13 June 2019
Sidestrand: a complex coastal cliff landslide captured on video
Many media outlets have been featuring a video from Sidestrand in Norfolk, eastern England, showing a complex landslide event. This video was posted to Facebook by Brad Damms. It can be accessed here:
There is a good guide to the geology of this area on the Norfolk Project webpages, and the BGS have a good page about this site as well. In essence, the cliffs are composed of highly complex glacial deposits, mostly consisting of glacial till but including rafts of chalk. There are classic, and very beautiful, glaciotectonic folds found in the materials. The combination of weak, deformed materials and the storminess of the North Sea (which has a very high tidal range) means that these cliff are very susceptible to landslides. The BGS note the following in terms of landslides in this section of coastal cliffs:
The landslides at the Sidestrand test site are complex, consisting partly of large-scale, deep-seated landslides and partly of mudslides and debris flows. The deep-seated movements tend to have a dominant rotational component, but are in part translational. In some cases these extend to depths several metres below platform level, but are more usually entirely within the cliff. The landslides form deeply incised embayments which are arcuate in plan. The backscarps at the cliff-top tend to be sharply defined vertical features which persist after the landslide event. Deep-seated landslides tend to rotate to angles of 10 to 20 degrees and break up during failure transport, producing large debris aprons which spread across beach and platform. These are short-lived as the debris is readily removed by the sea. Such large events are followed by many mudslides and mudflows.
The event captured in the video above shows many of the characteristics of these landslides, with sequential cliff collapses transitioning into flows that have, in some cases, quite high mobility:-
The video was captured by Brad whilst undertaking scanning of the cliffs for the scanLAB project. It will be fascinating to see what the scans show. The UK has just experienced a prolonged period of unusually heavy rainfall. This is likely to have been the immediate trigger for this event, although of course the underlying cause is wave erosion at the toe of the slope. This section of coast loses up to 3 m of land per year on average.