Coastal chalk cliff instability in NW France: role of lithology, fracture pattern and rainfall

Abstract

Coastal retreat has been studied along 120km of French Channel chalk coast from Upper Normandy to Picardy. During the investigation period, 1998–2001, 55 significant collapses were recorded. Of these 5.5% were very large-scale, 34.5% large-scale, 34.5% medium-scale and 25.5% small-scale collapses. Observations indicate that the larger the collapse size the greater the coastal cliff retreat. Four types of cliff failure were observed: (1) vertical failures in homogeneous chalk units; (2) sliding failures where two superimposed chalk units were present; (3) wedge and plane failures mainly recognized in the UK in formations with stratabound fractures; (4) complex failures in cliffs with more than one style of fracturing. Rainfall in relation to the timing of cliff collapse indicates two periods that trigger a collapse. The first occurs about one month after heavy rainfall within poorly fractured chalk and the second occurs when a dry period is interrupted by sharp rainfall in cliffs with major karst features (pipes etc). Medium to small-scale cliff collapses were, in some cases, caused by marine erosion at the base of the cliff creating a notch. A key factor controlling the type of collapse is the lithostratigraphic unit, while the extent of the collapse scar may be controlled by fracture type.

A geotechnical and GIS based method for evaluating risk exposition along coastal cliff environments: a case study of the chalk cliffs of southern England

“…Although McLaughlin (2002) noted that the utilization of socioeconomic factors is of great importance to future coastal studies, he also indicated that the collection of such data presents many difficulties including the reliability of the sources and the complexity of the analysis. Other methodologies evaluate coastal failures considering a number of natural factors such as heavy rainfall events, tidal ranges and wave energy (Del Rio and Gracia, 2009;Duperret et al, 2004;Hutchinson, 1971). However, the collection and the analysis of large amounts of data for such methodologies produce complexity and require plenty of time, experience as well as exhaustive field work.…”

Micro-seismic precursory cracks prior to rock-fall on coastal chalk cliffs: a case study at Mesnil-Val, Normandie, NW France

“…Unfortunately, the micro-seismic system was not working during the large-scale collapses and therefore the behaviour and characteristics of micro-seismic signals that would be recorded in the case of a very large-scale collapse, where rupture initiation occurs on pre-existing large-scale fractures remains unknown. However, Duperret et al (2004) suggested that the main mechanism triggering cliff failure processes are meteorological (rainfall, temperature), wave action on the shore platform and/or wave impact on the cliff base, cyclical tidal effects and fatigue caused by stress relief. These external mechanisms act on the rock-mass structure to trigger failures.…”

“…On coastal chalk cliffs, this is producing catastrophic collapses (e.g. Mortimore et al, 2004;Duperret et al, 2004), which raise problems of security, land-use planning and management of coastal areas. Investigations along 120 km chalk coastline in Normandie and Picardie, France identified 55 significant collapses with volumes ranging from about 100 m 3 to 85 000 m 3 and the larger collapses were capable of taking a bite out of the cliff face between 10-20 m deep during a single event (Duperret et al, 2002.…”

“…Unfortunately, understanding the actual mechanisms of coastal cliff retreat is poorly developed. Some elements are well-established, for example the role of the pre-existing fractures in the generation of block failure from the cliff (Hoek and Bray, 1981;Priest, 1993;Duperret et al, 2004;Busby and Jackson, 2006;Freyssines and Hantz, 2006). However, whilst the fractures clearly provide weaknesses along which many failures develop, the triggering processes including fracture development leading to failure remains poorly understood because it is associated with the difficulty of long-term monitoring and data collection.…”

Cliff Collapse Hazards Spatio-Temporal Modelling through GIS: from Parameters Determination to Multi-scale Approach

“…As defined in UK (Mortimore, 1983), chalk type units are defined on the basis of a lithostratigraphic concept and are more representative of the geotechnical properties of the chalk than the stratigraphic scale traditionaly used in France (Mortimore, 2001 ;Duperret et al, 2004). The Chalk lithostratigraphy layer is composed of six chalk units of various characteristics detailed below :…”