# A decade of erosion monitoring of coastal chalk cliff faces at a seasonal frequency (2010–2021) using terrestrial lasergrammetry and photogrammetry (Varengeville-sur-Mer and Dieppe, Normandy, France): Technical evolution and main results

Pauline Letortu, Stéphane Costa, Robert Davidson, Olivier Maquaire, Bastien Peuziat, Guillaume Thirard, Laurence David, Pauline Clain, Benjamin Troadec

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Pauline Letortu, Stéphane Costa, Robert Davidson, Olivier Maquaire, Bastien Peuziat, et al. A decade of erosion monitoring of coastal chalk cliff faces at a seasonal frequency (2010–2021) using terrestrial lasergrammetry and photogrammetry (Varengeville-sur-Mer and Dieppe, Normandy, France): Technical evolution and main results. Geomorphology, 2025, 479, pp.109738. 10.1016/j geomorph.2025.109738. hal-05045723

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https://hal.univ-brest.fr/hal-05045723v1

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1 A decade of erosion monitoring of coastal chalk cliff faces at a seasonal frequency

2 (2010-2021) using terrestrial lasergrammetry and photogrammetry (Varengeville-sur-

3 Mer and Dieppe, Normandy, France): Technical evolution and main results

4 Pauline Letortu<sup>a*</sup>, Stéphane Costa<sup>b</sup>, Robert Davidson<sup>b</sup>, Olivier Maquaire<sup>b</sup>, Bastien Peuziat<sup>b</sup>,

5 Guillaume Thirard<sup>b</sup>, Laurence David<sup>c</sup>, Pauline Clain<sup>a</sup>, Benjamin Troadec<sup>a</sup>

6

7 <sup>a</sup>Univ Brest, CNRS, LETG, IUEM, F-29280 Plouzané, France; pauline.letortu@univ-brest.fr,

8 pauline.clain29@gmail.com, benjamin.troadec@etudiant.univ-brest.fr

9 <sup>b</sup>Normandie Univ, UNICAEN, CNRS, IDEES, F-14000 Caen, France;

10 stephane.costa@unicaen.fr, robert.davidson@unicaen.fr, olivier.maquaire@unicaen.fr,

11 bastien.peuziat@gmail.com, guillaume.thirard@hotmail.fr

12 <sup>c</sup>CNRS, LETG, IUEM, F-29280 Plouzané, France; laurence.david@univ-brest.fr

13

14 * Corresponding author. Tel.: +33 (0)290915545, pauline.letortu@univ-brest.fr

15

16 Highlights:

17 - Cliff face erosion has been monitored at two coastal sites in Normandy for a decade;

18 - Seasonal monitoring confirms the importance of marine actions in the erosion process;

19 - Rock falls accounted for 68% of total erosion, and debris falls accounted for 32%.

20

21 Abstract:

22 Every four to five months between October 2010 and July 2021, cliff face topography was

23 monitored with centimeter-scale precision at two sites in Normandy, each with a chalk cliff

24 linear length of several tens to hundreds of meters. These sites (6.5 km apart as the crow

25 flies) have relatively similar lithological and meteorological contexts (the latter is affected by

26 seasonal variations of oceanic temperate climate) but are affected differently by marine

27 agents: Varengeville-sur-Mer is an active cliff (influenced by both marine and subaerial
28 factors), while Dieppe is an abandoned cliff (affected only by subaerial factors). Thanks to
our seasonal monitoring using terrestrial lasergrammetry and then terrestrial photogrammetry, our main outcomes are: 1) the cliff face erosion rate (2010-2021) is approximately 50 times higher in the active cliff context than in the abandoned one (0.51 ± 0.02 m/year and 0.00 ± 0.01 m/year, respectively), reinforcing the importance of marine actions in the erosion of Norman chalk cliffs; 2) unsurprisingly, along the active cliff, erosion peaks generally occur in winter, with periods of stability preferentially in summer; 3) in Varengeville-sur-Mer, of the 103 gravity movements identified on the cliff face, rock falls accounted for 68% of total erosion, compared with 32% for debris falls, the latter value being higher than reported in the scientific literature (from 2 to 25%); 4) nine cases of cliff foot departure generating a basal notch, which later triggered rock falls over the entire cliff face by instability propagation in Varengeville-sur-Mer, have been identified. This monitoring continues, providing a valuable database for understanding cliff erosion (rates, rhythms, and modalities).

Keywords: Coastal erosion; chalk cliffs; seasonal monitoring; Normandy

## 1. Introduction

Scientific knowledge is an essential prerequisite for the effective management of coastal areas in the context of increasing coastal development and climate change (IPCC, 2023). Despite the high spatial representativeness of cliffs on a global scale and the increasing risks associated with erosion, erosional coasts have been studied far less than depositional coasts, whether on a global, national, or regional scale (Trenhaile, 1980, 2002; Naylor et al., 2010; Mercier, 2022). According to Elsevier's Scopus and Clarivate Analytics' Web of Science™ (WoS) Core Collection databases, from 1960 to 2021 with the key words "rock* coast*", "cliff*", "falaise*" or "côte* rocheuse*", there were globally 1309 publications on cliff coasts (both soft and rocky cliffs) and rock coasts (including both rocky cliffs and low-lying rocky coast). From 1960 to 2003, the number of publications amounted to around ten per year and from 2004 to 2021, the number of publications increased sharply, peaking at 84 in
2020 (Fig. 1a). The first authors were predominantly from Europe, North America and Asia (Fig. 1b), mostly in developed countries, according to the list of the United Nations Development Programme (https://worldpopulationreview.com/country-rankings/developed-countries). This limited scientific output (1309 publications) is likely due to several factors, including the later development of these coastal areas by human populations and consequently more circumscribed and delayed management issues, the lack of data on vertical and often high, dangerous, and difficult-to-access features, and the complexity of these coastal systems (for example, the interactions between nearshore/platform/foreshore/cliff/hinterland).
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Fig. 1: a) Annual distribution of the 1309 publications identified on cliff coasts (both soft and rocky cliffs) and rock coasts (including both rocky cliffs and low-lying rocky coast) with the key words "rock* coast*", "cliff*", "falaise*", or "côte* rocheuse*" between 1960 and 2021, according to Elsevier's Scopus and Clarivate Analytics' Web of Science™ (WoS) Core Collection databases (institute for scientific and technical information (INIST-CNRS) - research support unit (UAR76)), and b) the number of publications by the country of the first author in the same databases (INIST-CNRS - UAR76).

Retreat rates on erosional coasts (most often expressed in m/year, in 2D) are well-documented and commonly used data (e.g. Prêcheur, 1960; Moore et al., 1999; Benumof et al., 2000; Lahousse et al., 2000; Costa et al., 2004; Greenwood and Orford, 2008; Del Río and Gracia, 2013; Bezore et al., 2016; Prémaillon et al., 2018; Swirad and Young, 2021; Rink et al., 2024). However, they do not accurately describe the geomorphological evolution of cliffs because (1) the rate smooths out the retreat, which occurs in jerks, and (2) it is primarily derived from the "cliff top" proxy, even though some studies have incorporated volumetric change (e.g. Rosser et al., 2013; Swirad and Young, 2022). This erosion proxy is incomplete (e.g. changes at the foot or in the middle of the cliff are not considered) compared to the cliff face, which captures all changes (Young et al., 2009). The location, intensity (eroded volume and types of falls, corresponding to modality), and frequency (regular or irregular patterns of erosion, corresponding to rhythm) of gravity movements are of utmost importance for the inhabitants, managers, and elected representatives of these at-risk areas.

The joint study of erosion rates, rhythms, and modalities is valuable and has been developed worldwide (e.g. Young and Ashford, 2006; Kennedy et al., 2014b; Kuhn and Prüfer, 2014; Ogawa et al., 2016; Williams et al., 2018). In Normandy, where the chalk coastline has been extensively studied by the French scientific community because of rapid changes (e.g. Prêcheur, 1960; Costa, 1997; Hénaff et al., 2002; Costa et al., 2004, 2019; Dewez et al., 2013; Letortu et al., 2014; Duguet, 2019), the research also contributes to the debate on the
bedrock control of retreat rates and modalities (Costa, 1997; Letortu, 2013), as well as the processes and agents responsible for chalk cliff fatigue and the triggering of cliff failure along the coast. Indeed, along this chalky coast, characterized by an oceanic temperate climate, intense rainfall events appear to be one of the main drivers of erosion: heavy winter rainfall (Duperret et al., 2002; Letortu et al., 2015a), particularly during wet years, raises the water table in the chalky massif (Lageat et al., 2006) or heavy summer rainfall during dry periods (Duperret et al., 2004). Although rare, freeze/thaw cycles are a highly effective erosion factor (Moses et al., 2006; Dewez et al., 2015; Letortu et al., 2015a). While Lageat et al. (2006) concluded that 1) subaerial processes were the primary triggers for chalk cliff falls and 2) marine influence was limited to the removal of falls, numerous researchers have highlighted the role of marine factors both as an eroding agent and as a factor that reactivates erosion through removal (Hénaff et al., 2002; Letortu et al., 2015a, 2015c, 2019). Exploratory work has also focused on the fatigue of the chalk massif through continuous, multiparameter instrumental monitoring over 13 months (Letortu et al., 2022).

To better understand the regressive dynamics of cliffs, it is recommended to acquire: 1) 3D data of the cliff face (from the foot to the top) to observe all erosion events; 2) data at high spatial resolution (inframetric) to visualize both small and large falls; 3) data at very high temporal frequency (from seasonal to daily surveys, or even more frequently) to identify erosion-prone periods or triggering factors; 4) data over large areas, such as the hydro-sedimentary cell scale, for a systemic view that supports effective coastal management; and 5) data over several decades to distinguish between variation and variability. Currently, meeting the last two criteria is particularly challenging.

