The Rangitata River 2019 floods; the braided river reclaims its stolen braids. The B&W image is late 1960. Images: Canterbury Maps Historical Aerial Imagery CanterburyMaps & partners licensed for reuse CC BY 4.0
From the ground, it would have seemed like chaos; floods of water rampaging over the plains, damaging anything in its path. But from above, a different picture was emerging. Environment Canterbury (ECan) staff were photographing the floods from the air, later stitching together the images to create a mosaic of the event. It showed the floodwaters were following a predetermined pattern. The flood was itself a river, with twists and braids and tributaries, much like the Rangitata itself. A zombie river, long ago buried beneath asphalt and housing and irrigators, had been revived. – The Rewilding Project, Stuff 2021
Home > Climate wiki > Impacts >
Summary
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. – NIWA 2025
It is not possible to ‘fix flooding’ and some level of flood risk would be present even if investment were significantly increased. There will always be a bigger flood event, or areas that cannot be practicably remedied. As described in the 8 September 2022 report that preceded this report: ‘Managing flooding is challenging in Christchurch as it is flat and low lying. Pipes, drains and waterways only have limited capacity so the city also relies on overland flow paths and flood ponding to deal with extreme events. We design our networks to direct stormwater and flooding towards parks and roads ahead of properties and homes. However, past practices have left a legacy of risk in some locations and there are still some very low lying buildings at high flood risk…’ – p181 Agenda, Christchurch City Council April 2023
Glossary of terms:
- Fluvial: flooding from rivers, primarily rainfall in the river’s catchment and/or snow melt raising levels to where it breaches banks, stopbanks, levees, dams etc; and/or partial glacier collapse (‘outburst flooding’; for example the 2003 Franz Josef event).
- Pluvial: flooding when rainfall that can’t drain quickly enough due to the intensity of the rain and impermeability of the surface (e.g. concrete or dry compacted earth, high water table, aquifers already saturated etc. (Video 3) and/or drainage capability and capacity (natural, i.e. streams, rivers, wetlands, and /or engineered structures such as ditches, drains, culverts etc).
- Flash flooding: Occurs quickly due to sudden outburst of intense rain over short catchments and steep slopes. For example, the 2023 Auckland Anniversary flood and the 2026 thunderstorm over Wellington.
- Surface flooding: where rainwater can’t drain away quickly enough, it pools on the surface due to blocked drains, storm water network overwhelmed.
- Coastal flooding: low-pressure weather systems raise the height of the ocean are often accompanied by storm waves. Exacerbated by rising sea levels (link to coastal flood tool) floodwaters from the land may be unable to drain into the ocean.
- 1%AEP (Annual Exceedance Probability) has a 1%, or 1-in-100 chance of occurring in any one year, and a 10% chance of occurring in any 10 year period.
- ARI (Average Recurrence Interval) is the average time period between floods of a certain size. For example, under the previous climate, a 100-year ARI flow occurred on average once every 100 years.
- Stationarity: term used by river engineers, hydrologists, economists and insurance companies to work out the statistical likelihood of flooding based on how often it happened in the past, hence term such as 1-in-100-year-flood which arguably are of little relevance as the past is no longer a useful guide to predict the future:
Hydrologists have recognized for some time that climate change has undermined stationarity in water management—indeed, they have declared that stationarity is dead. But economists have by and large not recognized that this applies to climate effects. They approach climate damages as minor perturbations around an underlying path of economic growth, and take little account of the fundamental destruction that we might be facing because it is so outside humanity’s experience… In effect, economists have assigned them a value of zero, when the risks are decidedly not. – Profs. Oreskes & Stern
- Evaporation occurs more frequently at higher temperatures because the water molecules are moving faster
- Condensation is the opposite. Water molecules bring heat energy with them, so the surface of the dust warms slightly while the temperature of the surrounding air cools slightly, allowing the droplets to condense. This is due to a fundamental law of thermodynamics: see the
- Transpiration is the process by which plants ‘exhale’ water vapour through their stomata. Plants lose more than 90% of their water through transpiration. However over the last 150 years as carbon dioxide has been increasing, the density of stomata in some plants has dropped 34%. This is restricting the amount of water vapour the plants release. This has implications for the water cycle, especially in tropical rainforests, which by definition create rain largely through transpiration. This could also lead to more flooding:
Plants get more water-efficient and leak less underground soil moisture out through their pores in a carbon-rich atmosphere. Add this up over billions of leaves in very sunlit, leafy places, especially the tropics, and it means there is a bunch more soil moisture stored up underground, so much so that climate models predict rainfall events will saturate the ground and more rain will run off into rivers. – Ass. Professor Mike Pritchard, UCI
- Atmospheric Rivers Flooding from rivers that originate in the mountains will likely increase in intensity, although not necessarily in frequency. Intensity is often associated with numbers and duration of atmospheric rivers (Video 1). NIWA’s online tool estimates the magnitude and frequency of high intensity rainfall at any point in New Zealand: High Intensity Rainfall Design Systems (HIRDS).
