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 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.
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 Lowestoft Formation (formerly the Lowestoft Till) with the so-called Second Cromer Till.
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 Happisburgh Till Member of the Happisburgh Formation. 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.
The deposits in the mid part of the cliff are largely obscured by landslides. They consist of:
- Ostend Clay Member (Happisburgh Formation)
- Walcott Till Member (Lowestoft Formation)
- the lower and central components of the Sheringham Cliffs Formation:
In the upper part of the cliff the exposure is good. Here the uppermost part of the Sheringham Cliff Formation (Trimingham Clay and Weybourne Town Till members) is seen. This is overlain by the Stow Hill Sand and Gravel Member of the Briton’s Lane Formation.
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.
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.
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.
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.
Change in cliff profile
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