The shoreline at one of South Australia's most picturesque beaches has retreated by up to 135 metres due to the ongoing effects of coastal erosion.
Flinders University researchers are studying the dune system at Beachport and hope to track the movements which are predicted to lead to the flooding of road infrastructure and the saline body of water, The Pool of Siloam.
Third-year PhD student Samuel Davidson said a GPS-controlled robotic vessel would use an attached sonar to give depth and position of Salmon Hole Bay and allow for a digital elevation model with data to be made.
"From that, we'll have a greater understanding of the transverse dunes that are in the bay and their movements, and, using that information, we'll then know where the sediment is going," Mr Davidson said.
He says seawater is expected to eventually break through the dune at Post Office Rock.
"The Pool of Siloam certainly will be inundated with seawater within the next 10 to 15 years," Mr Davidson said.
"There's quite a low-lying area in the middle of the dune system where storm waves are already splashing in and out."
Dunes moving at 'great rate'
Coastal studies academic at Flinders University Patrick Hesp said the bay had retreated hundreds of metres since the 1940s and particularly since the 1970s at "phenomenal rates".
"The reason for looking at this is to try and understand what the processes are of erosion and sediment transport," Professor Hesp said.
"It's an analogue for what's going to happen with future sea-level rise.
"We'll start to see far more beach erosion, dune erosion, dune translation landwards as sea levels rise, and, potentially, as waves get bigger because of climate change."
In the earliest photograph of the region, Salmon Hole beach is pictured almost attached to the aeolianite reef which extends to each point of the bay.
By 2016, the shoreline had retreated 120 to 135m. The reef is now located westwards of the shoreline, out to sea.
Anecdotally, Mr Davidson has heard fishermen recount tales of being able to walk out to the reef along the previously existing beach and fish straight off the reef.
Council plans to 'retreat' from coast
Wattle Range Council chief executive Ben Gower says the council has been surveying the situation for some time with monitors of their own.
"What we know is whatever you put in place to hold Mother Nature back, there'll be a cause and effect and there'll be a consequence further downstream," Mr Gower said.
"So, we need to understand what changes we can and cannot make and what the secondary effects of those changes will be."
For the time being, the council's plan is to "fundamentally retreat".
"Scenic Drive, which is our picture-postcard coastline road is likely to be lost," he said.
"We're already planning to move that back further inland and provide a natural barrier to protect the township when we do invest in that infrastructure."
Mr Gower said the expense of protecting the township and road infrastructure would have the biggest impact on the council's actions.
"It's incredibly expensive, whatever we do," he said.
"As soon as you put something on the coastline, like a large rock wall, the ocean tends to try and undermine the wall.
"It hits it hard in a storm event, it churns the sand up underneath it and drags it away again.
"So, you get further erosion happening so the solutions aren't easy, they're all very expensive and we want to get the best value for money."
'Millions and millions' needed
Professor Hesp said the money needed to stop the flooding event would outweigh any benefits.
"You could stop it if you wanted to spend millions of dollars on sea walls … millions and millions," he said.
"It would probably be an utter waste in this case, as luckily there's only a road at risk which can be rerouted.
"Other than that, [it's] best to leave it alone and watch and see what happens."
