# Beach-dune dynamics and coastal management along the North-Médoc coastline, southwest France: a decade of morphological change and erosion mitigation

Marine Vandenhove, Bruno Castelle, Alexandre Nicolae Lerma, Vincent Marieu, Vincent Mazeiraud

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Marine Vandenhove, Bruno Castelle, Alexandre Nicolae Lerma, Vincent Marieu, Vincent Mazeiraud. Beach-dune dynamics and coastal management along the North-Médoc coastline, southwest France: a decade of morphological change and erosion mitigation. Geomorphology, 2025, 482, pp.109758. 10.1016/j geomorph.2025.109758. hal-05078050

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# Beach-dune dynamics and coastal management along the North-Médoc coastline, southwest France: a decade of morphological change and erosion mitigation

Marine Vandenhove<sup>a,b,*</sup>, Bruno Castelle<sup>a</sup>, Alexandre Nicolae Lerma<sup>b</sup>, Vincent Marieu<sup>a</sup>, Vincent Mazeiraud<sup>c</sup>

## ARTICLE INFO

**Keywords:**

Coastal erosion

Shoreline variability

Morphological changes

Coastal management

LiDAR data

## ABSTRACT

The North-Médoc coastline, exposed to energetic waves and meso-macro tides and exhibiting major erosion hot-spots, has experienced notable changes in beach and dune volumes over the last decades due to a combination of natural forcings and human interventions, including coastal defenses and beach nourishment. Annual LiDAR data (2011–2023) combined with interspersed annual GNSS surveys were used to quantify spatial and temporal variations in beach-dune topography along approximately 14.5 km of coastline. Beach-dune changes along the North-Médoc coast over the past decade were influenced by multiple factors at varying scales. (1) Long-term changes, such as chronic erosion or accretion of up to 7 m/yr, are mainly inherited from large-scale shoal welding originating from the estuary mouth (internal forcing). (2) Coastal structures have localized impacts on shoreline dynamics, sometimes altering trajectories. While alongshore structures stabilize the shoreline, they may cause issues at their ends. Cross-shore structures, like groynes, have variable effects; for instance, extending one of the groyne after winter 2013–14 led to rapid accretion, widening beach, and raising elevations updrift. This accreting area is now used as a sediment extraction area to nourish the eroding southern beaches. (3) Interannual shoreline variability, linked to winter wave height fluctuations driven by climate modes of atmospheric variability, overlaps with trends from (1) and (2). However, this external forcing is more pronounced farther from the estuary, indicating a continuum from external forcing on open coasts to internal processes near the estuary.

## 1. Introduction

Since the late 19th century, the growing interest in sandy coasts has driven demographic growth and urbanization (Creel, 2003). Sandy coasts, which are directly exposed to storms, continuously adjust to wave energy through phases of erosion and accretion (Hunt et al., 2023). However, in some locations, the loss or degradation of natural habitats, often linked to anthropogenic structures or activities and referred to as coastal squeeze (Pontee, 2013), limits the ability of sandy coasts to adapt to changes in wave conditions. In such areas, chronic erosion can threaten coastal infrastructure (Williams et al., 2018) and even lead to the disappearance of sandy beaches (Cooper et al., 2020). To manage chronic erosion, coastal management strategies offer a variety of

solutions, including the construction of coastal defenses, beach nourishment, the promotion of foredune growth, and, in some cases, population relocation (Hapke et al., 2013; Rulleau and Rey-Valette, 2017; Dundon and Abkowitz, 2021; de Schipper et al., 2021). These measures vary in their durability and often induce adverse effects on adjacent coastlines (Hamm et al., 2002; Ells and Murray, 2012; de Schipper et al., 2021; Velasquez-Montoya et al., 2020). An in-depth understanding of the physical processes driving morphological changes, from the time scale of individual storms to the decade, is essential for the effective implementation of these coastal management strategies and for safeguarding the increasing interest in these coastal environments.

The morphological changes of sandy coasts occur on short-term (days to weeks), medium-term (months to years), and long-term (for

<footnote><sup>a</sup> Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, F-33600 Pessac, France</footnote>

<footnote><sup>b</sup> BRGM (French Geological Survey), Direction Régionale Nouvelle-Aquitaine, Parc Technologique Europarc, 24 avenue Léonard de Vinci, Pessac 33600, France</footnote>

<footnote><sup>c</sup> Communauté de Communes Médoc Atlantique, 9 rue du Maréchal d'Ornano, Soulac-sur-Mer 33780, France</footnote>

