A new report puts the risks of sea level rise in stark terms: numbers of people, roads, buildings, electric wires and water pipes exposed after every 10cm. Hundreds of thousands of New Zealanders may already be living in areas that could be at risk of flooding — and they’re not necessarily on the coastline.

Every 10cm of sea level rise puts several thousand more New Zealanders in a position they might rather avoid: living in an area that could be flooded by an extreme storm.

Modelling has found that each added 10cm puts at risk another 7,000 New Zealand buildings, worth an estimated $2.48b to replace, 133km of roads and 10km of railway line.

To put this in context, seas have already risen almost 20cm globally since pre-industrial times and 1m of sea level rise is considered a middling projection for 2100.

NIWA’s Ryan Paulik and Rob Bell have finished a two-year project for the Deep South National Science Challenge, where they added varying amounts of sea level rise to today’s worst-case scenarios for coastal flooding and laid that over maps of people, buildings, roads, electrical wire, airports and water pipes to see what would potentially be exposed.

The coastal floods they modelled were the worst-case scenario, like the one that hit the coast of the Firth of Thames in 2018, the kind that happens when a storm surge coincides with a big tide, waves and other factors to cause extreme damage.

Any particular area of New Zealand might expect to see a flood that big only once a century, but planners and governments are required to prepare for them: the researchers noted the New Zealand Coastal Policy Statement requires councils to plan ahead for coastal hazards for the next 100 years. 

It is not only roads and buildings that might be more exposed to extreme storms as the sea creeps inland.

After 0.8m of sea level rise – a reasonably likely outcome by the end of the century — almost 150,000 people living near New Zealand’s coasts could find themselves living in areas that could be inundated by an extreme storm flood. That’s double the number of people in the risk zone from the same-sized flood today, Bell and Paulik’s modelling shows.

The researchers added increments of 10cm at a time of sea level rise to today’s worst-case flooding until they reached 3m higher seas, to see how much worse the damage could be.

Yesterday, Paulik explained the study to a seminar arranged by Deep South, saying the results probably overestimated the area of flooding, since they don’t take into account real-life obstacles that would block seawater from flowing freely inland. The modelling is a broad indication of exposure, not a precise list of buildings, roads or farmland that would flood in any real, big storm.

Now, researchers hope to repeat the exercise looking at smaller, more frequent storms.

The large numbers weren’t a complete surprise: Paulik’s co-author Rob Bell told the Insurance Council conference last year that the pair’s draft estimates showed 125,000 buildings worth $38 billion could be at risk of flooding in a huge storm if sea levels rose 1m. That rose to roughly 235,000 buildings exposed after 3m sea level rise, meaning most of the risk was stacked inside the first 1m.

But the starkest numbers Paulik presented yesterday weren’t about coastal flooding.

For the first time, Bell and Paulik modelled how much property and how many people might be exposed to the risk of extreme river and surface flooding caused by storms and heavy rain.

The estimates were bracing: 674,534 people and $135b worth of buildings could be in the risk zone. That doesn’t include the areas at risk of flooding from the coast, which were modelled separately. 

Some $27b of the at-risk buildings from non-coastal floods are in Auckland. Another $40b are in Canterbury.

Also potentially exposed are thousands of kilometres of roads, spread across many regions. 

All up, adding different kinds of floods together, hundreds of thousands of New Zealanders — and potentially up to a million — could be at risk from different kinds of extreme floods at different times, should an extreme event hit their region. 

Paulik said the river and surface flooding estimates were rougher than the coastal estimates, because New Zealand does not have a national flood model. The researchers had to create a patchwork of flood maps using local government and other sources, meaning the numbers were only a broad first estimate. 

The findings are potentially sensitive because of the enormous value of housing and rural land that could be revealed as being exposed when the mapping is released.

Full details of the studies and how various regions could be affected won’t be released publicly until Paulik and Bell have canvassed their findings with local councils. Full public release of both the coastal and other modelling is scheduled for December.

One things that is clear already is that many of the coastal risks are stacked within the first 1m of sea level rise.

Bell and Paulik’s study estimated what would happen up to 3m of sea level rise, but the biggest spike in potential coastal costs and exposure came before 1m was reached.

For example, Canterbury, Otago and Bay of Plenty regions already had more than 10,000 residents each who could be exposed to an extreme coastal flood today, the study found. In Canterbury, after 1m of sea level rise, that number would rise to 51,000 people.

Christchurch has the dubious distinction of being the city with the most people already living in potentially flood-exposed coastal areas. The city’s population potentially at-risk if an extreme storm happened would more than triple after just 80cm of sea level rise, from 11,941 to 33,640.

In New Zealand as a whole there were 49,709 buildings (worth $12.5b) already in areas that could be exposed to a drenching from an extreme storm flood. That figure would increase by another 7,043 buildings, worth a collective $3.48b, for every additional 10cm the sea rose from now, Paulik told people at the seminar. .

The potential costs in each region rise at different rates, depending where the valuable assets are.

For example, cities with CBDs by the coast would see serious leaps in financial exposure as soon as the high-value central city buildings entered the flood risk zone. In Wellington and Canterbury, the replacement value of property exposed to a risk of coastal flooding jumped by about $5b – per city – for every 1m the sea rose, said Paulik.

At just 50cm higher seas, Christchurch and Napier would have twice as many exposed buildings as they have today. (Each city already has more than 10,000 exposed already).

Waikato and Canterbury would have 500km of roads exposed after 1m of sea level rise, while more than 1,000 km of water pipeline would be exposed in Hawkes Bay, Wellington and Canterbury.

The modelling makes it easier to grasp what’s at stake from people’s efforts to control greenhouse emissions.

Globally, the average sea level has already risen 19cm since pre-industrial times, but emissions play a huge role in what happens next.

Seas could be 30cm higher in 2045, or not until 2075, depending what trajectory we follow. The range of possible sea levels next century and beyond gets wider and wider the further ahead you look, depending on greenhouse gas emissions. 

If topping 30cm could be delayed for a few decades, that represented 22km of roads in Auckland alone that wouldn’t be exposed to flooding, Paulik and Bell’s modelling showed.

The latest exposure numbers are much larger than the 2015 ones the former Parliamentary Commissioner for the Environment calculated. That’s because Jan Wright’s report took the normal high tide-line and added sea level rise, without looking at what would happen in an extreme storm flood. 

As of today, just under 50,000 buildings worth $12.5b could be potentially exposed to an extreme coastal flood — even without any further sea level rise, Bell and Paulik found. 

Up to 72,065 people live in these exposed areas — based on the 2013 census — and that number would more than double after 80cm of sea level rise, they discovered.

That’s a population bigger than Nelson whose risks from the sea could change within 60cm.

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