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Inskip 'sinkhole' nothing to do with underwater caves

GIANT BITE: The 150-metre long and 50-metre wide chasm which opened up at at Inskip Point last month.
GIANT BITE: The 150-metre long and 50-metre wide chasm which opened up at at Inskip Point last month. Higgins Storm Chasing

A UNIVERSITY of Queensland researcher says there is an explanation for the Inskip Point "sinkhole'' and it has nothing to do with underwater caves or landslides.

North Stradbroke Island-based University of Queensland Master of Science student Konrad Beinssen said the science behind the massive cavity that opened up at Inskip Point on September 26 was clear.

Mr Beinssen has published two papers in reputable, peer reviewed engineering journals, including Australian Geomechanics, based on his studies of "retrogressive breach failures" (RBFs) at both Amity Point on North Stradbroke Island and at Inskip Point.

He said these events were well researched and understood in countries like the Netherlands and the US.

RBFs are triggered offshore and retrogress upslope towards the shore as a sand cliff sometimes more than seven metres high.

Sand grains cascading off the edge of this cliff generate a density current which carries entrained sand offshore.

This sand then resettles as the density current loses momentum.

Mr Beinssen said these events often go completely unnoticed until they reach the shore line and affect the beach, leaving behind a large semi-circular scar in the sand.

"There's been a lot of misinformation and this event at Inskip has been very timely in that its cause has been hotly debated," he said.

Mr Beinssen said events like this, although often on a much smaller scale, were frequent occurrences at Amity Point.

"They're quite common on the east coast of Australia," he said.

"They happen at Amity Point about once a fortnight but are not always big and don't always cause damage."

He estimates the Inskip Point event would have had a vertical sand wall of about seven to nine metres in height when it reached the beach and would have started about 100 metres offshore in about 10 to 15 metres of water.

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Mr Beinssen estimated it would have been moving at about 0.8 metres a minute.

He said the most likely trigger of RBFs events was ground water running under the beach out into a channel, often from tidal draw down, which can cause segments of the sand to liquify.

Mr Beinssen said a minor event could have set the event off, as can some man-made activities, and that these events, while starting small, can rapidly gather momentum and size.

He believed park rangers should restrict campers from setting up on the vegetation line of the beach and shift them about another 20 metres further into the bush to protect against any future incidents.

Mr Beinssen said he was certain there would be future events like that which occurred last month.

Topics:  inskip sinkhole sunshine coast


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