Nine of the ten most damaging floods in New Zealand between 2007 and 2017 occurred during AR [atmospheric river] events. – Reid et al. 2021
Background photo: Giovanni Arechavaleta/Unsplash
Flood risks in a warming world
You’ll often come across the statement that for every 1°C of warming, the atmosphere can hold about 7% more moisture. This figure comes from research undertaken by the French engineer Sadi Carnot and published 200 years ago this year. Yes, a hotter atmosphere has the capacity to hold more moisture. But the condensation of water vapour to make rain droplets releases heat. This, in turn, can fuel stronger convection in thunderstorms, which can then dump substantially more rain. This means that the intensity of extreme rainfall could increase by much more than 7% per degree of warming. What we’re seeing is that thunderstorms can likely dump about double or triple that rate – around 14–21% more rain for each degree of warming. – Dowdy et al, May 2024
In 2024 there was a record amount of water vapour in the atmosphere; 4.9% above the 1991–2020 average, markedly higher than in 2016 (3.4%) and 2023 (3.3%), the years with the second and third highest values (Fig. 2)
The record was again broken in 2026:
According to the Copernicus Climate Change Service (C3S), the amount of water vapour in the atmosphere through August was 27.35 kg per square metre, beating the previous record from July 2024 by 0.04 kg m². The value describes how much water vapour is in a column of air sitting above a square metre of the Earth’s surface, extending up through the atmosphere.
The 0.04 kg m² increase on the previous record, one climate scientist calculated, was the equivalent of about 20 trillion litres of additional water, or 20 cubes of water, each a kilometre wide, high and deep. – The Guardian Sept. 2026
The oceans have also warmed much faster than climate models predicted. This enables even more water to be evaporated. Taken together, this means that Aotearoa New Zealand will experience more frequent and higher intensity rainfall along the western coasts, particularly in Te Waipunamu South Island (Fig. 3). While Canterbury is projected to get less rain, the catchments of large braided rivers are up in the mountains, so they are more likely to flood, particularly as less snow is accumulating over winter due to higher overall temperatures.
-
By 2080, under a ‘best case’ scenario (SSP2-4.5) compared to the period 1986-2005, annual average rainfall is projected to increase 32.8% on the West Coast, with a similar reduction in rainfall across much of Canterbury. However, more in alpine river catchment are likely to experience a concurrent higher flood risk.
Higher temperatures is also leading to a rapid decline in our mountain glaciers, increasing the risks of glacial outburst floods. For example the 2003 Franz Josef outburst flood. Cyclones are projected to track further south as the ocean and atmosphere continues to warm, and may strike the eastern coasts of both islands.
This new rainfall pattern can lead to more dangerous floods, particularly across Waitaha Canterbury because soils dry out during droughts, making the ground less permeable. When heavy rain follows drought (see the drought forecasting tool below), rather than being absorbed, it flows off soils as if they were concrete. This can lead to flash floods (Video 2).
-
Demonstration showing how long it takes water to soak into parched ground. Video: Dr Rob Thompson
Interactive flood hazards mapping tool Aotearoa
- It uses 1% AEP but explains this in the context of rising temperatures rather than specific times in the future.
-
It does not take into account coastal flooding from a simultaneous 1%AEP storm tide event.
ECan: catchments, mapping tools, and live feeds: river flows & rainfall
Under the Canterbury Water Management Strategy, the catchment of each waterway—their wetlands, groundwater, springs, lakes and rivers that flow down to estuaries—is considered together. Environment Canterbury manages 59 river and drainage rating districts (i.e. areas where ratepayers contribute to the cost of flood protection). Areas outside these zones are not protected from floods.
-
This composite screen grab shows three river catchments by name. Click the image to be taken to the the website, then run the mouse over each river to reveal its name and catchment area.
-
Click the image to be taken to the the website for the latest riverflow data.
-
Click the image to be taken to the the website for the latest rainfall data.
-
The screen shot below shows flood modelling. Click on it to go the mapping tool, then click on an area or type in an address to open a dialogue box that shows a number of extreme scenarios over different time periods, with links to technical reports and storymaps.
-
Click on the image to open the live feed. In this example, a screengrab of the the mapping tool, selecting the monitoring station at Aoraki/Mt. Cook was taken at 1.10pm, 14 January, 2025 showing live rainfall and flow/water level data.
High Intensity Rainfall Design System V4 (HIRDs): Earth Science New Zealand
-
Click either of the screen shots below to be taken to the online portal where you can log in and learn how to use the HIRDs. You do need to log in, can zoom in to locations on the map or type in an address, a site name, or a site ID to look at rainfall records. ‘Generate report’ creates a choice of spreadsheets: depth or intensity. In this example, Round Hill, which is behind Colac Bay / Oraka, west of Riverton, Southand, was selected.Knowing past rainfall patterns (depth and intensity) may be a useful guide to future rainfall, however as extreme rainfall events are increasing, this is a guide only.
Flood risk learning and decision-making tools
Earth Sciences New Zealand (previously NIWA) have developed a number of educational resources (Fig. 10) including a ‘Township flood challenge game’ to help everyone understand and prepare. Click any of the images below to be taken to their website. If you live in a flood-prone area, you can help by contributing photos to a national database to support understanding of flood hazard and flood risk.
See also:
Home > Climate wiki > Impacts >