<footnote>* Corresponding author at: Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, F-33600 Pessac, France.
E-mail address: marine.vandenhove@u-bordeaux.fr (M. Vandenhove).</footnote>
decades) temporal scales (Stive et al., 2002; Hunt et al., 2023). On open sandy coasts, unaffected by anthropogenic activities and/or away from the influence of tidal inlets or estuary mouths, these temporal changes are driven by variability in hydrodynamic conditions operating on similar time scales (Castelle and Masselink, 2023). Specifically, the interplay of rapid storm-driven erosion and slower recovery periods leads to seasonal to multi-annual changes in beach morphology (e.g. Davidson and Turner, 2009). These longer-term changes are often related to atmospheric climate modes (e.g. Barnard et al., 2017; Mas-selink et al., 2023; Vos et al., 2023; Castelle et al., 2024). In contrast, along coasts near tidal inlets or estuary mouths, the morphological changes of sandy beaches become more complex as additional driving factors overlay external wave forcing. This additional forcing, often referred to as internal forcing, arises from the internal dynamics of the inlet (Hayes, 1975, 1980; De Swart and Zimmerman, 2009). These self-organized processes typically result in phenomena such as shoal migration and welding (FitzGerald, 1984; Gaudiano and Kana, 2001;

Robin et al., 2009; Ridderinkhof et al., 2016; Elias et al., 2019; Lenstra et al., 2019; Burvingt et al., 2022) and pronounced nearshore bathymetric changes like channel creation or deepening (Elias et al., 2019; Vandenhove et al., 2024). These morphological changes can considerably impact shoreline evolution along adjacent coasts (e.g. FitzGerald et al., 1984; van Heteren et al., 2006; Elias et al., 2019), which explains why maximum shoreline variations are typically observed in these areas (Castelle et al., 2022). The amplitude and time scales of these alternate periods of erosion and accretion are highly site-specific, generally increasing with the size of the estuary or inlet system (e.g. Ridderinkhof et al., 2016; Burvingt et al., 2022; Vandenhove et al., 2024). Furthermore, the respective contributions of internal and external forcing are often poorly understood and vary substantially alongshore. The situation is further complicated by human interventions, such as coastal structures and beach nourishment, which obscure the distinction between these driving factors. To effectively disentangle the contributions of these drivers, long-term (over 10 years) and reasonably high-

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) Location map of the study area showing the location of the four profiles surveyed (P1 to P4), the four groynes, the nine sectors showing contrasting behaviors (#1 to #9), and the sediment source area (in blue), where sand is sourced to nourish eroding sectors updrift (in red, with different locations for different dates), overlaid on the October 2023 orthophotography (source: PIGMA). (b-f) Implementation since October 2012 to September 2023 of major coastal structures: (b) Groyne 1, (c) Groyne 2 north of the 3 km seawall, (d) Groyne 3 extended in 2014–2015, (e) seawall constructed in 2009 and dismantled in 2019, and (f) Groyne 4 north of a dike extended in 2014–2015 and a seawall progressively enlarged since its construction in 2010.</figcaption></figure>
frequency (e.g., pre- and post-winter) morphological data are needed.

The North-Médoc coastline, a 14.5 km long sandy stretch in southwest France located near the large-scale Gironde estuary mouth, has undergone major and complex changes over the past century. These long-term changes have been attributed to the internal dynamics of the adjacent inlet and the implementation of coastal defenses (Vandenhove et al., 2024). However, on shorter time scales (i.e. years to decades), the influence of external forcing on multi-annual variability remains virtually unknown. Further south, along the open sandy coast, previous studies have shown that this variability predominantly drives shoreline changes, with fluctuations related to climate modes of atmospheric variability (Castelle et al., 2022; Masselink et al., 2023). The primary objective of this paper is to address the respective contributions of external forcing, internal forcing, and coastal management strategies to coastal changes at the beach-dune system neighboring the estuary mouth and to assess how these contributions vary along the coastline. To address these objectives, we combine annual large-scale LiDAR data, yearly GNSS topographic surveys at specific transects, wave data, and records of coastal management interventions.

## 2. Methods

### 2.1. Study site

The North-Médoc coast is situated in southwest France along the Atlantic coastline (Fig. 1a). This sector exhibits a complex sedimentological history shaped by coastal erosion and varying depositional environments, with a transition from Pleistocene deposits to Holocene marine sands and presence of ancient inlets (Stéphan et al., 2019). The sector experienced alternating coastal dune activity/stability periods during the late Holocene, which are mostly synchronous to those documented for the wider southwest coast of France Clarke et al. (2002). Beaches are made of fine to medium quartz sand with a median grain size around 300 µ m. The dune system, when present, is perched on Pleistocene paleosols, which are more resistant to marine erosion than sand and locally outcrop at the dune toe and/or upper part of the beach, particularly in chronically eroding sectors and/or after high-energy winters Stéphan et al. (2019); Laporte-Fauret et al. (2022). Coastal dunes are characterized by a relatively large range of plant species, with foredune and incipient dune systems dominated by Ammophila arenaria and Elymus farctus, respectively (Laporte-Fauret et al., 2020). Wind conditions are strongly seasonally modulated with an average hourly mean wind speed and direction of 4.5 m/s and 311° (NW) in summer and 5.2 m/s 263° (W) in winter, potentially exceeding 36 m/s during extreme storms (Laporte-Fauret et al., 2021). This region is exposed to energetic and seasonally-modulated wave conditions, predominantly from the W-NW direction in July (boreal summer) and from the W in January (boreal winter), with an average significant wave height of 1.1 m during summer and 2.4 m in winter. Due to the prevailing wave direction and the N-NW orientation of the coastline, the longshore drift along the North-Médoc coast is primarily directed northeastward (Fig. 1a). South of Cape Négade, however, it shifts direction and is directed southward, creating a divergence zone (Idier et al., 2013; Howa, 1997). The study area is also adjacent to the large macrotidal Gironde estuary, where alongshore tidal currents are notably strong, with tides modulating wave transformation and flow in both the nearshore and inner shelf region of the North-Médoc coast (Vandenhove et al., 2025), with flow velocities offshore exceeding 1 m/s (Howa, 1987). External forcings, such as waves and tides, interact with the internal dynamics of the estuary mouth. This interaction drives substantial multi-decadal morphodynamic changes in the nearshore region, including channel deepening and the attachment of sandbanks. The resulting bathymetric changes have had profound effects on large-scale and long-term shoreline evolution along the North-Médoc coast over the past century. These processes are explored in detail in Vandenhove et al. (2024).

