New flood modelling shows that 63,200 Waikato residents are at risk of one-in-100-year rainfall events.
This rises to 75,500 residents with a further 3C of global warming, the modelling shows.
The modelling, by Earth Science New Zealand, shows that an estimated $20.3 billion in building damages is forecast, rising to $24.9b if global temperatures increase by a further 3C.
Nationally, 750,000 people live in areas vulnerable to one-in-100-year rainfall events, putting about $235b worth of buildings at risk.
In Waikato, nearly 3260km of roads, 1380km of stormwater pipes and 16% of electricity sites on the national grid and substations are at risk of flood damage.
The Paris Climate accord has tried to cap global warming at 1.5C.
A recent United Nations report cautions that there is an 86% likelihood global temperatures will exceed 1.5C above pre-industrial levels within the next five years, with a 1% chance of reaching 2C.
Predictions by the United States-based National Oceanic and Atmospheric Administration show the Earth is tracking towards 2.7C above pre-industrial levels by 2100.
University of Waikato professor Iain White and associate professor Silvia Serrao-Neumann convened river managers, iwi, government agencies, financial institutions and other key stakeholders to address the flooding hazard.
Flood risk management varies by region, and there is no consistent method to assess current or future flood risk under climate change, Serrao-Neumann said.
“Local and regional governments also vary in how they manage and reduce these risks.”
A project outcome is the country’s first consistent flood hazard viewer.
Research helped provide a consistent and co-ordinated approach to understanding, measuring and communicating flood risk at a national level, Serrao-Neumann said.
Nearly 26,800km of the nation’s roads, 14,100km of stormwater pipelines and 21% of national substations are also exposed to flooding under New Zealand’s current climate, programme leader and principal hazards scientist at Earth Sciences New Zealand, Emily Lane, said.
It could rise to 30,800 km, 15,400km and 29% respectively.
“Our country’s flood risk is increasing and not just in places where we can remember floods occurring.”
Rapid urban intensification is another contributing factor, Lane said.
The research showed notable regional differences in exposure, with 8% of people in Taranaki affected by one-in-100-year rainfall events under current climate conditions, compared with 34% on the West Coast.
The findings are the culmination of a five-year national flood risk research programme.
It involved wide-ranging collaboration with other research organisations, universities, councils, central government agencies and industry.
Dr Lane says the flood risk viewer complements localised maps already developed by many of New Zealand’s regional and unitary councils.
It is the first time that rainfall flood maps covering different regions of the country can be viewed in a single online tool.
The mapping workflow was carried out for New Zealand’s current climate and then repeated for the three climate-change scenarios and compared with the current climate, Earth Sciences principal climate scientist Sam Dean said.
“Increasing extreme rainfall due to climate change is one of the biggest and most impactful hazards faced by New Zealand.”
The building of towns and flood defences has been shaped by historical floods that are no longer a reliable guide to the future, Dean said.
“Predicting how flood risk is going to change is important to ensure we can protect people and places that we value in the most cost-effective way.”
The flood hazard viewer provides a bird’s-eye view to identify high-risk areas, aiding policy development, risk assessment, and investment prioritisation.
The Ministry of Business, Innovation and Employment’s Endeavour Fund paid for the research.
Tom Eley is a multimedia journalist at the Waikato Herald. He previously worked for the Weekend Sun and Sunlive.