Coastal Chch under water with 100 years - report

The Tonkin & Taylor report for the city council says areas of the eastern suburbs may need to be abandoned in a "managed retreat". Photo / Thinkstock

Experts have delivered a dire warning that rising sea levels will put some suburban areas of coastal Christchurch under water within 100 years.

The city council is now making plans for what will be a vastly different looking Canterbury coastline, caused by warming seas, melting ice in Greenland and Antarctica, and more storm surges.

South New Brighton, South Shore, Sumner, Brooklands and even parts of Linwood will become water logged if current sea level increase predictions eventuate.

This is will lead to planned abandonment of residential areas, not too unlike what happened to the worst affected earthquake areas, which were red zoned.

The Insurance industry says that houses in those areas will over a long period of time become uninsurable.

The warning comes in a $90,000 report carried out by environmental consultants Tonkin & Taylor for the city council.

They key factor in the report is that it supersedes previous estimations of sea level rises.

Instead of sea levels rising to half a metre by 2115 as predicted by international authority Intergovernmental Panel on Climate Change (IPCC) the Tonkin & Taylor report says it will be double this.

The report also says the worst case scenario of a sea rise of 2 metres by 2100 cannot be ruled out also.

The sea rises will affect all of New Zealand's coastline, it says.

The report's findings will now be taken into account when the city council formulates its district plan this year, deputy-mayor Vicki Buck, said yesterday.

Cr Buck said stop banking and other flood protection methods would be considered in the district plan.

"We need to identify these areas and say here are the options - and there are options," she said.

"Climate change has a reasonably grunty implications for everyone and countries and cities across the world are facing this challenge including Bangladesh, the Pacific Island and even New York," she said.

The Tonkin & Taylor report for the city council says areas of the eastern suburbs may need to be abandoned in a "managed retreat".

Managed retreat means a strategic decision is made by authorities to abandon or relocate properties and assets in the area.

The report warns if this cannot be done then authorities will have to undertake "forced retreat".

Avon-Otakaro Network co-chairman Evan Smith, who last year co-wrote a submission with now Mayor Lianne Dalziel on sea level rise, said he was surprised by the Tonkin & Taylor report.

"It has serious implications for the city because there is more land vulnerable in storm surge situations. The Christchurch public need to be briefed on what that means," he said.

The Tonkin & Taylor report also recommends the city council increase their minimum floor levels from 11.08m to 12.3m to compensate for a sea level rise of 0.5 metres.

Mr Smith said this aspect has big implications for people currently having their earthquake damaged houses rebuilt or repaired.

Those fixed or rebuilt before new standards are adopted could have issues, he said.

Cr Buck said the city was in a fortuitous position as the red zone covered much of the area at risk.

"Although the consequences of the red zone have been horrific this is actually one of the benefits," she said.

Mr Smith said the Government would not consider extending the red zone to cover more of the flood management area.

"For the red zone it had to be clear that the damage was due to the earthquakes. To include sea level rise would have enormous consequences because it affects all coastal area in the country so they would be setting a precedent."

The Canterbury Earthquake Recovery Authority confirmed this saying some of the flood management areas met the criteria for red zone due to earthquake damage but not all parts met their criteria.

Cr Buck said people in this situation were most at risk.

"This is why stop banking and protection from flood are so important. We need to identify these areas and say here are the options - and there are options," she said.

NIWA climate scientist Brett Mullan said storms tracks were more "generally south" from New Zealand which made it difficult to predict if the city would see more severe storms.

"But at best guess we will continue to see the same level of storm surge but on top of rising sea levels which means overall there will be an increase of the base level," he said.