Coastal defense structures were first implemented along this coastline in the mid-19th century. To address the challenges of coastal dynamics and safeguard human and environmental interests, these strategies have evolved over time, resulting in the current coastal configuration shown in Fig. 1. In the northern half of the study area, coastal defense structures were established over 30 years ago, with no subsequent construction. These defenses include Groyne 1 (Fig. 1b), followed by Groyne 2, and a 3-km-long seawall (Fig. 1c). In contrast, the southern half has seen recent interventions. Notably, Groyne 3 was extended in 2014–2015 following the severe 2013/14 winter storms that caused extensive erosion along Europe’s Atlantic coast (Masselink et al., 2016) (Fig. 1d). A 100-m-long seawall, constructed in 2009 to protect a camping ground, was dismantled in 2019 (Fig. 1e), followed by a rapid realignment of the shoreline the same year. Further south, Groyne 4 and associated dikes were constructed in 2004–2005, incorporating older infrastructure, with the dikes extended southward in 2014–2015 (Fig. 1f). The southern seawall, built in 2010, has been progressively extended landward at its southern tip in response to localized erosion. Since 2018, beach nourishment has been introduced to complement these hard defense measures. Sand is sourced from the accreting beach updrift of Groyne 3 (Fig. 1a) and transported to nourish eroding areas further south. The nourishment volume increased from 40,000 m³/year in 2018 and 2019 to 60,000 m³/year between 2020 and 2023, reaching 85,000 m³ in 2024. The nourished zones of this sand “back-passing” system are indicated in Fig. 1a.

### 2.2. Dataset acquisition and treatment

#### 2.2.1. LiDAR dataset

Airborne LiDAR surveys were conducted along the entire southwest coast of France in 2011, 2014, and annually from 2016 to 2023 (Nicolae Lerma et al., 2019). These campaigns were carried out by the Observatoire de la Côte de Nouvelle-Aquitaine (OCNA) in collaboration with the French National Institute of Geography (IGN). The surveys produced 10 digital terrain models (DTMs) with a 1-m grid resolution and vertical errors below 0.15 m (Nicolae Lerma et al., 2019), using the Lambert 93 French national projected coordinate system. Elevation values are provided in NGF, referencing the French national geodetic datum. The DTMs were primarily acquired in early autumn (September–October), after the summer season, at spring low tide. An exception was the first campaign in 2011, conducted in spring (March/April).

A local cross-shore/alongshore coordinate grid (with y representing the longshore coordinate) was generated, extending 15,000 m alongshore and 1000 m cross-shore, with resolutions of 10 m and 2 m, respectively (Fig. 2a). The grid is aligned to a smoothed shoreline baseline of the North-Médoc coast (Fig. 2b). Each of the 10 LiDAR-

<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) Local cross-shore/alongshore coordinate system grid and (b) applied on a shoreline baseline (in black) of the North-Médoc coast (in gray).</figcaption></figure>
derived DTM s was interpolated onto this grid, using a NNI (Natural Neighbor Interpolation) method, enabling direct computation of differences between DTM s. These differences were analyzed by dividing the study area into sectors and quantifying volumetric changes in cubic meters per unit beach width at each 10-m spaced cross-shore transect. These sectors were delimited by major coastal structures (i.e. the four groynes in Fig. 1) and/or designed to discriminate areas with contrasting coastal change trajectories. To ensure accurate volume calculations, a data cleaning procedure was implemented. The inland limit of the computational domain was manually defined where morphological changes to the east became negligible (<0.2 m overall). The offshore limit was set at the 0.5-m NGF contour which approximately represents the lowest elevation covered by all LiDAR datasets and roughly corresponds to mean sea level (Port Bloc tide station, SHOM - Service Hydrographique et Océanographique de la Marine (2022)).

