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Coastal chalk cliff instability in NW France Role of lithology, Fracture
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.
... Furthermore, an extensive literature documents the extent to which a myriad of OPEN weathering processes contribute to, and in some cases, dominate the rate of cliff erosion 14,15 . For instance, cliff failure frequently occurs without waves impacting directly on the cliff 16,17 . Recently, Dietze et al. 18 showed that during the monitoring of a cliff on the Jasmund Peninsula of Rügen, Germany, marine processes did not trigger cliff failure, and wetting of the chalk was the most significant process. ...
... weathering processes contribute to, and in some cases, dominate the rate of cliff erosion 14,15 . For instance, cliff failure frequently occurs without waves impacting directly on the cliff 16,17 . Recently, Dietze et al. 18 showed that during the monitoring of a cliff on the Jasmund Peninsula of Rügen, Germany, marine processes did not trigger cliff failure, and wetting of the chalk was the most significant process. ...
It is often assumed that future coastal cliff retreat rates will accelerate as global sea level rises, but few studies have investigated how SLR (sea level rise) might change cliff-front wave dynamics. Using a new simple numerical model, this study simulates the number and type (breaking, broken, or unbroken) of cliff-front waves under future SLR scenarios. Previous research shows breaking waves deliver more energy to cliffs than broken waves, and unbroken waves generate minimal impact. Here, we investigated six cliff-platform profiles from three regions (USA, New Zealand, and UK) with varied tidal ranges and wave climates. Model inputs included 2013–2100 hindcast/forecast incident wave height and tidal water level, and three future SLR scenarios. Results show the number of both cliff-front breaking and broken waves generally increase for a high-elevation (relative to tide) cliff-platform junction. In contrast, breaking/broken wave occurrence decrease by 38–92% for a near-horizontal shore platform with a low-elevation cliff-platform junction under a high SRL scenario, leading to high (96–97%) unbroken wave occurrence. Overall, results suggest the response of cliff-front waves to future SLR is complex and depends on shore platform geometries and SLR scenarios, indicating that future cliff retreat rates may not homogeneously accelerate under SLR.
... The retreat of chalk cliffs is primarily a function of episodic cliff collapses, where the type, volume and frequency of collapse are controlled primarily by the chalk type and structure and cliff height (Mortimore et al., 2004a;Dornbusch et al., 2008;Robinson, 2020). The porosity of chalk means that groundwater saturation, rainfall and storms also play an important role in cliff collapse occurrence at these vulnerable coastlines (Duperret et al., 2004(Duperret et al., , 2005Mortimore et al., 2004a). One of the largest recent cliff collapses occurred near Beachy Head in 1999 with a total chalk volume loss of up to 150 000 m 3 , which produced a debris apron that extended up to 130 m across the shore platform (Mortimore et al., 2004a;Moses and Robinson, 2011;Robinson, 2020). ...
... One of the largest recent cliff collapses occurred near Beachy Head in 1999 with a total chalk volume loss of up to 150 000 m 3 , which produced a debris apron that extended up to 130 m across the shore platform (Mortimore et al., 2004a;Moses and Robinson, 2011;Robinson, 2020). In contrast, smaller collapses of volumes < 1000 m 3 occur much more frequently and often multiple times within a year (Duperret et al., 2004;Williams et al., 2004;Robinson, 2020). The removal rate of fallen cliff debris is widely unquantified (Moses and Robinson, 2011). ...
The white chalk cliffs on the south coast of England are one of the most iconic coastlines in the world. Rock coasts located in a weak lithology, such as chalk, are likely to be most vulnerable to climate-change-triggered accelerations in cliff retreat rates. In order to make future forecasts of cliff retreat rates as a response to climate change, we need to look beyond individual erosion events to quantify the long-term trends in cliff retreat rates. Exposure dating of shore platforms using cosmogenic radionuclide analysis and numerical modelling allows us to study past cliff retreat rates across the Late Holocene for these chalk coastlines. Here, we conduct a multi-objective optimisation of a coastal evolution model to both high-precision topographic data and 10Be concentrations at four chalk rock coast sites to reveal a link between cliff retreat rates and the rate of sea-level rise. Furthermore, our results strengthen evidence for a recent acceleration in cliff retreat rates at the chalk cliffs on the south coast of England. Our optimised model results suggest that the relatively rapid historical cliff retreat rates observed at these sites spanning the last 150 years last occurred between 5300 and 6800 years ago when the rate of relative sea-level rise was a factor of 5–9 times more rapid than during the recent observable record. However, results for these chalk sites also indicate that current process-based models of rock coast development are overlooking key processes that were not previously identified at sandstone rock coast sites. Interpretation of results suggest that beaches, cliff debris and heterogenous lithology play an important but poorly understood role in the long-term evolution of these chalk rock coast sites. Despite these limitations, our results reveal significant differences in intertidal weathering rates between sandstone and chalk rock coast sites, which helps to inform the long-standing debate of “wave versus weathering” as the primary control on shore platform development. At the sandstone sites, subaerial weathering has been negligible during the Holocene. In contrast, for the chalk sites, intertidal weathering plays an active role in the long-term development of the shore platform and cliff system. Overall, our results demonstrate how an abstract, process-based model, when optimised with a rigorous optimisation routine, can not only capture long-term trends in transient cliff retreat rates but also distinguish key erosion processes active in millennial-scale rock coast evolution at real-world sites with contrasting rock types.
