Flow Structure Determination in French Pass NZ

Understanding marine habitat requires a good understanding of the flow and dynamics of the region.  Observations of tidal flows in Te Aumiti (French Pass), the fastest such flows in New Zealand, were conducted using a vessel-mounted acoustic Doppler current profiler (ADCP), pressure sensors, a current meter mooring, drifters and microwave radar.  The suite of measurements revealed spring tide currents exceeding 4 m s^-1, but only in the throat of the pass.   The pressure sensors either side of the pass indicated a phase-shift in the tide of 26 min.  Drifters illustrate eddying motion, with the deep scour hole on the NE side of the narrows forming a focal point on both phases of the tide.  Notably, the drifters also provided evidence of an upstream eddy in the same location during south-westward flows.  The ADCP recorded vertical flows of up to 0.6 m s^-1.   The flow was subcritical as defined by external and internal Froude numbers, and scaling analysis suggests that the driving pressure gradient was balanced by the acceleration.  The area in the French Pass region suitable for tidal stream turbines is relatively constrained.  The analysis suggests in the order of 25 MW of total energy exists in the flow and the flow variability is characterised by large phase-dependent quasi-permanent eddy features.