Science Centres: Oceans

Mapping Marlborough's complex currents

NIWA recently employed a range of sophisticated technologies to measure and map the currents of Tory Channel and French Pass. This builds on our expanding knowledge about flow complexity in the Marlborough Sounds. The most recent study was for Land Information New Zealand (LINZ,) where the results will be used to update existing navigational maps of these complex, and sometimes treacherous, stretches of water.

An acoustic Doppler current profiler (ADCP) mounted on a launch gave us a near-continuous picture of the currents through a range of tidal conditions and depths. The flow in French Pass is complicated by whirlpools, upwellings, and underwater waterfalls. To measure some of this variability, we also used GPS-equipped drifting floats to trace surface water flows and an ocean wave radar to map patches of white water. These tools revealed, among other things, flow patterns associated with a major eddy swirling over a 100 m deep hole at the northeastern end of the Pass.

Combining these different methods allows us to build up a better picture of the huge variability in currents in the Marlborough Sounds, one of the most complex marine systems in New Zealand. The work provides essential baseline information for a multitude of needs, including navigation, aquaculture, water quality, and ecological appraisal.

Doppler what?

Our key instrument in this project, the acoustic Doppler current profiler (ADCP), makes use of a well-known physics phenomenon: the Doppler effect. This is the apparent change in sound frequency perceived by an observer moving relative to the source of a sound (think of how the sound of an ambulance’s siren changes as it passes you). The ADCP sends out high frequency pings of sound which are reflected off suspended matter, such as sediment particles and plankton, in the water. By carefully listening at the right times, the instrument can measure current speed based on the frequency-shift of reflected sound.