Maps reveal NZ’s hot spots for extreme coastal flooding

Just-released maps have revealed which areas of the country may be hit hardest by extreme coastal flooding.

Last year, a team of researchers warned that main centres like Auckland, Wellington, Napier, Marlborough and Nelson might be facing more than half a metre of rise by mid-century – and more than a metre by 2150.

With just 30cm of rise enough to make 100-year coastal storm surge and flooding an annual occurrence, fast-sinking parts of our coastal cities could cross that threshold by 2040.

Now, maps used with a recently-published study offer a yet-closer look at how sea level rise under climate change could compound coastal flooding problems.

Again, the work found that even small amounts of sea-level rise could drive a rapid increase in cumulative flooding from increasingly frequent events across the country.

“By 2065, there could be 40cm of sea-level rise, based on the current trajectory, or sooner where the landmass is subsiding,” Niwa scientist Dr Scott Stephens said.

The maps, produced as part of the Deep South National Science Challenge, showed large swathes of our coastline were vulnerable to flooding with extreme sea levels right now.

At present mean sea levels, some 1593sq km of area was exposed under a 10-year annual recurrence interval – or where there was a 10 per cent chance of the flooding recurring in any year.

Under a 100-year annual recurrence interval – or where there was a one per cent chance of extreme coastal flooding in a given year – the area was estimated at 1774sq km.

But another 30cm of sea level rise could greatly widen the area at risk - by another 713sq km (or 31 per cent) and 645sq km (27 per cent) under each of the intervals, respectively.

While more than 90 per cent of the exposed area was rural land, the modelling found urban spots were also at risk.

Of urban inundation areas where there were more than 10,000 people living, half were located in Bay of Plenty, Hawkes’ Bay and Canterbury.

This map shows inundation areas in Hawke’s Bay, under a one-in-100-year annual exceedance probability and assuming 30cm of mean sea level rise. Image / Niwa

“Coastal flooding is a global hazard that impacts New Zealand, with rising sea levels already causing more frequent and intense flooding along many coasts,” Stephens said.

“We have created maps that help identify the changing risk to land, property, and infrastructure from rising seas, at a regional and national scale.

“We hope this information will raise awareness and help councils and government to know where to conduct detailed investigations when developing adaptation strategies to protect our coastal communities.”

The maps were also intended to help the financial industry, national infrastructure, and service providers to risk assess their portfolios.

“It’s difficult to make decisions about the future if you can’t see it - these maps provide a window into the future.”

Coastal flooding was particularly likely when high tides, storm surges and large waves occurred at the same time, sending water over barriers in low-lying spots.

“With much of our major infrastructure and roughly 65 per cent of New Zealanders living within 5km of the coast, this is an issue that will impact many of us, so we must be aware of what’s coming and be prepared to adapt.”

Stephens added the maps weren’t designed for assessing exposure to individual properties.

“The advantage is that they fill in gaps for areas of the country that don’t currently have this data available, they provide nationwide information, and they include many scenarios of sea-level rise.”

According to New Zealand’s most recent risk assessment, 72,065 people live in areas exposed to that once-a-century risk, while about 675,500 people live in areas prone to flooding.

But those figures were almost certainly under-estimates, given more recent modelling.

Considering one estimate of the current rateable value of exposed residential property, $17 billion worth of homes could be at risk.

Add together all private and public properties and assets lying in coastal floodplain areas - and that sum climbed to nearly $150b.

Researchers have also warned that homeowners living in seaside areas at risk of one-in-100-year flood events today could lose insurance within their lifetimes – and perhaps within the next 10 years.

Over the longer term, insurance retreat from some locations was seen as inevitable, and at least one major insurer was already looking at pricing for coastal erosion risk.