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Waikato west coast tsunami hazards

Report - Numerical modelling of tsunami effects at Port Waikato, Raglan and Aotea Waikato West Coast (4MB)

Factsheet - Tsunami on Waikato's west coast (1.5MB)

A 2015 study to identify the risk of tsunami affecting Port Waikato, Raglan Harbour and Aotea Harbour has found wave heights will be relatively small, but inundation (flooding) may affect low-lying areas.

Despite the low likelihood of a tsunami event in these areas, tsunami may produce strong surges and currents, particularly at the entrance to the harbours, making it dangerous to be on or in the water.

The study focused primarily on tsunami generated by very large (Magnitude 9) earthquakes on known fault systems in and around the Tasman Sea and South West Pacific. 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 to the south and west of the South Island.

The study has also considered 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.

The study's findings

The tsunami study has found that for the Waikato west coast:

  • tsunami wave heights from all sources modelled are relatively small
  • flooding of low lying areas may occur if the tsunami occurs during a high tide
  • tsunami arrival times for the most common sources are between three and six hours, but with the peak tsunami activity occurring several hours after the first arrival
  • tsunami arrival times for distant sources from South America are 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 continued for many hours.

The study, funded by Waikato District Council, Waikato Regional Council and WEL Networks, was carried out by Jose Borrero of Raglan-based international marine and freshwater experts, eCoast.