Bedding-controlled coastal landslides in Southeast Britain between Axmouth and the Thames Estuary

Abstract

The coastline of Southeast Britain is formed in sedimentary rocks of Jurassic, Cretaceous and Tertiary age, the majority of these deposits containing thick strata of mudrocks, which have very low angles of dip. Where these strata are appropriately exposed, particularly at the foot of a coastal slope, they give rise to landslides where all or part of the sliding surface follows a single bed of mudrock. Where the dip of the bedding is steep, such landslides are referred to as dip-slope failures. However, landslides where the basal sliding surface is controlled by the location and orientation of a single argillaceous bed in the sequence are better termed bedding-controlled landslides. In addition, where coastal slopes contain several layers of mudrocks, geometrically similar landslides may occur with perched slide surfaces breaking out at a higher level within the slopes. Landslides with strong bedding-controlled basal shear surfaces are the predominant form of instability along the southeast coastline of Britain. Many of the individual landslide cases used in this paper have been studied separately over many years. The general similarity of the different records is discussed, drawing important inferences on a number of aspects of these landslides with a variety of basal sliding surface geometries.Keywords Landslides · Bedding-control · Southeast Britain · Coast IntroductionThe coastal zone of Southeast Britain has been a proving ground for landslide research for decades and, in some respects and locations, for centuries. The combination of sequences of sedimentary rocks within which there are appreciable thicknesses of mudrocks (i.e. stiff fissured clays), relatively low angles of dip, generally wet climate, and active coastal erosion leads to the occurrence of particular landslide types. These are generally bedding-controlled landslides with rotational and translational elements (Bromhead and Harris 1999). They are sometimes formed either wholly or mainly of large, displaced blocks of the parent rock mass, a situation which is commonest where the landslides have occurred comparatively recently. Alternatively, the landslide mass may be primarily formed from debris, and the landslide may represent the sum total of many instances of failure over long periods of time. Mudslides are also very common along the coastline and are usually associated with strongly localised water inputs to the slope. Less commonly, coastal slopes exhibit toppling behaviour, and there are stretches of coastal cliffs (usually in a single lithology) where rockfalls occur. Other common morphologies found along this stretch of coastline are low-lying tracts of land with coastal mudflats and marshes (sometimes reclaimed), shingle spits and beaches, as well as drowned valleys. Figure 1 identifies the major locations mentioned in the text and the region of the southeast coastline considered, between the

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A review of landsliding and coastal erosion damage to historic fortifications in South East England

“…Bromhead and Ibsen (2004) describe this stretch of coastline, and reference is made to that earlier paper for details of the underlying geological structure and the nature of the weak rocks present (Fig. 1).…”

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“…Section in green identifies those sediments of Tertiary or younger age. Red dots signify the chain of Saxon Shore Forts (afterBromhead and Ibsen 2004) …”

A combined geomorphological and geophysical approach to characterising relict landslide hazard on the Jurassic Escarpments of Great Britain

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