Tsunami are frequent visitors to New Zealand shores

Friday, 12 July 2019

We can't afford to take our minds off our tsunami risk for a moment, even if not all of them are devastating. PAUL GORMAN reports.

Cantabrians should know by now that damaging tsunami here are no idle threat.

The latest tsunami report for the Christchurch City Council offers jaw-dropping scenarios of foaming, angry streams of rapidly surging seawater and some city suburbs wallowing metres deep in it.

The National Institute of Water and Atmospheric (Niwa) study released this week investigates how one-in-500-year and one-in-2500-year South American tsunami – generated by magnitude 9.28 and 9.49 quakes respectively – might affect the city when combined with a range of climate change-induced sea level rises and shoreline changes.

Its findings offer pretty much the worst-case outcomes from a tsunami attacking the Canterbury coast, and conjure images of devastation in Indonesia, Japan and Samoa in the past couple of decades.

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Unfortunately for us, as Environment Canterbury natural hazards senior scientist Helen Jack points out, Banks Peninsula acts like a 'magnet' for tsunami while the shape of Pegasus Bay allows tsunami waves to repeatedly 'slosh backwards and forwards'.

The Niwa report concluded that in an absolute worst-case scenario, a one-in-2500-year tsunami on top of 1.06m of sea-level rise may swamp 73 square kilometres of Christchurch, about 5 per cent of the city.

The water could be 9.5m deep near the Waimakariri River mouth and the speed of the waves in the Avon-Heathcote Estuary may exceed 40kmh, with the largest tsunami wave 8.6m high, from crest to trough, at the Waimakariri mouth, and about 7.25m high at the mouth of the estuary.

The city council is reviewing its tsunami evacuation zones. In a briefing paper for councillors at Wednesday's infrastructure, transportation and environment committee staff said the information in the Niwa report would 'not necessitate a widespread update' of householders' land information memoranda (LIM).

'However, a review of tsunami-related LIM comments is currently under way and this may lead to updates to coverage and comments,' the briefing paper said.

Twelve properties at the head of Lyttelton and Akaroa harbours will be added to the tsunami evacuation zone, based on the Niwa report.

Not all tsunami that affect New Zealand's shores are major, of course. Most are only at worst minimally damaging, although they still contain capricious, dangerous currents, even if the surface waves look innocuous.

A Youtube clip of a small tsunami travelling up the Avon-Heathcote Estuary on February 28, 2010, after a magnitude 8.8 earthquake off the coast of Chile the day before, shows just how quickly a small surge can turn into powerful waves when it hits shore.

Tsunami are also surprisingly frequent along the South Island's east coast, as are tsunami warnings for other parts of New Zealand.

Those generated by massive, and not infrequent, magnitude 8-plus earthquakes in the subduction zone offshore of Peru and Chile are a significant threat. New Zealand has plenty of notice about the arrival of these tsunami, which take about 15 hours to cross the Pacific Ocean. Distance does little to diminish the energy unleashed and its inexorable movement towards us.

A tsunami from a megathrust quake along the Hikurangi subduction zone east of the North Island poses a considerable danger to Canterbury, while local faults offshore in Pegasus Bay may also generate smaller tsunami, if they rupture in a certain manner.

Two notable tsunami have affected the Canterbury coast in the past 10 years: the February 2010 event from Chile, and the magnitude 7.8 November 14, 2016, Kaikōura quake.

When that hugely complicated quake tore apart more than 20 faults onshore and offshore through North Canterbury and Marlborough soon after midnight, it generated tsunami waves up to 7m above normal sea level along the Kaikōura coast. These waves crashed about 250m inland at Ōaro and 170m inland at Goose Bay.

A few hours later further south, on the northern side of Banks Peninsula, long-period tsunami waves forced their way into Little Pigeon Bay. There the funnel shape of the bay pushed the first wave up more than 4m above mean sea-level and 140m inland, with debris in the seawater swamping and wrecking a historic weatherboard cottage.

The larger Pigeon Bay and Port Levy also experienced tsunami surges and some inundation.

The February 28, 2010, tsunami gave Christchurch residents some idea of what could happen with an even larger quake off the coast of Peru or Chile.

