Giant hole near Robe Obelisk triggers safety warnings as erosion causes cliff to collapse

A South Australian coastal council is warning the public to watch out for further cliff collapses around a giant hole developing near a popular tourist attraction.

What was once a small vent opening near Robe’s Obelisk has expanded into a sizable gap after the surrounding cliff collapsed for the second time in recent months.

The hole was the size of half a tennis court when it suddenly appeared last year, and has since widened, with locals calling it the ‘Woe Hole’ due to concern it caused.

Robe District Council chief executive James Holyman said the collapses were “part of the natural cycle”.

“From time to time we have pieces of the coast that fall off,” he said.

a man in a cardigan standing in front of a cliff hole
James Holyman says public safety is paramount.(ABC South East SA: Grace Whiteside)

But Mr Holyman said ‘the main thing’ was that the public was safe.

“I want them to be very aware that we’ve closed the Woe Hole,” he said.

“So that’s no reason to jump the fence and go take a look, because most people wouldn’t know where the major weak spots are.

“Don’t walk too close to the cliff anywhere in Robe because the worst outcome for us is someone getting hurt or killed for not behaving properly.”

Mr Holyman said he saw a person go over the barrier and walk towards the edge of the hole.

“Because I was there at the time, I was able to tell the individual, ‘Please sir, get away from here, this area is extremely dangerous,'” Mr. Holyman.

“But there’s not someone there all the time.

“Individual decisions and individual behavior must be made from a safety perspective.”

A man's hands holding a brochure with 3D images
Mr. Holyman with the results of aerial LiDAR surveillance of the Robe vent.(ABC South East SA: Grace Whiteside)

He said the limestone cliffs made it “very difficult” to erect more permanent security barriers.

“We would struggle because everything is chalky and porous in places, so it’s very difficult to put up posts and things,” Mr Holyman said.

“We are also reasonably certain – but we will continue to monitor – that it will continue to erode.

“So you could put in a reasonable amount of infrastructure, but it would have to be replaced pretty quickly.”

The council used light detection and ranging (LiDAR) technology to determine the locations of weak spots in the cliffs.

“We are very lucky to have LiDAR, which mapped the whole cave there,” Holyman said.

“We know it’s reasonably close to the coastal path, so we want to be proactive and plan where the path will go in the future.”

More erosion likely

Patrick Hesp, Flinders University professor of strategy for coastal studies, said these types of collapses were “likely” to continue.

“The continuous limestone erosion along the limestone coast occurs due to wave activity, as well as solution by rainwater and groundwater,” he said.

“Although there is a surface calcrete – which is this very cemented, very lithified, very hard layer on top of the limestone – it has been very significantly carved out below in a cave by the waves over time.”

A laminated paper sign indicating
Mr Holyman says the limestone cliffs make it difficult to erect permanent security barriers. (ABC South East SA: Grace Whiteside)

Prof Hesp said continued monitoring was essential.

“We have, for example, [a] 2018 LiDAR flight along the coast and it is essential to maintain this type of monitoring,” he said.

“Unfortunately, it is extremely expensive.

“It gives you incredible digital detail on the coast and so it’s a brilliant monitoring mechanism to be able to see how the coast is changing.

“Certainly the rate of erosion that we see, for example, at Cape Dombey [in Robe] and other areas where we have some monitoring would indicate that, yes, erosion is continuing everywhere.”

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