Ebb/Flood Asymmetry in Current Velocity and Turbulence Observed During Tidal Energy Resource Characterization of the Western Passage, Maine, USA

As the tidal energy sector begins to commercialize, resource measurements that conform to International Electrotechnical Commission standards are needed to understand the resource, have confidence in power production estimates, and to obtain project financing. Additionally, accurate measurements of turbulence are needed to accurately estimate fatigue loads and device lifetimes. The Western Passage near Eastport, Maine is a top-ranked U.S. tidal energy site. Characteristic flow and turbulence statistics from data collected at three stations at this site are presented. Bottom-mounted acoustic Doppler current profilers (ADCPs) were used at two stations and captured flow data throughout the 50 m water column from April - July 2017. Between those stations along a cross-channel transect, a Stable Tidal Turbulence Mooring (STTM) positioned ~ 10 m above the seabed was deployed for one week in May during the spring tide. The STTM was outfitted with an acoustic Doppler velocimeter with an inertial motion unit and a bottom-tracking down-looking ADCP to provide motion-corrected flow and turbulence characteristics at high temporal resolution. Asymmetry in current velocity and turbulence is observed at all stations. The principal axis of flow during flood is toward 343 °TN while ebb currents flow toward 142 °TN, 21 ° off a symmetrical flow. Peak flood velocities at the STTM reach 3.2 ms^-1, while peak ebb velocities reach only 1.7 ms^-1. The depth-averaged current speed at one ADCP station is 1.4 ms^-1 during flood and 0.9 ms^-1 at ebb. The majority of the tidal energy is therefore available during flood. Turbulent statistics (e.g. turbulence intensity (I), turbulence velocity auto spectra, turbulent kinetic energy, Reynolds stresses, and turbulence production and dissipation rates) are generally larger in magnitude on flood tide as well. For example, I is 13% during peak ebb and 15% for the same current speeds during flood. However, at the higher speeds of peak flood I is 10%. These results provide relevant flow conditions at this potential tidal energy site and will help device simulation tools estimate realistic loads on tidal turbines to improve device robustness and accelerate device design.