Three 2m-plus waves washed up Lyttelton Harbour that afternoon, alternately sucking out seawater from many bays and then flooding them. The head of the harbour around Teddington was inundated, with the sea also covering the Governors Bay jetty at times and rising up through the wharves at Lyttelton. Water levels continued to wobble for a couple of days.

Fortunately, the tsunami waves hit at low tide, ensuring no major damage in the harbour, the Avon-Heathcote Estuary and for coastal Christchurch.

Then Prime Minister John Key blasted people who ignored Civil Defence warnings.

'The reality is that if it had been a significant tsunami, New Zealanders would have died, and maybe in quite large numbers, because they weren't taking the warnings seriously,' he said.

The May 1960 Chilean tsunami had a much larger impact on Christchurch and Banks Peninsula, having been generated by the world's biggest recorded earthquake of magnitude 9.5.

Sea level around New Zealand oscillated for days, with eight recorded tides that rose and fell beyond normal levels. One of these was a 5m tsunami on May 23, with waves of 25kmh rushing into the estuary ,flipping a cabin cruiser, breaking yachts' moorings and pushing dinghies up on to the main road to Sumner.

Across the estuary, South New Brighton's Rocking Horse Rd was swamped and the Heathcote River rose by 1m as far upstream as the Opawa bridge.

In Lyttelton Harbour, water poured into the dry dock on two occasions, a Charteris Bay home got washed from its foundations, more than 100 sheep in an Allandale farmer's paddocks drowned and 1m-deep water streamed through the Wheatsheaf Hotel at Teddington.

Another great seafloor quake off Peru in August 1868 generated New Zealand's only documented fatal tsunami, with 20 people believed to have been killed on the Chatham Islands when three waves struck within an hour, the first 6m high.

It is the 'once or twice in a lifetime' tsunami that evacuation zones are created for, Jack said.

'That's when we do need to get out of the way. If one is coming our way, we want to know how far inland we need to go.

'The zones are drawn around many different possible scenarios – there is no one tsunami that would flood the entire tsunami evacuation zone.'

Tsunami behaviour and their effects are complicated to predict, she says.

'It depends on lots of different factors – what caused the tsunami, where it is coming from, the wavelength of the tsunami, the shape of the coast at your particular location, and the tide level when the largest tsunami waves arrive.

'The way tsunami behave around Banks Peninsula is totally different to the open coast on Pegasus Bay, which presents challenges for the likes of the city council and how it puts out tsunami messages. One bay can be badly affected by a tsunami, while the neighbouring bay may not be.

'Banks Peninsula is a bit of a tsunami magnet because of the way the Chatham Rise focuses waves towards it and then how the waves are funnelled up the long harbours and bays. We've seen this in historic tsunamis in 1868, 1870, 1960 and 2010.

'The open Christchurch coast is less vulnerable to tsunamis than Banks Peninsula, but can still get large tsunami wave heights in some scenarios because of the focusing effect of the Chatham Rise and the way waves can slosh about backwards and forwards within Pegasus Bay.'

City council natural hazards senior advisor Marion Schoenfeld​ says ​many of the findings of the Niwa report, commissioned to help with flood plain management decisions, are unsurprising.

However, what is 'somewhat' surprising is that the one-in-500 year modelling for Lyttelton and Akaroa harbours resulted in more land being inundated, in the innermost, landward, bays of the harbours, than in the one-in-2500 year tsunami.

'This meant that in a few instances the tsunami evacuation zone needed to be extended. This is because the particular wave height and length interacts with the depth, length and shape of the harbours and their bays in a way that makes the wave build up bigger. It funnels the water in and 'excites resonance'.

'The few affected property owners have already been informed of this, and that they should evacuate if an event happens.'

Council head of Civil Defence and emergency management Rob Orchard says the team is continually reviewing information about tsunami risk to keep its evacuation planning up to date. 

Any changes would follow detailed analysis of all technical information and the public – especially those in areas directly affected – would be notified.

'At this point there is no timeframe for when tsunami evacuation zones may be updated.'

In the meantime, Schoenfeld says Christchurch residents near the beach need to remember the key Civil Defence advice on tsunami.

'The best message remains: If it's long and strong, get gone.'