NIWA’s Report on New Zealand’s River Flooding Exposure

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NIWA’s Report on New Zealand’s River Flooding Exposure

The National Institute of Water and Atmospheric Research (NIWA) report titled, “New Zealand Fluvial and Pluvial Flood Exposure” provides flooding profile and study of the country’s flood hazard.

It stated that the number of insurance claims indicates that flooding events in New Zealand are increasing. Flooding is the most frequent and most damaging natural hazard since the late 20th century.

It is expected that climate change will bring in more intense and frequent rainfall and sea-level rise that can give rise to flood inundation risks across New Zealand, the NIWA report says.

According to the report, New Zealand does not have yet a nationally consistent mapping of flood hazard map of populations and assets in at risk for to identify populations and assets at risk in fluvial and pluvial flood plains.  

Importance of the flood hazard area map (FHLA)

The report addresses this challenge by creating a composite flood hazard area map (FHLA) from modelled and historic flood hazard maps and flood-prone soil maps that are publicly available in August 2018.

Through this composite flood hazard area maps, elements at risk where identified and enumerated in the report. This includes populations, buildings, transport infrastructure, electricity infrastructure, three-waters infrastructures and land cover.

These maps show the areas and populations with the highest exposures are those in populous regions like Auckland, Waikato, Wellington, and Canterbury. With Canterbury the having the highest exposed assets and population.  The report also shows the production land most exposed in key dairy and pastoral production regions including Waikato, Canterbury, and Southland.

The national flood hazard area (FHLA) map shows estimated population, built asset, land cover, and transport infrastructure exposure by each region, and New Zealand building and replacement value of those assets exposed to flooding hazards.

Climate change and flood hazards

NIWA has previously assessed possible effects of climate change on agricultural water resources and flood flows in New Zealand.

The report mentions the links between climate change to flood hazards. To determine this, the mean annual flood (MAF) discharge is analysed under 4 representative pathway scenario (RCP) for 2036 to 2056, and 2086 to 2099 periods to determine the level of potential flood hazards to populations and assets in catchments areas sensitive to changes in mean annual flood (MAF) discharges. The results of future hazards from variations in MAF are discussed in the report.

Overall the report shows the following assets and population within New Zealand’s potential flood hazard area.

These includes 20 thousand square km of approximately 675 thousand resident-occupied area, 411 thousand buildings with a replacement value of NZD 135 billion as of 2016 replacement values, 19 thousand km of roads, over 1,500 km of railway, 20 airports, and 1,397 km of transmission lines, and more than 21 thousand km of three-water pipelines.

Recommendation of the study

The study recommends a national-scale exposure assessment for both present-day and future exposures. This includes:

  • improving and extending flood hazard model using high-resolution topographic data that incorporate important features like stopbank, culverts, bridges which influence flood hazard characteristics; and
  • updated and improved population spatial data sets, building spatial data set, infrastructure spatial datasets, and land cover spatial datasets that include more categories or layers of information for example buildings are categorized as rest homes, schools, etc in building spatial data sets.

Browse the entire report by clicking on the button below:

 Source citation:

Paulik, R., Craig, H., & Collins, D. (2019, June). New Zealand Fluvial and Pluvial Flood Exposure. National Institute of Water & Atmospheric Research Ltd. The Deep South Challenge. Retrieved from https://www.deepsouthchallenge.co.nz/sites/default/files/2019-08/2019118WN_DEPSI18301_Flood%20Exposure_Final%20%281%29.pdf

BACKGROUND PHOTO CREDIT: Kat2Kat2 via Flickr Creative Commons License

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