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Coastal chalk cliff instability in NW France Role of lithology, Fracture

Authors:
  • Université Le Havre Normandie

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.
... Though the geology of the coastal chalk cliffs of Normandy has been fully explored by many authors (Mortimore and Pomerol, 1987, 1991Costa, 1997;Mortimore, 2001Mortimore, , 2011Costa et al., 2004Costa et al., , 2006Duperret et al., 2002Duperret et al., , 2004Duperret et al., , 2005Lageat et al., 2006;Senfaute et al., 2009;Letortu et al., 2015Letortu et al., , 2019, the subtidal shore platform morphology remain largely unknown, except through large-scale bathymetry surveys (Augris et al., 1993;Augris et al., 2004) and local studies of intertidal shore platforms at Eletot and Les Grandes Dalles Foote et al., 2006;Hénaff et al., 2006;Moses et al., 2006), and Mesnil-Val (Regard et al., 2012;Dewez et al., 2015;Duperret et al., 2016). ...
... The petrophysical and geomechanical properties (porosity, permeability, Uniaxial Compressive Strength (UCS), and diagenesis index (DI)) indicative of the cementation of each chalk formation of the Paris basin have been extensively studied and measured in the laboratory (Table 1) (Mortimore and Duperret, 2004;Duperret et al., 2005;Faÿ-Gomord et al., 2016). Recently, Faÿ-Gomord et al. (2016) demonstrated the impact of chalk microtexture on its physical and mechanical properties and defined six types of chalk microtextures (MT1 to MT6) describing the texture of the chalk matrix observed using a scanning electron microscope. ...
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Submerged marine terraces potentially provide crucial information on past sea-level variations and paleo-coastline locations that may be used to estimate long-term coastal erosion rates. The Normandy coastline has recently been surveyed using a shallow water high-resolution mapping system. We identified a new continuous submarine platform, called the inner platform, limited by a shore parallel edge located between -9 m and -10 m (NGF) along the Normandy chalk coastline. A lower rock platform, called the outer platform, ranging from about -14 m to -17 m (NGF) appears locally. This corresponds to inherited preserved submarine terraces created during a past sea level highstand. The high cosmogenic ¹⁰Be concentration measured at the end of Mesnil-Val inner shore platform (including intertidal and subtidal shore platforms) is attributed to the last glacial cliff location at 6.5 ky ± 1 ky. From the spatial edge location of the inner platform in Normandy, we estimated cliff retreat rates since 6.5 ky ± 1 ky ranging from 0.051 ± 0.008 m/y to 0.090 ± 0.014 m/y from place to place. Comparisons with the current coastal chalk cliffs indicate a mean retreat rate estimated over the contemporary period suggesting such long-term retreat rates are 33% to 57% lower than the contemporary ones (0.10 m/y to 0.18 m/y). This confirms a contemporary acceleration of chalk cliff system retreat rates.
... Coastal cliff evolution proceeds as an episodic process, wherein cliff failures are commonly linked to intense storms ( Komar and Shih, 1993;Hapke and Richmond, 2002). Cliff erosion is influenced by various marine and subaerial processes, including: waves (Norris, 1968;Lim et al., 2011;Young et al., 2011), groundwater flow ( Pierre and Lahousse, 2006), beach geometry (Sallenger Jr et al., 2002;Trenhaile, 2004), cliff lithology (Sunamura, 1992;Benumof et al., 2000;Collins and Sitar, 2008), mechanical and chemical weathering (Sunamura, 1992;Porter and Trenhaile, 2007), rainfall (Collins et al., 2007;Young et al., 2009a;Brooks et al., 2012), anthropogenic agents (Viles and Spencer, 2014), and structural characteristics ( Duperret et al., 2004). These processes are considerably complex owing to multiple factors that are highly variable in space and time. ...
... Near-shore topography and changes in beach morphology determine the frequency and style in which waves attack the cliff base. In addition, fracture networks play a considerable role in creating planes of weakness ( Duperret et al., 2004). Though all processes work in conjunction to modulate cliff evolution, the predominate driving mechanisms of cliff retreat are broadly attributed to wave attack and rainfall, where rainfall modulates the rate at the cliff top, and wave action drives retreat at the cliff base via undercutting from abrasion and mechanical wave action. ...
