7 January 2020
Trimingham: a large landslide in complex glacial deposits
Yesterday morning (6 January 2020) a large landslide occurred on the sea cliffs at Trimingham in north Norfolk, on the east coast of the UK. ITV News has some spectacular images of the aftermath of the landslide, and a nice drone video of the site too. This is one of the images:-
An alternative view shows that the landslide had quite high mobility across the beach:-
Substantial landslides on the east coast of the UK are not unusual – note the multiple landslides of various types shown on the images above for example. This is an area of exceptionally high rates of coastal erosion – indeed the British Geological Survey has an ongoing project in this area estimating the rates of loss of land. The data suggests that over the period between 1966 and 1985 this section of coast lost an average of 1.5 to 2.5 metres per year.
The geology of the cliffs at Trimingham is particularly interesting, and accounts for the style of landsliding that occurs at this site. The cliffs are formed from very large blocks of chalk sitting in series of glacial and periglacial sediments. This has been interpreted as a large push moraine created by the Middle Pleistocene ice sheet. It is draped by large amounts of outwash sand and gravel.
In a short field guide, available online, Lee et al. (2011) describe how these glacial and periglacial deposits control the landsliding on this coast:-
Landslide development along the coastal traverse is strongly controlled by glacitectonic structure. Thrusts (i.e. synclines) offer natural failure planes for landsliding as well as acting as planes that enable groundwater to infiltrate and migrate around the sequence. Thrusting also alters the geometry of discontinuities such as bedding and jointing ‐ at Trimingham, subvertically aligned beds of diamicton are highly prone to failure, generating earth flows and falls. The movement of sediment blocks of different permeability relative to each other during thrusting (and landsliding) leads to the development of highly localised and confined groundwater conditions which often accentuates landslide hazards. Synclines within the sequence also offer topographic lows within which groundwater seepage is focussed causing failure along bedding planes and the generation of large deep‐seated rotational and translational slides.
The UK has suffered an exceptionally wet few months, so coastal landsliding is not surprising. As rainfall intensities increase as the effects of global heating continue to accumulate, it is likely that we will see many more landslides in these highly vulnerable geological deposits.
Lee, J.R.; Pennington, C.V.L.; Hobbs, P.R.N.. 2011. Trimingham : structural architecture of the Cromer Ridge Push Moraine complex and controls for landslide geohazards. In: Phillips, E.; Lee, J.R.; Evans, H.M., (eds.) Glacitectonics : field guide. Quaternary Research Association, 218-227. (QRA field guides).