Rising seas: NZ must 'put brakes on' coastal development

Lower Hutt was one of the areas most immediately threatened by flood risks from climate change. Photo / NZ Herald

Scientists have warned against developing in coastal areas – even in places not yet threatened today – as new data shows how hundreds of thousands of Kiwis are living in climate change's firing line.

In two new reports, released today by Niwa and the Deep South National Science Challenge, researchers Ryan Paulik and Dr Rob Bell investigated how buildings and infrastructure would be hit by river and coastal flooding from storms and sea level rise.

One of the reports looked at how one-in-100-year flood events would be compounded with various sea levels up to 3m higher than today.

Around 72,000 Kiwis were currently exposed to present-day extreme coastal flooding, along with about 50,000 buildings worth $12.5 billion.

The risk exposure increases markedly with sea level rise, particularly during the first metre of rise, which meant long-term planning to address the risk was urgent, they found.

Modelling showed those areas most at risk initially included Napier, Wellington and Christchurch.

There was near certainty that the sea would rise 20cm to 30cm by 2040 and, by the end of the century, depending on whether global greenhouse gas emissions were reduced, it could rise by between 50cm and 1.1m.

That added an additional 116,000 people exposed to extreme coastal storm flooding.

Bell said it was important for councils addressing coastal adaptation to prioritise the areas most at risk but also to undertake regional 100-year risk assessments to inform where land-use planning restrictions are put in place to limit future intensification – even if coastal flooding was some years away.

"We need to put the brakes on development in coastal areas even if areas may not be impacted for a few decades - given sea level will continue rising."

The other report explored what could happen when rivers were flooded by heavy rain and storms.

Across the country, almost 700,000 people and 411,516 buildings worth $135b were exposed to river flooding in the event of extreme weather events.

With climate change, more extreme rainfall events are expected to occur – but weren't covered in this report. Also exposed were 19,098km of roads, 1574km of railways and 20 airports.

The information was put together using a range of flood maps and data published by local authorities, central government organisations, Crown Research Institutes and Land Information New Zealand.

While the information used was the best publicly available, it varied in detail and age, which meant the findings are general and should only be used as a first nationwide estimate, Paulik said.

He added the research has revealed an urgent need for national flood risk maps.

"National flood risk maps are essential because we need accurate and comprehensive information about the impact and costs of flooding today and under different climate change scenarios so everyone can plan and adapt."

The new research comes after Local Government New Zealand's own assessment, released earlier this year, showed as much as $14b of ratepayer-owned infrastructure was at risk of sea level rise – and recommended New Zealand set up a new national adaptation fund and risk agency to help councils address the problem.

Around $2.7b of roading, water, and building infrastructure was at risk from as little as half a metre rise in sea level.

The value of at-risk infrastructure ramped up sharply at each increment of sea level rise, with the data showing that $5.1b worth was at risk at 1m of sea level rise, with $7.1b at risk at 1.5m and $14.1b at risk at 3m.

Another recent Auckland Council report showed more than 43,000 Aucklanders were directly threatened by rising seas - up from 34,700 people in 2001.

The Government was responding to the threat by setting up a new national risk assessment system.