Flooding at Waihī Beach in May 2023. Photo / Alex Cairns
Almost 63,000 Bay of Plenty residents live in locations exposed to flooding from “one-in-100-year” rainfall events, new research led by Earth Sciences New Zealand reveals.
This number could increase to 72,500 with a further 3C of warming.
Researchers modelled scenarios on the current climate and with an additional 3C of warming for all parts of the country as part of a five-year research programme studying flood risk.
Nationally, more than 750,000 New Zealanders live in locations exposed to flooding, increasing to more than 900,000 in the warmer scenario.
In the Bay of Plenty, about $18.8 billion worth of buildings across the region are exposed, potentially rising to $22.4b.
The research showed 1770km of roads, 1140km of stormwater pipes, and 23% of the region’s electricity sites are also exposed to flooding in the current climate, potentially rising to 1990km, 1230km and 46%.
An Earth Sciences NZ statement said the findings came from a project that involved collaboration with other research organisations, universities, councils, central government agencies and industry.
It delivered the “first nationally consistent flood hazard viewer”.
Researchers applied a consistent method for flood modelling from 1% annual exceedance probability rainfall - a “one-in-100-year rainfall” - to create new maps for 256 floodplains nationwide.
Earth Sciences NZ principal hazards scientist and programme leader Dr Emily Lane said the country’s flood risk was increasing partly because of climate change, with more rain falling in shorter periods, and because of rapid urban intensification.
The research revealed significant regional variations in exposure, ranging from 8% of people in Taranaki exposed to one-in-100-year rainfall events under the current climate to 34% on the West Coast.
Earth Sciences NZ principal climate scientist Dr Sam Dean told NZME the Bay of Plenty had 18.8% of people living in exposed areas, and the region was “quite a flood-exposed place”.
“You have a reasonably large amount of people living at the mouth of large rivers … and lakes.”
He said the Bay of Plenty Regional Council invested a lot of money in understanding flood risk.
He added that New Zealand towns had “often” been built on floodplains.
“As you build more houses and more buildings in those floodplains, you’re reducing space for the water and you’re increasing the depth of the flooding … ”
The maps did not account for what towns might look like in the future.
“We might retreat in some places and that might help, or we might continue to intensify in others and that would worsen the flood beyond the impacts that we’ve looked at.
“If we’re going to be invested in protecting people from flooding … and central Government’s going to be involved in that, then this kind of national information is really useful.”
One example of a managed retreat is Matatā, near Whakatāne, where the Government and councils bought out homeowners in the Awatarariki fan, an area highly exposed to destructive debris flow and hit by devastating flooding in 2005.
University of Waikato associate professor Silvia Serrao-Neumann said flood risk management was handled differently in different regions and there had been “no consistent way” to assess flood risk or how climate change could impact it.
Local authorities varied in managing and reducing risks.
“Our research helps to address this by providing a consistent and coordinated approach to understanding, measuring and communicating flood risk at a national level.”
Lane said Earth Sciences NZ’s tool complemented localised maps already developed by many councils.
“It does not replace them. Local and regional flood models can provide the precision needed to understand property-level risk and to design infrastructure.”
Bay of Plenty Regional Council integrated catchments general manager Chris Ingle said the national flood tool was a useful addition to flood risk management resources.
“It supports consistent assessments across regions, especially in areas where local data is limited.
“It also enhances our ability to understand flood risk at a national scale.”
The tool would help to inform planning, emergency management and climate adaptation work, “while complementing our existing property-level decision-making practice”.
“This collaborative effort takes us one step closer to a more climate-resilient Aotearoa.”
The research was funded by the Ministry for Business, Innovation and Employment’s Endeavour Fund.
The regional council has secured almost $30 million of central Government funding to support critical flood protection projects across the region, through two rounds of the Before the Deluge programme.
The latest round includes work for the Rangitāiki-Tarawera Rivers Scheme, Whakatāne-Tauranga Rivers Scheme, Kaituna Catchment Control Scheme, and Ōpōtiki township flood protection infrastructure.
Megan Wilson is a health and general news reporter for the Bay of Plenty Times and the Rotorua Daily Post. She has been a journalist since 2021.