Waikato’s west coast at low risk of tsunami: study

08 March 2016

A study has found that Port Waikato, Raglan Harbour and Aotea Harbour is at a low-risk of inundation for a tsunami, but waves could cause strong surges and currents.

The findings are the result of a study, funded by Waikato District Council, Waikato Regional Council and WEL Energy, carried out by Jose Borrero of Raglan-based eCoast.

The study found that wave heights will be relatively small, but inundation may affect low-lying areas. It also concluded that tsunami may produce strong surges and currents, particularly at the entrance to the harbours, making it dangerous to be on or in the water.

“While west coast residents and visitors can have some confidence the hazard is relatively low, people still need to remain diligent and listen to advice from civil defence and council authorities,” said Merv Balloch, local civil defence controller for Waikato District Council.

“Our local civil defence staff will be using this information in working with communities to develop their emergency response plans for all hazards, not just tsunami.

Mr Balloch said the study’s results had been presented to the council, and Onewhero/Tuakau and Raglan community boards.

Regional hazards team leader Adam Munro, from Waikato Regional Council, said: “It was really important to us to better understand the tsunami hazard on Waikato’s west coast, so we can work with communities to manage the risk.

“Extensive modelling of a number of scenarios was carried out, but the study focused primarily on ‘near source’ tsunami generated by very large earthquakes on known fault systems in and around the Tasman Sea and South West Pacific,” Mr Munro said.

This included tsunami generated by earthquakes in the Solomon Islands, along the New Hebrides trench directly north of New Zealand, along the Tonga-Kermadec trench to the east of the North Island and along the Puysegur Trench south and west of the South Island.

The study also considered ‘distant source’ tsunami generated along the west coast of South America, focusing on the largest known historical events of 1868 and 1960 – each with earthquake magnitudes greater than 9 – in northern and southern Chile respectively.

For Waikato’s west coast, the tsunami study found that based on current information:

  • tsunami wave heights from all sources modelled are relatively small
  • inundation of low lying areas may occur if the tsunami occur during a high tide
  • tsunami arrival times from a ‘near source’ is between 3 and 6 hours, but with the peak tsunami activity occurring several hours after the first arrival
  • tsunami arrival times from a ‘distant source’ is between 15 and 17 hours.

Despite the relatively small tsunami heights, all of the scenarios produced potentially dangerous currents, particularly at the entrance to each harbour. In each case, these dangerous currents persisted for many hours after the arrival of the largest waves.

For more information, view the West Coast tsunami study factsheet.

To read the full technical report, visit www.waikatoregion.govt.nz/tsunami.

For further information, contact:

Wendy Valois
Waikato Regional Council, Communications advisor
07 859 2721 or 021 369 815