It is important to highlight that, after 2020, due to large accretion, the LiDAR surveys no longer fully covered the entire sandbank located north of the study site. This limitation resulted in flawed volume computations, as the minimum elevation of 0.5 m NGF was not reached on the seaward side. In addition, total volume differences were calculated using post-summer data, which account for beach recovery processes and, in some areas, beach nourishment. Consequently, the volume differences computed in this study are not entirely representative of natural winter-storm-driven erosion processes (2.1).

### 2.2.2. Beach-dune profile dataset

Post-winter beach-dune elevation profiles (P1 to P4) were surveyed annually from 2003 to 2024 by OCNA. These profiles were combined with post-summer data from the LiDAR surveys (see transect locations in Fig. 1a and details in Table 1). For more information on the OCNA GNSS survey dataset, readers are referred to (Nicolae Lerma et al., 2022).

To analyze the temporal evolution of the dune and upper beach at each profile (P1 to P4), elevation indicators (isocontours in NGF) were selected. Spatio-temporal trends of isocontours ranging from 5 NGF to 10 NGF were compared (Fig. 3). For profile P3, as shown in Fig. 3b, shoreline isocontours from 5 NGF to 10 NGF exhibit similar patterns, indicating that any of these contours can effectively characterize dune foot evolution. Profiles P1 and P4 show similar behavior (not displayed) given the relatively steep and homogeneous dune scarp. At profile P2 (Fig. 3a), however, the trends do not align due to the absence of a dune, as the seafront is managed for the town of Soulac-sur-Mer. Consequently, the 5.5-m NGF elevation contour was chosen to track upper beach evolution at P2 and dune foot evolution at P1, P3, and P4.

In order to address the impact of winter-wave conditions on inter-annual shoreline variability, winter shoreline change $dS/dt$ was systematically compared with the winter(DJFM)-mean wave height. Offshore wave conditions were retrieved from C3S ERA5 reanalysis (Hersbach et al., 2020) in approximately 45-m depth. As shown in Fig. 4 and in previous work, this winter-mean variability is largely controlled by climate modes of atmospheric variability. The Western Europe Pressure Anomaly (WEPA) index, explains most of the winter-mean wave height variability in the Bay of Biscay (Castelle et al., 2017). This climate index is defined as the normalized sea level pressure gradient anomaly between Ireland and the Canary Islands. A positive phases of WEPA corresponds to a southward-shifted and intensified

<table><caption>**Table 1**

Typology of beach/dune topographic profiles available at each transect, P1 to P4.</caption><thead><tr><td>Transect</td><td>Temporal coverage (yr)</td><td>Total number</td><td>Extracted from LiDAR</td><td>Measured by GNSS</td></tr></thead><tbody><tr><td>P1</td><td>2003–2024</td><td>28</td><td>9</td><td>19</td></tr><tr><td>P2</td><td>2010–2024</td><td>23</td><td>9</td><td>14</td></tr><tr><td>P3</td><td>2003–2024</td><td>30</td><td>10</td><td>20</td></tr><tr><td>P4</td><td>2003–2024</td><td>30</td><td>10</td><td>20</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Temporal evolution of elevation references ranging from 1 m NGF to 10 m NGF at the beach profile (a) P2 and (b) P3.</figcaption></figure>

Icelandic Low pressure/Azores High pressure gradient, which directs high-energy waves and storm winds towards the Bay of Biscay and most of the Atlantic coast of Europe. As shown in Fig. 4, WEPA explains approximately 79 % of the winter-mean wave height variability offshore of the Médoc coast.

## 3. Results

### 3.1. Large-scale spatial variability and management mode effects

Fig. 5 illustrates the topographic changes across the entire study area from 2011 (Fig. 5a) to 2023 (Fig. 5b), highlighting elevation variations exceeding 1 m in most parts of the study domain (Fig. 5c). Overall, a decrease in elevation is observed in the southern part, while an increase is seen in the northern section. This pattern is further emphasized in Fig. 6, which also provides insight into the temporal evolution of total volume (Fig. 6a), dune volume (Fig. 6b), and beach volume (Fig. 6c) changes since 2011. Volume differences for each sector from 2011 to 2023 are detailed in Table 2. The data show that substantial volume changes occurred overall, except in areas protected by seawalls in Sectors 2 and 6. Notably, at the seawall north of Sector 3, much of the “dune” volume loss reflects the dismantling of the seawall in 2019 (Fig. 1e). In the north, at Sector 9, where the beach extended beyond the LiDAR survey coverage, the dune volume dramatically increased by 550 m³/m between 2011 and 2023 (Fig. 6b, Table 2).

