29 September 2015

The Inskip sinkhole in Australia – undoubtedly a landslide

Posted by dr-dave

The Inskip Sinkhole

The Inskip Sinkhole, as described in various media reports, developed on the Inskip Peninsular at MV Beagle Point, north of Rainbow Beach, on Saturday night:-

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Descriptions of the event sound quite exciting, with a caravan, a trailer a car and some tents disappearing into the sea.  Fortunately there was no loss of life.  The best gallery of images of the aftermath can be found on the Sunshine Daily Coast website, from which this image is taken:

Inskip Sinkhole

Inskip Sinkhole from the Sunshine Coast Daily

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In the last couple of days the media have realised that this is not in fact a sinkhole, but is undoubtedly a submarine landslide.  This report gets it right:

A Queensland Parks and Wildlife spokeswoman said the event was unlikely to be related to or caused by earthquake activity.  “Rather, it’s most likely a natural phenomenon caused by the undermining of part of the shoreline by rapid tidal flow, waves and currents,” she said.  “When this occurs below the waterline, the shoreline loses support and a section slides seaward, leaving a hole, the edges of which retrogress back towards the shore.”

This rather breathless report suggests that such events are not particularly uncommon on this section of coastline:

“A similar “sinkhole” event in the area was reported in 2011, and Rainbow Beach locals recalled “numerous” events.”

It would be fascinating to see some high resolution bathymetry data for this area as these slope failure events should leave a very distinctive deposit on the seabed.  The above report quotes a local geologist, Ted Griffin, in explaining the phenomena:

“He said a large channel between Inskip Pt and Fraser Island regularly builds up a “shoulder” of sand, and falls away.  “This is a big channel, perhaps 50 or 100 metres. It’s just a very unstable cliff of sand,” he said.  “It seems to me very poor planning, that they’ve allowed development so close to such a vulnerable area.”

However, the exact mechanism of failure, and its causes, are not clear to me and would be worthy of further investigation.