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Wake measurements are critical for quantifying the hydrodynamic effects of tidal energy extraction and for designing arrays of tidal turbines. In this investigation field measurements from the wake of PLAT-I, a commercial-scale floating array of four tidal turbines, are presented. Instrumented surface-following drifters are used to measure velocity and turbulence in the wake of PLAT-I while deployed in Grand Passage, an energetic tidal channel in the Bay of Fundy, NS, Canada. Field measurements were obtained when the turbines where fully operational under optimal conditions during spring tides. The collected data are used to construct tridimensional maps of acoustic backscatter, mean velocity, vertical velocity variance and turbulent kinetic energy dissipation rate downstream of the platform at different stages of the tide. The operational turbine array wake was captured for ebb and flood tides. For both ebb and flood, reduced velocities are observed in the upper water column at the depths spanned by the turbine rotors. Vertical profiles of velocity mix quickly during flood tide but velocities remain slower than the undisturbed flow outside of the wake up to 20 effective diameters downstream of the platform. In contrast, during ebb, the velocity deficit persists farther downstream, and the reduced velocity increases slightly. Elevated turbulence levels peak at 2 effective diameters downstream of the turbines and recover to undisturbed flow levels at approximately 10 effective diameters downstream of the platform for both ebb and flood tides. Total energy dissipated through turbulence within the wake is estimated to be less than 10% of the energy extracted for electricity production. • Field measurements of the wake from an operational commercial scale floating tidal turbine array. • The wake is confined to the upper water column during both ebb and flood tides. • Wake velocity deficit recovers faster during flood tide than during ebb tide • Wake recovery occurs beyond 20 effective diameters downstream of the turbines. • Wake elevated turbulence intensities recover quickly for both ebb and flood tides [ABSTRACT FROM AUTHOR]

In this investigation, instrumented mobile platforms are used to spatially map the turbulent flows in Grand Passage, one of the Bay of Fundy's more energetic tidal channels in Nova Scotia, Canada. The aim is to characterize the flow around the PLAT-I floating tidal energy platform developed by Sustainable Marine Energy Canada (SMEC). GPS-tracked surface drifters equipped with fast-sampling acoustic Doppler current profilers (ADCPs) provide turbulence-resolving vertical profiles of velocity and turbulent kinetic energy dissipation rate along drifter trajectories, while vessel-mounted ADCP transects complement the mean flow velocity measurements. These data are used to construct tridimensional quasi-synoptic maps of mean velocities and turbulence parameters for several stages of the tide around PLAT-I's location including peak ebb and flood currents. The data set includes measurements under natural flow conditions, and while the turbines installed on PLAT-I were both not operational and operational. The measurement techniques and resulting maps successfully capture the spatial and temporal structure of the flow for unsteady conditions for various tidal conditions. The combined wake of the four 6.3 m operational PLAT-I turbines was measured for a single tidal stage. Closer to the turbines, the vertical extent of the wake is about 6 m (approximately one turbine diameter), increasing with distance downstream, while the maximum observed velocity deficit is 26% relative to not operating conditions, decreasing with distance downstream. [ABSTRACT FROM AUTHOR]

This study characterizes the turbulence in a 3.6 m s −1 tidal channel in the Bay of Fundy, Nova Scotia that has been identified for development as a tidal power resource. A horizontally aimed fast-sampling single beam acoustic Doppler profiler was deployed on a subsurface buoy, and the flow-aligned profile is used to test the cross-spectral (Garbini 1982a,b) and the spatial structure function methods for estimating dissipation rates of turbulent kinetic energy. Second and third order structure functions computed over separation distances in the mean flow direction vary with spatial scale as predicted by the Kolmogorov hypotheses. Cross-spectra from adjacent spatial bins have a noise floor an order of magnitude lower than the corresponding auto-spectra and phase lags over increasing spatial ranges demonstrate the advection of coherent structures by the mean flow. The spectral and structure function estimates of dissipation rate vary linearly with U 3 and agree to within a factor of 2.5 on ebb and 6 on flood, the spectral estimates being larger. Dissipation rates are an order of magnitude higher on ebb than flood tide, likely due to asymmetry in the local topography. [ABSTRACT FROM AUTHOR]

Highlights • BBL environment in the axis of Barkley Canyon (−985 m) is stable on time scales >1d and <3y. • Epibenthic megafaunal community is composed of species adapted to low oxygen levels. • Two major species of the community, C. tanneri and A. fimbria , are of fishery interest. • Seasonal variations are observed in the epibenthic megafauna of the axis of the canyon. • Inter-annual trends in abundance of major species is linked with "the blob" event. Abstract Understanding the impact of the environment on temporal trends in the composition and abundance of deep-sea species is essential for forecasting evolution of the community in the context of climate change. The recent development of deep-sea observatories enables multidisciplinary studies of long duration and high temporal resolution. We used a platform at the Ocean Networks Canada NEPTUNE Observatory located in the axis of Barkley Canyon between June 2012 and January 2015 to: (1) characterize the megabenthic community and the environmental conditions in the canyon; (2) investigate temporal patterns in the faunal community; and (3) determine the influence of environmental conditions on the observed patterns. The megafaunal epibenthic community, which included commercially important species, was composed of species known to be able to adapt to low oxygen conditions. A strong seasonal pattern characterized wind speed and direction, wave height, temperature, and chlorophyll concentration at the sea surface. The megafaunal epibenthic community exhibited seasonal patterns which were influenced by in-situ benthic boundary layer currents and temperature, weather conditions and chlorophyll concentration at the sea surface. At inter-annual scales, the surface temperature anomaly event observed in 2013 in the region could potentially impact the community structure with significant changes in species' densities, highlighting the need for long-term monitoring in the region. [ABSTRACT FROM AUTHOR]