Latest tsunami-modelling report for Christchurch grim for coastal suburbs

Tuesday, 9 July 2019

A worst-case scenario South American tsunami combined with higher sea levels could swamp 5 per cent of Christchurch, a report from mid-2019 shows. (Video first published December 2019.)

Christchurch suburbs as far inland as Avonside and Waltham could be inundated by seawater in a worst-case scenario South American tsunami combined with higher sea levels, new modelling shows.

A report commissioned by the Christchurch City Council says a one-in-2500 year tsunami on top of climate-change induced sea-level rise of 1.06 metres 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.

Tsunami surges flooded the Governors Bay jetty during the February 28, 2010, event.

There is also a chance the force of the seawater over sand dunes on the Brighton spit will erode them, causing even more widespread inundation.

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The February 28, 2010, Chile tsunami swamped the head of Lyttelton Harbour, with flooding at the Wheatsheaf tavern, Teddington.

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The May 1960 tsunami caused mostly minor but widespread damage around Banks Peninsula and disrupted Lyttelton Port operations for weeks, with many boats ripped from their moorings. Seawater is seen here pouring into the dry dock. The magnitude 9.5 earthquake which generated this tsunami remains the largest in recent times.

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Warnings were issued of possible tsunami along South Island coasts, and State Highway 1 from Waipara north and to the West Coast was closed, following the November 2016 magnitude 7.8 earthquake.

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The biggest tsunami wave may be 8.6m high, from crest to trough, at the Waimakariri mouth, and about 7.25m high at the mouth of the estuary.

At Lyttelton, the tsunami could dislodge shipping containers and logs from wharves and spread them around the harbour, significantly increasing damage.

The National Institute of Water and Atmospheric Research (Niwa) report's best-case scenario tsunami – a one-in-500 year event at the current sea-level – would still cause 39 sq km of the city to be flooded, up to 5.4m deep near the Waimakariri mouth.

That would inundate the floodplains of the Brooklands Lagoon, the Styx River, Avon River, the mouth of the Heathcote River, low-lying areas of the estuary, including Southshore, parts of New Brighton near the pier, and the area around the Waimairi Beach surf club.

Tsunami warning sirens along Marine Pde in Christchurch's New Brighton.

The Niwa report models how tsunami generated by great earthquakes in the Peru subduction zone may flood the city and surrounds, and parts of Banks Peninsula.

Niwa considered one-in-500 year and one-in-2500 year South American tsunami from magnitude 9.28 and 9.49 quakes respectively and how they might affect coastal Christchurch given either the current sea-level or rises of 0.19 metres, 0.41m and 1.06m, along with predicted shoreline changes for 2065 and 2120.

A new city council report warns that even the best-case scenario tsunami would breach sand dunes at New Brighton near the pier.

Part of the report also covers those faults in Pegasus Bay with a high risk of rupture causing a significant tsunami between Sumner and the Waimakariri River mouth including the North Canterbury, Leithfield and Pegasus faults.

Major inundation from both distant source tsunami scenarios will be worse with higher sea-levels and Banks Peninsula bays are expected to flood more than previously thought.

The council's infrastructure, transportation and environment committee will discuss the findings at its Wednesday afternoon meeting.

In the summary paper for councillors, council staff say the findings will help with long-term planning, civil defence and emergency management procedures.

'The information within these reports will not necessitate a widespread update of LIM comments. However, a review of tsunami-related LIM comments is currently under way and this may lead to updates to coverage and comments,' it says.

Twelve more properties at the head of Lyttelton and Akaroa harbours will be included in the tsunami-evacuation area, based on the Niwa report.

Council senior advisor, natural hazards, Marion Schoenfeld said in a statement most of the report was 'not surprising'.

'As expected, when sea-level rise was added to the modelling, the flooded area got bigger and water deeper.'

Infrastructure, transportation and environment committee chairwoman Cr Pauline Cotter told Stuff Christchurch residents were now well aware it might not be 500 years before a one-in-500 year event hit.

'The fact we have now endured so many disasters means we know it is possible. We no longer put this into the 'well, that will never happen' category.'

The different sea-level rise and tsunami-size scenarios in the report would allow the city to more accurately determine the risk and its response. The work was part of the multi-hazard analysis project, she said.

'From this we can develop a strong evacuation plan. We don't want everyone heading for the same part of the [Port] hills.'

There was important information in the report for other agencies too, Cotter said.

The prediction that logs and shipping containers could float dangerously around Lyttelton Harbour during a tsunami should be factored into the Lyttelton Port Company's planning.

'That could be life-threatening.'

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