In the southern region, from Sector 1 (y = 0 m) to Sector 4 (y = 5110 m), there is an extensive loss of dune volume, amounting to -3,090,000 m³ between 2011 and 2023. However, at Sector 5, Fig. 6a shows a net gain in total volume compared to 2011, with most of the contribution occurring since 2016. A closer examination of the elevation changes in Fig. 7 reveals a general increase in beach elevation following the extension of Groyne 3, which was constructed in 2014–2015 (Fig. 1d). This overall volume variation is a combination of gains in both beach and dune, due to an incipient dune formation (Fig. 6b and c). Since 2016, the beach volume south of Groyne 3 has increased by 190,000 m³ over a distance of 1700 m, reaching up to the middle of Sector 4 (y = 4480 m). The increase in dune volume amounts to 270,000 m³ and spans 1120 m on both sides of Groyne 3 (from y = 5310 m to y = 6430 m). It is noteworthy that since 2018, a large volume of sand (beach) has been extracted directly south of Groyne 3 to nourish eroding areas in the southern part of Sector 4 and Sector 3. Despite this substantial repeated extractions, there decrease of the accretion trend at the examined time scale. The locations and dates of these nourishment activities are shown in Fig. 1a.
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) Two-month moving average of significant wave height (Hs) (in black) overlaying corresponding non averaged Hs (in gray) from the ERA5 model (coordinates in WGS84: 44.5,-1.5; depth: ≈ 45 m), (b) anomaly of winter-mean wave height deviation from the mean from December to March (DJFM) superimposed onto the winter (DJFM) WEPA index, with R2 the corresponding coefficient of determination.</figcaption></figure>

At the transition between Sector 6 and Sector 7, a notable local beach volume loss of -140,000 m³ has been recorded since 2011 (from y = 8950 m to y = 9800 m in Fig. 6c). This specific site, corresponding to P1 in Fig. 8, exhibits its maximum beach loss between the end of the 3 km-long seawall in Sector 6 and the seawall south of Sector 7, which is delimited by Groyne 2 (Fig. 1c). The dune volume in the northern area of Groyne 2 has been strongly impacted, with a loss of -390,000 m³ between 2011 and 2023 (from y = 9310 m to y = 10,110 m) (Fig. 6b and c).

In total, between 2011 and 2019, -146,000 m³ of sand has been lost over the North-Médoc coast. This value corresponds to a dune volume loss of -986,000 m³ and a beach volume gain of 840,000 m³ (Table 2).

## 3.2. Coastal inter-annual variability

Fig. 9 illustrates the changes in the beach-dune profiles at the four transects, with their locations shown in Fig. 1a. Except for Profile P2 (Fig. 9b), which covers the beach at Soulac-sur-Mer with an urbanized seafront, the other profiles (P1, P3, and P4) span the entire beach-dune system (Fig. 9a, c, and d).

The corresponding time series of the 5.5-m NGF shoreline S are shown in Figs. 10a, 10c, 10e, and 10 g for P1 to P4, respectively. The corresponding winter changes $dS/dt$ are compared with the anomaly of the winter-mean wave height deviation from the mean time series in Figs. 10b,d,f and h. At all four points, a long-term trend is observed, with three of the four profiles experiencing erosion of up to -6.6 m/yr at P4, and accretion of +3.1 m/yr at P2. Importantly, substantial interannual shoreline variability, superimposed on the linear trends, is evident at all transects. This interannual variability is strongly correlated with winter-mean wave height variability at P3 and P4, the two profiles furthest from the estuary mouth. However, the winter-mean wave height variability explains less than 5 % of the observed interannual variability at P1 and P2. Interestingly, the severe winter of 2013–14 drove strong erosion at all transects, except at P2, which experienced slight accretion.

## 4. Discussion

The morphological changes of the North-Médoc coast are influenced by long-term, medium-term, and short-term processes, which include the effects of the Gironde Estuary internal dynamics, coastal management practices, and interannual variability in incident winter wave energy. Shoal migration and welding (e.g. FitzGerald, 1984; Gaudiano and

Kana, 2001) resulting from the self-organized internal dynamics of inlets (e.g. Hayes, 1980; De Swart and Zimmerman, 2009) can strongly affect shoreline evolution along adjacent coasts (e.g. van Heteren et al., 2006; Elias et al., 2019). As demonstrated by Vandenhove et al. (2024), the large-scale Gironde Estuary plays a dominant role in long-term coastal dynamics, which underpin the trends observed in Fig. 10. The long-term erosion at P3 and P4 reflects a major shoal attachment event that occurred in the 1920s south of Soulac-sur-Mer, near P3-P4. While this event initially led to considerable beach widening and elevation, the subsequent diffusion of this sediment-starved shoreline bulge has been observed over time. This process has resulted in the formation of a steep erosion scarp and a low dune toe at P3 and P4. At these locations, shoreline and dune erosion primarily occur due to the collision regime, as described in the storm impact scale of Sallenger (2000), where wave runup exceeds the threshold at the base of the dune scarp. As shown in Fig. 9c,d, the dune toe elevation at P3 and P4 is at or even below the Highest Astronomical Tide (HAT), indicating that wave runup reaches the dune toe even during moderate-energy wave conditions at higher tide levels. Further from the shoal attachment site, the erosion trend decreases, as observed at P1 (Fig. 9a,b). The only exception to this pattern is profile P2, where the accretion observed since 2014 is primarily due to the extension of Groyne 3 trapping more sand from the northerly longshore drift. On shorter time scales, interannual shoreline variability, typically ranging from 1 to 10 m, was observed at all transects. The relatively small erosion rate at P1 or accretion at P2, combined with longshore processes, means the shoreline at these locations (elevated at 5.5 m NGF) is relatively protected from wave action. This explains the weak correlation between shoreline interannual variability and winter-mean wave height variability. In contrast, at P3 and P4, erosion occurs when wave runup exceeds the threshold at the base of the dune scarp, explaining the stronger correlation with winter-mean wave height variability.