... The apparent friction coefficient H f /L f and the equivalent friction coefficient H cm /L cm are metrics widely utilized by scholars to assess the landslide mobility (Corominas 1996;Legros 2002;Lin et al. 2020). Figure 9 was constructed by combining research on chalk landslide mobility by Hutchinson (2002) and Duperret et al. (2004), loess landslide mobility by Ma et al. (2014), rockfall mobility by Crosta et al. (2017), and experimental studies on gravel mobility, along with 36 sets of data from simulation experiments. Figure 9a shows the apparent friction coefficient data, while Fig. 9b shows the equivalent friction coefficient data. ...
This study uses discrete element method models to simulate the fragmentation and deposition of landslides with varying volumes on terrains with different slopes and heights. The slope motion process during the numerical simulations of the landslide movement can be divided into three stages based on changes in the kinetic energy. The variations in the kinetic and frictional energies throughout the mass motion are used to establish pertinent parameters to analyze the dynamics of the slider fragmentation characteristics. Building on prior research, the impact of the slope on the mobility and deposit morphology, including the apparent and equivalent friction coefficients and the ratio of the width to length as a deposit morphology model, is examined using motion models. Concurrently, the three experimental variables (the slope gradient, slope height, and sliding block volume) are analyzed and discussed in conjunction with the motion and deposit morphology models. Previous studies indicate that the quantification of landslide fragmentation is only applicable to rock landslides and has limitations. In the numerical simulations, distinct contact models for pre- and post-fragment particles are defined to enumerate the total number of intact particles. Subsequently, a dimensionless parameter is formulated to quantify the degree of slope fragmentation. The relationship of this parameter with the motion and deposition models is subsequently explored. The results show that increased fragmentation reduces the landslide mobility, indicating that fragmentation is an energy-consumptive process that hinders landslide motion. These findings provide insights into the mechanisms of long-runout landslides and contribute to the reproduction of landslide dynamics.
... Due to climate change and global warming, these factors are escalating in frequency and intensity in recent decades (Bonaldo et al., 2019;Bray and Hooke, 1997;Dickson, Walkden and Hall, 2007;Gómez-Pazo, Pérez-Alberti and Trenhaile, 2021;Lollino et al., 2021;Pang et al., 2023;Theodore et al., 2020). As a consequence, the occurrence and severity of coastal erosion-induced landslides (often caused by the growth of cliff notches) and the associated losses of coastal land and properties have significantly increased (Andriani and Pellegrini, 2014;Budetta, 2011;Duperret et al., 2004;Kogure et al., 2006;Ružić et al., 2015;Theodore et al., 2020;Wolters and Gerald, 2008;Young, 2018;Young and Ashford, 2008;Zhang et al., 2023), becoming a serious challenge for many coastal communities that are not resilient enough to extreme weather events. ...
Cliff erosion is an unstoppable natural process increasingly occurring due to climate change and frequently causing crucial georisks on rocky coasts throughout the world. The resilience of a cliff depends on a variety of environmental, geometrical, geological and geotechnical conditions that have been included in several heuristic coastal hazard assessment approaches. In order to provide new quantitative insights into the relationship between some geometrical and structural characteristics of a sea cliff (height, basal erosion, discontinuities) and its stability conditions (failure mechanism, safety factor) this paper investigates numerically how the progressive undermining of a soft rock cliff affects its mechanical behaviour. It has been found that the undermining depth plays a significant role on cliff stability, whose mechanism of collapse changes according to the overhang slenderness. When a vertical joint is present, the higher is its persistence the lower is the global safety factor. Moreover, as the joint moves away from the cliff face the safety factor decreases, the worse condition being found when it is above the notch end. The results obtained can contribute to a deeper understanding of the failure mechanisms of seacliffs, helping in a reliable assessment of coastal risk and a proper design of the mitigation measures
... The Maronti cliff undergoes significant erosive action from the sea as it lacks adequate protection from a sufficiently wide beach. The wave action plays a critical role in the cliff retreat process by exerting stress at the cliff base and by infiltrating within the fracture system and pore space of the rock mass [34][35][36]. The wave motion creates beach notches of various depths, particularly evident in tuffaceous deposits where some of them can reach several meters in depth, which highlights the influence of the mechanical properties of the lithology in the erosion process [1,3]. ...
The morphoevolution of coastal areas is due to the interactions of multiple continental and marine processes that define a highly dynamic environment. These processes can occur as rapid catastrophic events (e.g., landslides, storms, and coastal land use) or as slower continuous processes (i.e., wave, tidal, and current actions), creating a multi-hazard scenario. Maronti Bay (Ischia Island, Southern Italy) can be classified as a pocket beach that represents an important tourist and environmental area for the island, although it has been historically affected by slope instability, sea cliff recession, and coastal erosion. In this study, the historical morphoevolution of the shoreline was analysed by means of a dataset of aerial photographs and cartographic information available in the literature over a 25-year period. Furthermore, the role of cliff recession and its impact on the beach was also explored, as in recent years, the stability condition of the area was worsened by the occurrence of a remarkable landslide in 2019. The latter was reactivated following a cloudburst on the 26th of November 2022 that affected the whole Island and was analysed with the Dem of Difference technique. It provided an estimate of the mobilised volumes and showed how the erosion and deposition areas were distributed and modified by wave action. The insights from this research can be valuable in developing mitigation strategies and protective measures to safeguard the surrounding environment and ensure the safety of residents and tourists in this multi-hazard environment.
... Landslides are the most predominant risk on rocky coast. It provide, in some cases, protective deposits accumulated in the base of the cliff and thus slow down mechanical waves erosion [8] Mass movements on steep cliffs are instantaneous phenomena that provide virtually no warning, they are triggered and amplified by a variety of marine and subaerial processes [9], including waves [10], groundwater flow [11], mechanical and chemical weathering [12], and precipitations [13]; while cliff lithology [14], coastal geometry [15] and structural features [16] are acting as conditioning factors. Over the past decade, we have witnessed several accidents caused by cliff collapses and landslides, which have killed or injured people and destroyed several buildings. ...
The coastal area of Safi between Cap Beddouza and Jorf Lihoudi is dominated by different forms of marine cliffs forming the coastline, with rock formations ranging from Jurassic to Quaternary. The main coastal geological risk is the instabilities of these cliffs due to the falls and collapses of the blocks, as well as landslides. The aim of this paper is to give detailed information on the rate of the cliff summit recession for the last 66 years, obtained by applying the DSAS model, in parallel with the identification of unstable elements, including their different parameters such as size, abundance, activity in time and their effect on the recession of the cliff top, which is fundamental information for the evolution the models of sea cliffs. These phenomena constitute a significant economical, social, and environmental risk.
Geology and geomorphology link starkly in coastal environments, exemplifying the importance of Fookes' geo model approach to understanding site dynamics. Cliffs, and the shore platforms that front them, are subject to both marine and subaerial processes. Throughout his work, Fookes emphasises the role of climate as a weathering agent, influencing material properties and requiring consideration in engineering projects. Comparatively, more is known about the operation of rock weathering processes on shore platforms than on cliffs. Yet, with changing climate, it is becoming apparent that winter salt and frost weathering, and summer salt and wetting-drying weathering contribute to more frequent occurrences of coastal rockfalls. Increasing storm activity and rising sea levels are enhancing marine and subaerial processes. Rock coasts are amongst the most rapidly eroding coastlines in Europe. New applications of technologies, including seismometers, Lidar and InSAR, provide a fuller understanding of rock coast process-response geo model behaviour and the resilience of rock coasts on engineering timescales. Communicating scientific understanding of rock coast dynamics to policy-makers, planners and the public remains a challenge. Assessments of hazard, risk, resilience and vulnerability of rock coasts associated with climate change provide useful communication tools. Databases such as the British Geological Survey GeoCoast geohazard data product, and EMODnet Geology (European Marine Observation and Data Network) coastal behaviour data products integrate and visualise available data and information to communicate situational awareness to a broad audience.
The Moroccan coastline extends for 2,130,80 km on the East Atlantic an Western Mediterranean coasts. It shows a high diversity of estuaries, bays, beaches and steep coasts. Despite its ecological, economic, and recreational importance, Morocco does not have a comprehensive inventory of its coastal landscapes. This study attempts to develop a primary classification of Moroccan steep coasts, based on a descriptive inventory of these landscapes, using 12 morphodynamic criteria that we mainly measured on satellite images. To achieve the said classification, we organized the steep coasts and the criteria in a binary matrix, which we treated with the hierarchical ascending classification method (CAH) and Factorial Correspondence Analysis. The results of this treatment provides a clustering scheme where we distinguish six different groups of cliffs, each of them being subdivided to 2 or 3 subgroups. Three categories of criteria appear as determinant in this classification: morphometric (length, area, and elevation), geologic (dominant rock and geologic eras), hydrodynamic (swells), and aerodynamic (wind). However, the two first criteria have the most significant influence on the classification.
This classification, mainly based on satellite data, remains preliminary and requires improvements such using some field data. Finally, the raw and compiled data collected in this study constitute database composed of 175 cliffs, described using parametric criteria; this database is a contribution to the national wetlands inventory and is essential to promoting their conservation as well as management.
Chalk is the most important source of fresh water in North-West Europe: more than eight million cubic metres of water are pumped daily from the aquifier. This work presents an authoritative survey of the origin, sedimentology, and hydraulic properties of this unique rock, and the chemical characteristics of the water it contains. The factors influencing the flow through the chalk and its role as a hydrocarbon reservoir in the North Sea are reviewed along with the threats that industrial chemicals, fertilizers, and pesticides present to the supply. The long-term implications of pollutants in this vital resource and the continuing ignorance of the factors that control the flow of water through the rock are highlighted.
Chalk is the most important source of fresh water in North-West Europe: more than eight million cubic metres of water are pumped daily from the aquifier. This work presents an authoritative survey of the origin, sedimentology, and hydraulic properties of this unique rock, and the chemical characteristics of the water it contains. The factors influencing the flow through the chalk and its role as a hydrocarbon reservoir in the North Sea are reviewed along with the threats that industrial chemicals, fertilizers, and pesticides present to the supply. The long-term implications of pollutants in this vital resource and the continuing ignorance of the factors that control the flow of water through the rock are highlighted.
Chalk is the most important source of fresh water in North-West Europe: more than eight million cubic metres of water are pumped daily from the aquifier. This work presents an authoritative survey of the origin, sedimentology, and hydraulic properties of this unique rock, and the chemical characteristics of the water it contains. The factors influencing the flow through the chalk and its role as a hydrocarbon reservoir in the North Sea are reviewed along with the threats that industrial chemicals, fertilizers, and pesticides present to the supply. The long-term implications of pollutants in this vital resource and the continuing ignorance of the factors that control the flow of water through the rock are highlighted.
The distribution of physical properties and the engineering performance of the Chalk in the Anglo-Paris Basin is investigated in both time (stratigraphically) and space (palaeogeographically). The retention of the traditional names for the Lower Chalk and its lithological subdivisions in southeast England is recommended but the Asham Zoophycos Beds and the Falling Sands Member are formally introduced to identify units in the Lower Chalk of East Sussex. A combined Middle and Upper Chalk forms one White Chalk Formation subdivided using the lithological units and terms of Mortimore (1986) and Mortimore and Pomerol (1987). Six features of the Chalk lithologies, hardness, alternating layers, large flints, fractures in relation to lithology, fracturing in channel-forming hardgrounds and block size are identified as crucial to engineering performance. The results demonstrate that none of the traditional major divisions of the Chalk (e.g. Lower, Middle & Upper Chalk in England or Cenomanian to Campanian in France) can be treated as uniform and consistent units for engineering purposes. -from Authors
Geological maps of the Chalk country of England have been revolutionised by the British Geological Survey. The initial impetus for developing a new lithostratigraphy for the Chalk was, however, the need to provide a framework for major construction projects, particularly earthworks and tunnels, in relation to physical properties and rock mass character. Out of this research arose divisions appropriate for mapping because the geomorphology reflected the material and mass properties. Latterly, the detailed marker bed stratigraphy has successfully been employed to construct ground models for tunnels along the south coast and for the Channel Tunnel Rail Link. The same detailed stratigraphy is also being used to develop a hydrogeological stratigraphy for the Chalk aquifer. The conspicuous 'stratigraphic' variation in the rock mass character of the Chalk reflects both lithological variation and tectonic controls. Understanding this variation and its causes makes the development of a mechanical or fracture stratigraphy possible. Such a fracture stratigraphy provides the means for modelling the Chalk as an aquifer, as an oil reservoir and aids prediction of ground conditions for slope, tunnel and earthwork design. Karst development in the Chalk is also closely related to its material and mass character, especially the lithostratigraphy and associated fracture styles. A key factor in the development of the detailed lithostratigraphy was the recognition of marker beds, including marl seams, flint bands and macrofossil and trace fossil horizons, that could be consistently recognised in rock-core as well as field sections. Training geologists and engineers to recognise these features has been an important part of the successful application of this geological research to a wide range of industries.