The evolution of the cliff face is often observed through mass movements, as their large dimensions make them easily detectable in various data sources (such as aerial photographs, satellite images, whether vertical or oblique, etc.). However, these mass movements represent only a portion of the material transfers induced by cliff erosion. According to Varnes (1978), Dikau et al. (1996) and Hungr et al. (2014), movements of
coherent rock in relation to fallen volume can be described by two terms. Debris fall refers to the detachment of small blocks, stones, or flakes from the cliff face (Fig. 2a), while rock fall describes large-scale movements (mass movements) that detach from all or part of the cliff face (Fig. 2b). According Hénaff et al. (2002), block falls range from a few tens of cm³ to a few tens of m³, and stone falls range from a few cm³ to dm³. These debris falls are often overlooked because they are invisible if the scale of observation is too coarse and the frequency of observation is low. However, these debris falls (blocks and stones) should be considered, as all such movements contribute to the evolution of the cliff face. On the chalky coasts of Sussex (UK), these individual movements account for almost 11% of total erosion (May and Heeps, 1985), 10% near Fécamp in Seine-Maritime (France, Hénaff et al., 2002), and 2 to 25% at Varengeville-sur-Mer in Seine-Maritime (Letortu et al., 2015c, 2019).

Moreover, the agents triggering these individual movements may differ from those causing mass movements (Costa, 1997; Letortu et al., 2015a). To observe these phenomena on cliff face, whether soft or rocky cliff face, monitoring should be precise to within 2-3 cm³ and conducted at a high temporal frequency (e.g. Kuhn and Prüfer, 2014; Williams et al., 2019) over many years, which terrestrial photogrammetry and lasergrammetry techniques can achieve.
<figure><img src="image_4.png" /><figcaption>Fig. 2: a) Debris falls and b) a rock fall observed at Varengeville-sur-Mer (Seine-Maritime).</figcaption></figure>

This paper updates the results of previously published work (Letortu et al., 2015c, 2019), with a temporal coverage of a decade and a change in cliff face monitoring technique (from terrestrial lasergrammetry to terrestrial photogrammetry). To provide insights into: 1) the departure zones of any movement when its size exceeds the error of acquisition and processing methods (which is here 2-3 cm³); 2) the quantification of debris falls versus rock falls in cliff face erosion; and 3) the debate on the prevalence of marine versus subaerial agents and processes in the erosion of coastal chalk cliffs in Normandy, we used cm-scale precision monitoring of cliff faces with terrestrial lasergrammetry and terrestrial photogrammetry. This monitoring was conducted every four to five months between 2010 and 2021 at sites representing both active cliff (influenced by subaerial and marine factors) and abandoned cliff (affected by subaerial factors only).
## 2. Study area

### 2.1. Regional setting

From Mont-Saint-Michel Bay to Le Tréport (Normandy), the coastline comprises 30% estuarine, 29% rocky cliffs, 26% sandy and gravel beaches, 9% artificial coastline, 5% low rocky coastline, and 2% soft cliffs (ROLNP, 2012). The chalk cliffs, which extend over 100 km from cap d'Antifer in the southwest to Le Tréport in the northeast (Seine-Maritime), are 60-70 m high on average (Fig. 3) and are retreating at a rate of 0.15 m/year (1966-2008), with spatial variability ranging from 0.09 m/year to 0.23 m/year depending on the hydro-sedimentary sub-cells (Letortu et al., 2014).

At their foot is a shore platform from 150 to 700 m wide (Costa et al., 2019; Duguet, 2019), which is covered at the top of the beach by a gravel barrier. Gravel barriers are generally thin (a few meters thick at most) and have a variable sand fraction (Costa et al., 2006a). The distribution and movement of these gravels are disrupted by obstacles to transport, such as mass movements, jetties, and groins. The littoral drift in the study area is oriented from SW to NE (Fig. 3). Jetties and groins are often found at the mouths of valleys that incise the plateau. At these points, updrift of the obstacle, the gravel barrier is often wide (30 to 100 m), while downdrift it is very thin.

Located at the northwestern edge of the sedimentary Paris Basin and in contact with the Channel, the cliffs of the Pays de Caux plateau consist of flinty chalk dating from the Upper Cretaceous (Mégnien and Mégnien, 1980; Pomerol et al., 1987). This highly karstified structure (Rodet, 1991) lies on residual flint formations and loess from the Quaternary (Lautridou, 1985). The Pays de Caux plateau appears tabular, but major structures break up the monotony of the chalky deposits. In addition to main deformations, shaped as a succession of anticlinal and synclinal undulations with an Armorican orientation (N150-N160) and sometimes extended by flexures or faults such as those of Fécamp-Lillebonne and Bray, there are Variscan-oriented structures (N60-N70) such as the Villequier fault, or structures with an N090 orientation (Cavelier et al., 1979; Mégnien and Mégnien, 1980; Cazes et al., 1985; Pomerol and Feugueur, 1986; Cavelier and Lorenz, 1987; Duperret et al., 2004, 2012;
181 Genter et al., 2004; Meire et al., 2019). These tectonic features have brought various Upper
182 Cretaceous geological stages to the surface (Fig. 3).

183 In terms of lithology, the different chalk stages exhibit slight variations in facies and detailed
184 sedimentary discontinuities, which result in subtle contrasts in strength (Juignet and Breton,
185 1992; Laignel, 1997). From the oldest to the most recent, we find Cenomanian chalk, which
186 outcrops for a few meters at the base of the cliffs at cap d'Antifer and cap Fagnet (Fécamp).
187 The latter are heterogeneous, sometimes rich in detrital elements (clays and quartz) and may
188 be glauconitic or nodular. The overlying stage is Turonian chalk. It consists of greyish to
189 whitish clayey chalk, with frequent nodular beds and little or no flint. This stage outcrops from
190 cap d'Antifer to the west of Yport, from Fécamp to the west of Veulettes-sur-Mer, and from
191 Puys (east of Dieppe) to Le Tréport. It reaches its maximum extension at Penly where there
192 is the Berneval-Londinières anticline. The Turonian is overlain by the Coniacian chalk, which
193 outcrops between cap d'Antifer and west of Veules-les-Roses, as well as between Puys and
194 Belleville-sur-Mer (Petit-Caux) and from Biville-sur-Mer (Petit-Caux) to Le Tréport. The
195 overlying Santonian stage appears in the central part of the study area, between Saint-
196 Pierre-en-Port and Saint-Aubin-sur-Mer, and between Sainte-Marguerite-sur-Mer and
197 Dieppe. Finally, Campanian chalk, the most recent chalk, is only present to a limited extent
198 between Saint-Aubin-sur-Mer and Sainte-Marguerite-sur-Mer (Genter et al., 2004; Duperret
199 et al., 2012; Duguet, 2019). According to geotechnical studies, the latter layers (Santonian
200 and Campanian) appear to be more susceptible to weathering than the other Upper
201 Cretaceous ones (Laignel, 2003). Above these chalky strata, a cover of clayey and sandy
202 sediments around 10 to 30 m thick, of Paleogene age (Bignot, 1962), replaces the usual
203

residual flint formation (Laignel, 1997; Costa et al., 2006b), notably at cap d'Ailly and

204

Sotteville-sur-Mer (Fig. 3).

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206

Fig. 3: Structural characteristics in Seine-Maritime and the locations of the study sites at Varengeville-sur-Mer and Dieppe.

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The cliffs are in contact with the Channel, an epicontinental sea characterized by a semi-diurnal, macrotidal environment (average spring high tide of 9.35 m at Dieppe (SHOM,
211 2020)). The eastern Channel (from the Bay of Seine to the Strait of Pas-de-Calais) has little relief, and depths rarely exceed 50 m. In front of the chalky cliffs, the seabed is characterized by bedrock, generally limestone, covered by thin, loose sediments (except at Le Tréport, where the ridges can reach 8 m). From seaward to depths of 6 to 10 m, the loose cover exhibits a gradient of refinement related to tidal current fields, ranging from cailloutis (from east of Antifer to east of Saint-Valéry-en-Caux) to gravelly zoogenic sands (from east of Saint-Valéry-en-Caux to east of Petit-Caux), coarse sands (from 2 to 0.5 mm, from east of Petit-Caux to east of Criel-sur-Mer), and fine sands (less than 0.5 mm, from east of Criel-sur-Mer to the bay of Somme) (Augris et al., 2004).

220 Located near the 50th parallel north, Dieppe has a temperate oceanic climate. Based on the 1991-2020 climatological normal of Dieppe (Météo-France, 2024), winters are mild, with average winter temperatures above freezing (5.6°C in January), although frost may occur (an average of 24 frost days), with the minimum temperature recorded at -16.6°C in February 1956. Average summer temperatures are also mild (17.8°C in August). Total precipitation is 805.2 mm, fairly evenly distributed throughout the year, with 129 days of rain (minimum monthly rainfall of 49.4 mm in March and a maximum of 94.3 mm in December), associated with the frequent low-pressure systems at these mid-latitudes. Daily rainfall can reach 100.8 mm in August 1966 during stormy episodes. The location is windy, with frequent and strong winds, averaging 4.8 m/s, particularly from November to January (40% of the days have gusts over 16 m/s), with a predominantly westerly direction (southwest to northwest). At Dieppe, an average temperature increase of +1.9°C between 1970 and 2023 and a cumulative change of +1.7% in total precipitation between 1991-2020 and 1971-1990 are observed (Cantat et al., 2024).

234 Based on tide gauges along the Seine-Maritime coast, the most frequent and strongest significant swells occur between October and March, linked to wind data. The swells are predominantly from the W to SW, and then from the NW, while N and NE swells are weaker. 235 236 237 Based on swell data, the predominant direction of sediment transfer is from the NE (65% of
the time), compared to the SW (35% of the time). The annual return period swell reaches 4.3 m at Dieppe. The H₁/₃ swell remains below 1 m for 70% of the time and below 2 m for 95% of the time. The swell exceeds 3 m only 0.3% of the time, which is approximately 25 h/year (Teisson, 1986). Swell periods are relatively short due to the limited fetches caused by the shallow and semi-enclosed shape of the Channel. Agitation conditions do not seem capable of setting gravels in motion below hydrographic zero (only coarse sands) (LCHF, 1972; Augris et al., 2004). Current gravel barrier natural nourishment can only come from cliff erosion.

