Oceanographers say growth and erosion of Western Australia’s coastline intensifies in La Niña weather conditions, due to a stronger Leeuwin Current and higher sea levels.
Australia has experienced La Niña weather conditions for the past two years, and the Bureau of Meteorology this week declared it will form again this summer.
Associate professor at the University of WA Oceans Institute Jeff Hansen said the Leeuwin Current, which flows south from the tropics, is stronger in La Niña, which pushes more water against the coastline.
“The Leeuwin Current is weakest in the summer and strongest in the winter,” Dr Hansen said.
“With that seasonal strength variation, we have about 20 centimeters of higher sea level in the winter than what we do in the summer, and then when we have a La Niña case you add another 10 to 15cm on top of that.”
Dr Hansen said it was the opposite during El Niño when the sea levels dropped 10 to 15cm.
CSIRO’s senior principal research scientist for oceans and atmosphere Ming Feng said La Niña would likely result in more storm events which would compound coastal erosion.
“If we have a 1cm sea level rise, roughly 1 meter of the coastline will be affected,” Dr Feng said.
“For example, if we have a 10cm sea level rise, then 10 meters of the coastline will be affected.
“I think when you have a sea level rise whether it be a steady rise or during an extreme event the coastline will be more impacted.”
Dr Feng said the ocean response to La Niña and El Niño weather events was a relatively new focus for researchers, assisted by more extensive ocean observation systems that did not exist 20 years ago.
Since 2020 the state government has spent $13 million attempting to stabilize WA’s coastline.
In Geraldton and Bunbury beaches were significantly impacted by erosion this year.
In Geraldton, a Marine Rescue Building which had 50 meters of dunes and sand in front of it now has less than 10 meters.
However, Dr Hansen said observing one section of a beach against the entire coastline was important.
He said while some beaches were eroded away, others grew.
“Coastlines are a very dynamic landform, they act as a buffer between the ocean and the land, and so they respond to the constantly changing primarily ocean conditions,” Dr Hansen said.
“If you go to the beach every day, in most instances the waves that you see, the tidal level, it’s going to be a bit different every day.
“And that constant change in the ocean conditions means that the beach is always changing.”
The ever-changing coastlines of Australia can be observed on the Digital Earth Australia map.
Geoscience Australia coastal earth observation scientist Robbi Bishop-Taylor uses fortnightly satellite data from the past 30 years to understand how coastlines have changed through time.
“An interesting thing that’s come out of this study, this product and research, is that when you zoom down to the beach level of Australia there’s a huge amount of variability,” Dr Bishop-Taylor said.
“So you get beaches that are eroding really rapidly, but then what you do find is that you start to zoom out and just up the shore of an eroding beach, you’ll often find a beach is growing just at the same rate as the sand slowly moves along the coastline.
“I think people, when they hear about sea level rise they kind of think every beach in Australia will go underwater and have erosion everywhere, but climate change operates on top of local factors.”
Clint Doak is the principal coastal engineer at MP Rogers and Associates.
“The beach is an absolutely dynamic system and small changes can affect where the sand is,” Mr Doak said.
“You can have fluxes of up to 100,000 cubic meters per year [of sand] that moves one way in summer and the other way in winter, and if there’s something that interrupts that flow such as a calmer summer and not much breeze, it will leave sand accumulating in some areas and erosion in other areas.”
Mr Doak said there was probably a lot more focus on erosion today but there were not any more areas eroding than when he first started observing coastline changes 20 years ago.
“I think the public focus on [erosion] is much higher now through the media and it’s more widely advertised when there is an issue,” he said.
“But I think the challenge with it is the local governments and the coastal managers probably now have a greater challenge that they’re got to satisfy a number of different stakeholders within the community.”