NHRP / Hazard themes / Weather, Flood and Coastal Hazards / Research Highlights 2015-16

Quantifying New Zealand's Coastal Risk Exposure

Photo: Dave Allen, NIWA

In November 2015, the Parliamentary Commissioner for the Environment (PCE) released her report ‘Preparing New Zealand for Rising Seas(LINK). NIWA contributed to the report by providing a ‘risk census’ on the regional and national risk exposure in low-lying coastal areas.

NHRP marae Anne Te Wake

Many marae and historical sites are located near the coast on low-lying land. This photo shows Mātihetihe marae on the coast north of Hokianga harbour. The hapū of Te Tao Mauī from Mitimiti are working with NIWA to understand how sea level rise might affect their marae. Photo: Anne Te Wake.

This census is the first attempt to consistently quantify the risk of exposure to rising seas in coastal areas across New Zealand. It was made possible because of the availability of both high-resolution LiDAR datasets and the national repository of buildings, roads and railways within RiskScape. These two critical geospatial datasets represent the two sides of the risk coin: hazard exposure (which, for inundation by sea level rise, is governed by land elevation), and who or what (‘assets’) is likely to be exposed. 

A national stocktake was completed looking at increments of narrow coastal elevations up to 3 metres above the mean high water spring (MHWS). Risk exposure was expressed simply as counts of normally-resident population, areas of land parcels, and numbers or lengths of built assets or infrastructure within these incremental elevation bands. The stocktake also provided a comparison of risk exposure between regions, territorial authorities and urban areas.

Resolving areas of coastal plains with such narrow increments in land elevation was mostly undertaken with LiDAR, but there are stretches of the New Zealand coast not represented by LiDAR. For these areas, we used the lower-accuracy national digital elevation model (DEM), with counts undertaken for a single 0–3 metre elevation zone. Comparisons between the two revealed that DEM underestimates the risk exposure by about half. Our recommendation is that coastal risk studies should only be done using LiDAR data. A summary of the risk-exposure across regions is shown. 

At 0 −1.5 metres elevation above MHWS, regions with the highest risk exposure include Canterbury, Hawke’s Bay, Waikato (roads especially), Wellington and Otago. Some key findings in relation to residents and buildings:

  • Of the regions with LiDAR data  available, two-thirds of people living in 0–1.5 metres coastal elevation zones are in Canterbury (23%), Hawke’s Bay (19%), Bay of Plenty (13%) or Auckland (12%), based on the 2013 Census.
  • Canterbury and Hawke’s Bay, followed by Waikato (mainly Hauraki-Coromandel), have the most buildings of all types in the 0–1.5 metres elevation zone.
  • Canterbury, Auckland, Wellington and Hawke’s Bay dominate the building replacement cost national totals. Across all areas with LiDAR available, the total replacement cost for all buildings comes to NZ$19 billion (2011) for land below 1.5 metres, rising to $52 billion for coastal land below 3 metres.
  • Dunedin (2683), Napier (1321) and Christchurch (901; excluding the Red Zone) have the most dwellings in the vulnerable lowest elevation band less than 0.5 metres above spring tide mark.

These findings do not necessarily mean that people and assets will be directly affected but it does mean they are potentially exposed to coastal hazards and sea-level rise over differing timeframes − with residents and buildings in lower elevation bands more likely to be impacted in the shorter term.

By Rob Bell, Ryan Paulik and Sanjay Wadwha (NIWA)

Contact: Rob Bell, NIWA

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Last updated 19 Sept 2016