Our results show notable alongshore variability in coastal dune morphology and behavior along the North-Médoc coast. Such large variability, on scales ranging from a few kilometers to tens of kilometers, has already been observed along shorelines adjacent to tidal inlets (Fenster and Dolan, 1996). A critical factor influencing coastal dune development on decadal time scales is beach width (Galiforni-Silva et al., 2019), whether near or far from the influence of tidal inlets and estuary mouths (Guillén et al., 1999; Aagaard et al., 2004; Keijsers et al., 2014; Ruessink and Jeuken, 2002). Shoal migration and welding
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** LiDAR DTM in (a) 2011, (b) 2023 with (c) corresponding elevation difference superimposed onto the 2023 orthophotography. For clarity, in (c) only elevation changes larger than 1 m are shown. The horizontal dashed lines delimitate the nine sectors.</figcaption></figure>

(Ridderinkhof et al., 2016), as well as channel formation or deepening (e.g. Elias et al., 2019), at tidal inlets and estuary mouths drive substantial shoreline changes along adjacent coasts at these time scales, thereby strongly impacting coastal dune system morphology. Along the North-Médoc coast, the coastal dune landscape varies considerably. In some areas, large dune systems are forming and increasing in volume at an average rate of approximately 50 m³/m/year (Sector 9, Table 2). In contrast, in severely eroding areas (Sector 3), dunes are absent, and the forest is collapsing onto the beach. Other sections exhibit scarped foredunes or incipient foredunes, which can alternate in both time and space. This pattern of coastal dune development, linked to shoal attachment phases, has been documented elsewhere (Galiforni-Silva

et al., 2020, 2022). While previous research on the North-Médoc coast has highlighted the influence of estuary mouth dynamics on secular shoreline trends (Vandenhove et al., 2024), the present work shows that these dynamics also strongly influence coastal dune landscapes and morphological changes on decadal time scales.

In line with recent work along the Atlantic coast of Europe (Masselink et al., 2023; Castelle et al., 2024) and more specifically in southwest France (Castelle et al., 2022), interannual shoreline variability was mostly driven by interannual variability in winter-mean wave energy linked with climate index WEPA. Such interannual shoreline variability, superimposed on long-term trends, has been observed in many coastal areas, where long-term trends are often due to major
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Alongshore distribution of (a) total, (b) dune and (c) beach volume difference (m³/m) with respect to 2011. Alongshore positions of the 9 sectors dividing the coastline (dashed lines), groynes (black lines), profiles (red lines) and seawall coverage (gray areas) are indicated.</figcaption></figure>

**Table 2**

Values of total, beach and dune volume differences (in m³/m and m³) at each sectors (#1 to #9) between 2011 and 2023. In addition, values of total, beach and dune volume differences (in m/m and m³) at southern erosion sectors (#1–5.5) and over the entire studied coastline (#1–9) and restricted to from 2011 to 2019 (due to LiDAR coverage limitations, see Section 2.2.1) are given.

<table><thead><tr><th>Sector (N)</th><th>Local along. cov. (m)</th><th>Vol. beach and dune (m³/m)</th><th>Vol. dune (m³/m)</th><th>Vol. beach (m³/m)</th><th>Vol. beach and dune (m³)</th><th>Vol. dune (m³)</th><th>Vol. beach (m³)</th></tr></thead><tbody><tr><td>#1</td><td>0-1250</td><td>-1190</td><td>-1210</td><td>20</td><td>-1,447,000</td><td>-1,475,000</td><td>29,000</td></tr><tr><td>#2</td><td>1250-2250</td><td>-90</td><td>-90</td><td>0</td><td>-90,000</td><td>-90,000</td><td>0</td></tr><tr><td>#3</td><td>2250-3690</td><td>-730</td><td>-750</td><td>20</td><td>-1,058,000</td><td>-1,084,000</td><td>26,000</td></tr><tr><td>#4</td><td>3690-5110</td><td>-280</td><td>-320</td><td>50</td><td>-394,000</td><td>-463,000</td><td>69,000</td></tr><tr><td>#5</td><td>5110-6180</td><td>320</td><td>180</td><td>150</td><td>350,000</td><td>191,000</td><td>160,000</td></tr><tr><td>#6</td><td>6180-9090</td><td>-40</td><td>-30</td><td>-10</td><td>-130,000</td><td>-87,000</td><td>-42,000</td></tr><tr><td>#7</td><td>9090-10,120</td><td>-510</td><td>-400</td><td>-110</td><td>-527,000</td><td>-415,000</td><td>-112,000</td></tr><tr><td>#8</td><td>10,120-10,980</td><td>180</td><td>200</td><td>-20</td><td>158,000</td><td>176,000</td><td>-18,000</td></tr><tr><td>#9</td><td>10,980-14,000</td><td>-</td><td>550</td><td>-</td><td>-</td><td>1,660,000</td><td>-</td></tr><tr><td>#1-5.5</td><td>0-5480</td><td>-410</td><td>-430</td><td>20</td><td>-2,217,000</td><td>-2,332,000</td><td>116,000</td></tr><tr><td>#1-9</td><td>0-14,000</td><td>-10</td><td>-70</td><td>60</td><td>-146,000</td><td>-986,000</td><td>840,000</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** DTM difference exceeding 0.5 m around Groyne 3 over (a) 2011–2014 and (b) 2014–2023, superimposed onto the orthophotography of 2023. Locations of sector's limits and profile P2 are specified.</figcaption></figure>