Human actions have altered the natural configuration through the installation of infrastructure (weight, vibrations, changes in water flow, etc.). These infrastructures are mostly located in valleys and dry valleys (for easy access to the sea), and sometimes imprudently on the plateau near the edge of cliffs, as seen on the Côte aux Hérons in Dieppe or in Criel-sur-Mer. At the foot of the cliffs, on the shore platform, structures such as groins and harbor jetties can disrupt or halt SW-NE sediment transit of the gravel barrier. The stock and spatial distribution of gravels have been – and continue to be – strongly modified along the coast and foreshore by 1) the collection of gravels and offshore sediments 2) the aforementioned structures, as well as by the nuclear power plants managed by Electricity of France (EDF, for example the site of Penly in Petit-Caux, Fig. 3) and by boat ramps. The absence or scarcity of gravels at the foot of cliffs is likely to alter the effectiveness of marine actions at the base of chalky cliffs, and consequently, the intensity and mechanisms of cliff erosion (Costa, 1997; Costa et al., 2006a; Letortu et al., 2014, 2025).

259

## 2.2. Study site selection and characteristics

To study differences in erosion rates, rhythms, and modalities, and to identify the respective contribution of marine and subaerial agents to cliff erosion, two neighboring sites (6.5 km apart as the crow flies, Fig. 4a) with relatively similar meteorological and lithological contexts
264

(cliffs carved from Coniacian and Santonian chalk), but differing in exposure to marine factors, were selected (Fig. 4b):

265

266

1) The first site is in an abandoned cliff, cut into Coniacian chalk (Fig. 4c), and affected only by subaerial agents and processes. The cliff was abandoned when an embankment was constructed in the early 1980s for the cross-Channel ferry landing and the storage of offshore sediments. The abandoned cliff face is located in Dieppe. The vertical front (simple cliff (Costa, 1997)) has been divided into two sections due to the different orientations of the cliff faces:

267

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269

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271

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- Dieppe 1 west (1 W) corresponds to a 45 m linear stretch (35 m high) oriented at 310°N (49°56'00" N; 01°05'18" E) (Fig. 4c);

273

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- Dieppe 2 north (2 N) corresponds to a 80 m linear stretch (35 m high) oriented at 010°N (49°56'00" N; 01°05'25" E) (Fig. 4c);

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276

2) The second site is located 6.5 km SW of the first, within an active cliff context at cap d'Ailly in Varengeville-sur-Mer (Fig. 4a), where it evolves under the combined actions of subaerial and marine agents. This site is characterized by an active cliff composed of Santonian chalk and Tertiary strata, including sand and clay (complex cliff, Costa, 1997). There are perched aquifers in the Tertiary terrain. Specifically, this cliff face (70° to overhang) lies on either side of the Petit Ailly dry valley (49°55'00" N; 01°00'27"E) at Varengeville-sur-Mer (250 m long, with a maximum height of 40 m, oriented approximately 010°N) (Fig. 4b).

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282
<figure><img src="image_6.png" /><figcaption>Fig. 4: Characteristics of monitored sites: a) location, b) and c) lithology, surveyed length in 2010 (L), height (H), orientation, and slope, d) fracturing, e) frequency of surveys between</figcaption></figure>
286

October 2010 and July 2021 (from Mortimore, 2001; Hoyez, 2008; Dewez et al., 2016;

287

Letortu et al., 2019).

288

At the monitored sites, the regional tectonic feature (N110-N130E) is evident in the

289

stereograms, with joints present at Varengeville-sur-Mer and Dieppe 2 N (Fig. 4d). At Dieppe

290

1 W, the orientation is approximately N025E, corresponding to the set of secondary fractures

291

observed by Genter et al. (2004). The dips of the layers are very shallow (0.5-1%) (Bignot,

292

1971). These fracture orientations and the lithology result in rock falls with a predominantly

293

vertical fracture pattern at both sites. In Varengeville-sur-Mer, a type of sliding failure may

294

occur between the Lower-Middle Santonian and the Upper Santonian (Duperret et al., 2004).

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296

## 3. Monitoring methodology

297

### 3.1. Frequency of cliff face data acquisition

298

To address our research questions regarding 1) the departure zones of any movement; 2)

299

the quantification of debris falls versus rock falls in cliff face erosion; and 3) the debate on the

300

prevalence of marine versus subaerial agents and processes in erosion, high-frequency

301

monitoring over at least a decade, combined with very high spatial resolution, is essential.

302

Since October 2010, the cliff faces at Dieppe (Dieppe 1 W and 2 N) and Varengeville-sur-

303

Mer have been monitored every four to five months, resulting in 32 data acquisitions at

304

Dieppe and 34 at Varengeville-sur-Mer (Fig. 4e). We have defined the cliff face extent (Fig.

305

4b and 4c) in function of two limits: the beach/ground and the top of the cliff face. These

306

limits are defined by morphology, not by elevation. For example, the junction between the

307

cliff face and the beach depends on the location of the gravel barrier for Varengeville-sur-Mer

308

(which is seasonally variable). It depends on the grass on the ground for Dieppe. The top of

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the cliff face is the junction between the rocky cliff face and the vegetation or soil.

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311

### 3.2. Evolution of terrestrial data acquisition techniques
From 2010 to 2014, we used a RIEGL LMS390i Terrestrial Laser Scanner (TLS), and from 2014 to 2021, a RIEGL VZ-400 (Fig. 5). This enabled us to quantify changes with a precision of ± 0.02 m at Dieppe and ± 0.03 m at Varengeville-sur-Mer (Letortu et al., 2015; 2019).

In France and abroad, during the 2000s and the early 2010s, cliff face monitoring using TLS, with cm-scale precision in evolution results, was a novelty (Lim et al., 2005, 2010; Rosser et al., 2005, 2013; Quinn et al., 2010; Dewez et al., 2013; Letortu, 2013; Kuhn and Prüfer, 2014; Letortu et al., 2015b, 2015c). Subsequently, the technique became more widely accessible as processing software (such as the free CloudCompare software) became available. Our cliff face monitoring in Normandy, with cm-scale precision, continued and progressed until the end of 2020 (Letortu et al., 2019). For example, reducing the number of 10-cm high cylindrical reflective targets from a dozen to three (Jaud et al., 2017) saved time both on site and during processing.

A previous paper comparing Unmanned Aerial Vehicle (UAV) photogrammetry, terrestrial photogrammetry, and terrestrial lasergrammetry (with instruments based on the shore platform in front of the cliff face) for monitoring cliff face erosion in Varengeville-sur-Mer in 2018 demonstrated that: 1) the average error of each technique is less than ± 0.03 m, meeting our precision requirements for erosion monitoring; 2) UAV photogrammetry, in addition to offering greater spatial coverage, appears to be less sensitive to variance than conventional terrestrial photogrammetry, though it remains dependent on weather conditions and pilot availability; 3) despite these differences, terrestrial techniques can be used interchangeably when a UAV pilot is unavailable, weather conditions are poor, or a high level of reactivity is required; 4) the low cost of terrestrial photogrammetry is a major advantage for the reproducibility of monitoring (Letortu et al., 2018).

Despite these encouraging results, we continued the monitoring protocol with a TLS until 2021, as several obstacles remained: 1) data acquisition time for terrestrial photogrammetry remained lengthy due to the set-up of numerous targets, and 2) photogrammetric data processing time (for both terrestrial or UAV techniques) was long and highly variable in quality. This was mainly due to the photo alignment stage, which remained challenging due
to the lack of photo georeferencing. The use of Real Time Kinematic (RTK) Global Navigation Satellite System (GNSS) assisted terrestrial Structure-from-Motion (SfM) photogrammetry (Jaud et al., 2020) has addressed these issues while maintaining data precision of ± 0.03 m or better. The photo acquisition protocol (which involves using either a Nikon D800, with a 35 mm focal length to capture 36 Mpix photos, or a Nikon D3500 with a 35 mm focal length to capture 24.8 Mpix photos) is similar to classic terrestrial photogrammetry. To avoid distortions, 10-20 photos per station are taken from various angles towards the cliff face, with stations positioned 15 to 20 m apart in a quincunx arrangement. Additionally, the camera tripod is equipped with a frame for attaching a GNSS receiver (Trimble R6 or R8S, Fig. 5). With a known offset (D distance in Fig. 5) between the center of the GNSS antenna and the optical center of the camera, the position of each photo station is determined, and therefore the positions of the photos are also known (Fig. 5). This eliminates the need for targets. Additionally, RTK GNSS-assisted terrestrial SfM photogrammetry allows the cartographic coordinates of the photos to be integrated into Agisoft Metashape software before the three processing stages (photo alignment, alignment optimization, and creation of a dense point cloud). These coordinates greatly facilitate the alignment stage and can be used to integrate a large number of photos, thus improving the resolution of the point cloud. The time saved is substantial: processing around 600 photos to create a dense point cloud (with centimeter resolution or better) of a cliff over 350 meters takes less than 3 hours on a computer with a graphics processing unit, compared to ten or more hours without coordinate integration. Moreover, non-georeferenced photos can also be incorporated into the data processing.
<figure><img src="image_7.png" /><figcaption>Fig. 5: Changes in data acquisition and processing at the Varengeville-sur-Mer and Dieppe sites.</figcaption></figure>

Thus, in February 2021, we switched from three-target TLS to RTK GNSS-assisted terrestrial SfM photogrammetry. This change reduced material costs, limited the field team to two people, and made deployment easier due to the lightweight equipment. Additionally, we estimated a time savings of around 30 minutes per site when extending the surveyed linear area in Varengeville-sur-Mer (Fig. 5). From October 2010 to July 2021, the monitored cliff surface area in Varengeville-sur-Mer gradually increased due to the optimization of surveys (expanding from 6,000 m² to 24,000 m², which corresponds to a length of 250 to 530 m).

