin - at 2.1 m height - was outside the constant stress layer. The pronounced asymmetries between flood and ebb (i.e. in boundary layer thickness, turbulence intensity, and (C_d) are attributed to differences in upstream bathymetry. Index Terms-turbulence, bottom stress, tidal power, broadband acoustic Doppler profiler, acoustic time-of-flight sensor.