Rising seas threaten Christchurch homes

GLENN CONWAY
Last updated 12:44 08/01/2014
Land elevation map
Tonkin & Taylor

MOVEMENT: Changes in ground elevation between 2003 and 2011.

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The insurance industry is warning it could pull cover for parts of Christchurch if the findings of a sea level rise report are not seriously considered.

Large chunks of Christchurch's eastern suburbs could be submerged by rising sea levels within the next 100 years, a report commissioned by the Christchurch City Council warns.

The full report can be read here.

Warming seas, the melting of Antarctic ice and the likelihood of increased tsunamis all raised the chances of some coastal communities going under water by 2115 and the council will need to start talking to residents in some areas about the level of "tolerable risk'' both are prepared to take as ongoing climate change dictates more influence on those parts of the city.

Areas most likely to be affected were South New Brighton, Southshore, Sumner, Brooklands and parts of Linwood and the report said the city council will need to talk to residents in affected communities about when the "degree of risk'' becomes unacceptable in certain areas.

Main roads to Akaroa could also be cut off if projected flooding hits.

Some of those areas are already in the Government-owned red zone.

Responding to the Tonkin & Taylor report this morning, the Insurance Council of New Zealand spokesman Samson Samasoni said the challenge would be to translate its recommendations into action.

"Without adaptation, there will be increased claims which led to higher premiums "or even insurance cover being withdrawn in some areas of Christchruch and throughout New Zealand''.

Every dollar spent in pre-disaster mitigation and adaptation measures would save many more dollars after the event, he said.

Samasoni said this sort of forward-thinking research was vital to improve Christchurch's resilience to sea level rise and extreme weather from climate change.

COASTLINE TO CHANGE DRAMATICALLY

The Tonkin & Taylor report, which cost ratepayers $90,000, said future planning of housing and development in those areas should plan for at least a one-metre sea level rise meant the Christchurch coastline could look dramatically different this time next century.

At best, protection measures could minimise the impact rising sea levels could have but the report warned a "retreat'' from some coastal areas may be needed over time.

The greatest impact of sea level rise on the city will be the raised risk of storm inundation and "the greater frequency of extreme tidal levels.''

The other major impact will be the "progressive shoreline retreat of low-lying areas,'' the report warned.

Hundreds of hectares of land could be submerged and "shoreline retreats'' ranging from 40m up to 200m-plus in some areas would be needed.

Detailed impacts along the coastline include:

- Christchurch dunes: A 1m sea level rise increases tsunami risk and the shoreline retreat would impact on the New Brighton Community Library and the North New Brighton Memorial and Community Centre and car-parking areas.

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- Beaches south of estuary: More flooding for Sumner and Taylors Mistakes surf clubs needing a 40m-60m shoreline retreat and the flooding of 70 hectares of land.

- Avon-Heathcote estuary: More than 530 hectares of land flooded and shoreline retreats of 370m at Southshore and 560m at South New Brighton.

- Lower Avon and Heathcote Rivers: A combined 2400 hecrates of land flooded and more flooding predicted in lower reaches.

- Brooklands lagoon and Styx River: About 1640 hectares of land flooded and a shoreline retreat that would extend the Brooklands lagoon shoreline by 700 hectares due to "passive inundation.''

- Port Levy: Shoreline retreat of 200m impacting Fernlea Point Road and Wharf Road.

- Okains Bay: Shoreline retreat of 60m impacting Okains Bay Road.

- Akaroa: The loss of 13 hectares of land from flooding with shoreline retreats of 70m for the northern area and 170m for the southern area impacting Jubilee Park and Beach Road.

- Takamatua: The loss of seven hecrates of land under water and a 200m shoreline retreat impacting Takamatua Bay Road and Old French Road.

- Duvauchelle: Increased flooding with about 10.5 hectares of land expected to be flooded and a 100m shoreline retreat impacting State Highway 75, Onewa Flat Road, Seafield Road and the local school.

- Wainui: Increased flooding with about 3.4 hectares of land under water and a 10m shoreline retreat impacting Wainui Road.

The report said a key issue for the city will be to determine what degree of risk is acceptable for property and people already located in areas vulnerable to the impacts of natural hazards (tolerable risk).

Tolerable risk is defined as the level of risk individuals and communities are prepared to tolerate under certain circumstances in return for a specific benefit.

"The (council) needs to consider when the degree of risk becomes unacceptable, at what level of cost (economic, cultural, social and environmental) they are prepared to undertake protection responses.''

This would require "focused discussion'' with affected people.

The report also recommended the council develop a city-wide sea level rise strategy which would create specific local plans "to increase the communities' resilience to sea level rise.''

A spokesperson for the Canterbury Earthquake Recovery Authority (Cera) said flood management areas were the responsibiity of local councils.

Christchurch City Council Flood Management Areas were identified in a City Plan change before the Canterbury earthquakes.

"CERA continues to work closely with all agencies involved in flood-prone areas to ensure the provided information on flood risk and floor level requirements is shared through its own channels.''

- The Press

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