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... Defined as a steeply sloping surface where elevated land meets the shoreline (Hampton and Griggs, 2004), constitute about 80% of the world seashore (Emery and Kuhn, 1982). Rising above the geological outcrops developed on a wave-cut platform, upon beaches or directly over the sea (Trenhaile, 2010;Sunamura, 2015) are affected by different marine and subaerial processes (Alessio and Keller, 2020), including waves (Young et al., 2011), groundwater flow (Pierre and Lahousse, 2006), mechanical and chemical weathering (Porter and Trenhaile, 2007) and rainfall (Collins and Sitar, 2007); while acting as conditioning factors the cliff lithology (Sunamura, 1992), coast geometry (Hampton and Griggs, 2004) and structural characteristics (Duperret et al., 2004). ...
Article
Coastal cliffs provide a high landscape value to many natural sites around the world. This means that an ever-increasing number of people are attracted to them. At this point, there is a growing need to manage these spaces from the safety of visitors, but with a view to preserving the environment. With this aim, this paper presents an approach to analyze and manage instabilities in these environments, particularly those subjected to significant anthropic activity, which has been implemented in the cliffs of the Basque Coast Geopark. The starting point is a detailed topographic information, obtained from UAV flights, and the identification on site of unstable elements, including their typology, active source areas, dynamics and reach. From this information, the simulation of rockfall processes, which basically correspond to toppling and infinite slope instabilities favored by differential erosion along the coastline, is approached in two and three dimensions. Results allow the design of precise actions by sectors, according to the energy, height and reach of the detached blocks, including barriers, middle slope actions, ditches and information strategies, depending on the different uses of the sectors. Therefore, this approach leads to a more detailed and environmentally friendly management of these environments.
... Such nested collapse-triggered landslide hazard phenomenon has rarely been documented in regions of chalk cliffs surrounding the English Channel ( Duperret et al., 2004) and it does not fall within the chalk cliff collapse classification defined in Mortimore et al. (2004). While understanding and modeling the specific processes involved in the two-step slope failure are beyond the scope of this paper, a first order interpretation of the processes were formulated in a technical report (Pannet et al., 2015). ...
Article
In this study, we present a complete and successful case study where gravity and seismic refraction surveys detect and map previously poorly known sand and clay-filled depressions within the top chalk layers in a costal context, near Dieppe, Normandy, France. This study was commissioned by local authorities after a coastal chalk cliff collapse exposed a sand and clay-filled depression which turned into a > 100,000m3 landslide. This resulted in a massive clifftop retreat exceeding 40 m, which threatened infrastructure and amenities. For risk and safety assessment, coastal managers commissioned BRGM to (i) determine the depth and extent of the sand and clay-filled depression, and (ii) map the presence of similar cliff-top depressions in a 2-km-long and 400-m-wide band inland of the coast. Both geophysical methods allow the detection and mapping of the sand and clay-filled depressions, which are characterised by a co-localized deepening of the first seismic horizon and a positive gravity anomaly. Seven auger holes confirm the geophysical interpretation, with depth to the top of the chalk>60m in some instances. A map of the depth to the top of chalk is inverted using the gravity residuals. The successful mapping of the previously poorly-documented sand and clay-filled depressions on the Dieppe clifftop, using both gravity and seismic refraction tomography, was used in part to generate a Coastal Landslide Hazard Zonation map, which is a useful tool for coastal managers who need to make hazard-mitigating decisions.
... Figure 2 shows certain features of the coastal rocky cliffs on the Ryukyu Islands (Kogure et al. 2006). Duperret et al. (2004) reported that the instantaneous collapses observed in Upper Normandy and Picardy, France, were caused by the instability of coastal chalk cliffs. Bruun (1962) generated the Bruun rule model to simulate the effect of the rate of sea-level rise on coastline erosion. ...
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