### 3.3. Cliff face data processing evolution

While stabilizing the error, the change in technique from terrestrial lasergrammetry to terrestrial photogrammetry has allowed us to reduce the number of software packages from five to two: the pay-to-use Agisoft Metashape software (educational license) and the free CloudCompare software, developed by EDF, which includes several tools and plugins such as the Multiscale Model to Model Cloud Comparison (M3C2) Distance (Lague et al., 2013)
for result visualization (Figs. 5 and 6). This distance algorithm has been widely used for 380 evolving rock masses (e.g. Earlie et al., 2015; DiFrancesco et al., 2020; Acharya-Chowdhury 381 et al., 2024) to measure erosion and deposition (with uncertainty estimates). It is based on a 382 locally oriented search cylinder from each point from the observed (Cloud 1) to the compared 383 cloud (Cloud 2), to compute the average distance between points captured by each cylinder 384 (Lague et al., 2013). Compared to TLS with around ten targets, field acquisition time has 385 been reduced by half (from 3 h to 1 h 30), while the length of cliff face monitoring has 386 increased. However, processing time remains similar: approximately 10 h for the 387 Varengeville-sur-Mer site and approximately 5 h for Dieppe 1 W and Dieppe 2 N. 388
<table><tbody><tr><td><strong>TLS data processing</strong></td><td>RIEGL-RiSCAN PRO (pay-per-use)</td><td>CloudCompare (free)</td><td>Fledermaus (pay-per-use)</td><td>Matlab (pay-per-use)</td><td>3DReshaper (pay-per-use)</td></tr><tr><td>1<sup>st</sup> STEP<br>Point cloud georeferencing and alignment</td><td>Point cloud geo-referencing (Lambert-93, IGN69)<br>Station merging and aligning of previous point clouds</td><td></td><td></td><td></td><td></td></tr><tr><td>2<sup>nd</sup> STEP<br>Point cloud cleaning</td><td></td><td>Manual cleaning of the point cloud with a sliding window</td><td></td><td></td><td></td></tr><tr><td>3<sup>rd</sup> STEP<br>Mesh (Delaunay triangulation) and creation of 2.5D Digital Elevation Model (DEM)</td><td></td><td></td><td>Possible rotation of the cliff face perpendicular to the measurement axis and its tilting perpendicular to z</td><td></td><td>From point cloud to mesh (resol. 0.02)<br>Creation of 2.5D DEM (resol. 0.02 in Dieppe, 0.06 in Ailly)</td></tr><tr><td>4<sup>th</sup> STEP<br>DEM of difference (DoD) and used for result visualization</td><td></td><td></td><td></td><td>DoD</td><td></td></tr></tbody></table>

<table><tbody><tr><td><strong>RTK GNSS-assisted terrestrial SfM photogrammetry data processing</strong></td><td>Agisoft Metashape (pay-per-use)</td><td>CloudCompare (free)</td></tr><tr><td>1<sup>st</sup> STEP<br>Station georeferencing, creation and alignment of point clouds</td><td>Georeferencing of photo stations (Lambert-93, IGN69)<br>Photo alignment<br>Alignment optimisation<br>Creation of a dense point cloud</td><td>Alignment with previous point clouds</td></tr><tr><td>2<sup>nd</sup> STEP<br>Point cloud cleaning</td><td></td><td>Manual cleaning of the point cloud with a sliding window</td></tr><tr><td>3<sup>rd</sup> STEP<br>2.5D mesh</td><td></td><td>2.5D mesh (resol. from 0.02 to 0.10) (volume calculation)</td></tr><tr><td>4<sup>th</sup> STEP<br>Calculation of mesh of difference</td><td></td><td>Difference between meshes (x, y, z)</td></tr><tr><td>5<sup>th</sup> STEP<br>Result visualization</td><td></td><td>Distance of erosion with M3C2 plugin</td></tr></tbody></table>

<caption>Fig. 6: Changes in data processing steps between TLS and RTK GNSS-assisted terrestrial</caption>

<caption>SfM photogrammetry at the Varengeville-sur-Mer and Dieppe sites.</caption>
To evaluate measurement quality achieved for this monitoring, we can use accuracy and precision. Accuracy is "closeness of agreement between a measured quantity value and a true quantity value of a measurand" (JCGM, 2012, p. 21) whereas measurement precision is "closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions" (JCGM, 2012, p.22). Our results are based on point clouds which are aligned in comparison with a reference, so as described in Letortu et al. (2018, 2019), precision is the most important parameter in our monitoring. All point clouds are aligned to the initial point cloud if the cliff face has not changed significantly (as in Dieppe) or to the relevant previous point cloud if the cliff face has undergone substantial changes (as in Varengeville-sur-Mer) using best fit alignment algorithms in CloudCompare as the iterative closest point algorithm (Besl and McKay, 1992; Chen and Medioni, 1992). The registration error tends to decrease during user defined iterations (Lague et al., 2013). This adjustment reduces the error margin considering only the instrumental error and the cloud fitting error. In the context of repeated TLS surveys, fixed areas of several hundred or thousands of square meters in different locations are compared across point clouds using standard data processing methods to assess precision.

The planimetric precision assessed by the Root Mean Square Error (RMSE) is ± 0.03 m for Varengeville-sur-Mer and Dieppe 2 N, and ± 0.02 m for Dieppe 1 W (Table 1). The precision value per site is used to assess the volumetric precision by multiplying by the area studied.

The volume precision is ± 163.7 m³ in Varengeville-sur-Mer for a surface area of 5455.5 m², ± 10.8 m³ in Dieppe 1 W for a surface area of 539 m², and ± 37.5 m³ in Dieppe 2 N for a surface area of 1249.7 m² (Table 1).

<table><caption>Table 1: Precision in planimetry and in volumetry (based on a reference surface) for Varengeville-sur-Mer, Dieppe 1 W, and Dieppe 2 N.</caption><thead><tr><td>Site</td><td>Precision in planimetry (m)</td><td>Reference surface for volumetric estimate (m²)</td><td>Precision in volumetry (m³)</td></tr></thead><tbody><tr><td>Varengeville-sur-Mer</td><td>0.03</td><td>5455.5</td><td>163.7</td></tr><tr><td>Dieppe 1 W</td><td>0.02</td><td>539</td><td>10.8</td></tr><tr><td>Dieppe 2 N</td><td>0.03</td><td>1249.7</td><td>37.5</td></tr></tbody></table>
416

417

Point clouds with imprecision greater than ± 0.03 m (such as the TLS sinking in wet sand on 24/02/2011 and 26/02/2015 in Varengeville-sur-Mer), are marked with an asterisk in tables 4 and 5 and figures 7 to 9 and are not used for calculations. Some positive changes might appear due to vegetation growth (grass on the cliff face) and localized reconstruction errors.

418

419

420

421

It should be noted that in Dieppe, anthropogenic erosion occurred in March 2012 during cliff stabilization work. This involved the removal of overhanging sections (estimated at 2 m³ over 18 m² in Dieppe 1 W and 82 m³ over 152 m² in Dieppe 2 N) and the installation of protective nets. These actions reduced the study area at Dieppe 1 W from 1,000 m² to 500 m² (equivalent to a linear length reduction from 45 m to 25 m) and at Dieppe 2 N from 1600 m² to 1100 m² (equivalent to a linear length reduction from 80 m to 50 m). These anthropogenic changes have been identified and excluded from our erosion analysis.

422

423

424

425

426

427

428

429

### 3.4. Error quantification of cliff face erosion rates over one decade between sites and of cliff top retreat rates over five decades at the same sites

430

431

Since our cliff face monitoring is conducted on a decadal basis, it is interesting to compare these rates of change with those of the cliff top (edge of the plateau), as obtained through the area-based approach from 1966 to 2008 every 50 m (the used cliff top proxy and method are detailed in Letortu et al., 2014). This data has been updated to 2022 using the orthophotograph with a 0.2 m resolution from the French geographic institute (IGN, https://geoservices.ign.fr/bdortho).

432

433

434

435

436

437

Obviously, since the analysis periods, methods, and data quality differ, these comparisons should be interpreted with caution. However, they can still provide insights into the consistency of the results obtained (and thus the representativeness of the chosen 10-year period) as well as highlight discrepancies between the rates derived from the two proxies.

438

439

440

441

The error in the cliff top retreat rate (± 0.02 m/year) was estimated by fitting a root sum-of-squares method over time (T) (Fletcher et al., 2003; Gorokhovich and Leiserowiz, 2011; Bernatchez et al., 2021) to the area-based approach. The mean annual error (Ea) is the

442

443
square root of the cumulative error divided by time, where the cumulative error includes the
square of the data georeferencing or orthorectification error (Egeor<sub>d1d2</sub>) and the square of the
operator-related digitization error for each coastline (Enum op<sub>d</sub>), which is estimated as one
pixel and thus depends on data resolution.

$$
E_a = \frac{\sqrt{E_{geor_{d1d2}}^2 + Enum\ op_{d1}^2 + Enum\ op_{d2}^2}}{T}
$$

For estimating the error in erosion rates from cliff face monitoring, we used the following data
from Varengeville-sur-Mer: over the common surface between 08/10/2010 and 09/07/2021
(5,455.5 m²), the eroded volume of the cliff face is 29,711.5 m³. This results in an average
erosion of 5.45 m, which, when including the error of ± 0.03 m, leads to erosion rates ranging
between 0.50 and 0.51 m/year, corresponding to an error of ± 0.01 m/year (Table 2).

