| Cook Strait, the channel separating New Zealand’s North and South Islands, is at it’s
narrowest around 22 km across with flows driven by a semidiurnal tide, wind and a
baroclinic pressure gradient. Water depths are around 250-300 m in the main part of
the channel, with shoals to the south and the submerged Fishermans Rock (aka
pinnacle) in the centre northwest of the Strait. The substantial tidal flow speed is due
to the tide being nearly out of phase comparing the ends of the strait and further
enhanced by a narrowing of the strait. It has significant potential for a tidal energy
resource suitable for extraction due to both its significant energy levels but also its
proximity to electricity infrastructure and nationally high uptake of renewable energy in
general. Here we describe recent flow and turbulence data and contextualise them in
terms of scales relevant to marine energy extraction. With flow speeds reaching 3 m
s−1 in a water column of > 200 m depth the setting is heuristically known to be
highly turbulent. Turbulent energy dissipation rates are modest but high for oceans,
around 5x10−5 W kg−1. Thorpe scales, the observed quantity representing the
energy-bearing scale, are often as much as one quarter of the water depth. This means
eddy sizes can potentially be larger than blade length. A boundary-layer structure
was apparent but highly variable. This has implications for both operation of tidal
turbines, as well as modulating their effect on the environment. Fishermans Rock
itself is interesting as if can be considered a proxy for a larger array of turbines. |