Sidestrand is situated 1.3 km south-east of the village of Overstrand on the Norfolk coast. The cliff faces north-east and the beach is very wide and sandy.
Sidestrand, Norfolk, location map. BGS © 51.
Sidestrand is part of a programme of work monitoring coastal erosion and landsliding at several sites around the coast of Great Britain. Active, deep-seated rotational landslides and mudslides predominate and BGS has been collecting data from this site since 2001. This is National Landslide Database ID 16350/1.
Sidestrand coastal section. © Mike Page, SkyView.
Geology
A considerable amount of work has been carried out on the local geology (Banham, 1988; Hart and Boulton, 1991; Lunkka, 1994; Hamblin, 2000; Lee et al., 2004), and, to a much lesser extent, the geotechnical properties of the tills of North Norfolk (Kazi and Knill, 1969; Hutchinson, 1976).
The current geological interpretation is that of Lee et al. (2004). This scheme equates the (formerly the Lowestoft Till) with the so-called Second Cromer Till.
Coastal section; taken from Lee et al., 2004.
B = Briton’s Lane Formation, S = Sheringham Cliffs Formation, L = Lowestoft Formation, H = Happisburgh Formation, Ch = Chalk/pre-glacial, hachuring = obscured by defences; black arrows = thrust faults.
Important features of the landslide site and neighbouring cliffs are the glaciotectonically controlled syncline and large-scale shears (Lee et al., 2004). The deposits within the platform and lower part of the cliff consist of the matrix-dominant, dark grey of the . This till has been subjected to small-scale,, glaciotectonic folding and, as with most tills of this type, is regularly jointed. The folding is observed in freshly eroded or landslide-exposed sections in the cliff and in the platform.
Happisburgh Till Member showing chevron folding. The ranging rod is 1 m long. BGS © 51.
The deposits in the mid part of the cliff are largely obscured by landslides. They consist of:
- Ostend Clay Member (Happisburgh Formation)
- (Lowestoft Formation)
- the lower and central components of the :
In the upper part of the cliff the exposure is good. Here the uppermost part of the Sheringham Cliff Formation ( and members) is seen. This is overlain by the of the .
The Happisburgh Till Member is a 3–6 m-thick, massive, yellow-brown. sandy till, while the Walcott Till Member is a stiff, blue-grey, chalky, flinty till.
Trimingham Clay Member (TCM), Weybourne Town Till Member (WM) and Stow Hill Sand and Gravel Member (SHM). BGS © 51.
Virtually the entire thickness of the Briton’s Lane and Sheringham Cliffs formations has been subject to glaciogenic thrusting (Lee et al., 2004). Distinguishing syndepositional and postdepositional thrust/shear features from modern landslide features is difficult in some cases. Some shear features visible in marine-eroded cliff sections are clearly the basal and side-shears of modern mudslides.
Landslides
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 backscarps at the cliff top tend to be sharply defined, vertical features that persist after the landslide event. The landslides form deeply incised embayments that are arcuate in plan.
Deep-seated landslides tend to rotate to angles of 10–20° and break up during failure transport, producing large debris aprons that 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 level of activity during the survey period was high, particularly during the winter of 2000/01. It included various types of movement at beach level and a large-scale debris flow, which ran out across the beach and persisted for two years. Active beach thrusting and deep-seated rotation have been observed periodically.
Deep-seated rotational landslide thrusting up through the beach at Sidestrand. BGS © 51.
Large debris flow of winter 2001/02. BGS © 51.
The BGS Landslide Response Team carrying out a survey of the large debris flow of winter 2001/02 (photograph taken in April 2002). BGS © 51.
Survey results
As part of a cliff monitoring programme at BGS, data was collected from the cliffs at Sidestrand annually for six years. The principal method of survey is long-range terrestrial laser scanning (terrestrial LiDAR).
The annual surveys’ results were processed to provide data for models of coastal recession. The data collected in the field by laser scanning and GPS were entered into a modelling package and the resulting computer model enables volume calculations and observations as to the way in which the coast is eroding.
Gallery
Unusual shapes created by marine erosion of the Happisburgh Formation. BGS © 51.
A flow at Sidestrand that overtopped the wooden palisade coastal defences. BGS © 51.
Chevron folding in the Happisburgh Till Member at Sidestrand. BGS © 51.
Chevron folding in the Happisburgh Till Member at Sidestrand. BGS © 51.
Terrestrial LiDAR in action at Sidestrand. BGS © 51.
Change in cliff profile
How the central cliffs changed from 2005 to 2006. BGS © 51.
How the eastern cliffs changed from 2005 to 2006. BGS © 51.
More information
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