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)
Nine of the ten most damaging floods in New Zealand between 2007 and 2017 occurred during AR [atmospheric river] events. – Reid et al. 2021
Under the RCP8.5 warming scenario, there will be a global doubling or more of the occurrence, integrated water vapor transport and precipitation associated with EAR (extreme atmospheric rivers), and a more concentrated tripling for the landfalling EARs, by the end of the 21st century. – Wang et al, 2023
Terminologies
- 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’ see here for example).
- 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.) and/or drainage capability and capacity (natural, i.e. streams, rivers, wetlands, and /or engineered structures such as ditches, drains, culverts etc).
- Flooding on coastal areas: low-pressure weather systems raise the height of the ocean and are often accompanied by storm waves. This can inhibit floodwaters from draining into the ocean; a problem exacerbated due to rising sea levels.
- 1-in-100 year flood: The terms AEP (Annual Exceedance Probability) and ARI (Average Recurrence Interval) describe the probability of a flow of a certain size occurring in any river or stream.
- ARI is the average time period between floods of a certain size. For example, a 100-year ARI flow will occur on average once every 100 years.
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Alternatively, AEP is the probability of a certain size of flood flow occurring in a single year. A 1% AEP flood flow 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. Therefore, the 100-year ARI flow and 1% AEP flow are different terms to describe a flow of the same size in any given river.
- However, terminologies like ‘1-in-100 year flood’ is arguably of little relevance as it uses the past to predict the
future, something that has evident in our rapidly changing climate (see Stationary).
Environment Canterbury manages 59 river and drainage rating districts (i.e. areas where ratepayers contribute to the cost of flood protection). This map shows each rating district around rivers. Areas outside these zones are not protected from floods.
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. Managing catchments in this integrated way means problems can be considered based on each catchment’s unique attributes. These maps will help you identify which catchment zone you are in. Note that while there is overlap, these zone are not the same as river-ratings districts.
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.
Effects of climate change
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
It’s already warmed 1.36°C since the late 19th century (although in 2024 global average temperatures exceeded 1.5°C, averages are calculated in the previous 20-30 years). The oceans have warmed as well, much faster than models predicted.
This means New Zealand will experience more frequent and higher intensity rainfall along the western coasts, particularly in the South Island. Flooding from rivers that originate in the mountains will likely increase in intensity. Rivers that originate in the foothills of the eastern side of both islands are likely to receive less rain (Fig. 4). However, short periods of extreme rainfall may occur anywhere, resulting in an increased risk of pluvial flooding. This type of high rainfall is associated with numbers and duration of atmospheric rivers (Video 1).
Smaller foothill-fed rivers may also flood between extended periods of drought. Drought dries out soils making them less permeable, so heavy rain following a drought can be more damaging as the water flows off the soil rather than being absorbed (Video 3).
Flooding is New Zealand’s most frequent damaging natural hazard. Insurance claim statistics indicate damaging flood events have been increasing since the late 20th century. Future climate change will cause sea levels to rise and could increase heavy rainfall events potentially increasing flood inundation hazard. When coupled with urban development in or near active floodplains they would expose New Zealand to more frequent damage and disruption from flood hazard events leading to higher economic losses. – NIWA
NIWA tools for adapting to climate change
ECAN river and rainfall data tool
Stationarity is a 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.