Kaikoura tsunami waves measured in Wellington Harbour

The location of the wave meters on the eastern side of the entrance to Wellington Harbour.

The location of the wave meters on the eastern side of the entrance to Wellington Harbour.

The MetOcean Solutions science team made some unique wave measurements of the 14 November Kaikoura New Zealand tsunami event.

Oceanographer Florian Monetti has been studying the complex swell wave transformations into Wellington Harbour since 2015 as part of the environmental assessments for the Wellington Harbour Deepening Project. CentrePort, who operates Wellington Harbour, recently commissioned additional wave measurements at the popular surf breaks on the eastern side of the harbour entrance, with the intention to use these data to more precisely validate the numerical wave models of the harbour. MetOcean Solutions supplied three wave meters (RBRsolo) on seabed frames that were placed very close to the surf breaks. These meters record water levels continuously at twice per second, and as luck would have it they were deployed just a few days before the 7.8 magnitude earthquake. 

“The measurements are quite remarkable,” says Florian. “Because the meters were spaced along 3 km of the entrance coastline we can clearly see the progression of the tsunami waves as they enter the harbour.” 

The first noticeable change to the sea surface occurred only 2 minutes after the 12:02 am earthquake, with waves around 0.5 m occurring - likely radiating from the adjacent shoreline.  Then, after 24 minutes a mild disturbance was observed for about 10 minutes, which is probably the waves generated by the quake within the Wellington Harbour. Next, some 48 minutes after the quake, the water level first receded by about 0.8 m and then 12 minutes later it rose by 1.6 m. The tsunami waves moved relatively slowly through the harbour entrance, their speed limited to around 35 km/h by the shallow depth. The time lag between the first wave arriving at the Pipes and then Lion Rock surf breaks was 5 minutes. The two first waves were the largest but another 13 waves between 0.4 m and 1.0 m in size occurred over the following 6 hours. Notably, all these waves had a well-defined periodicity of (i.e. they recurred every) 27 minutes, and this distinctive tsunami signal could still be detected 19 hours after the arrival of the first wave. 

The tsunami waves started soon after the earthquake and continued until 19 hours after the quake. The graphs show the water level as measured (top) and with the tide and normal wave patterns removed (bottom).

The tsunami waves started soon after the earthquake and continued until 19 hours after the quake. The graphs show the water level as measured (top) and with the tide and normal wave patterns removed (bottom).

At Wellington, the size of the largest tsunami wave was quite close to the tidal range (which is 1.4 m), and the largest waves occurred when the tide was still low. This means the sea level changes were similar to what happens most days with the typical rise and fall of the tide. In fact, the highest water level occurred during the 9th tsunami wave, which coincided with the high tide. Some coastal flooding from the tsunami waves would certainly have occurred if the largest waves had arrived during the high tide.      

CentrePort has kindly made the data freely available for international tsunami researchers, and interested people should contact Florian for access (f.monetti@metocean.co.nz).