Planning on a sea-level rise of X?

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"Planning on a sea-level rise of X?" is the title of NIWA principal scientist Dr Rob Bell's paper at the New Zealand Coastal Society Annual Conference in Nelson this week. Dr Bell is a key-note speaker at the event. "It's a play on words," he says. "There is no single number to plan for."

The upper range of Intergovernmental Panel on Climate Change (IPCC) projections for a sea-level rise of 0.5 - 0.8 metres by the 2090s is seen as more likely now. "But rises of more than a metre can't be ruled out, and should at least be considered," says Dr Bell.

In New Zealand, 65 percent of our communities and major infrastructure are within 5km of the sea. Twelve of New Zealand's fifteen largest towns and cities are coastal: Whangarei, Auckland, Tauranga, Gisborne, Napier-Hastings, New Plymouth, Whanganui, Wellington, Nelson, Christchurch, Dunedin and Invercargill - home to 64 percent of the total New Zealand population.

Auckland's downtown area, Wellington's CBD, and the airports of Auckland, Wellington, Nelson, and Invercagill are also very close to the sea.

There has been unprecedented development of our New Zealand coastal areas, where housing, along with tourism and its associated facilities such as roads, have resulted in a dramatic rise in potential risk from storm-tide inundation, coastal erosion or wave overtopping. This, says Dr Bell, "will be put to the test as sea-level rise and climate change effects progressively increase our exposure to these coastal hazards."

Dr Bell's view is that the uncertainty in nailing down the rate of sea-level rise or future impact on wave heights or storms can largely be circumvented by having a range of flexible approaches to planning and engineering design.

"A lot can be achieved by mainstreaming adaption to climate change into everyday council processes and scheduling of asset upgrades. For example, when councils are upgrading footpaths and roads, they should consider elevating the levels while they do it, and installing larger replacement stormwater pipes, when opportunities arise.

Dr Bell looks at a variety of complementary approaches to adapting to climate change. "The key is to get councils and infrastructure operators, such as engineers and planners, planning ahead, rather than waiting for more scientific certainty," he says.

For future proofing and planning, NIWA advocates a risk-based approach, asking "what's at stake" in terms of exposure to coastal hazards and sea-level rise. The higher the stakes, the more precaution should be applied. For example, a new coastal suburb should be designed with much higher sea levels in mind at the outset.

Local community plans for responding to sea-level rise and development can also be done in stages, taking into account the amount of sea-level rise the community can withstand, and then focusing on what should be done at the next stage. Timing of staged plans can initially be based on a credible rate of sea-level rise, but plans can be advanced or delayed depending on results from monitoring of local sealevels. This avoids the need to decide, at the outset, what sea-level rise value to allow for.

A similar adaptive approach is being used in planning staged upgrades to the Thames River storm-surge barrier in London, where the stakes are incredibly high. The approach has also been used in planning the upgrade of the low Waterview causeway – part of Auckland's northwestern motorway – considering the merits and costs of staging increases in height or undertaking works in one go.

Dr Bell says we also need to treat greenfield developments separately from the legacy of existing coastal development. New suburbs or coastal resorts are built to last, and given that the sea level is expected to keep rising for at least the next century, it is prudent to factor in a sizeable safety margin for sea-level rise and changes in coastal erosion or inundation. The 2010 New Zealand Coastal Policy Statement backs up this precautionary approach, requiring that new developments avoid risks from coastal hazards and climate change.

As the 1996 IPCC Working Group III report stated, "The challenge is not to find the best policy today for the next 100 years, but to select a prudent strategy and to adjust it over time in the light of new information."

The New Zealand Coastal Society (NZCS) is a technical group affiliated to the Institution of Professional Engineers New Zealand (IPENZ).

Paekakariki sea wall repairs. Credit: Dave Allen
Seawalled house in Whitianga. Credit: Rob Bell
Indicative areas that will require risk analysis to establish their likely vulnerability to coastal inundation as a result of sea-level rise – either directly (eg, inundation during storm events) or by the impact of sea-level rise on the drainage of low-lying coastal lands. The shaded red and orange areas show approximate land levels less than 5 m and 10 m above sea level, respectively: they have been extracted from reprocessed topography data collected by the Space Shuttle Radar Topography Mission (NASA). Accuracy of the topography is around 5–8 m. Credit: Ministry for the Environment 2008

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