Source: Earth Sciences New Zealand
New research led by Earth Sciences New Zealand reveals that more than 750,000 New Zealanders live in locations exposed to flooding from one-in-100-year rainfall flooding events. And this number could increase to more than 900,000 with a further 3 degrees of warming due to climate change.
Around $235 billion worth of buildings across the country are exposed, which could rise to $288 billion if there is 3 degrees of additional warming.
26,800 kms of the nation’s roads, 14,100 kms of stormwater pipelines and 21% of national grid sites (e.g. substations) are also exposed to flooding under New Zealand’s current climate. This could rise to 30,800 kms, 15,400 kms and 29%, respectively, with 3 degrees of warming.
“Our country’s flood risk is increasing, and not just in places where we can remember floods occurring. This is partly due to climate change – we know rainfall intensity is increasing across Aotearoa, with more rain falling in shorter periods. Rapid urban intensification is another contributing factor,” says Dr Emily Lane, the programme leader and principal hazards scientist at Earth Sciences NZ.
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.
The findings come from the culmination of a five-year research programme looking at flood risk across Aotearoa New Zealand. The programme involved wide-ranging collaboration with other research organisations, universities, councils, central government agencies and industry.
A major output of the project is the country’s first nationally consistent flood hazard viewer. The researchers applied a consistent method for flood modelling from 1% annual exceedance probability (AEP) rainfall to create new maps for 256 flood plains around the country. It is the first time 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 (+1, +2 and +3 degrees of additional warming), compared with the current climate.
“Increasing extreme rainfall due to climate change is one of the biggest and most impactful hazards faced by New Zealand. The building of our towns and flood defences have been shaped by historical floods that are no longer a reliable guide to the future. 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,” says Dr Sam Dean, principal climate scientist at Earth Sciences NZ.
“Our new flood hazard viewer provides a clearer view across the country of which communities are at greatest risk from flooding, now and as the climate changes. The tool can be used to support risk assessments and adaptation investment decisions,” says Dr Dean.
The new flood hazard viewer provides a bird’s-eye view (to street level) to identify areas most at risk – supporting national policy development and risk assessment and helping to make a national case for prioritisation and investment planning. It also fills gaps for locations that don’t already have this information. As the first nationally consistent view of flood hazard available to New Zealanders, it is a significant first step towards a national flood map that will be able to provide property-level information about risk.
Dr Lane says Earth Sciences NZ’s tool is complementary to localised maps already developed by many of New Zealand’s regional and unitary 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,” she says.
Graeme Campbell, River Management Advisor for Te Uru Kahika – Regional and Unitary Councils Aotearoa, has been involved throughout the project.
“In New Zealand, detailed local flood hazard maps are made available by local government agencies and provide essential information for local decision-making. Regional and unitary councils will continue to make these maps available, and the new national tool produced by Earth Sciences NZ does not replace these local flood hazard maps.
“Instead, it adds value by providing information where there are currently gaps and improving the comparability of flood hazard assessments nationwide. Recognising the importance of local flood knowledge and expertise, Earth Sciences NZ has engaged closely with regional council river managers and scientists for the duration of this five-year research program,” he says
Another important part of this research programme, led by Professor Iain White and Associate Professor Silvia Serrao-Neumann of the University of Waikato, involved bringing together river managers, iwi, government agencies, financial institutions and stakeholders. This enabled the research team to ensure that programme outputs would be useable and useful, to explore adaptation options under changing climate conditions, and to design new ways to make decisions, taking into account climate and socio-economic projections.
“Flood risk management is handled differently in different regions, and, to date, there’s no consistent way to assess current flood risk or how this might change under climate change. Local and regional governments also vary in how they manage and reduce these 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,” says Associate Professor Serrao-Neumann.
“Having consistent information is crucial for others to act to manage their own risk – from business to communities to infrastructure providers. This tool helps enable consistent processes for modelling and provides the foundation on which to build national-scale resilience. Providing certainty to decision makers will help avoid passing liabilities to current and future generations,” says Professor White.
The five-year research programme, titled Mā te haumaru o ngā puna wai o Rākaihautū ka ora mō ake tonu, also included a detailed look at complex social interactions for communities affected by flooding, including with programme partners Wairewa Rūnanga.
More on this part of the project will follow.
Can I access the flood exposure data for New Zealand and regions?
Please note that this is part of a larger dataset that will be released later this year alongside a report into the exposure modelling methodology.
What does one-in-100-year rainfall or 1% AEP mean?
One-in-100-year rainfall is also called 1% annual exceedance probability (AEP). It means that in any year, there is a 1%, or one in 100, chance of an event that size occurring at that location. It’s important to note that if flooding from a 1% AEP rainfall event happens, it doesn’t mean you’re off the hook for the rest of the century – each year, the risk effectively resets, meaning 1% AEP events can happen several times in the same 100-year period. And as our climate warms and heavy rainfall becomes more intense, what is currently a 1% AEP event will occur more frequently.
When are +1, +2 and +3 degrees of additional warming expected to occur?
Timeframes for additional warming compared with our current climate are uncertain as we don’t yet know how future global greenhouse gas emissions will track. Based on the latest climate projections for New Zealand, 1 degree of additional warming may occur by 2029 (under a higher emissions scenario; SSP585) and by 2042 (under a low scenario; SSP126). Two degrees of additional warming could occur by 2056 under SSP585 or by 2074 under SSP245. Three degrees of additional warming could occur by around 2076 under SSP585. Lower emissions scenarios (SSP245 and SSP126) do not reach 3 degrees of extra warming through to the end of the century. If the world was to achieve the lowest future emissions scenario (SSP126) – which is possible but unlikely based on current trajectories – we would stay within around 1 degree of additional warming by the end of the century. You can find an explainer on Shared Socioeconomic Pathways (SSPs) here: https://www.carbonbrief.org/explainer-how-shared-socioeconomic-pathways-explore-future-climate-change/
How does this tool differ from flood maps already available from councils?
Flood hazard can be assessed in a variety of ways and using a range of variables, all of which can give different results. For instance, many council maps provide information at individual property level, which this tool does not. Some maps incorporate flooding from the sea, such as sea-level rise and storm surge inundation (our viewer only shows flooding from rainfall events). Furthermore, flood maps can be created at different resolutions and using different AEPs, and models may include bespoke calibrations that individually address details such as missing culverts or flood defences.
How does this tool differ from the National Flood Map announced recently by the Minister for Climate Change?
The National Flood Map will be based on a national flood model, a national coastal inundation model, and regional and local flood information. It will be the definitive source of information for understanding flood risk both now and in the future.
The work done to date on this flood hazard viewer can provide an important stepping stone towards this National Flood Map. We hope that this work will underpin and vastly speed up the process, so that all New Zealanders get access to a single source of information they can rely on when understanding their hazard risk.
