Hidden risk: What scientists just discovered about NZ’s coastal flooding danger

• Analysis has explored how many Kiwis are threatened by “indirect” risks of coastal flooding, such as being cut off from vital services.
• It shows 217,000 people are in danger of being isolated, with some of our most deprived communities at disproportionate risk.
• The new figures add to the 750,000 people already shown to be vulnerable to extreme flooding from rivers and storm surge events.

A first-of-its-kind analysis has revealed a large but previously hidden chunk of the population at risk from flooding – with our most deprived disproportionately endangered.

Modelling suggests some 750,000 people and 500,000 buildings near rivers and beaches are exposed to extreme flooding today, and those figures will only rise as our planet continues to heat.

While estimates like these tell us about who’s at risk of direct impacts, less has been known about indirect ones, like residents being cut off from vital services amid disasters.

A new study offers a first glimpse at this picture – and finds some troubling trends.

Its lead author, Mitchell Anderson of Christchurch-based Urban Intelligence and the University of Canterbury, cited the example of a house in a small coastal community during a storm surge event.

“In some cases, a home that is high and dry during the storm event may be detoured to an alternative route or to a different facility entirely,” Anderson said.

“In other cases, we see residents unable to get out of their community and unable to reach one or a combination of healthcare, emergency services or supermarkets.”

That very scenario played out in isolated East Coast communities during last year’s Cyclone Gabrielle, to which the effects of climate change were estimated to have added about 10% more rainfall.

The bridge over Hikuwai River between Tolaga Bay and Tokomaru Bay was taken out during last year's Cyclone Gabrielle, leaving communities cut off.

When Anderson’s team modelled the impacts of nearly 200 different coastal flood scenarios to every property in New Zealand, they found indirect impacts increased the number of people at risk by several orders of magnitude.

He pointed to Thames-Coromandel, where around 4000 residents were at risk from coastal flooding today.

When the risk of isolation was added, that figure climbed by a further 22,000 people.

In national terms, the figures increased from 61,000 to 217,000 when considering those directly exposed and those likely to become isolated, respectively.

“And consistently, we see low-income and high-deprivation populations carrying more than their fair share of the impacts – that is, the percentage of these population groups within the at-risk group is relatively much higher than those with higher income, etc..”

In a present one-in-100-year coastal flooding event, for instance, accounting for indirect impacts on top of direct ones increased the number of Māori people at risk from just over 13,000 to more than 30,300.

Anderson and his colleagues said the results highlighted the need for planners and policymakers to think beyond direct damage and consider who might be left behind in future disasters – especially when it came to the tricky issue of managed retreat.

That meant making adaptation strategies more inclusive and equitable – and ensuring isolated communities were part of discussions.

“Without inclusive engagement or understanding the distribution of risk within specific communities, we risk perpetuating existing inequalities and implementing ineffective or potentially harmful solutions.”

Anderson’s team, from Urban Intelligence and the University of Canterbury, were now exploring the measurement of other indirect risks from other climate-worsened hazards like landslides and wildfires.

Their study, published in the journal Risk Analysis, comes as National Institute of Water and Atmospheric Research (Niwa) scientists are using cutting-edge artificial intelligence to forecast flood inundation within a fraction of the time needed to run current models.

Ahead of intense storms, forecasters and agencies typically turn to physical models: those that predict impacts based on physics-based calculations like rainfall and river flows.

Niwa scientists have been simulating cutting-edge new flood models on 2021's real-world deluge in Westport. Photo / George Heard

While such models can take as long as 24 hours to run, Niwa scientists working on a new project have found using machine-learning can turn out detailed maps in a matter of minutes.

“What people really want to know is not just whether the river is running high, but what areas will be flooded, and what’s at risk from that potential flooding,” said Niwa climate, atmosphere and hazards platform manager Nava Fedaeff.

“We’re exploring how AI will help us to move from weather forecasts to inundation forecasts quickly enough so that useful information gets to those who need it.”

In testing the AI model, a team led by Niwa data scientist Dr Deidre Cleland successfully simulated the impacts of 2021′s disastrous Westport flooding.

“Our next step is operationalising this machine learning capability so that rapid flood map forecasting is available for a real incoming flood event in Westport,” Cleland said.

“We are also working on extending the machine-learning approach to other locations around New Zealand, starting with those at highest risk of flooding.”

Jamie Morton is specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

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