Waves of change
in Greener Homes on 19 May 2013
King tides give residents a view into the future of our coasts
TWICE a year, the tides reach their peak. And when they do, the sea washes over piers and paths, and inundates parks. It’s a prelude to a coastline with higher seas all year round – one in which seaside real estate will be at increasing risk.
“King tides are a great proxy,” says Caitlin Calder-Potts, from Green Cross Australia. “They’re a way of bridging the gap between an abstract projection for sea level rise, and actually seeing what the impacts are in your local area. By observing them, we can understand how our coasts might change.”
To that end, she’s coordinating a project called Witness King Tides, in which citizens photograph while the waters rise. You can register online, then upload your snaps of the seaside.
The next big tide during daylight hours will sweep the Victorian coastline from May 26 to 30. It peaks in Portland at lunchtime on the 26th, for example, or Port Welshpool at dusk on the 30th. (Find the exact tide times on the Bureau of Meteorology website.)
The project started in New South Wales in 2009, and since then, it’s been held in Queensland and southern Tasmania. So far, wave watchers have uploaded 4000 images. Many show coastlines coping well, but elsewhere, infrastructure is already at risk.
“We’ve had feedback from lots of surf life saving clubs saying they’re vulnerable, and community spaces like parks and foreshores too. Erosion and estuarine flooding are fairly common during king tides,” Ms Calder-Potts says.
The project’s popularity isn’t surprising: more than 8 out of ten Australians live near the coast, and there are over 700,000 homes within 3 kilometres of the sea.
According to the Climate Commission, sea levels are likely to rise by between half a metre and one metre by 2100. Even at the lower end, that could increase the frequency of flooding by several hundred times.
But last year, the Victorian government last year scrapped a requirement to plan for 80 centimetres sea level rise by the end of the century (except for new “greenfields” developments).
Professor Bruce Thom, former chair of the federal Coasts and Climate Change Council, says the planning system should adopt higher-end thresholds for developments that are expected to last.
He was part of a 2009 government study into the climate change risks to Australia’s coast. It found that hundreds of thousands of buildings would be at risk of flooding and damage under a high sea level rise scenario that coincides with a storm surge.
Prof Thom says king tides – which aren’t connected with human-caused global warming – help us understand sea level rise because they make local impacts clear.
(“King tide” isn’t a scientific term. It refers to the biggest of the regular “spring tides”, which occur when the Moon is full or new, and aligned with the Earth and the Sun.)
Local knowledge is useful, because sea level is more complex than you might think. Firstly, the ocean isn’t flat – it’s constantly in flux, in the same way as the atmosphere moves according to high and low pressure systems. Secondly, tidal levels depend on the shape of the shoreline.
“What happens at a particular place can vary enormously because of the nature of the bays, inlets, lakes and lagoons,” Prof Thom says.
Climate change causes sea level rise in two main ways: by increasing ocean temperatures (water expands as it warms) and by melting glaciers, ice caps and ice sheets.
“In certain parts of Australia we’ve had a very small amount of sea level rise going on for some time,” he says. “The fear scientists have is that the rate of rise will increase – that it isn’t linear and could be exponential. The big concern is disturbance to the Greenland ice sheet or the West Antarctic ice sheet.”