18 July 2013

Toppling failure – a spectacular time lapse video from coastal cliffs in France

Posted by Dave Petley

A new video has appeared on Youtube showing a time lapse sequence of photographs of a toppling failure that occurred at 12:10 pm on 15th July 2013 at St Jouin Bruneval in Normandy, France:

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The photographs were taken by Pierre Pannet of BRGM, the French Geological Survey.  Fortunately, no-one was injured in this event.

The accompanying text to the Youtube video is as follows:

This sequence of still photographs were captured just as a cliff section was collapsing on the beach of St Jouin Bruneval in Normandy (49.63907°N, 0.1490°E). My colleague, Pierre Pannet from the BRGM (French Geological Survey) office of Normandy had just been called in by the Maire of the district as a collapse had occurred the day before. When he got onto the beach, a new cliff section collapsed in front of his camera.

At this location, the cliff is made of something like 50 m of chalk resting on the Gault clay horizon. This clay unit is weak and impermeable. It must have kept a perched water table weakening the base of the chalk face. Rains have been particularly heavy in the last 12 months, though on July 15 the weather was fair and sunny. Note that the tide was just starting to rise again. Neither the rain nor the sea beating the base of the cliff can be considered as the direct cause of collapse. It is more likely that it is the slow flow of saturated clay that has initiated the rupture and a sudden acceleration of that slow slip that caused the chalk “towers” to collapse.

Looking at the detail of the collapse, two towers of chalk collapsed head first. Their back has detached as thick wedges. Such motion indicates that the cliff foot had lost all resistance up to a depth behind the center of equilibrium of both towers. The towers fracture mid-way through their fall before hitting the ground. Then blocks bounced off the ground reaching further out. The final scree apron reaches a probable elevation of 25 m (to be confirmed), with a regular slope and the outflow may be something of the order of 50 to 100 m out to sea.

An insight into toppling failure?

One interest aspect of this sequence is the initiation of the topppling failure.  The very first image of the sequence is the one below.  I have highlighted a small cloud of dust on the downslope side of the failing block:

toppling failure

I am not sure what this cloud of dust represents – any thoughts?  My immediate hypothesis would be that during the toppling failure the lower block is pivoting on a small area of rock and debris in this region, which as a result must be under huge compressive stress.  It could be that this is area of rock is failing in a spectacular fashion, generating the dust.  Later in the sequence this area of dust becomes larger before being overwhelmed by the collapse.