<table><caption>Table 2: Area, volume, changes, and rates of change obtained from monitoring the cliff face (from the base of the cliff face, just above the gravel barrier for Varengeville-sur-Mer and the grass on the ground for Dieppe to the top of the cliff face, delimited by the vegetation or soil) between the first survey (October 2010) and the last survey (July 2021) in Varengeville-sur-Mer and Dieppe.</caption>
   <thead>
    <tr>
     <td>
      Site
     </td>
     <td>
      Date of the first survey
     </td>
     <td>
      Date of the last survey
     </td>
     <td>
      Number of months between surveys
     </td>
     <td>
      Area (m²)
     </td>
     <td>
      Volume (m³)
     </td>
     <td>
      Change (m)
     </td>
     <td>
      Rate of change (m/year)
     </td>
    </tr>
   </thead>
   <tbody>
    <tr>
     <td>
      Varengeville-sur-Mer
     </td>
     <td>
      08/10/2010
     </td>
     <td>
      09/07/2021
     </td>
     <td>
      129
     </td>
     <td>
      5455.5
     </td>
     <td>
      -29711.5
     </td>
     <td>
      -5.45
     </td>
     <td>
      -0.51
     </td>
    </tr>
    <tr>
     <td>
      Dieppe 1 W
     </td>
     <td>
      07/10/2010
     </td>
     <td>
      09/07/2021
     </td>
     <td>
      129
     </td>
     <td>
      539.0
     </td>
     <td>
      -5.0
     </td>
     <td>
      -0.01
     </td>
     <td>
      0.00
     </td>
    </tr>
    <tr>
     <td>
      Dieppe 2 N
     </td>
     <td>
      07/10/2010
     </td>
     <td>
      09/07/2021
     </td>
     <td>
      129
     </td>
     <td>
      1249.7
     </td>
     <td>
      31.2
     </td>
     <td>
      0.02
     </td>
     <td>
      0.00
     </td>
    </tr>
   </tbody>
  </table>

4. Results

4.1. *Comparison of cliff face erosion rates over one decade between sites and cliff top retreat rates over five decades at the same sites*

Using the same proxy (cliff face), the erosion rate in Varengeville-sur-Mer is 0.51 ± 0.01 m/year (2010-2021), whereas Dieppe shows very little change over the same period (0.00 ± 0.01 m/year), with a change that is not statistically significant and falls within the margin of error of our method (Table 3). In other words, the change observed in the cliff face of Dieppe
is so slight that it does not exceed the margin of error of our method after 10 years of monitoring. The erosion rate over a decade is 50 times greater on an active cliff face than on an abandoned cliff face, compared to being 36 times greater based on 7 years of monitoring (Letortu et al., 2019) and 3 to 4 times higher based on 3 years of monitoring (Letortu et al., 2015c).

<table><thead><tr><th>Varengeville-sur-Mer</th><th>Monitoring period</th><th>Length of cliff (m)</th><th>Detailed erosion rate (m/year, every 50 m)</th><th>Average erosion rate (m/year)</th></tr></thead><tbody><tr><td>Cliff face monitoring</td><td>2010-2021</td><td>250</td><td>0.51 ± 0.02</td><td>0.51 ± 0.01</td></tr><tr><td rowspan="6">Cliff top monitoring</td><td>1966-2022</td><td>50</td><td>0.41 ± 0.02</td><td rowspan="6">0.34 ± 0.02</td></tr><tr><td>1966-2022</td><td>50</td><td>0.40 ± 0.02</td></tr><tr><td>1966-2022</td><td>50</td><td>0.24 ± 0.02</td></tr><tr><td>1966-2022</td><td>50</td><td>0.17 ± 0.02</td></tr><tr><td>1966-2022</td><td>50</td><td>0.32 ± 0.02</td></tr><tr><td>1966-2022</td><td>50</td><td>0.53 ± 0.02</td></tr></tbody></table>

<table><thead><tr><th>Dieppe 1 W</th><th>Monitoring period</th><th>Length of cliff (m)</th><th>Detailed erosion rate (m/year, every 50 m)</th><th>Average erosion rate (m/year)</th></tr></thead><tbody><tr><td>Cliff face monitoring</td><td>2010-2021</td><td>25</td><td>0.00 ± 0.01</td><td>0.00 ± 0.01</td></tr><tr><td rowspan="2">Cliff top monitoring</td><td>1966-2022</td><td>50</td><td>0.06 ± 0.02</td><td rowspan="2">0.05 ± 0.02</td></tr><tr><td>1966-2022</td><td>50</td><td>0.05 ± 0.02</td></tr></tbody></table>

<table><caption>Table 3: Comparison of erosion rates obtained from monitoring the cliff face and the cliff top at Varengeville-sur-Mer and Dieppe.</caption><thead><tr><th>Dieppe 2 N</th><th>Monitoring period</th><th>Length of cliff (m)</th><th>Detailed erosion rate (m/year, every 50 m)</th><th>Average erosion rate (m/year)</th></tr></thead><tbody><tr><td>Cliff face monitoring</td><td>2010-2021</td><td>50</td><td>0.00 ± 0.01</td><td>0.00 ± 0.01</td></tr><tr><td>Cliff top monitoring</td><td>1966-2022</td><td>50</td><td>0.02 ± 0.02</td><td>0.02 ± 0.02</td></tr></tbody></table>

When comparing the proxies (cliff face and cliff top) in Dieppe, the erosion rates for the cliff face are lower than those for the cliff top (0.00 ± 0.01 m/year versus 0.02-0.05 ± 0.02 m/year for Dieppe 2 N and Dieppe 1 W, respectively). However, the results are consistent for Dieppe 2 N within the margin of error. For Varengeville-sur-Mer, the results differ. Monitoring of the cliff top between 1966 and 2022 shows a lower average erosion rate compared to monitoring of the cliff face (respectively 0.34 ± 0.02 and 0.51 ± 0.01 m/year) (Table 3). Here, our cliff
481 face monitoring may be influenced by frequent mass movements that are not smoothed out
482 over such a short period, and the cliff face proxy captures all movements, unlike the cliff top
483 proxy.

484

485 4.2. Seasonality of erosion at Varengeville-sur-Mer: Peak erosion periods in winter and relatively calm periods in summer

487 Due to their seasonal frequency, it is not possible to precisely determine the agents and
488 processes responsible for cliff failure in our monitoring. However, a diachronic approach to
489 the Varengeville-sur-Mer active cliff face shows that surveys conducted during all or part of
490 the winter period (from early September to late February) are associated with high erosion
491 rates (ranging from 0.01 to 1.69 m/year). Outside these periods (from early March to late
492 August), erosion rates appear to be lower, though not negligible (up to 0.65 m/year) (Table
493 4).

493

494 The period from 18/09/2012 to 11/02/2013 is characterized by very significant erosion
495 (4138.9 m³). Relative to the studied surface area (5643.8 m²), this represents a total retreat
496 of 0.73 m (Table 4). This four-and-a-half-month period of high erosion would correspond to
497 an average erosion rate of 1.96 m/year.

496

498 From fall 2012 to spring 2013, erosion was particularly significant in both volume and
499 frequency (Table 4). Exceptional rainfall conditions between October 2012 and March 2013,
500 with a cumulative total of 575.5 mm in Dieppe (Météociel), represent a 29% increase over
501 the 1971-2000 average. Notably, rainfall in October and December was much higher than
502 average (+159%, +56%, respectively), which may explain the increase in erosion.

501

503 The winter of 2013-2014 is also highly instructive, marked by a succession of low-pressure
504 systems that brought significant marine agitation and rainfall (Blaise et al., 2015). This period
505 saw the highest volume of erosion in the decade studied, with 16651.3 m³ lost between
506 25/09/2013 and 11/03/2014. This five-and-a-half-month period of intense erosion

504

506
corresponds to an average erosion rate of 6.07 m/year, based on an average surface area of 507
508 5990.2 m² (Table 4).

