15 March 2012

The Dover chalk cliff rockfall in the context of other such events

Posted by Dave Petley

Several newspapers in the UK are reporting a large coastal chalk cliff rockfall late last week in Kent in SE. England.  The main reason for the interest is that the “White Cliffs of Dover” have a rather romantic appeal for the British people, especially those who remember the tribulations of the major conflicts of the 20th Century.  The landslide was certainly large and quite spectacular, judging by this Daily Mirror image:




This article, in common with many others, speculate on the cause of the landslide:

It is thought rain and freezing conditions over winter may have weakened the cliffs, which caused cracks in the chalk.

In fact chalk cliff collapses such as this are both quite common and well-studied.  The best source of information about them is a review paper by the sadly missed Professor John Hutchinson, which through the miracle of Google Books, can be accessed online.  The article points out that some of these landslides develop into flows that travel across the wave-cut platform.  It is interesting to note that this fall does not appear to have done so to any great extent.  The paper (see for example the map on page 266) identifies landslides in this area (just west of the South Foreland Lighthouse) in 1891, 1896, 1910, 1911 and 1979, although clearly these are affecting different sections of the cliff.  John hypothesised a recurrence interval of about 80 years for falls at any particular location, which seems to be about right in this case.

The triggering is intriguing as the UK has had an exceptionally mild and unstormy winter, and rainfall levels are at a record (and worrying) low.  John noted in Fig 45 of his paper that there is a strongly seasonal trend in observed falls, with all recorded events occurring in the period October to April (the dark grey here shows the fall events in Kent, the light grey (mostly covered by my edit) is for NW Europe):

This implies that there is an environmental triggering process, but it is unclear as to exactly how this operates at present.

The news articles pick up on the obvious point that such failures do present a risk to those on the foreshore.  The tragic event at Ruegen in Germany in December, which killed a child, is testament to the hazard.  Being close to high coastal cliffs is undoubtedly associated with some risk.