Sea levels rising twice as fast as expected in Christchurch

Thursday, 7 December 2023

Sea levels are rising quicker than expected in some parts of Christchurch, leaving authorities with little time to plan how best to adapt to the changing coastlines.

Sea levels are rising up to twice as fast as expected around Christchurch’s coast, a study has revealed, with some land sinking at more than five times the rate it did before the earthquakes.

Yet under the Christchurch City Council’s current rules, properties are still able to be built in at-risk areas.

The council has known for years that land along the coast has been eroding and flooding more since the 2010, 2011 and 2016 Canterbury and Kaikōura earthquakes.

It was why the council and Environment Canterbury commissioned GNS Science to look at how the land was moving, and decipher what that meant for local sea-level rises.

A study by GNS Science found that land in some parts of Christchurch is sinking five times faster than it was before the 2010 and 2011 earthquakes. In the graph above, the darker the red, the greater rate the area is sinking by.

The resulting study found land across the city and Banks Peninsula is sinking at a greater rate than before the earthquakes, and shows no sign of slowing down.

It could take decades before the rate of sinking returns to pre-quake levels, the study said, depending if there are more large earthquakes in the meantime.

Following its release, councillor Sara Templeton called for the council to speed up its work on community adaptation planning - which may include managed retreat - and to finalise a coastal hazards district plan change, which could stop people from building on at-risk land.

The greatest rates of sinking are happening around the edges of the Ihutai Avon Heathcote Estuary, at Bexley, South New Brighton and Southshore, and at Sumner, Redcliffs, Whakaraupō Lyttelton Harbour and Koukourarata Port Levy.

The New Brighton spit and Southshore areas are sinking nearly 1cm per year, or five times faster than before the quakes.

Lyttelton Harbour and Port Levy are sinking at an average annual rate of 6.6mm.

Instead of experiencing the projected global sea-level rise of 14cm to 23cm by 2050, the study found that those two areas could see sea levels rise to 34cm to 43cm by 2050.

Potential options for communities to adapt to rising sea levels include managed retreat of assets like roads, the council says.

Jane Morgan, Christchurch’s team leader for coastal adaptation planning, said it was possible other parts of the peninsula had already met previously projected sea-level rises for 2050, as some spots had sunk 10 to 20cm post-quake, but data for the whole peninsula wasn’t available in time for the study.

Morgan said the study highlighted the importance of early planning, and the council’s initial adaptation planning was well under way for some of the most affected communities.

“We don’t have all the answers about what life is going to look like in the future, but we know there are some important decisions we can all be making now to make sure we’re better prepared,” she said.

Templeton said residents and their homes were being put at risk under the council’s current regime, which allows new homes to be built in at-risk areas.

“I’m frustrated and concerned at the delay,” she said.

“We don’t know exactly when the impacts [of sea levels] will be felt, but minimising risk and having plans in place will help our communities plan for their futures and stop the ad hoc post-disaster responses that we have seen in the past.”

John Higgins, the council’s head of planning and consents, said development was already limited in some areas required for flood management.

He said any decision to speed up work on adaptation plans would be considered during the long-term plan process.

Councillors agreed to proceed with addressing coastal hazards with urgency, but a time frame is not yet available.