<table><caption>Table 4: Erosion of the cliff face at Varengeville-sur-Mer between 08/10/2010 and 09/07/2021 (surveys with imprecision greater than ± 0.03 m are marked with an asterisk).</caption><thead><tr><th>Date of the first survey</th><th>Date of the last survey</th><th>Number of months between surveys</th><th>Name of DEM (digital elevation model) or mesh of difference</th><th>Area (m²)</th><th>Volume (m³)</th><th>Change (± 0.03 m)</th><th>Rate of change (m/year)</th></tr></thead><tbody><tr><td>08/10/2010</td><td>24/02/2011*</td><td>4.5</td><td>A</td><td>5850.0</td><td>117.0</td><td>0.02</td><td>0.05</td></tr><tr><td>24/02/2011*</td><td>27/09/2011</td><td>7.0</td><td>B</td><td>5995.2</td><td>-546.4</td><td>-0.09</td><td>-0.16</td></tr><tr><td>27/09/2011</td><td>18/12/2011</td><td>3.0</td><td>C</td><td>6010.1</td><td>-532.4</td><td>-0.09</td><td>-0.35</td></tr><tr><td>18/12/2011</td><td>21/03/2012</td><td>3.0</td><td>D</td><td>5367.1</td><td>-346.5</td><td>-0.06</td><td>-0.26</td></tr><tr><td>21/03/2012</td><td>18/09/2012</td><td>6.0</td><td>E</td><td>5855.8</td><td>-482.2</td><td>-0.08</td><td>-0.16</td></tr><tr><td>18/09/2012</td><td>11/02/2013</td><td>4.5</td><td>F</td><td>5643.8</td><td>-4138.9</td><td>-0.73</td><td>-1.96</td></tr><tr><td>11/02/2013</td><td>25/09/2013</td><td>7.5</td><td>G</td><td>6046.8</td><td>-1885.3</td><td>-0.31</td><td>-0.50</td></tr><tr><td>25/09/2013</td><td>12/02/2014</td><td>4.5</td><td>H</td><td>5992.9</td><td>-6538.5</td><td>-1.09</td><td>-2.91</td></tr><tr><td>12/02/2014</td><td>11/03/2014</td><td>1.0</td><td>I</td><td>5987.4</td><td>-10112.8</td><td>-1.69</td><td>-20.27</td></tr><tr><td>11/03/2014</td><td>09/07/2014</td><td>4.0</td><td>J</td><td>6645.9</td><td>-88.6</td><td>-0.01</td><td>-0.04</td></tr><tr><td>09/07/2014</td><td>24/09/2014</td><td>2.5</td><td>K</td><td>6891.2</td><td>-196.1</td><td>-0.03</td><td>-0.14</td></tr><tr><td>24/09/2014</td><td>26/02/2015*</td><td>5.0</td><td>L</td><td>7088.1</td><td>400.0</td><td>0.06</td><td>0.14</td></tr><tr><td>26/02/2015*</td><td>08/06/2015</td><td>3.5</td><td>M</td><td>7231.3</td><td>-73.3</td><td>-0.01</td><td>-0.03</td></tr><tr><td>08/06/2015</td><td>25/09/2015</td><td>3.5</td><td>N</td><td>7183.7</td><td>-182.8</td><td>-0.03</td><td>-0.09</td></tr><tr><td>25/09/2015</td><td>28/01/2016</td><td>4.0</td><td>O</td><td>7039.7</td><td>-115.2</td><td>-0.02</td><td>-0.05</td></tr><tr><td>28/01/2016</td><td>21/06/2016</td><td>5.0</td><td>P</td><td>7128.4</td><td>-39.4</td><td>-0.01</td><td>-0.01</td></tr><tr><td>21/06/2016</td><td>27/10/2016</td><td>4.0</td><td>Q</td><td>7402.3</td><td>-144.0</td><td>-0.02</td><td>-0.06</td></tr><tr><td>27/10/2016</td><td>25/01/2017</td><td>3.0</td><td>R</td><td>8011.7</td><td>-460.4</td><td>-0.06</td><td>-0.23</td></tr><tr><td>25/01/2017</td><td>26/06/2017</td><td>5.0</td><td>S</td><td>9903.8</td><td>-1387.8</td><td>-0.14</td><td>-0.34</td></tr><tr><td>26/06/2017</td><td>02/11/2017</td><td>4.5</td><td>T</td><td>14649.9</td><td>-3004.8</td><td>-0.21</td><td>-0.55</td></tr><tr><td>02/11/2017</td><td>16/01/2018</td><td>2.5</td><td>U</td><td>14475.1</td><td>-4035.5</td><td>-0.28</td><td>-1.34</td></tr><tr><td>16/01/2018</td><td>29/03/2018</td><td>2.5</td><td>V</td><td>14605.4</td><td>459.4</td><td>0.03</td><td>0.15</td></tr><tr><td>29/03/2018</td><td>06/07/2018</td><td>3.0</td><td>W</td><td>15194.2</td><td>-9804.9</td><td>-0.65</td><td>-2.58</td></tr><tr><td>06/07/2018</td><td>17/10/2018</td><td>3.5</td><td>X</td><td>15959.8</td><td>288.1</td><td>0.02</td><td>0.06</td></tr><tr><td>17/10/2018</td><td>17/01/2019</td><td>3.0</td><td>Y</td><td>16174.1</td><td>-419.7</td><td>-0.03</td><td>-0.10</td></tr><tr><td>17/01/2019</td><td>21/06/2019*</td><td>5.0</td><td>Z</td><td>16201.8</td><td>1159.8</td><td>0.07</td><td>0.17</td></tr><tr><td>21/06/2019*</td><td>02/09/2019</td><td>2.5</td><td>AA</td><td>15795.1</td><td>-227.2</td><td>-0.01</td><td>-0.07</td></tr><tr><td>02/09/2019</td><td>16/01/2020</td><td>4.0</td><td>AB</td><td>15742.6</td><td>-2654.4</td><td>-0.17</td><td>-0.51</td></tr><tr><td>16/01/2020</td><td>14/04/2020</td><td>3.0</td><td>AC</td><td>15923.2</td><td>-5609.5</td><td>-0.35</td><td>-1.41</td></tr><tr><td>14/04/2020</td><td>22/10/2020</td><td>6.5</td><td>AD</td><td>16197.9</td><td>138.8</td><td>0.01</td><td>0.02</td></tr><tr><td>22/10/2020</td><td>02/02/2021</td><td>3.5</td><td>AE</td><td>24019.1</td><td>-243.4</td><td>-0.01</td><td>-0.03</td></tr><tr><td>02/02/2021</td><td>13/04/2021</td><td>2.0</td><td>AF</td><td>23337.7</td><td>145.1</td><td>0.01</td><td>0.04</td></tr><tr><td>13/04/2021</td><td>09/07/2021</td><td>3.0</td><td>AG</td><td>24025.6</td><td>137.2</td><td>0.01</td><td>0.02</td></tr><tr><td>Total</td><td></td><td>129.0</td><td></td><td></td><td></td><td>-51254.5</td><td></td></tr></tbody></table>

509

510

511
Calm summer weather does not imply an absence of rock falls. During the summer, the beach and shore platform are heavily used for leisure activities, which can increase the risk of incidents, sometimes resulting in fatalities.

### 4.3. Eroded volumes, proportion of rock falls, and debris falls: debris falls are not negligible

With the new data processing using CloudCompare software, the volume of material lost in Varengeville-sur-Mer is estimated at 51255 m³, based on the 27 DEMs or meshes of difference with good precision (Table 4). The cm-scale precision of the measurements has allowed us to morphologically differentiate rock falls (deeper than 1 m and with a diameter greater than 1 m) from debris falls, such as block falls and stone falls (shallower, up to ≈ 1 m, and with a diameter of around 1 m) (respectively A and B in Figs. 7 and 9). In the case of stone falls, we must also account for the departure of thin material (such as flakes) over large areas, which is visible due to the very high spatial and temporal resolution of the monitoring. These latter movements, which are difficult to isolate, are quantified by deduction.

Between 08/10/2010 and 09/07/2021, the cliff face at Varengeville-sur-Mer experienced significant erosion, with losses reaching up to 15 m over the decade (Fig. 7). Over the 103 identified falls, 68 rock falls accounted for an eroded volume of 34841 m³ (± 914 m³), while 35 debris falls, including blocks and stones, were responsible for an erosion of 143 m³ (± 11 m³) (Table 5). Rock falls (mass movements) contributed 68.0% to the total erosion, compared to 0.3% from block and stone departures. The remaining debris falls, which are difficult to isolate, account for 16271 m³ or 31.7% of the total erosion. The proportion of debris falls (32.0%) is higher than the values reported by May and Heeps (1985), Hénaff et al. (2002) and Letortu et al. (2019) (10%, 11%, and 2%, respectively). But this value is close to the 25% results in Letortu et al. (2015c). Because the analysis periods, methods, and data quality differ, the comparisons with previous studies should be interpreted with caution.

<table><thead><tr><th>Rock fall (1/2)</th><th>Eroded volume (m³)</th><th>Rock falls (2/2)</th><th>Eroded volume (m³)</th></tr></thead><tbody><tr><td>A1*</td><td>24.7</td><td>A37</td><td>162.0</td></tr><tr><td>A2*</td><td>97.9</td><td>A38</td><td>334.7</td></tr></tbody></table>

<table><thead><tr><th>Individualized stone falls or block falls</th><th>Eroded volume (m³)</th></tr></thead><tbody><tr><td>B1*</td><td>(+)0.3</td></tr><tr><td>B2</td><td>14.8</td></tr></tbody></table>
<table><tr><td>A3</td><td>99.4</td><td>A39</td><td>1.5</td></tr><tr><td>A4</td><td>8.5</td><td>A40</td><td>2.2</td></tr><tr><td>A5</td><td>911.2</td><td>A41</td><td>977.8</td></tr><tr><td>A6</td><td>364.9</td><td>A42</td><td>37.5</td></tr><tr><td>A7</td><td>53.9</td><td>A43</td><td>732.0</td></tr><tr><td>A8</td><td>24.3</td><td>A44</td><td>2.3</td></tr><tr><td>A9</td><td>7.3</td><td>A45</td><td>31.4</td></tr><tr><td>A10</td><td>44.4</td><td>A46</td><td>30.3</td></tr><tr><td>A11</td><td>48.9</td><td>A47</td><td>82.5</td></tr><tr><td>A12</td><td>783.6</td><td>A48*</td><td>2.9</td></tr><tr><td>A13</td><td>1532.3</td><td>A49*</td><td>46.9</td></tr><tr><td>A14</td><td>3978.6</td><td>A50</td><td>2.5</td></tr><tr><td>A15</td><td>8509.2</td><td>A51</td><td>11.6</td></tr><tr><td>A16*</td><td>9.8</td><td>A52</td><td>19.8</td></tr><tr><td>A17</td><td>3.8</td><td>A53</td><td>31.1</td></tr><tr><td>A18</td><td>31.8</td><td>A54</td><td>2133.9</td></tr><tr><td>A19</td><td>71.8</td><td>A55</td><td>315.5</td></tr><tr><td>A20</td><td>31.5</td><td>A56</td><td>1.1</td></tr><tr><td>A21</td><td>48.3</td><td>A57</td><td>30.0</td></tr><tr><td>A22</td><td>720.3</td><td>A58</td><td>116.5</td></tr><tr><td>A23</td><td>1.6</td><td>A59</td><td>2341.3</td></tr><tr><td>A24</td><td>73.5</td><td>A60</td><td>101.7</td></tr><tr><td>A25</td><td>572.7</td><td>A61</td><td>1.4</td></tr><tr><td>A26</td><td>31.6</td><td>A62</td><td>11.9</td></tr><tr><td>A27</td><td>318.7</td><td>A63</td><td>336.8</td></tr><tr><td>A28</td><td>3763.1</td><td>A64</td><td>56.3</td></tr><tr><td>A29</td><td>343.8</td><td>A65</td><td>1326.6</td></tr><tr><td>A30</td><td>2836.4</td><td>A66</td><td>1.3</td></tr><tr><td>A31</td><td>2.8</td><td>A67</td><td>113.6</td></tr><tr><td>A32</td><td>157.2</td><td>A68</td><td>20.2</td></tr><tr><td>A33</td><td>12.0</td><td>Total</td><td>34840.9</td></tr><tr><td>A34</td><td>1.5</td><td></td><td></td></tr><tr><td>A35</td><td>79.4</td><td></td><td></td></tr><tr><td>A36</td><td>5.3</td><td></td><td></td></tr></table>

