DW: People have been quick to say a sinkhole has emerged at Inskip Point on the Queensland coast. But you dispute that. Why?

Professor David Williams: There have been examples in Scandinavia and even in Germany, where you have slope instability, which mobilizes or fluidizes the material behind the failure, and it appears like a sinkhole. But in this case it is right on the coast. It's happened numerous times before and it's well-known by locals. I think it's more likely to be an undersea slip, which has left a scarp, which then keeps regressing back because it's below water.

And they repair - sometimes within weeks or months - they fill in again. The sea brings the sand back.

Well that's interesting, because most reports so far have suggested this hole will keep growing and the whole peninsula could disappear.

No, there's been a history of it. This one was at night. But in 2011, 2012, and 2013 they were all during the day and you can watch YouTube videos of them.

And they've all repaired.

So tell us more about the differences between this and a sinkhole.

A sinkhole has more to do with [a situation] where you've got sands that go quick, if you like. I mean, there are similarities between the two, because if you've got sand that's partially below water, it will collapse. You just go to the beach, dig a hole in the sand below the water table, and everything that's below the water table starts falling in. So it's a very similar phenomenon.

But a sinkhole is more where you've got subterranean flow - and I'm not saying it isn't in this case - I'm saying it's more likely, given that it's right on the coast, that it's probably a subterranean slip induced by wave action or whatever, that then creates a scarp which regresses back up the beach.

Freak event

And is it true they are impossible to predict?

Yes, I would say you can't predict them. And interestingly when I saw it, I tried to [search online] for more information about it, because I presumed that someone would have investigated the failures that have occurred before, and there doesn't seem to be anything there at all. No one has come in with a drilling rig or a cone truck, or even a barge and tried to test the soils to see whether, say, there's a weak layer at depth, or see just exactly what's there. So it's all anecdotal evidence. It happens, it repairs itself, people use the campsite again, and then it happens again. It doesn't seem to be related to any particular event - like it may have been a high tide, but I don't know.

The lack of investigation is ironic, given that nearby Fraser Island is entirely sand, which seems a very weak structure. Has there not been much study in this area at all?

No, but there are differences. The sand islands up the east coast of Queensland are there because usually there is a little volcanic cone, or something that traps the sand.

And the littoral drift is north, so sand is being transported north anyway, and if there's an obstruction of any sort, it will start gathering the sand.

So that's how those sand islands form and most of them are probably still growing. But periodically they will be washed out by king tides and things like that - you'll get a washout of sand, the sand will be taken further away, and then it will be brought back in again.

Sand beaches

And the beaches along the Queensland coast do the same thing. You'll have massive erosion from beaches, and scarps forming that are several meters high sometimes from a cyclonic type event. The sand will be dumped further out from the beach and then it'll be brought back again. Or, if we're too impatient, we'll pump it back, because we want to reinstate the beach. There are numerous beaches where we've done that. We also have problems where we take out breakwaters at river mouths, because that interrupts the littoral drift north, so sand accretes on the southern side and depletes on the northern side - you get this erosion on the northern side and an extra piece of land on the southern side.

There are a number of cases like that where they pump sand from the south to the north to compensate for the fact that it would have happened naturally but they've now put in these breakwaters that stop it.

So do you expect a surge in research after this latest event at Inskip Point? Are you interested in doing any research - or is it just one of those things - it happens, and it repairs?

I'd be quite interested to know more about it, but whether anyone's prepared to put some funding towards it, I somehow doubt it. No, I think this will repair and it'll probably happen again, and it'll repair again, and people just put up with it.

The fortunate thing is that - to my knowledge - no one has ever been hurt by one of these. They've lost some property - this time it was a caravan, a car and a tent, I think.

And you should watch the YouTube videos because you'll see people stand perilously close to an edge that is caving into the sea, progressively - most of them taking photographs or making movies, and you know what it's like when you get behind a camera, you ignore what's around you! I'm amazed that no one's fallen into it as it's forming, because people are really close to the edge, and you can see the back scarps form as a vertical cliff of sand falls in three or four meters above sea level.

Professor David John Williams is the Golder Professor of Geomechanics and the Director of the Geotechnical Engineering Centre within the School of Civil Engineering at The University of Queensland.