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Effects & Impacts: Floods – bigger and more often

The Rangitata River 2019 floods; the braided river reclaims its stolen braids. The B&W image is late1960. 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)

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Home > Climate wiki > Impacts > Canterbury flood risk

Floods: bigger & more often

Summary

“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:

“4.2 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, 05 April 2023

“New Zealand has a potential FLHA [flood hazard area] land area of over 20,000km2, occupied by a usually-resident population of approximately 675,000. The FLHA has over 411,000 buildings with a NZD$135 billion replacement value (2016 replacement values). FLHA infrastructure network components include more than 19,000 km of roads, over 1,500 km of railway, 20 airports, 3,397 km of electricity transmission lines and more than 21,000 km of three-waters pipelines.” – NIWA

“Nine of the ten most damaging floods in New Zealand between 2007 and 2017 occurred during AR [atmospheric river] events.” – Reid et al. 2021

  Impacts

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Home > Climate wiki > Impacts > Canterbury flood risk

Summary

“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:

“4.2 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, 05 April 2023

“New Zealand has a potential FLHA [flood hazard area] land area of over 20,000km2, occupied by a usually-resident population of approximately 675,000. The FLHA has over 411,000 buildings with a NZD$135 billion replacement value (2016 replacement values). FLHA infrastructure network components include more than 19,000 km of roads, over 1,500 km of railway, 20 airports, 3,397 km of electricity transmission lines and more than 21,000 km of three-waters pipelines.” – NIWA

“Nine of the ten most damaging floods in New Zealand between 2007 and 2017 occurred during AR [atmospheric river] events.” – Reid et al. 2021

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.
    • 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.

Video 1: If you live in, on, or anywhere near rivers and low lying areas that have been or you think might be subject to flooding, particularly in light of the potential risk of becoming uninsurable, then we would strongly recommend you either read NIWA’s report on this (the link at the top of this page) and/or watch this one hour presentation on the report.
Fig. 2. From 2019 NIWA's 2019 report, 'New Zealand Fluvial and Pluvial Flood Exposure' (page 8). Exposure to flood risk does not mean all of the areas on the map (Fig. 2) will flood. However, the risks are increasing as the climate changes as warmer air carries more moisture.
Fig. 2. From 2019 NIWA’s 2019 report, ‘New Zealand Fluvial and Pluvial Flood Exposure’ (page 8). Exposure to flood risk does not mean all of the areas on the map (Fig. 2) will flood. However, the risks are increasing as the climate changes as warmer air carries more moisture.

Effects of climate change

The atmosphere holds ~7% more water for every 1°C warming. It’s already warmed 1.2°C since 1850. The oceans have warmed as well, and faster than predicted in the 2013/14 IPCC report.

This means New Zealand is likely to 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 frequency and duration. Rivers that originate in the foothills of the eastern side of both islands are likely to receive less rain (Fig. 3; current rainfall; Fig. 5; projected rainfall). 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 an increasing number and duration of atmospheric rivers. 

Smaller foothill-fed rivers may also flood between extended periods of drought. Drought dries out soils making them less permeable, so a flood that follows a drought can be more damaging.

Fig. 3: Modelled annual mean rainfall average 1986-2005. Results are based on dynamical downscaled projections using NIWA's Regional Climate Model. Resolution of projection is 5km x 5km. (Image: NIWA)
Fig. 3: Modelled annual mean rainfall average 1986-2005. Results are based on dynamical downscaled projections using NIWA’s Regional Climate Model. Resolution of projection is 5km x 5km. (Image: NIWA)

“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

Fig. 4: South Island Flood Hazard Area (dark blue) (Image: NIWA).
Fig. 4: South Island Flood Hazard Area (dark blue) (Image: NIWA).

Projected changes to rainfall

Fig. 5: Projected annual mean rainfall changes under RCP8.5 climate change scenarios (see 'Explainer' at the end of this page). Time periods: 2031-2050 (left) and 2081-2100 (right). Changes are relative to 1986-2005 average (Fig. 4) based on the average of six global climate models. Results are based on dynamical downscaled projections using NIWA's Regional Climate Model. Resolution of projection is 5km x 5km. (Image: NIWA)
Fig. 5: Projected annual mean rainfall changes under RCP8.5 climate change scenarios (see ‘Explainer’ at the end of this page). Time periods: 2031-2050 (left) and 2081-2100 (right). Changes are relative to 1986-2005 average (Fig. 4) based on the average of six global climate models. Results are based on dynamical downscaled projections using NIWA’s Regional Climate Model. Resolution of projection is 5km x 5km. (Image: NIWA)
Fig. 6: Click on the image for up to date river flow information on the Environment Canterbury website.
Fig. 6: Click on the image for up to date river flow information on the Environment Canterbury website.
Fig. 7: Click on the image for up to rainfall data on the Environment Canterbury website.
Fig. 7: Click on the image for up to rainfall data on the Environment Canterbury website.
Fig. 7: Click on the image to be taken to the website. Please read all of the instructions first and then hit 'close' button on the instructions. Select any country and dates.
Fig. 7: Click on the image to be taken to the website. Please read all of the instructions first and then hit ‘close’ button on the instructions. Select any country and dates.

More information

  • Explainer: Atmospheric rivers
    This 90-second video explains what happens to California. The identical processes create atmospheric rivers that impact Aoteaora.