human interventions (e.g. Luijendijk et al., 2018), sediment supply (e.g. Ruggiero et al., 2010; Besset et al., 2019), or, in the future, sea-level rise (e.g. Hinkel et al., 2013). For the North-Médoc coast, the long-term trend is primarily driven by the estuary's dynamics due to its large spatial scales. While interannual shoreline variability will typically dominate the shoreline signal and its uncertainties over the next couple of decades on relatively stable coasts (e.g. Le Cozannet et al., 2016; Ibaceta et al., 2023), the substantial long-term trends (1–7 m/yr) observed along the North-Médoc coast suggest that the contribution of such interannual variability to medium- to long-term coastal change is increasingly negligible. In contrast, long-term trends inherited from the estuary mouth, combined with local coastal management strategies, will dictate the future evolution of the North-Médoc coast.

Over time, various methodologies have been developed to evaluate shoreline evolution, tailored to meet the specific requirements of coastal engineering and management projects. These methods range from empirical techniques to sophisticated numerical models, reflecting the inherent complexity and diversity of coastal environments. Within this framework, one-line theory (Pelnard-Considere, 1956) has played a pivotal role in improving the ability of coastal scientists and engineers to predict shoreline changes and assess the effects of coastal interventions (e.g. Hanson, 1989; Hanson and Kraus, 1989; Frey et al., 2012), with the possibility of considering the sedimentary characteristics of coastal slopes composed of loose, non-cohesive materials (Tomasicchio et al., 2020, 2023). Reduced-complexity shoreline models driven by a combination of cross-shore and longshore processes (Vitousek et al., 2017;
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** DTM difference exceeding 0.5 m around Groyne 2 over (a) 2011–2014 and (b) 2014–2023, superimposed onto the orthophotography of 2023. Locations of sector's limits and profile P1 are specified.</figcaption></figure>

Robinet et al., 2018; Antolínez et al., 2019) could also be used to address wave-driven coastal changes along the North-Médoc coast. However, in such models, the equilibrium approach should be replaced with alternative methods to account for cross-shore processes, given the prevalence of the collision regime under moderate- to high-energy waves during higher tides. Such models should therefore provide a continuum between equilibrium-based and collision-driven shoreline responses across relatively stable and chronically eroding sectors. Additionally, they should be capable of accounting for the strong alongshore variability in beach-dune profile characteristics. At the other end of the spectrum, process-based models using input reduction techniques can be employed to capture large-scale changes in the estuary mouth and the

resulting shoal dynamics (Cayocca, 2001). These approaches should also incorporate the effects of sea-level rise and, when possible, include inherited geological features, which are crucial for long-term evolution in this region (Stéphan et al., 2019). Between these extremes, shoreline change is influenced by a combination of these various driving factors, including coastal management strategies, making it challenging to predict the future of the North-Médoc coast at intermediate time scales.

A key characteristic of the North-Médoc coast is the extensive array of hard coastal structures that have been progressively implemented since the mid-19th century. The results demonstrate that such structures can have both positive and negative effects on shoreline dynamics. For instance, consistent with the expected outcomes of groyne extension
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Beach profile evolution (a) P1, (b) P2, (c) P3 and (d) P4. Corresponding water levels at the study site for 2022 are added (source: SHOM) with HAT: Highest Astronomical Tide, MSL: Mean Sea Level and LAT: Lowest Astronomical Tide. The right-hand panels show photographs of P1, P3 and P4 profiles on the 27th February 2024 @M.Branellec (BRGM). Photograph associated to P2 profile was taken by drone on the 24th April 2024 @V.Marieu (CNRS-EPOC).</figcaption></figure>

