Coastal cliff geohazards in weak rock: the UK Chalk cliffs of Sussex

Abstract

Abstract Geohazards related to chalk coastal cliffs from Eastbourne to Brighton, Sussex are described. An eight-fold hazard classification is introduced that recognizes the influence of chalk lithology, overlying sediments and weathering processes on location, magnitude and frequency of cliff collapses. Parts of the coast are characterized by cliffs of predominantly a single chalk formation (e.g. Seven Sisters) and other sections are more complex containing several Chalk formations (Beachy Head). Rock properties (intact dry density or porosity) and mass structure vary with each formation and control cliff failure mechanisms and scales of failures. The Holywell Nodular Chalk, New Pit Chalk and Newhaven Chalk formations are characterized by steeply inclined conjugate sets of joints which lead to predominantly plane and wedge failures. However, the dihedral angle of the shears, the fracture roughness and fill is different in each of these formations leading to different rock mass shear strengths. In contrast the Seaford and Culver Chalk formations are characterized by low-density chalks with predominantly clean, vertical joint sets, more closely spaced than in the other formations. Cliff failure types range from simple joint controlled conventional plane and wedge failures to complex cliff collapses and major rock falls (partial flow-slides) involving material failure as well as interaction with discontinuities. Other hazards, related to sediments capping the Chalk cliffs, include mud-slides and sandstone collapses at Newhaven, and progressive failure of Quaternary Head and other valley-fill deposits. Weathering, including the concentration of groundwater flow down dissolution pipes and primary discontinuities, is a major factor on rate and location of cliff collapses. A particular feature of the Chalk cliffs is the influence of folding on cliff stability, especially at Beachy Head, Seaford Head and Newhaven. A new classification for cliff collapses and a new scale of magnitude for collapses are introduced and used to identify, semi-quantify and map the different hazards. Climate (and climate change) and marine erosion affect the rate of development of cliff collapse and cliff-line retreat. This was particularly evident during the wet winters of 1999–2000–2001 when the first major collapses along protected sections of coastline occurred (Peacehaven Cliffs protected by an undercliff wall; Black Rock Marina the Chalk cliffs and the Quaternary Head). It is the geology, however, that controls the location and scale of erosion and cliff failure.