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Impacts: floods – bigger and more often

Floods: bigger and more often

(Images: Rangitata River CanterburyMaps & partners licensed for reuse CC BY 4.0)

Floods: bigger & more often

Summary

“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: from rivers, primarily rainfall in the river’s catchment and/or snow melt raising river levels to the point that it breaches riverbanks, 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 (eg concrete or dry compacted earth, high water table, aquifers already saturated etc.) and/or drainage capability and capacity (natural, ie streams, rivers, wetlands, and /or engineered structures such as ditches, drains, culverts etc).
  • Flooding on coastal areas: low-pressure weather systems raise the elevation 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.
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.

Environment Canterbury manages 59 river and drainage rating districts (i.e. areas where ratepayers contribute to the cost of flood protection). This map shows the extent of 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.

Fig. 1: The Rangitata River 2019 floods: the 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 | Google Earth)

“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)

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 more than 1°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 west coast, 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 kind of high rainfall is associated with an increasing number and duration of atmospheric rivers. These smaller 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)

“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).

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. 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.

Explainer

RCP8.5 ‘Worst Case Scenario’:

RCPs or Representative Concentration Pathways, represent the concentration of greenhouse gasses in the atmosphere based on how these gasses retain heat.

  • Heat is measured in watts per metre squared, written as W⋅m2
  • In most graphs, the numbers 2.6, 4.5, 6.0, and 8.5 are W⋅m2 however W⋅m2 is implied, and the four units are written as four scenarios: RCP2.6 being the lowest amount of heat (2.6 W⋅m2) and RCP8.5 being the most (8.5W⋅m2)
  • The projected rainfall maps on this webpage are limited to RCP8.5 (worst case scenario) for two reasons:

1. “Stage 1 of this NCCRA used projections based on RCP8.5, a high greenhouse gas emissions scenario. This is assumed to be a plausible upper level of risk. It supports the identification of the most significant climate-related risks, analysed in Stage 2 of the assessment.” – p36 National Climate Change Risk Assessment

2. Real-world events are outpacing several of these climate projections, which in turn has prompted this disclaimer: “More extreme scenarios are possible, and the sensitivity of the climate system remains uncertain.” (Op. cit.)


Atmospheric Rivers

Video 2: While this video explains what happens to California; the same processes create atmospheric rivers that impact Aoteaora.

References and further reading