Reinstatement costs of up to $380 million required if flood occurs in Christchurch

In the worst case scenario, a 1m sea-level rise leaves the Christchurch with $380 million worth of "reinstatement" damage - buildings repairs, vehicles and clean-up costs. Photo / Christchurch Star

Grim projections show the large cost of an extreme flood event and future sea level increases on buildings and homes.

In the worst case scenario, a 1m sea-level rise leaves the city with $380 million worth of "reinstatement" damage - buildings repairs, vehicles and clean-up costs.

The worst hit suburbs will be South Brighton, New Brighton, Avondale, Moncks Bay and Aranui. In lower sea level rise scenarios, river flooding will become a problem in Beckenham and St Martins.

It comes at a time when insurers may not provide cover in extreme cases if risk has not been not mitigated, because the frequency of flooding may deem it a "foreseeable" risk,said Parliamentary Commissioner for the Environment Dr Jan Wright.

With a 1m sea level rise, $380 million in reinstatement costs would be required and people would be displaced from 2337 buildings across the city. At 500mm, reinstatement costs would be $105.2 million with 507 buildings affected, and with no sea level rise, $98 million with 456 buildings affected.

The data was prepared for the city council by the National Institute of Water and Atmospheric Research. The report combined sea level rise with extreme flooding, known as a one in 500-year event.

But the real cost could be double if business interruption and infrastructure damage are included.

"A crude rule of thumb is that overall losses are often double the reinstatement costs," said the report.

If the residential red zone was rebuilt, losses would be between three and 10 times greater.

NIWA used hazard impact and loss modelling software called RiskScape. It overlaid last year's property and building data coupled with draft city council high flood hazard management maps, adding in sea level rise scenarios. Stop banks are expected to have failed and the flooding occurs at night with no warning or a short lead in time.

Dr Wright said the sea level is projected to increase by about 300mm by 2065.

This means "100-year events", like the flooding that struck the city in March 2014, will occur more often. In Christchurch, this means once a year by 2065, and with "every tide" when the sea rises 1m, she said.

The insurance industry was therefore considering whether the effects of sea level rise, erosion and flooding are becoming such a "foreseeable risk" that cover could be affected, said Dr Wright.

"[Insurance companies] insure against risk that is unforeseen, but this [flooding and sea level rise] is becoming a foreseen risk. Over time, insurers will go, well, this is getting pretty predictable and we are not going to cover it," she said.

Insurance Council of New Zealand chief executive Tim Grafton said if floods were "frequent", like in Flockton Basin, insurers could raise the excess and premiums or not provide cover in "extreme cases".

"[Insurers] are in the business of cover for the sudden unexpected occurrences. That's where you might see companies as a last resort not providing cover," he said.

But if mitigation measures are undertaken - similar to that in the Flockton Basin - to lessen the risk of flooding, insurers would respond accordingly because the risk is less, he said.