Wellington - Highest Rate of Sea-Level Rise in NZ

Two reports released today show Wellington has the highest rate of sea-level rise in New Zealand and highlight areas in the region that are vulnerable to coastal flooding over the next 100 years.

The reports, commissioned by the Greater Wellington Regional Council (GW), look at sea-level rise and coastal flooding from storm events in the Wellington region including current trends and future scenarios based on those trends. Wave heights, wind strength, storm-tide levels, sea-level variability and large-scale storm events are all assessed.

Findings from the studies and the implications they are likely to have on the Wellington region’s coasts, communities and local authorities were presented today.

It was found that the Wairarapa and south coasts are exposed to the largest waves, with average wave heights of over 6.0m in places during some of the computer-simulated storm events, but that these areas experience lower storm tides*. In contrast, the Kapiti Coast experiences smaller waves, but is subject to larger storm tide inundation due to shallower sea floors near the coast.

Another key finding in the report is that Wellington Harbour has experienced an average rise in sea level of about 2mm per year over the past 100 years. Most of this rise is due to climate change but is being exacerbated by subsidence of the city over the past decade, caused by slow-slip seismic events from deep tectonic plate movements. Projections for the end of this century indicate sea level in Wellington region could rise by 0.8m by the 2090s or 1.0m by 2115.

The reports will be useful for anyone involved in urban planning to help make decisions around the development of buildings, roads and other infrastructure in low-lying coastal areas.

“It will be especially important to have the reports to inform the new 10-year regional plan we are currently putting together,” says GW Senior Hazards Analyst Dr Iain Dawe.

“We need integrated coastal management in the region with authorities working together to plan for natural disasters in the future. It’s critical we have a long-term view of issues such as sea-level rise that will impact our coastal communities.”

The reports were written by NIWA scientists (Rob Bell, Emily Lane, Scott Stephens, Richard Gorman and David Plew) with consultants Prof. John Hannah and Dr Jeremy Gibb. Funding assistance was provided by Wellington City and Kapiti Coast District Councils.

The sea-level rise report is available at www.gw.govtnz (look under ‘Plans & Publications/Document library/Hazards’). The report on storm tide and coastal flooding will be available on this site soon.

* Storm tide results from a combination of low air pressure, high waves and strong onshore winds that push water up against the shoreline, causing a temporary rise in water levels for up to three days When this occurs at high tide, it can cause coastal flooding and damage in low-lying areas, such as that which occurred in Wellington’s Owhiro Bay a few weeks ago.

QUESTIONS & ANSWERS

1. Is the sea rising?
Yes. Tide gauge recordings from the mid-19th century and satellite measurements from the early 1990s all show that the sea is rising. In New Zealand there are tide gauge recordings from all the main ports, including Wellington, that date back over 110 years.
2. How fast is the sea rising?
The long term record from New Zealand shows that sea level has been rising at an average rate of 1.7mm/yr. However, in Wellington it is slightly higher due to subsidence and is currently increasing at 2.03mm/yr. This equates to a little over 0.2m over the last 100 years alone.
3. What is storm-tide?
Storm-tide results from a combination of low air pressure, strong winds and high waves that act together to raise the water level at the coast. The effect, which can last up to three days depending on the severity of the storm and when it coincides with high tide, can cause coastal flooding, erosion and damage from debris being thrown up by wave activity.
4. What areas are vulnerable to coastal flooding?
All low-lying areas around the coast are subject to storm-tide flooding but this vulnerability will increase due to sea-level rise. Areas at risk include:
• the mouths of rivers and streams including the Otaki, Waikanae, Hutt and Whakataki Rivers
• low-lying parts of Wellington and Porirua Harbours, as well as the lower Wairarapa Valley including Pauatahanui, Eastbourne Bays, Petone and Lake Onoke
• areas subject to severe erosion such as Paekakariki, Raumati South and Te Kopi.
5. Why is the sea rising?
Water expands when it is heated and this is what is happening to the oceans due to global warming. Temperature measurements show Earth has warmed by an average of 0.7oC over the past century and this in turn is warming the seas, causing them to expand. Global warming is also causing increased melting of the polar ice caps and glaciers that is increasing ocean volumes.
6. Will storms become more frequent due to climate change?
Current research indicates that while storms in New Zealand are not likely to become more frequent, the storms we do get may increase in severity. Thus, storm-tide events may become longer, increasing the likelihood of coastal flooding and erosion.
7. What are local authorities doing?
Greater Wellington Regional Council along with City and District Councils are in the process of identifying areas vulnerable to coastal hazards and are developing plans to manage high hazard areas. Guidance documents have also been produced by central government to assist local authorities in managing climate change and coastal hazards. See the Proposed Regional Policy Statement on www.gw.govt.nz (look under ‘Plans & Publications/Regional plans, policies & strategies’).

8. What can I do to be better prepared?
Have a household plan that includes how to contact each other in an emergency and how you would manage if you were cut off due to coastal flooding. Put together a emergency kit that contains enough food and water, essential medications and toilet paper to last three days, as well as a battery radio, torch, spare batteries and first aid kit. For more information check out: www.gw.govt.nz/emergencies or www.civildefence.govt.nz

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