Sea levels in Wellington have risen by 20 centimeters over the past century – and scientists predict a fivefold increase over the next 100 years.
Coastal hazard scientist Dr Scott Stephens says high tides will continue to reach new highs and waves will reach higher levels and carry more energy, causing more damage to roads and coastal infrastructure.
Sea level rise is caused by two factors — both the result of global warming, Stephens says. More and more water is entering the oceans as the Earth’s ice caps and glaciers melt and the oceans themselves take up more space, with water expanding as it warms.
In addition to raising the water level around land masses, it raises the water table inland. “Think of it like a sponge in a bucket of water,” Stephens says. “If you increase the water level in the bucket, the water soaks up higher and that sponge could stretch far inland.”
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According to natural hazard analyst Iain Dawe, Wellington has one of the longest tidal records in the country. The city’s first wharves, built in the 1890s, were fitted with tide gauges, meaning scientists now have more than 130 years of measurements to analyze.
Dawe works for the Greater Wellington Regional Council, the region’s flood protection agency. The data shows an average increase of 2.1 millimeters per year, or about 22 cm to 25 cm in total.
Sea level rise is exacerbated by tectonic subsidence – the natural sinking of the earth’s crust – meaning Wellington is suffering a ‘double whammy’ of factors. The area is subsiding about 1mm to 4mm each year.
Advice must consider the danger to assets and infrastructure, as well as people, Dawe says. “All of our stormwater flows to the coast” – and the water cannot flow upstream.
“There are basements [in the central city] that receive water during very high tides because groundwater cannot flow properly,” he says.
In Wellington, restrictions on where people can build are set out in Wellington City Council’s Draft District Plan. Developments in coastal areas and low-lying interior areas will need to achieve a higher benchmark in terms of resilience measures to obtain resource clearance.
The council’s climate change adaptation strategy foresees the need for improved stormwater infrastructure, maintenance of dykes, better access to information on future risks and engagement with high-risk communities to start plan for climate change.
Vulnerable areas include the city centre, Thorndon, Island Bay, Kilbirnie and Miramar. The plan also proposes mitigation measures, such as raising the ground level of any new buildings and creating floodwater retention areas around them.
Factors that may influence consent approvals include the type of building materials used and the proposed activities in the building.
A council-led project to support the development of the video game-like version of the Capital to educate people about sea level rise projections recently won a US$1 million prize (1, NZ$5 million) as part of the Global Mayors Challenge.
Wellington Airport is located in the low coastal area of Kilbirnie. Along with reducing operational emissions by 30% by 2030 and expanding to meet the needs of electric aircraft and sustainable fuels, it says resilience is a priority.
Sustainability manager Chris Vidal said they are upgrading the stormwater drainage to deal with heavy rains, but new higher levees are needed as the existing ones are nearing the end of their life.
“They are critical to protecting the runway and the airport as a whole from sea level rise, storm surges and major weather events.
“The West Dyke also protects council-owned roads and the main sewer lines to and from Moa Point, which carry most of Wellington’s sewage.”
Councilors voted last year to cut a previously agreed $76m loan for the works from its draft 10-year plan, after fears the money could also be used for a proposed 300m runway extension million, which they said ran counter to the council’s emissions reduction goals.
Coastal roads are increasingly at risk of washing out – already a factor as in Ōwhiro Bay during large storms. This was one of the arguments in favor of Transmission Gully north of town – an alternative to the coastal route, State Highway 59.
Stephens says the worst effects occur when a storm coincides with a high tide.
“At the moment, only the most extreme storms on the highest tides are a problem, but this will become more common as low tides become equal to high tides.”
In turn, flood damage will become more intense. Waves carry energy, which dissipates as they enter shallow water.
“With a higher sea level, a wave can approach [to the shore] before it starts to break and lose energy.
In December, the council removed a “wave ramp” in Ōwhiro Bay, which had formed against a sea wall, creating the perfect ramp to send waves down the road and into the properties beyond.
In addition to people and places, natural ecosystems and biodiversity are under threat.
“Ordinarily, a natural system is perfectly capable of absorbing impacts and moving inland,” says Dawe. But many built environments encroach on sand dunes, wetlands and estuaries, enclosing them.
As the seas rise, so do the rivers. More water in the rivers means more erosion on the banks, and faster and more dangerous flows which push the capacities of the banks of stop.
“As the sea level rises, it increases the height of the river at the mouth,” Dawe explains. “It puts more pressure on the stop banks to contain that water.”
The stop banks are planned with water levels for the next 100 years in mind, during which time it could rise between 0.8m and 1.6m.
“There are places on the Kāpiti coast where the water table is almost on the surface and prevents this water from flowing out,” Dawe explains.
Even up to a kilometer from the coast, the mouths of estuaries and wetlands will be higher and less able to absorb additional water.
There are, as always, limited resources. Dykes could cost tens of millions per kilometer. Dawe says that ultimately it’s about finding a balance.
“Where do you invest and defend? And where are you migrating from?