(van Rijn, 2011), the extension of Groyne 3 in 2014–2015 successfully limited erosion in Sector 5 and even induced accretion immediately updrift of the groyne. This process has reached the point where this accreting area now serves as a sediment extraction area in the “back pass” management strategy, contributing to nourish the eroding Sectors 3 and 4. However, the benefits driven by hard structures often come with negative consequences, such as toe scour and downdrift erosion due to the disruption of longshore sediment transport (Bruun, 1995). While further investigation is needed, the dramatic erosion observed downdrift of Groyne 2 (Fig. 8b) a few years after the extension of Groyne 3 (located approximately 3 km updrift) may reflect a delayed response to the interruption of the longshore drift. In contrast, no major erosion hotspots have been observed updrift or downdrift of the coastal structures further south in Sectors 2–4, suggesting full alongshore sand bypassing and that shoreline changes in these areas are primarily driven by cross-shore processes. Finally, the alongshore seawall constructed in Sector 6 during the 19th century has effectively stabilized the shoreline, preserving both the dune and the narrow barrier separating the ocean

beach from the estuary.

Despite regular beach nourishment efforts introducing 40,000 to 85,000 m³/year since 2018 through a “back-passing” system to sustain the beaches in Sectors 3 and 4, severe chronic erosion continues to be recorded. This persistent erosion threatens a major road which, as of fall 2024, lies less than 30 m from the cliffed shoreline. Consequently, these small-scale periodic nourishments are not considered a viable long-term solution, as shoreline retreat remains unabated. Mega-nourishments, such as the Sand Engine project involving 20 million m³ (Stive et al., 2013), present an intriguing alternative. However, such approaches must be carefully designed (e.g. Mulder and Tonnon, 2011; de Schipper et al., 2014, 2016; Luijendijk et al., 2017), with expectations for a considerably shorter lifespan than that observed for the Sand Engine, even with similar nourishment volumes (Vandenhove et al., 2024). Instead, adopting solutions along the gray-green spectrum of coastal protection (Pawel Waryszak and Macreadie, 2021; Singhvi et al., 2022) should be explored to enhance coastal erosion mitigation in these sectors.
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** (a,c,e,g) Time series of 5.5-m NGF shoreline *S* (gray: post winter; black: post summer) at P1 to P4 with their computed long-term trend. In (b,d,f,h) Corresponding yearly shoreline change *dS/dt* (black) superimposed onto anomaly of winter-mean wave height deviation from the mean (red), with *R*² the corresponding coefficient of determination.</figcaption></figure>

## 5. Conclusions

The present contribution documents the highly variable beach-dune morphological changes along the North-Médoc coast over the past decade, driven by a combination of factors operating across different time and spatial scales. (1) On longer time scales, chronic erosion or accretion, primarily influenced by major morphological changes in the estuary mouth and subsequent shoal welding and diffusion, dominate shoreline change along most of the coast (internal forcing). (2) Coastal structures have localized effects on shoreline variability, occasionally leading to shifts in shoreline trajectories. While alongshore structures effectively stabilize the shoreline, they can cause adverse effects at their extremities. The impact of cross-shore structures, such as groynes, is more variable. (3) Interannual shoreline variability, influenced by winter-mean wave height fluctuations driven by climate modes of atmospheric variability, superimposes on the longer-term trends identified in (1) and (2). However, this external forcing's impact is more pronounced further away from the estuary mouth. This suggests a continuum from coast sections dominated by external forcing, typical of open coast behavior, to areas adjacent to the estuary mouth where inlet/shoal processes dominate.

## CRediT authorship contribution statement

**Marine Vandenhove:** Visualization, Methodology, Investigation, Formal analysis, Writing – original draft. **Bruno Castelle:** Supervision. Project administration, Methodology, Formal analysis, Writing – review & editing, Writing – original draft. **Alexandre Nicolae Lerma:** Supervision, Project administration, Methodology, Funding acquisition, Writing – review & editing. **Vincent Marieu:** Software, Methodology, Formal analysis, Data curation. **Vincent marieu:** Writing – review & editing. **Vincent Mazeiraud:** Writing – review & editing.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This work is part of the project ESTOC, co-funded by the Region Nouvelle-Aquitaine, the Communauté de Communes Médoc Atlantique (CdCMA), the BRGM and EPOC (University of Bordeaux - CNRS). BC received funding from Agence Nationale de la Recherche (ANR) grant number ANR-21-CE01-0015 (SHORMOSAT). The study area comprises
the Demonstrator “Soulac”, which monitoring has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No 101135410 – The DuneFront Project. ERA5 wave data is produced by the Copernicus Climate Change Service (C3S) at ECMWF. The authors would like to thank the Eolissatoire de la Côte Nouvelle Aquitaine (OCNA) for the beach profile and LiDAR datasets. We acknowledge the sea level pressure data providers in the ECA&D project (http://www.ecad.eu Klein Tank et al., 2002) and the Irish Meteorological Service (http://www.met.ie/climate) for the Valentia Observatory data, which were used to compute the WEPA index.

## Data availability

Data will be made available on request.

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