Site Analysis

Understanding all the features of a site, using and protecting the best, and minimising the impact of the worst.

Rising sea levels

A rise in sea levels as a result of climate change is a real hazard for existing and new buildings in coastal locations. Sea levels around New Zealand have risen by up to 220 mm in the last century and are expected to rise by a further 300–400 mm in just the next 30–40 years.

The IPCC (Intergovernmental Panel on Climate Change) has projected a global average sea-level rise of up to 980 mm before the end of the century. Many houses being built today will still be standing then.

A joint report from the Ministry for the Environment and Statistics New Zealand in October 2017 found that together with rising seas, “We can expect tides, waves, and storm surges (extremely long, slow waves) to reach further inland more regularly, resulting in more frequent and serious flooding…”

Erosion will be a growing problem in many areas, particularly on the east coast of both main islands.

For new house construction this will mean that some locations that could have been built on in the past may no longer be suitable because of:

  • the threat of damage from flooding or erosion
  • inability to get insurance
  • local authority restrictions. (Under the Resource Management Act, local authorities are required to have particular regard to the effects of climate change.)

Where construction is permitted, higher floor levels may be required by local authorities or greater setbacks, such as longer erosion hazard setbacks for cliff-top or beachside homes.

New restrictions may apply in existing settlements. In Mapua and Ruby Bay, for example, there are Residential Closed Zones where further subdivision is prohibited, there can be no land filling, no new habitable buildings and no extension or replacement of existing habitable buildings closer to the shore.

For some existing settlements, choices will have to be made whether to restore natural defences to sea level rise (such as sand dunes or estuaries) or carry out (if possible) a managed retreat of existing buildings, giving up on land that is subject to high levels of flooding or erosion.

A 2015 study by the Parliamentary Commissioner for the Environment found that there were 43,680 homes and 133,265 people in the higher-risk areas – less than 3 metres above mean high water spring.

The hazard varies by location. Canterbury and Hawke’s Bay have the highest risk exposure, but the challenges are high in other locations too. In South Dunedin, for example, nearly 2,700 homes are less than 500 mm above the spring high-tide mark, with more than 70 percent of homes lying lower than 250 mm elevation. The council is examining how to deal with this, including the option of a “managed retreat”.

 

Updated: 16 January 2018