Dive contractors work to salvage a damaged boat at Tutukaka Marina on Wednesday. Photo / Michael Cunningham
Dive contractors work to salvage a damaged boat at Tutukaka Marina on Wednesday. Photo / Michael Cunningham
Tutukaka Harbour's unique shape is the reason it's so vulnerable to tsunami surges, experts say.
Waves triggered by Saturday night's eruption of the Hunga Tonga-Hunga Ha'apai volcano were felt around the Pacific but the only place in New Zealand to experience major damage was Tutukaka Marina.
The cost is stillbeing tallied but is likely to run into many millions of dollars with six vessels sunk, three piers badly damaged and a fuel pontoon wrecked.
The only other significant damage known was to a newly-built oyster farm in Whangaroa Harbour.
Jose Borrero, a tsunami hazard specialist from eCoast Marine Consultancy and Research, said Tutukaka was a known ''tsunami resonator''.
Tutukaka's unique shape — a narrow entrance, two lobes in the middle and a narrow end — meant waves of a particular wavelength resonated and were amplified as they entered the harbour.
The effect was not seen with normal wind-generated waves, which had periods measured in seconds, but was evident with much longer wavelengths such as those caused by tsunami.
Tutukaka Harbour's unique shape makes it vulnerable to tsunami surges. Photo / Michael Cunningham
Other tsunami resonators around New Zealand included Whitianga, Poverty Bay and Lyttelton Harbour.
Borrero said Tutukaka seemed to respond to a number of different wavelengths.
It had amplified waves from Chile and Japan, which had very long periods or wavelengths, but it had also amplified the tsunami from Tonga which likely had a shorter wavelength.
He suspected there was also another factor at play outside the harbour.
''Tutukaka is not that much different to other sections of the New Zealand coast but the amplification effect is much stronger. There must be something else offshore that is focusing energy as well.''
A tug boat fitted with a crane removes part of a damaged pier at Tutukaka Marina on Wednesday. Photo / Michael Cunningham
Northland harbourmaster Jim Lyle said other Northland harbours, such as Whangārei and the Bay of Islands, had many arms and estuaries which allowed surges to spread out and dissipate.
Tutukaka Harbour, however, had a narrow entrance, widened slightly in the middle, then narrowed again at the end with no estuary or rivers.
As a result surges did not disperse but were focused on the marina at the far end.
''That's why Tutukaka is always getting walloped.''
Lyle said now was a good time to consider how the marina's design could be modified, for example by altering breakwaters or building an attenuator, to reduce the effects of tsunami.
Jesse Owen from Salt Services at work inspecting boat hulls and salvaging at Tutukaka Marina. Photo / Michael Cunningham
Dive Tutukaka owner Jeroen Jongejans agreed a redesign and extra investment could be needed to help the marina better withstand tsunami.
''Whenever there's a tsunami in New Zealand we are the most spectacular spot to watch it. The marina's been here for 40 years and we've had tsunami before, but we've never had an effect like this.''
However, Borrero did not offer much hope the problem could be solved by building more breakwaters.
If the entrance to marina was made smaller the resonance inside could become even greater.
Map showing currents within Tutukaka Harbour after the 1976 Kermadec earthquake. Note the powerful currents surging through the marina entrance. Photo / Jose Borrero
''Part of the current issue could be the narrow entrance to marina. With a narrow entrance you get these incredibly strong current jets flushing in and out — that's what breaks moorings and sinks boats.''
While the wave heights last weekend were no higher than those in the March 2021 tsunami the currents were much stronger.
That could be due to the shorter wavelength of a volcano-generated tsunami compared to an earthquake tsunami, he said.
A catamaran which sank during Saturday night's tsunami surges was refloated on Tuesday and will be towed to Whangārei for repairs. Photo / Michael Cunningham
Borrero said Saturday's event may have been the first Pacific-wide tsunami triggered by a volcano in modern human history.
The only comparable example was the eruption of Krakatau, in the Indian Ocean between Java and Sumatra in what is now Indonesia, in 1883.
A catamaran which sank in Tutukaka Marina on Saturday night was refloated on Tuesday and is expected to be towed to Whangārei for repairs.
The last two sunken vessels were removed yesterday by salvage crews using divers, a barge and a crane.
