Infra gravity (IG) waves are long waves with periods between 25 and 120 s.
They are generated in the process of large swell waves breaking on adjacent beaches.
Often they are correlated with the tide, being up to twice as high at high tide than at low tide.
They are difficult to detect, and usually they are so small they cannot be felt, but they cause ships to surge at berth, breaking mooring lines and endangering people, other ships, and infrastructure.
Breakwaters are effective at reducing or eliminating swell, but not IG waves, which pass through them unabated.
In many cases the only way to avoid damage when IG waves are present is to send the ships out to sea until the IG waves pass. They usually last for only a few hours, up to a day.
The plot below shows the IG waves at Sumner Head, adjacent to Lyttelton Harbour. The data come from the tsunami gauge SUMT run by GeoNet.
The wave heights have three ranges: