# The contribution of LiDAR data and 3D analysis to the study of morphology and recent retreat of the coastal cliffs of Pas-de-Calais

Emmanuel Blaise, Olivier Cohen, Denis Marin, Audrey Vivier-Boudrier

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Emmanuel Blaise, Olivier Cohen, Denis Marin, Audrey Vivier-Boudrier. The contribution of LiDAR data and 3D analysis to the study of morphology and recent retreat of the coastal cliffs of Pas-de-Calais. Géomorphologie: relief, processus, environnement, 2025, Falaises et côtes rocheuses, 31 (3), pp.229-250. 10.4000/15qnv. hal-05521970

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# The contribution of LiDAR data and 3D analysis to the study of morphology and recent retreat of the coastal cliffs of Pas-de-Calais

Apport des données LiDAR et de l'analyse 3D à l'étude de la morphologie et du recul récent des falaises littorales du Pas-de-Calais.

Emmanuel Blaise, Olivier Cohen, Denis Marin and Audrey Vivier-Boudrier

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OpenEdition
Journals

**Electronic version**

URL: https://journals.openedition.org/geomorphologie/21288

ISSN: 1957-777X

**Publisher**

Groupe français de géomorphologie

**Printed version**

Date of publication: April 1, 2025

Number of pages: 229-250

ISSN: 1266-5304

Provided by Bibliothèque de l'Université du Littoral Côte d'Opale

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**Electronic reference**

Emmanuel Blaise, Olivier Cohen, Denis Marin and Audrey Vivier-Boudrier, "The contribution of LiDAR data and 3D analysis to the study of morphology and recent retreat of the coastal cliffs of Pas-de-Calais", *Géomorphologie : relief, processus, environnement* [Online], 31-3 | 2025, Online since 20 February 2026, connection on 20 February 2026. URL: http://journals.openedition.org/geomorphologie/21288

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# The contribution of LiDAR data and 3D analysis to the study of morphology and recent retreat of the coastal cliffs of Pas-de-Calais

## Apport des données LiDAR et de l'analyse 3D à l'étude de la morphologie et du recul récent des falaises littorales du Pas-de-Calais

Emmanuel Blaise *, Olivier Cohen, Denis Marin, Audrey Vivier-Boudrier

Université du Littoral Côte d'Opale, Laboratoire d'Océanologie et de Géosciences - LOG UMR 8187 - CNRS | ULille | ULCO | IRD.

### ABSTRACT

This study updates the data on the morphodynamic evolution of the Pas-de-Calais coastal cliffs, based on a diachronic analysis of cliff-top kinematics between 2000 and 2021, using the area-loss method. It also uses LiDAR datasets to conduct a three-dimensional topographic analysis, based on previously defined retreat zones. In analysing cliff retreat and morphological variations, 2D and 3D approaches complement each other, and help to identify recent erosion areas. The results reveal relatively high medium-term retreat rates across the study area (-0.26 m/year) and significant spatio-temporal variability. This variability is explained by the diversity of cliff types in terms of their hydrogeological characteristics, topomorphological configurations, and the magnitude and nature of anthropogenic impacts during the 20th century. Additionally, the low quality of the orthophotographs used in certain sectors may affect the accuracy of measurements and introduce significant uncertainty.

**Keywords:** coastal erosion, sea cliffs, LiDAR, morphodynamics, shoreline, 3D analysis.

### RÉSUMÉ

Dans cette étude, nous avons mis à jour les données concernant l'évolution topographique des falaises littorales du Pas-de-Calais sur la base, (i) d'une analyse diachronique de la cinématique des falaises entre 2000 et 2021 à l'aide de la méthode de calcul des aires perdues, (ii) d'une analyse diachronique de la topographie en 3 dimensions à l'aide de jeux de données LiDAR. Dans le cadre de l'analyse du recul et des variations morphologiques des falaises, les approches 2D et 3D se complètent et permettent de mettre en évidence des secteurs d'érosion récente. Les résultats obtenus montrent des valeurs de recul relativement importantes sur une période de 20 ans, sur l'ensemble du linéaire étudié (-0,26 m /an), mais également de fortes variabilités spatio-temporelles, qui s'expliquent par la grande diversité des falaises en termes de caractéristiques hydrogéologiques, de configurations topomorphologiques, ainsi que par l'ampleur et la nature des impacts anthropiques subis au cours du XXe siècle. Par ailleurs, certains secteurs présentent une incertitude importante en raison de la qualité parfois insuffisante des orthophotographies utilisées, ce qui peut affecter la précision des mesures réalisées.

**Mots-clés :** érosion côtière, falaises littorales, LiDAR, morphodynamiques, trait de côte, analyse 3D.

### ARTICLE INFORMATION

Received April 15, 2025.
Accepted Januray 30, 2026.

*Corresponding author. Tel: +33 328237614
ORCID: 0009-0001-7510-6489
E-mail adresses:
emmanuel.blaise@univ-littoral.fr

## 1. Introduction

Coastal cliffs have generally received less attention from the scientific community than depositional coasts, estuaries and deltas (Trenhaile, 1980; Fournier and Regnauld, 1991; Naylor et al., 2010). Yet, they represent a major component of the world's shorelines, with their proportion varying significantly according to the studies and methods used, from about 52 % (Young and Carilli, 2019) to nearly 80 % (Emery and Kuhn, 1982), with more recent estimates converging at around 53 % (Costa et al., 2025). Several factors may explain why cliffs have long been overlooked. Historically, research has focused primarily on low-lying sedimentary coasts, which are more densely populated and where issues of erosion and marine flooding are more pressing. In addition, the evolution of cliffs can sometimes be extremely slow or even imperceptible to the naked eye

over a human lifetime, making their monitoring less of a priority compared to more dynamic beach environments. The study of cliffs is also complicated by the rapid variation in lithostratigraphy and structural composition along the coastline, making it difficult to apply standardised methodologies and to compare results across different spatial and temporal scales. However, like low-lying coasts, some cliffs are experiencing increasing land-use pressures, leading to urban expansion, intensified coastal activities (Turner et al., 1996) and greater vulnerability to coastal erosion. In addition, cliff retreat is a significant, although rarely quantified, source of sediment for depositional coasts (Naylor et al., 2010; Costa et al., 2015; Peuziat et al., 2021; Regard et al., 2021; Luetzenburg et al., 2023).

Throughout the 20th century, and even more so in recent decades, research on the morphodynamics of coastal cliffs has increased significantly worldwide (Naylor et al., 2010). Two main objectives
have generally structured these studies: (i) to quantify recent cliff-top retreat and analyse associated mass-movement processes; and (ii) to estimate the volume of debris released by cliff erosion that contributes to longshore sediment transport.

In France, some sites have been the focus of an increasing number of studies over time, to the point of becoming true open-air laboratories, thanks in particular to the gradual accumulation of observational series, especially topomorphological data. This is particularly true for the chalk cliffs of Upper Cretaceous age in Normandy (Costa et al., 2001, 2003, 2019; Hénaff et al., 2002; Le Cossec, 2010; Letortu, 2013; Elineau, 2013; Letortu et al., 2014, 2019, 2025; Roulland et al., 2019; Hassen et al., 2020; Duguet et al., 2021; Roulland, 2022). To a lesser extent, but with increasing research activity in recent years, attention has also been given to the unconsolidated cliffs of Brittany, composed of a wide range of Quaternary deposits (palaeosols, head, loess, aeolian silt, and littoral sediments; Basara et al., 2018, 2019; Basara, 2019), and to the cliffs of Charente-Maritime, which display a marked geological diversity: chalky limestones of Cretaceous age, Jurassic clay-marl formations, and cliffs made up of sandstones, sands, clays, limestones, and marls dating from the Tithonian as well as the Upper Cretaceous (Jean-Baptiste, 2021; Béranger et al., 2024, 2025; Martins and Bernon, 2022). Further south, recent studies have increasingly focused on the Cretaceous calcareous flysch cliffs of the Basque Country (Prémaillon, 2018; Prémaillon et al., 2018a, 2018b; Martins et al., 2022; Guillen, 2024), the cliffs of southern France (Giuliano, 2015), and, more sporadically, other coastal sectors.

At the scale of northern France, the Pas-de-Calais cliffs are among the best studied from a geological perspective, owing to the exceptional nature of their sedimentary sequences, which span from the Jurassic to the Cretaceous periods. Their unusual thickness results from a specific tectonic context marked by a network of normal faults, which led to the formation of a gulf that favored sediment accumulation, unlike in neighbouring regions. The Cretaceous formations rest on a major erosional surface and display continental Wealden deposits, whose thickness varies according to fault activity, particularly around Wissant, where the deposits thicken towards the west under the influence of major structures. While the geology of the Boulonnais cliffs has long been well documented, their short-term morphodynamics remained poorly studied until the late 1990s. It was only through the work of Guillaume Pierre and Philippe Lahousse that significant progress was made in the understanding of the forcing factors and processes shaping cliff evolution (Lahousse and Pierre, 2003; Pierre and Lahousse, 2003, 2004, 2006; Pierre, 2005a, 2005b, 2006, 2007, 2008a, 2015). Conducted initially within the framework of the Coastal Risk Prevention Plans (Plans de Prévention des Risques Littoraux - PPRL) and at the request of local authorities, these studies combined the analysis of Napoleonic cadastral maps and aerial photographs with extensive field observations and measurements. The so-called “transect method,” based on the comparison of cliff-top positions along profiles perpendicular to the coast (every 10 m or every 5 m depending on the area), was central to their quantification of retreat. Although subsequently criticised for its limitations in capturing the complexity of cliff erosion dynamics (Costa et al., 2003; Letortu et al., 2014; Martins et al., 2022), this approach nonetheless produced a substantial body of quantified results that remains a reference today.

The research of the late 1990s and early 2000s shed light on the role of geological, morphological and hydrological forcing factors, both preparatory and triggering, in cliff evolution. Work concentrated mainly on Cap Gris-Nez (Battiau-Queney et al., 2003; Pierre, 2005b, 2006, 2015), Cap Blanc-Nez (El Khattabi et al., 2017; Lahousse and Pierre, 2003; Pierre and Lahousse, 2003, 2004, 2006; Pierre, 2007, 2015), and to a lesser extent around Équihen-Plage (Pierre, 2008a, 2008b) and the northern Bas-Boulonnais (Pierre, 2006, 2007, 2008a). The methodologies applied combined diachronic analyses of coastline position, based on cadastral maps (1812 onwards) and aerial photographs (from 1939), with *in situ* observations as well as hydrodynamic and geophysical measurements. These complementary approaches made it possible to document both average retreat rates and the processes driving erosion, whether continuous (e.g., gullying, surface runoff, small mass movements) or catastrophic (e.g., large rockfalls or collapses triggered by exceptional hydrometeorological events).

Despite their importance, these works remain the last comprehensive studies covering cliff morphodynamics at the scale of the entire Boulonnais coastline. Since then, only sporadic research has been undertaken, mainly by regional or governmental services such as CEREMA, without establishing an integrated monitoring framework. The potential of new tools such as 3D datasets from airborne LiDAR or drone-based photogrammetry, widely adopted since the 2010s (Brock and Purkis, 2009; Crapoulet et al., 2015; Roulland et al., 2019), has not yet been applied systematically to this coastline. Consequently, there has been no generalised, structured scientific monitoring over the past 25 years. A recent update by Blaise et al. (2022) provided new data on Boulonnais cliff evolution, but it relied essentially on the transect method, did not exploit the full set of available imagery, and excluded key areas such as Cap Blanc-Nez.

As in the case of research on sedimentary coasts, a variety of methods and tools have gradually been used, nowadays mostly in combination, drawing on a wide range of data sources: historical and recent iconographic documents (such as maps, cadastral plans, aerial photographs, orthophotographs, satellite images, and LiDAR-derived digital elevation models), as well as terrestrial and oblique photographs, postcards, and works of art. Field surveys were also carried out using measuring instruments such as total stations, GNSS (Global Navigation Satellite System), and drone-based digital elevation models (Cohen and Gardel, 2011; Cohen and Héquette, 2020).

The main objective of this paper is to provide an updated assessment of cliff retreat in the Pas-de-Calais department, a coastline that has not benefited from a comprehensive re-evaluation since the early 2000s. Beyond documenting recent changes, the article focuses on the methods and geomatic tools applied to coastal dynamics, with the aim of testing and illustrating their relevance at the scale of the Boulonnais cliffs. To this end, two complementary approaches were employed. First, a diachronic cartographic analysis of cliff-top retreat between 2000 and 2021 was conducted using three successive orthophotographs and the area-loss method, a technique now commonly applied to coastal cliff studies, but never before implemented for the Pas-de-Calais cliffs. Second, the study relied on LiDAR-derived DEMs (2008-2023), a type of dataset not previously exploited in this context, to carry out a morphological
analysis of erosion patterns. This approach allowed the identification of recent erosion sectors, six of which were investigated in greater detail through topographic transects, in order to characterise the gravitational processes responsible for their evolution.

## 2. Geological and geomorphological context of the cliffs of Pas-de-Calais department

The cliffs studied in this research are located on the coast of Pas-de-Calais, in the Hauts-de-France region, northern France (fig. 1). These cliffs extend for about 50 km (23 km of rocky coastline) (fig. 1C) and are located between two maritime lowlands: the Bas-Champs Picards to the south and the Flemish maritime plain to the north.

The cliffs of the Pas-de-Calais are of Jurassic age from Equihen-Plage to Cap Gris-Nez and of Cretaceous age in the area of Cap Blanc-Nez. In addition, there are a few more limited cliff sections cut into Pleistocene superficial deposits, notably at Sangatte and at the Dune Rousse cliff. The rocky coasts can be divided into four distinct zones (fig. 1C).

The first zone corresponds to the cliffs of Cap Blanc-Nez, which extend for about 6 km along the North Sea coast. Their height varies from about 15 m to more than 100 m at Cap Blanc-Nez (fig. 2A-C). These cliffs are characterised by a relatively steep and rugged relief (fig. 2A, B). In the central part, they are mainly composed of Cretaceous white chalk, while to the west near Strouanne, they have a clayey-chalk composition (fig. 2C). To the east near Sangatte, they are composed of sandy-gravelly detritus derived from slope deposits (fig. 2A). The contact between the two formations corresponds to an ancient Pleistocene marine cliff, which is obliquely cut by the current coastline (Daburon et al., 2017). These formations are highly porous and often traversed by groundwater flows, increasing their fragility and promoting the development of instabilities. The cliffs are also cut by some crans, a local French term for "hanging valleys" which are usually dry, although some may be temporarily drained by streams. At the base of the cliffs there is a wide coastal platform, usually covered with sand, although coarser boulders from cliff erosion can also accumulate there.

Zones 2, 3 and 4, extending over 7 km, 5.5 km, and 4.5 km, respectively, correspond to the cliffs of the Bas Boulonnais, which are incised into Jurassic sandstones and clays, particularly Kimmeridgian clays, making them highly susceptible to landslides and collapses. Except for the section between Pointe de la Courte dune and Cap Gris-Nez, which faces north, the cliffs of the Bas Boulonnais face the English Channel. Lower than the cliffs of Cap Blanc-Nez, they rise only a few tens of metres above sea level, reaching about 30 m at Cap Gris-Nez and up to 50 m at Cap d'Alprech (fig. 2J), Pointe de la Crèche and Cap Gris-Nez. The cliff line is interrupted in several places, notably by the Slack dune system, which extends for 2 km and blocks the estuary of the same name, and by several coastal towns: Audresselles, Ambleteuse, Wimereux and the port of Boulogne-sur-Mer. In addition, the incised valleys of the Wimereux and the Liane briefly interrupt the continuity of the rocky escarpment, but only over relatively short distances (fig. 1C). Here too, several crans follow one another, particularly between Cap Gris-Nez and Audresselles, and between Cap d'Alprech and Equihen-Plage.

Between Pointe de la Courte dune and Audresselles, the cliffs are steep and overlook a rocky foreshore composed of landslide debris (sandstone, limestone, marl) (fig. 2D-F). At the foot of the cliffs, large Jurassic sandstone boulders (the result of the calcareous cementation of quartz sands) form rock accumulations. Known locally as œufs (eggs), these blocks range in size from one metre to several metres (fig. 2E). The cliffs extending from north of Wimereux to Équihen-Plage have a lower relief, with elevations generally lower than those of Cap Gris-Nez and Cap Blanc-Nez. Composed of alternating Jurassic sandstones and marls, they have been subject to differential erosion, creating a landscape that varies greatly from one area to another (fig. 2G-L). The foreshore here is predominantly rocky (as in the case of Pointe aux Oies) (fig. 2H), composed of sandstone outcrops and clay-marl deposits resulting from cliff erosion. In some places, sandstone blocks, detached by erosion and sometimes cemented by calcium precipitation, accumulate at the foot of the slopes, partially protecting them from wave action. This area is also characterised by a variety of profiles, alternating between cliffs (to the north and south of Wimereux), benches (Pointe de la Crèche) (fig. 2I) and even rock steps (Équihen-Plage) (fig. 2L), influenced by the nature of the geological formations and by continental and marine weathering processes.

Cliff erosion is the result of a complex interaction between forcing agents of subaerial, marine and internal origin. These agents can be classified into two categories: preparatory factors, which contribute to the cliff's gradual weakening, and triggering factors, which cause its sudden collapse. Preparatory factors include physicochemical weathering processes - such as dissolution (Knight, 2025), frost shattering (Hansom et al., 2014; Letortu et al., 2014) for example), as well as water infiltration (Pierre and Lahousse, 2006), which alters the mechanical properties of rocks. Internal stress redistribution and decompression processes also play a role (El Khattabi et al., 2017). Triggering factors are relatively sudden events, such as heavy rainfall (Duperret et al., 2004; Pierre and Lahousse, 2006) or freeze-thaw cycles (Matsuoka, 2008; Letortu et al., 2019). In the short term, marine forces such as swells and storms remove or redistribute material accumulated at the base of cliffs, a process observed at Cap Blanc-Nez and in many other sectors. In some cases, storm waves can also directly attack the cliff toe, particularly where rocks are less resistant and when high water levels favour marine action. These factors are difficult to quantify, as they require repeated field campaigns and the instrumentation of pilot sites with diverse equipment, which makes predicting cliff retreat episodes particularly challenging. Over the longer term (spanning several years to decades), these processes gradually weaken the base of the cliffs, leading to the formation of basal notches and to the erosion and lowering of the rocky coastal platform. This erosion results from a combination of mechanical abrasion by waves and sediment, chemical dissolution of carbonate rocks and salt weathering linked to spray and tidal wetting-drying cycles.

Meanwhile, continental influences, particularly rainfall through runoff and infiltration and freeze-thaw cycles, play a key role in cliff-top erosion and in promoting mass movements such as translational and rotational landslides, as observed at Cap Gris-Nez (fig. 3C) and south of Équihen-Plage, as well as rockfalls and collapses, notably at Cap Blanc-Nez and near Wimereux (fig. 3E, F). These dynamics are closely linked to the geological composition and structural characteristics of the materials that make up the cliffs.

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 1** – **Location of Pas-de-Calais coastal cliffs.** A: national scale; B: regional scale; C: local scale. Topo-bathymetric map of the northern coast of Pas-de-Calais (BD Topo IGN - Institut national de l'information géographique et forestière, bathymetric DTM from the SHOM - Service Hydrographique et Océanographique de la Marine).</figcaption></figure>

1. Location of cliffs on the coastline; 2. Dunes; 3. Urban areas; D. Geologic map modified from Mansy (2003); 4. Quaternary sediments; 5. Tertiary; 6. Turonian-Senonian (upper Cretaceous - chalk); 7. Cenomanian (upper Cretaceous); 8. Upper Aptian - Albian; 9. Lower Cretaceous (Wealdian); 10. Upper Jurassic; 11. Middle Jurassic; 12. Palaeozoic; 13. Observed faults; 14. Hypothetical faults.

**Fig. 1** – **Localisation des falaises littorales du Pas-de-Calais.** A : échelle nationale ; B : échelle régionale ; C : échelle locale. Carte topo-bathymétrique de la côte nord du Pas-de-Calais (BD Topo IGN - Institut national de l'information géographique et forestière ; MNT bathymétrique du SHOM - Service hydrographique et océanographique de la Marine).

1. Localisation des falaises sur le littoral ; 2. Dunes ; 3. Zones urbaines ; D. Carte géologique modifiée d'après Mansy (2003) ; 4. Séiments quaternaires ; 5. Tertiaire ; 6. Turonien-Sénonien (Crétacé supérieur - craie) ; 7. Cénomanien (Crétacé supérieur) ; 8. Aptien supérieur - Albien ; 9. Crétacé inférieur (Wealdien) ; 10. Jurassique supérieur ; 11. Jurassique moyen ; 12. Paléozoïque ; 13. Failles observées ; 14. Failles supposées.
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**Fig. 2** – Diversity of geological and morphological features of the Pas-de-Calais cliffs. A: soft cliffs (slope deposits) west of Sangatte; B: high white chalk cliffs of Cap Blanc-Nez; C: marl chalk cliffs near Strouanne; D: clayey-sandy cliffs of the Pointe de la Courte dune, erupting large sandstone blocks called “eggs” because of their shape; E: Cap Gris-Nez cliffs composed of lower shales and sandstones intercalated with marls and clays; steep fronting ramp covered with collapsed sandstone boulders protecting the base of the cliffs; F: cliffs of Cran Noirda: Marls, clays in significant proportions and resistant limestones; G: clayey-sandy cliffs of Dune Rousse; H: cliffs of Pointe aux Oies: marls, clays and laminated limestones; under the effect of the run-off, ponds filled with silt are exposed on the upper rocky platform; I: cliffs to the north of Pointe de la Crèche, composed mainly of clays and sandstones; J: cliffs of Portel, composed of marls and clays, with sandstones and limestones in smaller proportions; K: Cap d'Alprech, composed of marls, siltstones and calcareous sandstones, with outcrops of resistant ferruginous sandstones; L: cliffs of Equihen-Plage, composed mainly of marls and clays, but also of siltstones, calcareous sandstones and a smaller proportion of ferruginous sandstones. Photos: E. Blaise (2024), except E: J. Braukmann (2019) and L: A. Vivier-Boudrier (2024).

**Fig. 2** – Caractéristiques géologiques et morphologiques diverses des falaises du Pas-de-Calais. A : falaises meubles (dépôts de versant) à l'ouest de Sangatte ; B : hautes falaises de craie blanche du Cap Blanc-Nez ; C : falaises marno-crayeuses (principalement cénomanienne aux abords de Strouanne ; D : falaises argilo-gréseuses de la pointe de la Courte-Dune libérant des macro blocs de grés en forme “d'oeufs” ; E : falaises du Cap Gris-Nez composées de schistes et de grès intercalées de marnes et d'argiles ; versant sub-horizontal et cordons de blocs protégeant le bas de falaise ; F : falaises du Cran Noirda : marnes, argiles en proportions importantes, et calcaires résistants ; G : falaises argilo-sableuses de la Dune Rousse ; H : falaises de la pointe aux Oies : marnes, argiles et calcaires étagés ; sous l'effet du ruissellement des flaques chargées de limons sont visibles sur le haut du platier rocheux ; I : Falaises au nord de la pointe de la Crèche composées en grande partie d'argile et de grés portant le même nom ; J : falaises du Portel composées de marnes et d'argiles, ainsi que de grés et de calcaire en proportions moindres ; K : Cap d'Alprech composé de marnes, siltites et calcaires gréseux, et affleurements de grès ferrugineux résistants ; L : falaises d'Equihen-Plage principalement composées de marnes et argiles, mais aussi de siltites, de calcaires gréseux ainsi que de grés ferrugineux dans une moindre mesure. Photos : E. Blaise (2024), sauf E : J. Braukmann (2019) et L : A. Vivier-Boudrier (2024).
<figure><img src="image_10.png" /><figcaption>**Fig. 3** – Examples of cliff erosion and retreat processes observed on the study cliffs. A: non-permanent basal notch in detrital soft rock at Sangatte; B: gully erosion of the upper part of the cliff at Sangatte; C: translational landslide on the clay-sandstone slope at Pointe de la Courte Dune with a frontal block accumulation; D: rotational landslide in the clay-sand cliff at Dune Rousse; E: progressive rockfall at Pointe aux Oies; F: major rockfall with debris cone at Cap Blanc-Nez. Photos : E. Blaise 2024 (except D: E. Blaise 2022).</figcaption></figure>

<caption>**Fig. 3** – *Exemples de processus d'altération et d'érosion sur les falaises étudiées.* A : encoche basale non pérenne dans la roche détritique meuble à Sangatte ; B : ravinage du haut de falaise à Sangatte ; C : glissement de terrain translationnel dans le versant argilo-gréseux à la pointe de la Courte Dune avec amas de blocs frontal ; D : glissement de terrain rotationnel dans la falaise argilo-sableuse de la falaise de la Dune Rousse ; E : éboulement progressif à la Pointe aux Oies ; F : éboulement majeur et cône de débris au Cap Blanc-Nez. Photos : E. Blaise 2024 (sauf D : E. Blaise 2022).</caption>

# 3. Data and methods

## 3.1. Diachronic analysis of cliff evolution between 2000 and 2009

### 3.1.1. *Selection of the morphological reference indicator*

This preliminary step is essential for any diachronic study using photo-interpretation of coastal kinematics, regardless of the type of coastline. The chosen indicator must be identifiable in all the documents being compared (Boak and Turner, 2005). For coastal cliffs, the morphological indicator commonly used can vary between study sites and authors, ranging from the base of the cliff to the top of the cliff, also known as the “cliff top” (Costa, 1997; Lim et al., 2005; Pierre and Lahousse, 2006; Young et al., 2009; Letortu et al., 2014). This choice is explained by the morphology of certain areas, where the slope profile, sometimes concave and very gentle, does not always

allow a precise representation of the actual variations in the position of the cliff face. As a result, the “cliff top” used as a reference indicator in this study corresponds to the most prominent summit break in slope of the landscape, and is not necessarily the highest. This ultimately introduces a variable and unpredictable margin of error inherent in the operator’s ability to identify this indicator on aerial photographs, the quality of which can also vary. The digitization of the cliff top was carried out directly on screen, at a scale between 1:500 and 1:1000.

### 3.1.2. *Data sources and estimation of uncertainties*

In order to estimate the distances and rates of retreat of the cliffs of Pas-de-Calais, three series of orthophotographs (2000, 2009 and 2021) were used as documents for the four study areas (23 km) (tab. 1), with a spatial resolution ranging from 20 to 50 cm/pixel.

The aerial photographs were delivered ortho-rectified and geo-referenced, and unfortunately, we had no control over the geo-
<caption>Tab. 1 – Overview of data and dates selected.</caption>

<table><caption>Tab. 1 – Synthèse des données et dates retenues.</caption><thead><tr><th></th><th>Year</th><th>Provider</th><th>Shooting date or period</th><th>Data type</th><th>Spatial resolution per pixel</th><th>Vertical error margin of the LiDAR surveys</th></tr></thead><tbody><tr><td rowspan="3">2D</td><td>2000</td><td>IGN</td><td>06/05/2000</td><td>Orthophotograph</td><td>50 cm</td><td></td></tr><tr><td>2009</td><td>Géo2France</td><td>June to August 2009</td><td>Orthophotograph</td><td>20 cm</td><td></td></tr><tr><td>2021</td><td>IGN</td><td>06/09/2021</td><td>Orthophotograph</td><td>20 cm</td><td></td></tr><tr><td rowspan="4">3D</td><td>2008</td><td>DDTM62</td><td>08/05/2008</td><td>LiDAR survey</td><td>50 cm</td><td>0.15 m</td></tr><tr><td>2013</td><td>CLAREC</td><td>06/12/2013</td><td>LiDAR survey</td><td>100 cm</td><td>0.1</td></tr><tr><td>2020</td><td>ROLNHdF</td><td>21/09/2020</td><td>LiDAR survey</td><td>100 cm</td><td>0.1</td></tr><tr><td>2023</td><td>IGN</td><td>15/02 to 02/05/2023</td><td>LiDAR survey</td><td>50 cm</td><td>0.04 m</td></tr></tbody></table>

IGN : French National Geographic Institute

Géo2France : Region Hauts-de-France Geographical database

DDTM : Departmental Directorate for Territories and Sea

CLAREC : Airborne Laser Scanning for Coastal Environmental Risk Assessment project

ROL : Coastal Monitoring Network of Normandy and Hauts-de-France administrative regions

referencing process, nor any information about the root mean square (RMS) errors induced by this rectification work. Despite the overall good accuracy, distortions caused mainly by cliff height (parallax errors on already rectified and georeferenced images), as well as other factors such as photo resolution and digitisation errors (Boak and Turner, 2005; Crowell et al., 1991; Durand, 1998; Moore, 2000; Robin, 2002; Juigner et al., 2012; Thior et al., 2021), can lead to significant error margins, potentially reaching several metres, with a maximum of ±3.6 m in certain areas (tab. 2). Parallax, in particular, is the main limitation of the data used in our study. This effect, which results from the fact that aerial photographs are taken from a particular angle, is particularly noticeable in landscapes with a pronounced relief, such as coastal cliffs. Landscape features at different heights appear to shift differently depending on the position of the camera, creating visible distortions from one photograph to the next. This parallax phenomenon complicates the analysis of

cliff position changes, as the apparent displacements may result from angular distortion rather than actual landscape evolution. For this reason, the orthophotographs from 2005 and 2012, which are available for our study sites, were not used in this study as the parallax-related distortions in these images would have affected the accuracy of the results. In order to minimise this distortion and ensure the reliability of the analyses, we have selected from among the most recent orthophotographs those that benefited from the best georeferencing and the most accurate rectification possible, while recognising the inherent limitations of this phenomenon. This choice meant that some recent orthophotographs had to be set aside.

The vertical error margins of the LiDAR surveys range from 0.15 m (2008) to 0.05 m (2023) according to the data providers. Therefore, the evolutions detected on the DEM comparisons will be considered as insignificant changes or stability when inferior to 0.15 m and will not be displayed on the DEMs of difference.

<caption>Tab. 2 – Overview of estimated errors on digitised shorelines.</caption>

<table><caption>Tab. 2 – Synthèse des erreurs estimées sur les traits de côte digitalisés.</caption><thead><tr><th colspan="4">ZONE 1</th><th colspan="4">ZONE 2</th></tr><tr><th>A - Year</th><th>2000</th><th>2009</th><th>2021</th><th>A - Year</th><th>2000</th><th>2009</th><th>2021</th></tr></thead><tbody><tr><th>Pixel error</th><td>0.50</td><td>0.20</td><td>0.05</td><th>Pixel error</th><td>0.50</td><td>0.20</td><td>0.05</td></tr><tr><th>XY offset error</th><td>0.70</td><td>0.40</td><td>-</td><th>XY offset error</th><td>1.20</td><td>1.30</td><td>-</td></tr><tr><th>Digitalisation error</th><td>1.00</td><td>0.40</td><td>0.10</td><th>Digitalisation error</th><td>1.00</td><td>0.40</td><td>0.10</td></tr><tr><th>Total error</th><td>1.32</td><td>0.60</td><td>0.11</td><th>Total error</th><td>1.64</td><td>1.37</td><td>0.11</td></tr><tr><th>B - Period</th><th>2000-2009</th><th>2009-2021</th><th>2000-2021</th><th>B - Period</th><th>2000-2009</th><th>2009-2021</th><th>2000-2021</th></tr><tr><th>Total error (m/yr)</th><td>0.16</td><td>0.05</td><td>0.06</td><th>Total error (m/yr)</th><td>0.24</td><td>0.11</td><td>0.08</td></tr></tbody></table><table><thead><tr><th colspan="4">ZONE 3</th><th colspan="4">ZONE 4</th></tr><tr><th>A - Year</th><th>2000</th><th>2009</th><th>2021</th><th>A - Year</th><th>2000</th><th>2009</th><th>2021</th></tr></thead><tbody><tr><th>Pixel error</th><td>0.50</td><td>0.20</td><td>0.05</td><th>Pixel error</th><td>0.50</td><td>0.20</td><td>0.05</td></tr><tr><th>XY offset error</th><td>3.70</td><td>0.30</td><td>-</td><th>XY offset error</th><td>3.30</td><td>0.40</td><td>-</td></tr><tr><th>Digitalisation error</th><td>1.00</td><td>0.40</td><td>0.10</td><th>Digitalisation error</th><td>1.00</td><td>0.40</td><td>0.10</td></tr><tr><th>Total error</th><td>3.87</td><td>0.54</td><td>0.11</td><th>Total error</th><td>3.48</td><td>0.60</td><td>0.11</td></tr><tr><th>B - Period</th><th>2000-2009</th><th>2009-2021</th><th>2000-2021</th><th>B - Period</th><th>2000-2009</th><th>2009-2021</th><th>2000-2021</th></tr><tr><th>Total error (m/yr)</th><td>0.43</td><td>0.05</td><td>0.18</td><th>Total error (m/yr)</th><td>0.39</td><td>0.05</td><td>0.17</td></tr></tbody></table>

### 3.1.3. Methodology used for calculating retreat rates

Two main approaches are generally used to quantify shoreline kinematics: the so-called transect (or baseline) method and the area-loss method. The transect method, widely used in the scientific literature (Dolan et al., 1978, 1991; Fenster et al., 1993; Faye et al.,

2008; Suanez et al., 2012; Stéphan et al., 2019), has been facilitated since the 2000s by the development of the Digital Shoreline Analysis System (DSAS) by the USGS (U.S. Geological Survey) (Moore, 2000; Thielier et al., 2009). Initially programmed as an ArcGIS extension, this tool is now available as a standalone version (v6.0). It allows the automatic generation of transects perpendicular to a user-defined

<figure><img src="image_11.png" /></figure>
baseline and then analyses the shoreline evolution at different dates using different statistical methods (e.g., EPR, NSM, LRR). Thanks to this automation, DSAS has significantly streamlined and accelerated the methodological steps involved in estimating shoreline retreat rates.

However, several studies highlight a major limitation of the transect-based approach (Costa et al., 2003; Letortu et al., 2014): it relies on point-based measurements carried out only along the transects, which means that shoreline variation is only quantified at specific locations, without taking into account the changes that occur between these transects. As a result, this technique does not allow for continuous analysis along the entire study area, and certain events, such as isolated rockfalls on cliffs, may be underestimated or even missed if they do not occur precisely along a transect. In

contrast, the area-based calculation method provides a continuous measure of cliff retreat by quantifying the area lost between two dates. This approach involves comparing two polylines representing the position of the cliff top at different times. Unlike the transect method, it provides a more comprehensive view of coastal dynamics and is particularly well suited to areas affected by irregular events such as gravitational movements. It therefore serves as a more representative indicator of coastal erosion (Costa et al., 2003; Letortu et al., 2014). This method involves the creation of polygons from two consecutive positions of the cliff top (fig. 4); 429 polygons were created (Zone 1: 112; Zone 2: 132; Zone 3: 100; Zone 4: 84). These polygons were based on a preliminary automatic generation of transects perpendicular to the shoreline, spaced every 50 m, using the StationLine extension available in QGIS. This makes it

<figure><img src="image_12.png" /><figcaption>Fig. 4 - Method for quantifying the retreat of the cliff crest by calculating lost areas and volumes. Example in the north of Pointe de La Rochette (Base aerial photograph: IGN 2021).</figcaption></figure>

<caption>Fig. 4 - Méthode de quantification du recul de la crête des falaises par calcul des aires et des volumes perdus. Exemple au nord de la Pointe de la Rochette (Photographie aérienne : IGN 2021).</caption>
possible to quantify the area lost over 50 m sections of the cliff. The area values obtained are then weighted according to the length of coastline contained within each polygon and the time interval between the two cliff-top positions, which is 21 years for the longest period (2000-2021). In this study, the area loss calculation method was employed due to its enhanced reliability and capacity for uninterrupted calculation of retreat rates along the cliffs.

## 3.2. 3D morphological analysis by comparison of LiDAR data

The analysis of LiDAR surveys makes it possible to obtain the full 3D picture of the cliff topography and identify the type of landslides, from its toe to its top surface and not only a schematic view of the cliff edge movements. In order to complement the aerial photographs diachronic analyses, LiDAR datasets from 2008, 2013, 2020 and 2023 (tab. 1) were compared computing DEMs of Differences (DoDs). These DoDs show elevation differences, highlighting negative values indicating erosion on the cliff face, but also positive values corresponding to areas where eroded material has accumulated. The analysis was carried out in three stages.

First, volume evolutions from 2008 to 2020 were computed for each of the 429 compartments extending between the same 50 m equidistant transects than these for the lost areas method but delimited by the cliff toe on the 2008 DEM and the cliff edge in 2021 DEM digitized from aerial orthophotographs (fig. 4). Consequently, the resulting polygons differ slightly in shape and surface from those used in the area loss method, since they span the entire cliff face. The aim of this first analysis was to assess whether the lost areas results were consistent with the volume evolutions. This cross-validation was carried out by correlating the areas losses computed on the aerial photographs and the volume evolutions extracted from the DEMs comparisons.

In a second stage, the changes from 2008 to 2023 were analysed to provide an overall view of the morphological changes along the cliffs. This DEMs comparison allowed the identification of recent intense erosion sectors, i.e., where the cliffs have eroded significantly more than along the rest of the studied zone. Six of these sectors were then studied in more detail using intermediate LiDAR datasets. To provide a clearer description of the cliff slope changes and characterise the mass movements affecting these sectors, cross-shore topographic profiles were extracted from the DEMs of 2008, 2013, 2020 and 2023 and compared. These six sites are also representative of the different geological formations of the study area.

In a third stage, a focus was made on the Dune Rousse sector. This is a very original cliff in the region (fig. 3D): the most rapidly eroding one, composed of a thick layer of clay (11 m) topped with thinner layers of stratified sands and gravels (7 to 8 m, Buchwald and Antoine, 2024). Its evolution was analysed using date-to-date DoDs, volume calculations and topographic profiles comparisons.

# 4. Results and discussion

## 4.1. Cliff erosion rates between 2000 and 2021 derived from area-loss method

Cliff erosion can be analysed at different spatial and temporal scales (fig. 5). Over the entire study period (2000-2021), the average rate of cliff retreat is -0.26 m/year (±0.12 m/year uncertainty). However,

this overall value masks significant temporal variability. In general, the average cliff retreat appears to have been higher during the period 2000-2009, with an observed average of -0.34 m/year, but the estimated average uncertainty for this period is also high and nearly equal to the retreat values (±0.31 m/year), suggesting that, in this sector, the cliff remained relatively stable over the study period, with minimal erosion. Between 2009 and 2021, retreat rates are more moderate, averaging -0.20 m/year (±0.07 m/year). Significant spatial variability is also observed from one zone to another. Zone 1 and especially Zone 3 show the highest retreat rates, while Zones 2 and 4 show more moderate values. These results are consistent with those of earlier research conducted since the 1990s (Battiau-Queney et al., 2003; Lahousse and Pierre, 2003; Pierre and Lahousse, 2003, 2004, 2006; Pierre, 2005, 2015), which reported average retreat rates generally ranging from -0.01 to -0.38 m/year depending on site characteristics.

By comparison, results obtained over longer study periods (59 to 74 years) indicate values between -0.01 m/year and -0.38 m/year (Pierre, 2005a) (tab. 3). This difference could be explained by the temporal scale: long-term averages tend to smooth out abrupt retreats, whereas shorter-term observations capture more extreme events. Short-term calculations therefore appear more contrasted, better highlighting spatial variability and the impact of intense erosive events, which are partially averaged out in the long-term means.

### 4.1.1. Zone 1 – Sangatte to Strouanne (including Cap Blanc-Nez)

Between 2000 and 2021, retreat rates average -0.42 m/year (±0.16 m/year) between 2000-2009 and -0.20 m/year (±0.07 m/year) between 2009-2021. Some values at Cap Blanc-Nez are missing due to parallax distortions in the 2000 orthophotograph, which led to artificial anomalies and necessitated the exclusion of a number of segments. Despite this limitation, the data demonstrate that erosion was strongest in the 2000s, then slowed in the following decade.

The observed retreat rates are higher compared with those reported in previous studies, where the maximum was around -0.20 m/year over a 59-year period (tab. 3). However, as noted by Pierre (2005a), exceptional, punctual events can cause significant localized retreat (up to -40 m in 1993), while periods of intense rainfall can trigger widespread cliff recession, as in November 2000. Moreover, studies of cliff dynamics over medium to long-term periods tend to smooth out the impact of such singular events, thereby reflecting long-term trends more accurately in this specific context. This can explain why previous studies reported lower retreat rates. These studies also emphasized the contrast between processes: near Sangatte and Strouanne, soft slopes undergo continuous degradation (gullying and repeated falls), whereas the chalk cliffs of Cap Blanc-Nez experience sudden collapses and rockfalls, strongly influenced by the fracture network (El Khattabi et al., 2017). Hydrological control is critical: intense rainfall and the large volumes of water infiltrating the soil generate high pore pressures within the chalk, which can trigger abrupt failures at the cliff face, as described by the piston flow mechanism (Pierre and Lahousse, 2006). Overall, our results confirm both the temporal variability of the retreat in this sector and the dual erosion dynamics already documented in the literature.

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 5 – Retreat rates (m/year) of the Pas-de-Calais cliffs (2000-2021), calculated using the area-loss method and averaged over 50 m of coastline. The table shows the mean values in each zone, for the periods 2000-2009 and 2009-2021; the graph represents the distribution of the raw values obtained for 2000-2021.</figcaption></figure>

1. Sangatte; 2. Mont Roty; 3. Cap Blanc Nez to Petit Blanc Nez; 4. Cran Barbier; 5. Cran Poulet; 6. Dune Rousse cliff; 7. Pointe aux Oies to Wimereux; 8. Wimereux to Pointe de la Crèche; 9. Equihen-Plage.

<caption>Fig. 5 – Vitesses de recul (m/an) des falaises du Pas-de-Calais (2000-2021), calculées à partir de la méthode des aires perdues, et moyennées pour 50 m de linéaire. Dans le tableau, les valeurs sont données par zone, pour les périodes 2000-2009 et 2009-2021 ; le graphique représente la distribution des valeurs brutes obtenues pour 2000-2021.</caption>

1. Sangatte ; 2. Mont Roty ; 3. Du Cap Blanc Nez au Petit Blanc Nez ; 4. Cran Barbier ; 5. Cran Poulet ; 6. Falaise de la Dune Rousse ; 7. De Pointe aux Oies à Wimereux ; 8. De Wimereux à la Pointe de la Crèche ; 9. Equihen-Plage.

<caption>Tab. 3 – Comparison between the results obtained in this study over short timescales and those previously reported and synthesised in Pierre (2005a) over the medium term (59-74 years).</caption>

<table><caption>Tab. 3 – Comparaison des résultats obtenus dans le cadre de notre étude, sur des temps courts, avec les résultats précédemment et synthétisés dans Pierre (2005a), sur le moyen terme (59-74 ans).</caption><thead><tr><th rowspan="2">Zone</th><th colspan="3">Present study</th><th colspan="2">(Pierre, 2005a)</th></tr><tr><th>Mean retreat 2000-2009 (m/yr)</th><th>Mean retreat 2009-2021 (m/yr)</th><th>Mean retreat 2000-2021 (m/yr)</th><th>Area</th><th>Mean retreat (m/yr)</th></tr></thead><tbody><tr><td rowspan="3">1</td><td rowspan="3">-0.42</td><td rowspan="3">-0.26</td><td rowspan="3">-0.35</td><td>dépôts de versant Sangatte</td><td>-0.29 (59 ans)</td></tr><tr><td>craie Blanc-Nez</td><td>-0.17 (63 ans)</td></tr><tr><td>craie/argile Strouanne</td><td>-0.20 (63 ans)</td></tr><tr><td rowspan="2">2</td><td rowspan="2">-0.14</td><td rowspan="2">-0.22</td><td rowspan="2">-0.15</td><td>grès Gris-Nez (N)</td><td>-0.01 (59 ans)</td></tr><tr><td>grès/argile Griz-Nez (W)</td><td>-0.07 (59/64 ans)</td></tr><tr><td>3</td><td>-0.54</td><td>-0.26</td><td>-0.38</td><td>-</td><td>-</td></tr><tr><td rowspan="3">4</td><td>-0.27</td><td>-0.07</td><td>-0.14</td><td>argile/grès Alprech</td><td>-0.17 (74 ans)</td></tr><tr><td colspan="2">Average for the entire study area</td><td rowspan="2">-0.26</td><td>argile/grès/argile Equihen</td><td>-0.38 (74 ans)</td></tr><tr><td>-0.34</td><td>-0.20</td><td>grès et calcatoire Le Portel</td><td>&lt;-0.04 (74 ans)</td></tr></tbody></table>

### 4.1.2. Zone 2 – Cap Gris-Nez to Ambleteuse

Our analysis shows lower retreat rates (-0.15 m/year on average, 2000-2021), with almost negligible values in many sectors when uncertainties are considered. This sector exhibits the lowest retreat rates. This observation had already been noted

in previous studies (Battiau-Queney at al., 2003; Pierre, 2005a, 2006, 2015), although the retreat values measured then were even lower (ranging from -0.01 to -0.07 m/year over 58 years) (Pierre, 2005a) (tab. 3). These studies also highlighted the role of the more resistant sandstone layers, as well as the protective effect of the steep rocky coastal platform, sometimes covered
with large rock blocks, which attenuate the impact of incoming waves during high water levels.

Nevertheless, some sectors of Zone 2 are more exposed to erosion. At Cran Poulet (fig. 6B), results from 2000 to 2021 show more pronounced erosion, consistent with the historical impacts of pebble extraction (1885-1945), which removed the protective ridge and reactivated a previously stable cliff (Pierre, 2006, 2008a). At Cran Barbier, structural weaknesses caused by World War II shelling (Pierre, 2005b) and highly fractured rocks make the sector prone to suffosion and subsurface water erosion (fig. 6A). These anthropogenic and structural factors explain the local deviations from the generally low retreat rates observed across Zone 2.

### 4.1.3. Zone 3 – Ambleteuse to Boulogne-sur-Mer

Zone 3 shows the highest rates of retreat along the studied cliffed coastline (-0.38 m/year, 2000-2021; up to -0.54 m/year during

2000-2009). Surprisingly, it has received little detailed attention in previous studies, which have only provided broad values from the French National Coastal Erosion Indicator (Hédou et al., 2018: -0.5 m/year on average). Our results provide new precision and confirm that this area is particularly vulnerable. Unlike other relatively stable sectors, such as the occasionally protected Cap Gris-Nez, or Cap Blanc-Nez proper, composed of resistant chalk, Zone 3 is mainly made up of clays, marls, and sands, which are far less cohesive and therefore prone to collapses, landslides, and gullying. Sites such as the Dune Rousse cliff illustrate this instability with recurrent rotational landslides (Buchwald and Antoine, 2024; Blaise et al., 2022). Moreover, the limited protection provided by littoral platforms or pebble ridges, which are only sparsely present, increases the exposure of the cliff base to marine attack, especially during storms.

In addition, Zone 3 includes a higher number of urban sectors exposed to erosion, notably between Pointe de la Rochette and

<figure><img src="image_15.png" /><figcaption>**Fig. 6** – Vertical aerial comparison of cliff segments in Zone 2. A: fragilization of the cliff crest due to Second World War bombings at Cran Barbier; B: narrow and low pebble ridge at the top of the beach, providing only limited natural protection to the cliff at Cran Poulet; C: presence of a protective pebble ridge at Cran Mademoiselle. Photos IGN (2021).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 6** – Comparaison aérienne verticale de certains segments de falaises au sein de la Zone 2. A : fragilisation de la crête de la falaise par les bombardements de la Seconde Guerre mondiale au niveau du Cran Barbier ; B : cordon de galets en haut de plage, étroit et peu élevé, offrant une protection naturelle limitée à la falaise au niveau du Cran Poulet ; C : présence d'un cordon de galets protecteur au niveau du Cran Mademoiselle. Photos IGN (2021).</figcaption></figure>
the northern part of Wimereux, where roads and buildings are located at the cliff edge or just a few meters from it, with retreat rates reaching up to 1.6 m/year between 2000 and 2009, and more generally around 0.80 m/year over the 2000-2021 period. Overall, Zone 3 emerges as one of the areas most affected by retreat, where geological fragility and the absence of natural protection coincide.

### 4.1.4. Zone 4 – South of Boulogne-sur-Mer (Le Portel to Équihen-Plage)

The observed retreat rates on the cliffs of Zone 4 are generally low (-0.14 m/year on average, 2000-2021) and often fall below the measurement uncertainty. These observations contrast with those reported in previous studies, where retreat at Équihen-Plage could reach -0.38 m/year (over a longer time span of 74 years). From a methodological perspective, it is important to highlight the limitations of photo-interpretation on irregular and vegetated slopes: dense vegetation often obscures the cliff top, and the slope is less sharply defined than in the case of vertical cliffs, making 2D analysis particularly challenging.

Over short periods, translational or rotational landslides affecting the cliffs of Zone 4, which give rise to a stepped and terraced morphology, primarily displace the intermediate or lower parts of the cliff without causing significant retreat of the cliff top. However, over longer periods or during major landslides, the cliff top can also retreat, contributing to the overall evolution of the cliff front. This differs from the spectacular collapses observed at Cap Blanc-Nez but nevertheless contributes to long-term retreat. The social consequences are significant: many houses have been abandoned or demolished, and World War II bunkers have disappeared (Pierre, 2008b). At Équihen-Plage, earlier estimates reached -0.42 m/year (1929-2003), confirming that even moderate long-term averages can conceal substantial episodic instabilities.

## 4.2. Analysis of eroded cliff volumes using LiDAR data

### 4.2.1. Results of the volume-loss analysis

Figure 7A provides a synthetic overview of the situation, showing the volumes of erosion within the 429 boxes previously defined. As with the area-loss method, there is a high degree of variability along the coastline. Zone 1, which includes the Cretaceous chalk formations of Cap Blanc-Nez and the softer Pleistocene deposits to the south-west (Petit Blanc-Nez) and the north-east of the cape (see below), is clearly the area most affected by erosion. As in the other zones, there is a significant variability along the length of the area: highly eroded areas may be adjacent to more stable ones. The sector n°40 (Mont Roty), between the Cap Blanc-Nez and Sangatte, with a large volume loss provides a clear example along a short stretch of coast: it experienced a localised landslide, whereas sectors 39 and 41 remained more stable. This sector was also previously identified by G. Pierre (2005a) as highly dynamic with sudden collapses and rockfalls, as mentioned above.

In Zones 2, 3 and 4 of the Jurassic Bas-Boulonnais, erosion volumes are generally lower. However, there is still significant spatial variability, with moderate erosion occurring in most sectors and more severe retreat in some others. These include the Dune

Rousse cliff, the southern part of Pointe de la Rochette and areas south of Cap d'Alprech and Equihen-Plage (fig. 7A).

The two datasets are linked in Figure 7B. The purpose is not to interpret eroded volumes as a function of erosion areas, but to compare them. The coefficient of determination ($r^2$ = 0.37) explains that, in only 37 % of cases, area-loss values are proportional to eroded volumes. If the correlation was perfect ($r^2$ = 1, $r^2$ can range from 0 to 1, or from 0 to 100 %), it could be assumed that the two results are fully linked, and therefore redundant and that the area-loss method is a convenient and precise way to assess cliff erosion, for example when 3D data are missing. However, in this case, the relatively weak relationship suggests that the two datasets offer distinct yet complementary insights. This relative low statistical relation can be partially explained by a one-year offset between the two measurement periods. But this is primarily because the first method only considers surface loss from the flat top of the cliff, whereas the second method considers the entire cliff. The volume depends on the escarpment's slope and height, not only on the changes occurring on the upper part of the cliff.

A simple analysis carried out in the four zones provides smaller scale results. Considering the two erosion indexes (area and volume losses, tab. 4), Zone 2 can be considered the most stable, as it has the smallest ranges of variation of all the zones. When excluding the data from the Dune Rousse (compartment n°248), Zone 3 is slightly more eroding. Zones 4 and especially Zone 1 are retreating more rapidly.

The graph on Figure 7B highlights the most eroded areas. These are sectors 37 and 40 between Sangatte and Cap Blanc-Nez (Zone 1) located in soft Pleistocene sandy-gravelly slope deposits, sectors 248 and 249 at the Dune Rousse (Zone 3) located in sand and clay deposits, and sectors 411 and 412 at Équihen-Plage (Zone 4) in cliffs composed of Jurassic marls and clays.

<caption>Tab. 4 – Minimum, maximum and ranges of variations of area and volume losses for Zones 1 to 4 (for Zone 3, data into brackets are calculated without compartment n°248, Dune Rousse sector).</caption>

<table><caption>Tab. 4 – Minimum, maximum et amplitudes de variation pour les aires et volumes perdus pour les Zones 1 à 4 (pour la Zone 3, entre parenthèses, résultats calculés sans prendre en compte le compartiment n°248, secteur de la Dune Rousse).</caption><thead><tr><th></th><th>Zone 1</th><th>Zone 1</th><th>Zone 1</th><th>Zone 1</th></tr></thead><tbody><tr><td>Min. area loss</td><td>12.98</td><td>0.12</td><td>4.91</td><td>0.01</td></tr><tr><td>Max. area loss</td><td>651.16</td><td>421.51</td><td>1219.92 (713.9)</td><td>489.29</td></tr><tr><td>Range of variation</td><td>638.18</td><td>421.39</td><td>1215.01 (708.99)</td><td>489.28</td></tr><tr><td>Min. volume loss</td><td>117.3</td><td>56.4</td><td>10.1</td><td>20.8</td></tr><tr><td>Max. volume loss</td><td>12280.5</td><td>5741.3</td><td>19868.1 (6435.3)</td><td>7533.6</td></tr><tr><td>Range of variation</td><td>12163.2</td><td>5684.9</td><td>19858 (6425.1)</td><td>7512.8</td></tr></tbody></table>

### 4.2.2. Comparison of the DEMs from 2008 and 2023

Comparing the 2008 and 2023 DEMs enables the analysis of cliff evolution at high spatial resolution (1 m²/pixel). This provides more details than the volume loss method. The graph at the bottom left of Figure 8 shows that 75 % of the cliffs face (in planar projected surface area) are experiencing erosion. A total of 37.8 ha (40 % of the cliff face's total area of 94.5 ha) are experiencing mild erosion (0 to -1 m³). On 26.8 ha (28.4 %), the erosion ranges from -1 to -5 m³. Only 6.2 ha (6.5 %) of the cliff face are affected by severe erosion (beyond
<figure><img src="image_17.png" /><figcaption>**Fig. 7** - Spatial distribution of volume losses (2008-2020) and correlation with areas losses (2009-2023). A: spatial distribution of volume losses between 2008 and 2020; B: correlation between volume losses (2008-2020) and area losses (2009-2023).</figcaption></figure>

<caption>**Fig. 7** - Distribution spatiale des volumes perdus (2008-2020) et corrélation avec les aires perdues (2009-2023). A : répartition spatiale des volumes perdus entre 2008 et 2020 ; B : corrélation entre les volumes perdus (2008-2020) et les aires perdues (2009-2023).</caption>

-5 m³). Around 25 % of the cliffs are experiencing accumulation, which corresponds to mass transfer along the slopes. This is detailed in the analysis of six specific examples below. The highest and most continuous erosion values are found in Zone 1, between Sangatte and the eastern part of Cap Blanc-Nez (fig. 2A), and between Cran d'Escalles and the Petit Blanc-Nez (fig. 2C). Severe erosion is also detectable in Zone 2, between the Pointe aux Oies (fig. 2H) and the Pointe de la Rochette. Elsewhere, erosion is more moderate, though there are some localised areas of significant settlement, such as near Cap Gris-Nez semaphore (fig. 2E), the Dune Rousse cliff (fig. 2G), and Equihen-Plage (fig. 2L). These results are consistent with the previous outcomes from the volume loss method.

### 4.2.3. Characterisation of cliff mass movements using topographic profiles

Comparing topographic profiles extracted from the 2008, 2013, 2020 and 2023 DEMs helps refine previous observations regarding slope morphodynamics. The profiles correspond to the transects defined for the area/volume-loss methods. Figure 9 illustrates the

topographic and volumetric evolution of six profiles in areas where significant erosion was identified. The small tables below the profile location maps (with differential DEMs from 2008 to 2023, colour captioned in Figure 8) illustrate simple and cumulative volumetric changes from one date to the next. Negative values indicate erosion, while positive values signify an increase in volume. This augmentation of volume can be explained by the accumulation of eroded material transferred from adjacent cliff segments. For example, during a landslide, material falling from the top of one segment can accumulate at the foot of a neighbouring segment. In some cases, this material may subsequently be redistributed by longshore current, which along the English Channel coast is oriented southward to northward during flood tide and northward to southward during ebb tide, as observed at the Dune Rousse cliff.

Profile A corresponds to transect 37 next to the Mont Roty. The upper part of the cliff here reaches an elevation of 50 m and is composed of easily erodable Pleistocene deposits, while the lower part consists of chalk. Moderate crest collapses are shaping the evolution of this area. The loose materials that accumulate at the base of the cliff appear to be quickly removed by wave action. At

<figure><img src="image_18.png" /></figure>
<figure><img src="image_19.png" /><figcaption>Fig. 8 – Comparison of the Digital Elevation Model of 2008 and 2023.</figcaption></figure>

<caption>Fig. 8 – Comparaison des Modèles Numériques de Terrain de 2008 et 2023.</caption>

Cap Blanc-Nez (transect 59), which is composed entirely of chalk, the cliff top reaches 100 m (profile B). It remained nearly stable from 2008 to 2020, losing only -40.5 m³. A significant collapse of the upper section occurred thereafter. The increase in profile volume in 2023 may be due to material being transferred from neighbouring segments. The role of longshore sediment transport associated with littoral drift has to be further investigated. A large talus cone has formed at the base. Evolution here occurs episodically through major landslides. This pattern is common along Cap Blanc-Nez, where smaller collapses are rapidly cleared by the sea while larger ones can persist for a long time, as observed after the 1993 collapse (Lahousse et al., 2000) and the 2000 event (Lahousse and Pierre, 2003).

The main area of erosion in Zone 2 (transect 145) is located next to Cap Gris-Nez lighthouse. The profile is characterised by a concave-convex geometry. The 48 m high cliff exhibited stability between

2008 and 2020, after which a translational slide occurred, resulting in the upper section of the cliff sliding downwards to form a ridge of rock blocks (fig. 2E). Profile D was extracted from transect 248 at the Dune Rousse. This cliff, as mentioned previously, is very distinctive in the study area, as it is composed of sands and clays. It seems to evolve through rotational sliding in multiple stages (fig. 3D) and is described more precisely in the next section.

At Pointe de la Rochette, in the north of Wimereux (profile E, transect 285), the cliff is made up of alternating layers of limestone and marl. This 37 m high cliff has been eroding continuously since 2008. The upper section, between 17 and 37 m, is eroding the most. The lower part is much more stable. Material losses occur through rockfalls, which are quickly cleared by the sea, thereby preventing the formation of talus cones.

At Equihen-Plage (profile F, transect 411), the cliff slope is more moderate than in other areas. The upper cliff section experienced
A

<figure><img src="image_20.png" /></figure>

C

<figure><img src="image_21.png" /></figure>

E

<figure><img src="image_22.png" /></figure>

B

<figure><img src="image_23.png" /></figure>

D

<figure><img src="image_24.png" /></figure>

F

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

**Fig. 9** – Comparison of topographic profiles extracted from DEMs calculated with LiDAR data. A: Mont Roty; B: Cap Blanc-Nez; C: Cap Gris-Nez; D: Dune Rousse; E: pointe de la Rochette; F: Equihen-Plage.

The tables under the profiles show the volume evolution in cubic metres by time periods and the cumulative evolutions from 2008.

**Fig. 9** – Comparaison de profils topographiques extraits de MNT calculés avec les données LiDAR. A : Mont Roty ; B : Cap Blanc-Nez ; C : Cap Gris-Nez ; D : Dune Rousse ; E : pointe de la Rochette ; F : Equihen-Plage.

Les tableaux sous les profils indiquent les évolutions volumétriques en mètres cube de ces profils dans chaque période de temps et les évolutions cumulées depuis 2008.

<figure><img src="image_27.png" /></figure>
a downslope movement between 2008 and 2023, which was likely a
translational slide.

The annual growth rates for profile volumes have been calculated by linear regression. They are shown on the graph for each profile as AAR (Average Annual Rate). These results clearly show that the Cap Gris-Nez site (fig. 9C) is the least affected by erosion, with an AAR of -0.1 m³/year. This is followed by Cap Blanc-Nez (fig. 9B, AAR = -1 m³/year), Mont Roty (fig. 9A, AAR = -4.4 m³/year) and Equihen-Plage (fig. 9F, AAR = -7.9 m³/year). The Pointe de la Rochette (fig. 9E) and Dune Rousse (fig. 9D) sites are experiencing severe erosion, with AAR values of -8.5 m³/year and -22.6 m³/year, respectively. This analysis was only conducted on topographical profiles. However, it would be worthwhile to apply it to larger areas in DEMs in three dimensions and refine it by studying not only the overall evolution of the cliffs, but also that of several areas on their slopes. This is illustrated by the following example of the Dune Rousse.

4.2.4. *The case of the Dune Rousse cliff*

The analysis of the profiles on Figure 9D shows several stages in the evolution of the rotational landslide. The first stage of sliding affects the upper section of the cliff, where retreat has been rapid. The edge has retreated by 23 m from 2008 to 2020 (with a cumulative volume loss of -244.94 m³ in the profile, at a rate of -19,7 m³/year), and by -12 m from 2020 to 2023 (with a volume loss of -142 m³ in only three years, at a rate of -54,4 m³/year). The sliding mass is temporarily stored on the mid-slope, where large ridges appear. Wave action clears the lower part of the slope, reducing the “toe” support effect. A second slide of the stored mass may then occur, with the aforementioned ridges descending toward the sea while deforming.

Parts A, B and C in Figure 10 are date-to-date DoDs displaying the cumulative evolution of the cliff from 2008 to 2023. A rapid translation of the severe erosion zone (shades of dark orange and red) to the east is clearly visible, thus corroborating the observations made on the topographical profiles and providing new information. There is an acceleration of erosion, but it is not uniform along the entire shoreline of this sector. The presence of an erosion lobe that is both deepening and shifting southwards is indicated on the maps by the straight arrows for eastward deepening and curved arrows for the south shift. Furthermore, small blue areas can be identified in these comparisons, corresponding to the effects of gravitational movements along the slope: sand deposits from the upper part of the cliff accumulate halfway down the slope. The lower part of the cliff is subject to ongoing erosion, albeit to a lesser extent than the upper section. This is confirmed by the analysis of the volumetric changes in the upper and lower parts of the cliff. In order to differentiate between these two upper and lower parts, a threshold was established at an altitude of 13 m (fig. 10E), situated just above the step located approximately halfway up the slope. The volumes were calculated using the DTM on either side of the threshold and within the area of the rectangle shown in the figure. In diagram F of the Figure 10, the raw values of change between each MNT are presented, while chart G illustrates the annual rates of evolution, a useful representation given the unequal lengths of the periods. It is obvious that the overall rate of change of the cliff (dark green

rectangles on the graphs) remained almost stable during the first
two periods and accelerated during the last one. It can also be
seen from the chart that erosion at the base of the cliff (light blue
rectangles) is slowing down, with even mild accretion occurring
during the last period. The next topographical surveys will help to
establish whether the lower section is stabilising or if the previous
stabilisation was only temporary. On the upper part of the cliff
(purple rectangles), erosion is accelerating. This decline in volume
is consistent with the deepening of the erosion lobe towards the east
and its extension towards the south. This accelerated erosion of the
upper part of the cliff results in a reduction in the overall slope of
the hillside.

This example of the Dune Rousse could therefore be conceptualised as a rotational landslide that is receding and extending. Given that the upper part of the cliff is wider than the lower part, it can be also deduced that this rotational landslide is not occurring on a homogeneous flat plane, but rather in an “inverted cone-shaped” lobe as illustrated in the block diagram (fig. 10D), showing the DoD from 2008 to 2023 overlaid on the 2023 DTM. The topographical features in the immediate vicinity of the site are of particular relevance to the present study. The northernmost extremity of the site is constrained by the dip and the outcrop of a limestone layer. It has been demonstrated that volumes erode at the upper end of the structure, consequently resulting in the concentration of the eroded material at the lower part of the cliff in a more confined area as indicated on the block diagram (fig. 10D) by the downward and converging arrows. Does the “inverted cone shape” contribute to blocking, accumulating and stabilising the lower part of the cliff? Are these processes due to less erosive marine conditions in recent periods, or did these forces get stronger to the point where almost all the material from the upper part of the cliff could be removed?

The observed differences in evolution between the upper and lower parts of this loose cliff illustrate the typical vertical partitioning of forcing factors on soft cliffs: atmospheric processes generally dominate the upper part, while marine influences prevail at the base. The evolution in time of these forcing conditions still needs to be investigated. In order to go beyond the definition of a conceptual model of evolution, the interactions between the two parts of the cliff also need to be better identified.

**5. Conclusion**

The coastal cliffs of Pas-de-Calais clearly demonstrate active
erosion. Area-loss calculations indicate that between 2000 and
2021, the cliffs experienced an average retreat rate of 0.26 m/year,
with an associated uncertainty of ±0.16 m/year. Although this
average value reflects the phenomenon's overall scale at a regional
level, it masks significant spatial and temporal disparities, which
are largely explained by the diversity of lithostratigraphic and
topomorphological characteristics. However, the results of this study
should be interpreted with caution. Methodological uncertainties
remain, particularly due to the variable quality of the orthophotos
used (notably those from 2000), some of which display significant
distortions that could bias shoreline identification and therefore
coastal retreat measurements. These limitations led to the exclusion
of some data (outliers labelled as "positive anomalies").
<figure><img src="image_28.png" /><figcaption>**Fig. 10** – Comparison of the DEMs calculated with LiDAR data on the Dune Rousse cliff. A, B, C: comparisons of the 2008 and 2013, 2008 and 2020, 2008 and 2023 DEMs; D: block diagram of the cliff; E: comparison of topographic profiles extracted from DEMs and 13 m elevation threshold; F, G: volume evolution and annual rates of change of the cliff for each time period.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Fig. 10** – Comparaison des MNT calculés avec les données LiDAR sur le site de la Dune Rousse. A, B, C : comparaisons des MNT de 2008 et 2013, 2008 et 2020, 2008 et 2023 ; D : bloc diagramme de la falaise ; E : comparaison des profils topographiques extraits des MNT et seuil à 13 m d'altitude ; F, G : évolution des volumes et des taux d'évolution de la falaise pour chaque période.</figcaption></figure>
Despite these issues, the implemented protocol provides robust quantitative insights into retreat rates. The 2D approach was based on the area-loss method, which is now considered more reliable than the traditional transect method, particularly when the cliff front is irregular or fragmented. This method was adapted for processing LiDAR data across the entire study area. LiDAR data are highly relevant today because they produce 3D results with a smaller margin of error than processing aerial photographs. These data also enable the study of the evolution of the cliff along the entire slope, from the base to the top, rather than just at the edge. This dual, complementary approach allows a shift from linear retreat values to quantifying volumes, thus providing a quantified morphodynamic analysis based on the calculation of volume losses (e.g., compartmentalised altitudinal differentials) and the identification of the types of gravitational movements involved.

The originality of this work lies in its local scale. The area-loss method had never before been applied to the cliffs of the Pas-de-Calais region. Furthermore, LiDAR data, which had been unavailable at the time of previous studies, are used extensively for the first time in this study. Beyond providing an updated assessment of cliff retreat, the study also demonstrates the complementarity of 2D and 3D approaches for documenting cliff dynamics at medium to short-term time scales. Future research should build on this foundation by combining volumetric analyses with a more detailed morphometric characterisation of the cliffs (height, slope, roughness). Furthermore, erosion-exposure indices similar to those developed by Prémaillon et al. (2018a) should be developed. The identification of vulnerable sectors and the anticipation of their future evolution under changing climatic and coastal conditions will be facilitated by this approach.

## Acknowledgements

We would like to thank the Service National d'Observation (National Observation Service) DYNALIT for its financial and material support, as well as the Réseau d'Observation du Littoral Normandie – Hauts-de-France (Coastal Monitoring Network of Normandy and Hauts-de-France administrative regions), providing the 2020 orthophotography and LiDAR data.

We would like to thank the undergraduate students in Geography at the Université du Littoral Côte d'Opale, as well as those in Life, Earth, and Environmental Sciences at the Université de Lille, who contributed, as part of their end-of-year internships, to producing some of the data used in the two-dimensional analysis.

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## Version abrégée en français

Les falaises du Pas-de-Calais (Hauts-de-France) (fig. 1) souffrent d'un déficit d'actualisation des connaissances et de données depuis les travaux de référence menés au début des années 2000. Ces falaises n'ont depuis pas fait l'objet d'une réévaluation globale intégrant les technologies géomatiques modernes. Cet article vise à combler cette lacune en proposant une nouvelle quantification du recul des falaises du Boulonnais. L'étude menée se distingue donc par l'utilisation de deux méthodologies complémentaires, inédites à l'échelle des falaises du Pas-de-Calais : (i) une analyse de la cinématique en 2 dimensions (2000-2021) utilisant la méthode des surfaces perdues, sur la base d'orthophotographies aériennes, permettant de dépasser les biais de la méthode linéaire traditionnelle ; (ii) une analyse morphologique en 3D (2008-2023) via l'utilisation de modèles 3 dimensions issus de levés LiDAR. Ce protocole permet non seulement d'actualiser les taux d'érosion, mais aussi de caractériser finement les processus gravitaires et les

volumes sédimentaires mobilisés sur les secteurs les plus actifs du point de vue de l'érosion.

Le littoral étudié se compose de 23 km de côtes rocheuses, marqué par une dichotomie lithostratigraphique marquée entre le Jurassique du Bas-Boulonnais et le Crétacé du cap Blanc-Nez. Des dépôts pléistocènes meubles (Sangatte, Dune Rousse) complètent cet ensemble divisé en quatre zones morphologiques. La Zone 1 (de Sangatte à Strouanne) (fig. 2A-C) se distingue par des falaises abruptes atteignant 100 m d'altitude au niveau du cap Blanc-Nez, composées de craie, d'argiles et de dépôts de versant dans certains secteurs. La porosité élevée et la présence de vallons suspendus (« crans ») y favorisent l'instabilité hydrogéologique. Les Zones 2 à 4 (du cap Gris-Nez à Équihen-Plage – le Bas-Boulonnais) (fig. 2D-L) incisent les alternances gréso-argileuses du Kimméridgien. Moins élevées (< 50 m) et localement urbanisées, ces falaises présentent des profils variés (gradins, banquettes). L'érosion différentielle y génère sur l'estran des accumulations de blocs gréseux (« œufs ») qui protègent partiellement le pied de versant. La dynamique érosive résulte d'une interaction complexe entre forçages marins et continentaux (fig. 3). Les facteurs préparatoires (altération physico-chimique, décompression, pressions interstitielles) affaiblissent la structure rocheuse, tandis que les événements météorologiques (pluies intenses, épisodes de gel-dégel) agissent comme déclencheurs des ruptures. L'action marine, notamment lors des tempêtes, participe au sapement basal et à l'évacuation des matériaux érodés, mais ne concerne cependant pas tous les secteurs puisque, lorsque l'inclinaison de la pente du platier est trop forte, la mer n'atteint pas le pied de la falaise, ou très rarement. La typologie des instabilités est particulièrement litho-dépendante : glissements dans les faciès argilo-marneux et éboulements plus soudains dans la craie ou les grès fracturés.

Cette étude met donc en lumière l'intérêt de combiner l'analyse de l'évolution du trait de côte en deux dimensions sur des photographies aériennes (de 2000 à 2021) avec une analyse tridimensionnelle réalisée grâce à la comparaison de Modèles Numériques de Terrain calculés à partir de levés par LiDAR (de 2008 à 2023) (tab. 1, 2). Cela permet de mieux appréhender la dynamique érosive récente de ce type d'environnements côtiers. La méthode des aires perdues a été utilisée pour mesurer les reculs de la crête de falaise (fig. 4A-D), tandis que les données LiDAR (fig. 4E, F) ont permis de calculer les volumes de matériaux érodés, permettant une lecture fine de la cinématique des falaises. Sur l'ensemble de la période, le taux de recul moyen du trait de côte est de -0,26 m/an (± 0,12 m/an) (fig. 5, tab. 3). Cette moyenne masque une importante variabilité temporelle, avec une érosion plus marquée entre 2000 et 2009 (-0,34 m/an) qu'entre 2009 et 2021 (-0,20 m/an). L'analyse met en évidence que les calculs sur le court terme capturent mieux les événements érosifs extrêmes, contrairement aux moyennes à long terme qui tendent à lisser ces ruptures. La variabilité spatiale est très forte aussi.

- Zone 1 (de Sangatte à Strouanne) : ce secteur présente des taux de recul élevés (-0,42 m/an sur 2000-2009 ; -0,35 m/an sur 2000-2021). L'érosion se manifeste de deux manières : dégradation continue des pentes meubles (Sangatte et Strouanne) et effondrements brutaux des falaises crayeuses (au cap Blanc-Nez), souvent déclenchés par de fortes pressions interstitielles (mécanisme de piston flow) ;

- Zone 2 (du cap Gris-Nez à Ambleteuse) : c'est la zone la plus stable
(-0,15 m/an sur 2000-2021), protégée par des grès résistants et un platier rocheux. Toutefois, des reculs ponctuels sont observés (Cran Poulet, Cran Barbier), exacerbés par des facteurs anthropiques historiques (extraction de galets, bombardements) ;

- Zone 3 (d'Ambleteuse à Boulogne-sur-Mer) : ce secteur enregistre les reculs les plus rapides (-0,54 m/an sur 2000-2009 ; -0,38 sur 2000-2021). La nature lithologique (argiles, marnes, sables) et l'absence de protection naturelle rendent ces falaises particulièrement vulnérables aux glissements rotationnels et au ravinement, menaçant directement les infrastructures urbaines ;

- Zone 4 (au sud de Boulogne-sur-Mer) : les taux mesurés sont faibles (-0,14 m/an sur 2000-2021), mais sans doute sous-estimés par la méthode 2D en raison de la végétation et de la morphologie en gradins des glissements, qui affectent davantage la mi-pente que le trait de côte sommital.

La comparaison des MNT (2008-2023) montre que 75 % de la superficie des falaises est en érosion. L'analyse des profils topographiques aide à caractériser la morphodynamique des types de falaise. Par exemple elle confirme la stabilité relative du cap Gris-Nez (Taux annuel moyen de -0,1 m³/an) et l'instabilité critique de la Dune Rousse (-22,6 m³/an) (fig. 9). Ce site est un bon exemple de glissement rotationnel. L'érosion s'accélère sur la partie supérieure de la falaise (perte de -142 m³ entre 2020 et 2023), fonctionnant comme un lobe d'érosion en « cône inversé » (fig. 10) qui s'approfondit vers l'est et s'étend aussi vers le sud. On observe un partitionnement vertical des forçages : les processus atmosphériques semblent dominer au sommet, tandis que l'action marine évacue les matériaux accumulés en pied de falaise, déstabilisant l'ensemble.

L'étude confirme le caractère actif de l'érosion des falaises du Pas-de-Calais, piloté par la diversité lithostratigraphique. Elle valide la pertinence du couplage méthodologique 2D/3D : l'usage du LiDAR permet de dépasser la simple mesure linéaire du recul pour caractériser les mouvements gravitaires sur l'ensemble du versant. Ces résultats soulignent la nécessité de développer des indices d'exposition à l'érosion pour anticiper l'évolution future de ces littoraux vulnérables.