<table><caption>Table 5: Type, number, and volume of falls observed between 08/10/2010 and 09/07/2021 in Varengeville-sur-Mer (falls resulting from imprecise surveys are marked with an asterisk).</caption><tr><td>B3</td><td>8.8</td></tr><tr><td>B4</td><td>8.7</td></tr><tr><td>B5</td><td>2.6</td></tr><tr><td>B6</td><td>7.4</td></tr><tr><td>B7</td><td>3.9</td></tr><tr><td>B8</td><td>2.3</td></tr><tr><td>B9</td><td>1.5</td></tr><tr><td>B10</td><td>2.6</td></tr><tr><td>B11</td><td>2.7</td></tr><tr><td>B12</td><td>1.6</td></tr><tr><td>B13</td><td>5.3</td></tr><tr><td>B14</td><td>7.6</td></tr><tr><td>B15</td><td>0.6</td></tr><tr><td>B16</td><td>9.3</td></tr><tr><td>B17</td><td>5.9</td></tr><tr><td>B18</td><td>4.1</td></tr><tr><td>B19</td><td>19.9</td></tr><tr><td>B20</td><td>0.4</td></tr><tr><td>B21</td><td>0.2</td></tr><tr><td>B22</td><td>0.3</td></tr><tr><td>B23*</td><td>14.9</td></tr><tr><td>B24</td><td>0.4</td></tr><tr><td>B25</td><td>0.2</td></tr><tr><td>B26</td><td>2.5</td></tr><tr><td>B27</td><td>0.2</td></tr><tr><td>B28</td><td>0.3</td></tr><tr><td>B29</td><td>5.3</td></tr><tr><td>B30</td><td>4.2</td></tr><tr><td>B31</td><td>2.3</td></tr><tr><td>B32</td><td>4.4</td></tr><tr><td>B33</td><td>5.0</td></tr><tr><td>B34</td><td>3.2</td></tr><tr><td>B35</td><td>4.4</td></tr><tr><td>Total</td><td>142.9</td></tr></table>

537

538

539

540
Varengeville-sur-Mer

East

West

East

West

1/2

11/03/2014-09/07/2014

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

09/07/2014-24/09/2014

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

24/09/2014-26/02/2015*

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

26/02/2015*-08/06/2015

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

08/06/2015-25/09/2015

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

25/09/2015-28/01/2016

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

28/01/2016-21/06/2016

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

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

21/06/2016-27/10/2016

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

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

27/10/2016-25/01/2017

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

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

25/01/2017-26/06/2017

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

<figure><img src="image_27.png" /></figure>

08/10/2010-24/02/2011*

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

24/02/2011*-27/09/2011

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

27/09/2011-18/12/2011

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

18/12/2011-21/03/2012

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

21/03/2012-18/09/2012

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

18/09/2012-11/02/2013

<figure><img src="image_38.png" /></figure>

<figure><img src="image_39.png" /></figure>

11/02/2013-25/09/2013

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>

25/09/2013-12/02/2014

<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /></figure>

12/02/2014-11/03/2014

<figure><img src="image_44.png" /></figure>

<figure><img src="image_45.png" /></figure>

26/06/2017-02/11/2017

A21

<figure><img src="image_46.png" /></figure>

A24

A25

A26

02/11/2017-16/01/2018

<figure><img src="image_47.png" /></figure>

A28

A29

A32

A33

16/01/2018-29/03/2018

<figure><img src="image_48.png" /></figure>

A36

B17

A37

B18

A38

29/03/2018-06/07/2018

<figure><img src="image_49.png" /></figure>

A42

A43

A44

A46

B19

0 40 m

<figure><img src="image_50.png" /></figure>

08/10/2010 Date of point cloud acquisition

24/02/2011* Date of point cloud with high imprecision (≥ ± 0.03 m)

A Rock fall (depth > 1 m and with a diameter > 1 m)

B Debris fall (depth < ≈ 1 m and with a diameter ≈ 1 m)
<figure><img src="image_51.png" /><figcaption>Fig. 7: Cliff face erosion at Varengeville-sur-Mer between 08/10/2010 and 09/07/2021.</figcaption></figure>
544

545

546

547

548

549

550

For Dieppe 1 W and 2 N, between 07/10/2010 and 09/07/2021, the cliff faces were not affected by rock falls (Figs. 8 and 9). For block falls and stone falls (debris falls), no stone falls and six stone falls (Figs. 9 and 10a) were observed, totaling $1.5 \text{ m}^3 \pm 0.3 \text{ m}^3$, and these were observed in all seasons. Note that erosion caused by anthropogenic actions to secure the cliffs has not been considered. The cliff face evolves very slowly, being affected only by subaerial agents and processes, which appear to be relatively ineffective in terms of erosion.
<figure><img src="image_52.png" /><figcaption>Fig. 8: Cliff face erosion at Dieppe 1 W between 07/10/2010 and 09/07/2021.</figcaption></figure>
<figure><img src="image_53.png" /><figcaption>Fig. 9: Cliff face erosion at Dieppe 2 N between 07/10/2010 and 09/07/2021.</figcaption></figure>

553

554

555
4.4. *Basal notch leading to complete cliff face instability*

Fig. 7 shows one of the erosion modalities previously described along the Seine-Maritime coastline (Costa et al., 2006a; Dewez et al., 2013; Letortu et al., 2019. 2015c): the creation of a basal notch (ranging from a few decimeters to just over 2 m deep) which, by generating an overhang in the upper part, progressively destabilizes the entire cliff face and results in a rock fall (mass movement). Nine such cases have been observed over ten years of monitoring at Varengeville-sur-Mer. On the eastern side, seven cases can be observed and on the western side, there were two cases (Table 6).

<table><thead><tr><th colspan="2">Eastern side of Varengeville-sur-Mer</th><th colspan="2">Western side of Varengeville-sur-Mer</th></tr><tr><th>Basal notch (number, volume estimate in m³)</th><th>Rock fall of the entire cliff face (number, volume estimate in m³)</th><th>Basal notch (number, volume estimate in m³)</th><th>Rock fall of the entire cliff face (number, volume estimate in m³)</th></tr></thead><tbody><tr><td>A2 (uncertain volume), A3 (99.4 ± 5.3 m³)</td><td>A5 (911.2 ± 27.6 m³)</td><td>A14 (3,979.6 ± 66.9 m³)</td><td>A15 (8,509.2 ± 60.3 m³)</td></tr><tr><td>A10 (44.4 ± 5.9 m³)</td><td>A13 (1532.3 ± 32.7 m³)</td><td>B11 (2.7 ± 0.4 m³)</td><td>A30 (2,836.4 ± 29.4 m³)</td></tr><tr><td>A16 (uncertain volume), B8 (2.3 ± 0.5 m³), A17 (3.8 ± 0.6 m³), A18 (31.8 ± 3.6 m³)</td><td>A22 (720.3 ± 21.9 m³)</td><td></td><td></td></tr><tr><td>A21 (48.3 ± 2.0 m³), A24 (73.5 ± 6.2 m³)</td><td>A28 (3763.1 ± 36.5 m³)</td><td></td><td></td></tr><tr><td>A29 (343.8 ± 14.3 m³)</td><td>A43 (732.0 ± 21.3 m³)</td><td></td><td></td></tr><tr><td>A37 (162.0 ± 9.6 m³), A47 (82.5 ± 7.1 m³)</td><td>A54 (2133.9 ± 33.1 m³)</td><td></td><td></td></tr><tr><td>A53 (31.1 ± 2.4 m³)</td><td>A59 (2341.3 ± 43.9 m³)</td><td></td><td></td></tr></tbody></table>
Table 6: Basal notch leading to rock fall of entire cliff face (numbers, volume estimates) observed in the eastern and western sides of Varengeville-sur-Mer between 08/10/2010 and 09/07/2021.

## 5. Discussion

### 5.1. Dieppe: Minimal erosion on the abandoned cliff

Between October 2010 and July 2021, the erosion observed on the cliff faces at Varengeville-sur-Mer and Dieppe shows significant differences. The abandoned cliff at Dieppe, composed of Coniacian chalk and affected exclusively by subaerial agents and processes (e.g. rainfall with runoff and infiltration (Duperret et al., 2002; Lageat et al., 2006; Letortu et al., 2015a), temperature variations leading to gelifraction (Dewez et al., 2015; Letortu et al., 2015a)) exhibits relative stability with anthropogenic actions removed (with six stone falls observed at Dieppe 2 N, Fig. 10a). In contrast, the active cliff at Varengeville-sur-Mer, composed of Santonian chalk overlying Tertiary terrain and affected by both marine and subaerial agents and processes, is eroding rapidly. This cliff face experiences frequent mass movements (68 rock falls, accounting for 68% of total erosion) and debris falls (32% of total erosion). The abandoned cliff face at Dieppe has changed little over the 10 years of monitoring. This relative stability is due to the ineffectiveness of erosion agents and processes, as well as the removal of potential erosion zones through anthropogenic actions when the cliff face was made safe. Weathering continues, and fractures can be observed on the cliff faces at Dieppe, notably indicating areas that may delimit future rock falls (e.g. Frayssines and Hantz, 2006; Collins and Stock, 2016) (Fig. 10b).
<figure><img src="image_54.png" /></figure>

Dieppe 2 N (P. Letortu, 20/06/2019)

<figure><img src="image_55.png" /></figure>

Dieppe 2 N (P. Letortu, 26/06/2022)

586

587 Fig. 10: a) Fallen stone at the base of the cliff at Dieppe 2 N; b) fractures delimiting a potential rock fall on the cliff face at Dieppe 2 N.

