Christchurch City Council says a recent study of vertical land movement has shed new light on the extent and speed of sea-level rise happening in parts of the city.
The study, completed earlier this year by GNS Science, has shown that sea-level rise in some parts of the district could happen up to twice as fast as previously thought, as a result of increased land subsidence following the Christchurch and Kaikōura earthquakes.
“We all knew that the land changed dramatically as a result of the Canterbury earthquakes, and this new data shows the extent to which some areas have continued to subside after the earthquakes,” says Council Team Leader Coastal Adaptation Planning Jane Morgan.
“It’s also confirmed that, since then, most parts of the district are subsiding faster than before the earthquakes.
“The greatest subsidence is happening around the edges of Ihutai Avon Heathcote Estuary, at Bexley, South New Brighton and Southshore, and at Sumner, Redcliffs, Lyttleton Harbour and Koukourarata Port Levy.
“We don’t yet have a good understanding of what’s happening in other parts of Banks Peninsula, due to the availability of data at the time of this work. But generally, where we have low-lying and coastal areas subsiding, it means the impacts of sea-level rise – coastal flooding, erosion and rising groundwater levels – will happen sooner, giving us less time to plan ahead and adapt.
“For example, in some locations where the land has sunk 10 to 20 centimetres since the earthquakes, the amount of sea-level rise previously projected to happen by 2050 could’ve already happened,” she said.
Vertical land movement is caused by movements in the earth’s crust. The upward or downward movements can be very fast, such as during an earthquake, or slow and gradual. Local land movements can happen from other processes too, such as liquefaction that might be triggered by an earthquake.
Global sea-level rise projections suggest Christchurch could see 14 to 23 centimetres of sea-level rise over the next 30 years. However, in places where land is subsiding at about 8 millimetres per year, such as parts of Brighton Spit and parts of Lyttelton Harbour and Koukourarata Port Levy, sea levels could rise by 38 to 47 centimetres – twice as much over the same 30-year timeframe.
“We understand it’s common for land to keep subsiding after large earthquakes,” says Ms Morgan. “While there’s no sign the subsidence has slowed down in Christchurch, evidence from other parts of the world tells us that, eventually, the speed of vertical land movement will return to normal. However, it might take another few decades and will depend on whether we experience more large earthquakes.”
Ms Morgan says the latest study, which was co-funded with Canterbury Regional Council (Environment Canterbury), highlights the importance of early planning.
It will help inform the Council’s work with communities to adapt public infrastructure and assets, such as roads, pipes and parks, she said.
“Our initial adaptation planning is already well under way with communities in the Whakaraupō Lyttelton Harbour to Koukourarata Port Levy area, which is one of the areas most affected by this new information.”
“We’ve been working closely with a Coastal Panel of community and rūnanga representatives to develop adaptation pathways for public assets, to make sure that local values and knowledge are a key part of these discussions.
“Over time, sea-level rise is going to have a big impact on how we live, use and move around our coastline and low-lying inland areas. In some places, we’re already seeing the impacts of more frequent and severe weather events.
“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.”