$2m tsunami sirens plan faces axe

A million dollar tsunami siren plan has been scrapped by the Tauranga City Council. Photo / File

A $2 million stand-alone plan that relied on sirens to warn Tauranga of an approaching tsunami looks likely to be axed.

The city council could, instead, revert to its previous philosophy where the focus was as much on mobile warning technologies, including text messaging and helicopters with loudspeakers.

A big mix of systems to alert residents living along Tauranga's vulnerable coastal suburbs has re-entered contention after a three-hour tsunami workshop yesterday.

The workshop was the latest step in a saga of twists and turns, which began when the council abandoned sirens 2 years ago in favour of a system that included remote-controlled household alarms. The next U-turn was provoked by the Japanese tsunami, with community pressure forcing the council back to looking at sirens.

In April this year, the council voted to stop its contract with electronic siren supplier Meerkat, effectively returning to square one after spending $190,000 trying to find a way forward for the $2 million siren plan it had budgeted for.

Councillors yesterday heard from some of the country's leading tsunami scientists, including an expert from engineering consultancy Tonkin and Taylor, who had just finished analysing the impact of a worst-case scenario tsunami on low-lying parts of the city.

The work of Richard Reinen-Hamill would feed directly into the controversial legal requirement on the council to insert tsunami warnings on to the land information files of every at-risk property in the city - nearly all of which were in Mount Maunganui and Papamoa. He based his work on a GNS Science prediction that the greatest risk to the city was from an earthquake causing a huge subterranean slip on the Kermadec Trench, northeast of East Cape. This subduction zone along two continental plates had the potential to generate a tsunami up to 14m high, depending on the fracture and size of the event.

If the slip happened on the section of the trench closest to the Bay of Plenty, it would take only 50 minutes for a tsunami to reach Tauranga. That barely gave time for people to reach the safety of higher ground once the magnitude of the risk had been assessed by the Ministry of Civil Defence. GNS scientist James Beban said it was not a matter of if, but when, a tsunami would strike the Bay of Plenty from the Kermadec Trench. He estimated that a tsunami from the trench would hit the Bay every 350 to 1500 years.

Mr Beban said there were very strong similarities between the subduction zone off Japan that triggered the tsunami two years ago and the Kermadec Trench.

The good news was that tsunami from close-in sources such as Astrolabe, White Island or the Volkner Faults would generate a wave height of less than one metre. The return period was 350 to 800 years.

The council did not make any decisions, although Councillor Larry Baldock supported the recommended options (outlined on this page). Using the existing network of fire brigade sirens and plugging the gaps would save ratepayers a lot of money, he said.