Floods: bigger and more frequent
Floods: bigger & more often
“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
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. This problem is increasing exacerbated by rising sea levels.
Effects of climate change
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. 2; current rainfall; Fig. 4; projected rainfall). However, short periods of extreme rainfall may occur anywhere, resulting in an increase risk of pluvial flooding. 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.
“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
RCP8.5 ‘Worst Case Scenario’:
- Heat is measured in watts per metre squared, written as W⋅m−2
- In most graphs, the numbers 2.6, 4.5, 6.0, and 8.5 are W⋅m−2 however W⋅m−2 is implied, and the four units are written as four scenarios: RCP2.6 being the lowest amount of heat (2.6 W⋅m−2) and RCP8.5 being the most (8.5W⋅m−2)
- 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.)
References and further reading
- Ministry for the Environment: First national climate change risk assessment for New Zealand
- NIWA/ECan : Climate change projections for Canterbury
- 2019 NIWA: New Zealand Fluvial and Pluvial Flood Exposure (part of the Deep South Challenge New Zealand)
- ECan: River flow data (updated frequently along with alerts)
- ECan: Rainfall data (updated frequently along with alerts)
- NIWA: New Zealand’s climate
- NIWA: National Climate Centre
- NIWA: Climate change
- NIWA: The impact of El Niño and La Niña on New Zealand’s climate
- LAWA NZ: interactive website for river flows and water quantity and quality
- 2020 NOAA: Global Climate Report March 2020
- 2019: WMO Statement on the State of the Global Climate in 2019, World Meteorological Organisation, WMO-No. 1248
- 2014 IPCC 5th assessment Report AR5: Australasia
- 2013/2014 IPCC 5th Assessment Report AR5 (full)
- 2005: Goodsell et al; Outburst flooding at Franz Josef Glacier, SouthWestland, New Zealand, New Zealand Journal of Geology and Geophysics, 48/1 pp95-104