Long waves in Port Taranaki

Wave model showing swell wave penetration to Port Taranaki. Longwaves are generated by these swell waves. 

Wave model showing swell wave penetration to Port Taranaki. Longwaves are generated by these swell waves. 

When an enormous storm swell was forecast to hit the west coast of the North Island of New Zealand this winter, Remy Zyngfogel, one of our keen oceanographers, decided this was the perfect opportunity to make a unique set of measurements within Port Taranaki.

Long (or infragravity) waves are water level variations with periods of more than 25 seconds. These waves create big problems in many ports around the world because their wavelength is similar to the size of commercial ships. For a moored vessel, the waves directly agitate the ship at the berth, which can cause vigorous motion that can break the mooring lines and present a high danger to personnel.

At Port Taranaki, along with many other ports, MetOcean Solutions provides a hyper-local harbour forecasting service which includes long waves and the vessel surging outcomes. Based on this forecast, Port Taranaki had a 5-day warning of closure due to the storm swell. So, Remy planned an experiment in the empty port. With the help from harbour staff he deployed 7 wave sensors (RBRsolo) on the seabed in a line across the main harbour basin. A remarkable set of data was obtained, as demonstrated by the animation right. Water levels for the different long wave frequencies can be visualised and the complex wave forms and reflection patterns easily identified.

Taranaki Harbourmaster, Neil Armitage, says ‘For the first time we can actually see these waves, how they are behaving, and better understand the effect they have on ships in our harbour.’    

The data will be used to further validate the numerical models of long wave generation and penetration.

Location of the sensors.

Location of the sensors.

Animation showing the long waves measured in Port Taranaki.

Animation showing the long waves measured in Port Taranaki.