How did the recent wave event in Wellington compare?

The recent large waves that hit the Wellington and Kaikoura coastlines were formed in the Southern Ocean and travelled thousands of kilometres before they hit these exposed coastlines. 

To answer how frequently events of this magnitude happen, you need to have information on the historical wave climate for the site of interest.

Within MetOcean we maintain numerous high-resolution model datasets that have been validated and calibrated, and then run back to 1979 (soon to be 1959!)  This allows us to answer the question of what the annual exceedance probability (AEP) of these magnitude events are.

Extreme value analysis of combined significant wave height (Hs) and peak wave period (Tp) based on 40 years of hindcast data offshore Wellington Harbour.

One of the interesting things about this recent event was that at its peak, it had a relatively large peak period (the time interval between waves) of around 15 seconds. Significant wave heights were recorded, reaching around Hsig = 6.5 m, with a maximum recorded wave height off Baring Head of almost 15 m,  see graph here.

Based on the modelling work of the MetOcean Team, extreme value analysis suggests that significant wave heights of this magnitude can be expected to occur with an AEP of 0.5 (or on average once every two-years).  However, if we consider the fact that the peak period was around 15 seconds at the peak of the event, then the recent waves that hit the Wellington Coast had an AEP of between 0.2~0.5 (or on average once every two - five years) – see figure.  This event proved to be larger and more energetic than the event last year. Through the modelling undertaken in the MetOcean team and the handling of warnings by MetService who worked closely with WREMO, a more structured response was in place to manage the event as it developed.

These data are available through our Hindcast portal in MetOceanView, while an opensource Python tool (soon to be released) allows people to analyse time-series data easily and quickly. These two tools provide critical information required for the management and design of key infrastructure and assets – and the ability to place these extreme events in context.