589

590 If further work is carried out to make the Dieppe site safe, it will be necessary to consider whether this site should be retained, or if other sites, such as Penly in Petit-Caux (9 km NE of Dieppe, Fig. 3) might be more suitable.

593 In contrast to Varengeville-sur-Mer, where 35 individualized debris falls were recorded (Table 5), there have been few debris falls (only 6 stone falls) on the cliff face at Dieppe. In addition to marine actions, the presence of perched aquifers in the Tertiary strata of Varengeville-sur-Mer facilitates the wetting of chalk through runoff from the cliff top, which can promote erosion processes (Dietze et al., 2020) such as gelifraction, leading to debris falls (Letortu et al., 2015a). Indeed, the cliff face is often shimmery in Varengeville-sur-Mer, whereas this is not the case in Dieppe. At Dieppe, the top of the cliff is very fractured due to karstic networks and tunnels built during the Second World War, which could drain surface water, causing high flow circulation into the cliff.

602

603 5.2. Varengeville-sur-Mer: More rock falls on the eastern cliff face than on the western cliff
604 face
At Varengeville-sur-Mer, the eastern cliff face appears to be more dynamic in terms of the number of rock falls (though not in total erosion) over the observed decade. It would be interesting to see if this trend continues in subsequent years. One hypothesis is that the chalk in this area may be more fractured, making it more susceptible to small but frequent rock falls, following the power law scaling properties of fall magnitude-frequency distributions (Dong and Guzzetti, 2005; Teixeira, 2006; Barlow et al., 2012).

### 5.3. Prevalence of marine actions in the erosion of Norman cliffs: Toward quantifying the respective contribution of agents and processes

Cliff face monitoring over the past decade confirms the intense dynamics at Varengeville-sur-Mer and highlights the prevalence of marine agents and processes over subaerial ones, supporting the findings of researchers in Normandy and beyond (e.g. Guilcher, 1954; Earlie et al., 2015; Rink et al., 2024). These results now need to be supplemented by monitoring the magnitude of subaerial and marine parameters (moisture and water availability, wave action...) as has been done recently on some coasts worldwide (Dietze et al., 2020; Bernatchez et al., 2021; Young et al., 2021), particularly in the context of climate change, and by improving the dating of rock falls thanks to social platforms and very high resolution and high revisit frequency satellite images as Pléiades (Letortu et al., 2020, 2021). To this end, two complementary approaches have been implemented:

1. The installation of a pressure sensor on the shore platform since January 2023, supported by the French coastal observatory DYNALIT, in conjunction with a camera positioned in Hautot-sur-Mer (located 1 km E of the studied site). This set up allows for the analysis of water levels (including storm surge, runup, and setup), spectral energy distribution, wave breaking types, and gravel barrier morphology (Soloy et al., 2021; Letortu et al., 2022);

2. Monitoring the meteorological parameters with a weather station installed on the plateau in Varengeville-sur-Mer, near the cliff edge since July 2024.

However, four main sources of complexity remain in determining the agents and processes responsible for the failures:
633

- Firstly, the non-linear evolution of the coastal environment can limit direct cause-and-effect relationships. Thresholds, hysteresis, synergies/combinations, positive and negative feedbacks and the "purge" effect (described in detail in Viles, 2013) complicate the analysis of these relationships (e.g. Inkpen and Stephenson, 2006). For example, the relative role of marine and subaerial agents and processes in cliff erosion may change over time as the shore platform gradually widens, which affects the amount of wave energy delivered to the cliff base (negative feedback);

634

635

636

637

638

639

640

- Secondly, boundary conditions (such as sea-level, climate, geology, tides, and wind waves) modulate the dominance of agents and processes in coastal evolution at specific locations and times, necessitating the integration of spatial and temporal scales. For example, the erosive power of the ocean depends on the relative balance between tidal and wave climates (Kennedy et al., 2014a);

641

642

643

644

645

- Thirdly, the cliff is also a legacy of past environmental changes (Trenhaile, 2002). In understanding current agents and processes and their impacts, it is important to consider long-term rock degradation as well (Griggs and Trenhaile, 1994; Swenson et al., 2006; Joyal et al., 2016; Duguet et al., 2021);

646

647

648

649

- Finally, studies often focus on the cliff face, whereas the cliff is part of a vast system that includes the entire slope from the nearshore to the hinterland (Trenhaile, 1987; Pinot, 1998). Such studies are likely to provide only a partial view of the behavior of the entire slope. Therefore, obtaining and analyzing simultaneous data on the whole system is a major scientific challenge.

650

651

652

653

654

655

### 5.4. Switching to terrestrial photogrammetry: a potential limitation of the technique

656

657

658

659

660

The shift from terrestrial lasergrammetry to terrestrial photogrammetry is certainly less cumbersome, more cost-effective, and flexible in the field (Letortu et al., 2018; Letortu, 2023), but it has some drawbacks, such as the variability of 3D reconstruction depending on light conditions (e.g. Burdziakowski and Bobkowska, 2021). This is because photogrammetry is a passive technique that does not generate its own measurement signal, unlike
661 lasergrammetry, which is an active technique. In our fieldwork at Varengeville-sur-Mer, with
662 the highly variable light conditions (cloudy with sunny spells) during the cliff line survey of
663 530 m, error may increase, limiting the precision of scree quantification (with an error around
664 0.05 m instead of 0.03 m). It is therefore important that the data acquisition period preferably
665 occurs during overcast weather.

666

667 **6. Conclusions**

668 For more than 10 years (2010-2021), cliff face erosion has been monitored by terrestrial
669 lasergrammetry and photogrammetry with cm-scale precision at two chalk cliff sites in
670 Normandy, located 6.5 km apart as the crow flies. The sites, which have similar lithology
671 (Coniacian and Santonian chalk), are affected by roughly the same subaerial agents but
672 differ in their exposure to marine agents: Varengeville-sur-Mer is an active cliff, while Dieppe
673 is an abandoned cliff. This configuration provides an opportunity to contribute to the debate
674 about the roles of subaerial and marine agents in coastal erosion along the Seine-Maritime
675 chalk coastline. Our research presents five main findings: 1) the cliff face erosion rate is
676 around 50 times higher in the context of the active cliff than in the abandoned one (0.51 ±
677 0.02 m/year vs. 0.00 ± 0.01 m/year). This confirms and reinforces the predominance of
678 marine actions in the erosion of Norman chalk cliffs compared to our previous results (Letortu
679 et al., 2015c, 2019); 2) erosion peaks for the active cliff tend to occur in winter, while calm
680 periods mostly occur in summer; 3) 103 rock falls accounted for 68% of total erosion at
681 Varengeville-sur-Mer, compared with 32% for debris falls. The latter value is higher than in
682 previous studies (Hénaff et al., 2002; Letortu et al., 2019, 2015c; May and Heeps, 1985),
683 which range from 2% to 25%. These material displacements, often overlooked due to their
684 large scale, are now quantified and can be better integrated into research. 4) Nine cases of
685 cliff foot erosion at Varengeville-sur-Mer generate instability through overhanging, which
686 leads to rock falls along the entire cliff face. This ongoing monitoring provides a valuable
687 database for understanding cliff erosion, complementing high-frequency monitoring over
688 shorter periods (less than a decade), such as that conducted by Pennington et al. (2024). It
689 contributes to both fundamental knowledge (erosion rates, rhythms, and modalities) and
690 applied knowledge (risk management, including basal notches generating overhanging
691 zones as precursory signs of failure and forecasting falls using probabilistic analyses)
692 through a multitude of proven and emerging methods and techniques. For example, this
693 dataset can be used to train deep learning algorithms, which can facilitate data analysis,
694 particularly for 1) identifying erosion zones (de Gélis et al., 2022) and 2) calculating eroded
695 volumes.

696

697 Acknowledgements

698 Dieppe – cap d'Ailly is a monitoring site of the French coastal observatory SNO-DYNALIT
699 (https://www.dynalit.fr). DYNALIT acknowledges funding from the National Centre for
700 Scientific Research (CNRS-INSU), the French coastal research infrastructure ILICO
701 (https://www.ir-ilico.fr/), the ministry for higher education and research and the agency for
702 research on the environment.

703 This work was supported by the French "Agence Nationale de la Recherche" through the
704 "Laboratoire d'Excellence" LabexMER [ANR-10-LABX-19-01] program, and co-funded by a
705 grant from the French government through the "Investissements d'Avenir" program and the
706 Brittany Region. It was also supported by ISblue project, Interdisciplinary graduate school for
707 the blue planet (ANR-17-EURE-0015), and co-funded by a grant from the French
708 government under the program "Investissements d'Avenir".

709 This research was also supported by the CNES (EROFALITT and HIRACLES projects). It is
710 based on observations with Pléiades satellites.

711 We extend our thanks to Marion Jaud, Emmanuel Augereau, and Stéphane Bertin for their
712 advice, and the reviewers for their detailed reading of the manuscript and their very helpful
713 and constructive comments.

714

715 Data availability
716 Part of the dataset is available online (https://doi.org/10.35110/9e8e66f9-7801-4c30-9486-

717 bdbcd584b577 for Varengeville-sur-Mer, https://doi.org/10.35110/19c1b837-a688-49ca-8b6f-

718 22f35911c609 for Dieppe 1 W and : https://doi.org/10.35110/7e8463d2-1643-4522-a571-

719 9832c4795c90 for Dieppe 2 N). Other data can be provided upon reasonable request.

720

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