# Field measurements of wave and flow dynamics along a high-energy meso-macrotidal coast adjacent to a large estuary mouth

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

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## ► To cite this version:

Marine Vandenhove, Bruno Castelle, Alexandre Nicolae Lerma, Vincent Marieu, Kévin Martins, et al.. Field measurements of wave and flow dynamics along a high-energy meso-macrotidal coast adjacent to a large estuary mouth. Estuarine, Coastal and Shelf Science, 2025, 317, pp.109205. 10.1016/j.ecss.2025.109205. hal-04988079

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Submitted on 12 Mar 2025

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# Field measurements of wave and flow dynamics along a high-energy meso-macrotidal coast adjacent to a large estuary mouth

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

## ARTICLE INFO

**Keywords:**

Sandy coast

Estuary mouth

Tide-driven current

Wave-driven currents

In situ measurements

## ABSTRACT

The North-Médoc coast, located to the south of the Gironde estuary, exhibits complex hydrodynamic conditions driven by high-energy incident waves and large tides, which are influenced by large-scale rocky outcrops, shoals, and tidal channels on the inner shelf. These wave- and tide-driven hydrodynamics result in rapid morphological changes, with shoreline erosion peaking locally at 5 m/year. In autumn 2022, an intensive two-week field campaign was conducted along three cross-shore transects distributed across the North-Médoc coast to, for the first time, document and analyze the hydrodynamics in this area. The campaign involved collecting measurements of waves and currents across both the inner shelf and nearshore regions, using six current profilers and 13 pressure sensors. Tides were found to predominantly modulate wave transformation and flow in both the nearshore and inner shelf regions of the North-Médoc coast. Strong alongshore currents were measured across the entire study site, increasing southward, which is linked to the overall morphology of a tidal channel. Tidal currents are flood-dominated in the northern part and ebb-dominated in the southern and central parts of the study area. Total currents and wave heights are modulated by tidal elevation, with, for example, increased wave height at the coast during higher tide levels. Nearshore waves, which undergo complex transformation across the inner shelf, such as wave refraction over shoals and rocky outcrops, also drive longshore currents in the surf zone, superimposed on the tidal current. These conditions result in a strong, net, northeastward longshore current, even during low-energy wave conditions. These observations provide new insights into the hydrodynamics of this rapidly evolving coastline and establish a comprehensive dataset that will be crucial for the development and validation of process-based and reduced-complexity models in this region.

## 1. Introduction

Coastal areas adjacent to inlets or estuary mouths are of significant interest for both human activities and environmental conservation. The recent growth of tourism and recreational activities has increased the attractiveness of these zones, driving urban development and population expansion (Allen et al., 1999; Le Berre et al., 2016). Compared to open coasts, sandy shorelines near inlets exhibit remarkably complex and rapid spatial and temporal variability (Bamunawala et al., 2021; Castelle et al., 2022). Managing such dynamic environments often requires specific coastal strategies, including dredging, coastal structures, and beach nourishment (Hapke et al., 2013; Rulleau and Rey-Valette, 2017), which, in turn, intensify anthropogenic pressure on these fragile ecosystems (Hapke et al., 2013; Nguyen et al., 2020).

Coastal morphological changes adjacent to large inlets result from the interplay between external factors, such as waves and tides and internal forcings linked to the dynamics of the ebb tidal delta (Hayes, 1975, 1980; Bertin et al., 2009; De Swart and Zimmerman, 2009), and possibly anthropogenic perturbations (Velasquez-Montoya et al., 2020). These interactions can drive the migration of channels and shoals (e.g. FitzGerald, 1984; Ridderinkhof et al., 2016; Elias et al., 2019; Lenstra et al., 2019), and ultimately to the welding of sandbars to adjacent coasts (e.g. FitzGerald et al., 1984; Gaudiano and Kana, 2001; Burvingt et al., 2022). This dynamic process often results in rapid and pronounced shoreline variations (Castelle et al., 2022), with potentially high erosion rates posing risks to infrastructures, alternating

<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><sup>d</sup> UMR 7266 LIENSs, CNRS - La Rochelle Université, Institut du Littoral et de l'Environnement, 2 rue Olympe de Gouges, La Rochelle, 17000, 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>
with localized periods of intense accretion (Anthony et al., 2006; Elias et al., 2019; Vandenhove et al., 2024).

Nearshore hydrodynamics are crucial to such complex morphodynamic systems, where both tide- and wave-driven currents co-exist, driving sediment transport and feedback mechanisms (Green and Coco, 2014). Tides drive currents through tidal inlets and estuary mouths, typically reaching maximum velocities in the channels during ebb and flood, with velocities in such environments potentially exceeding 1 m/s (e.g. Li et al., 1998; Wargula et al., 2014). In addition, tides can influence nearshore wave transformation (Davis and Fox, 1981; Davidson et al., 2008; Dodet et al., 2013; Lenstra et al., 2019), which in turn affects breaking wave conditions and wave-driven currents. Complex tide-modulated processes, such as depth- or current-induced wave refraction, bottom friction, wave steepening, dissipation, or even blocking and breaking by opposing tidal flows (Davidson et al., 2008), can co-exist and alternate in both time and space, ultimately controlling nearshore currents. Waves can also significantly impact flows (e.g. Orescanin et al., 2014; Wargula et al., 2018). Previous studies have primarily focused on the dynamics of smaller-scale inlets (e.g. Byrne et al., 1980; Davis and Fox, 1981; FitzGerald, 1982; Gaudiano and Kana, 2001; Wargo and Styles, 2007; Shaeri et al., 2013; Fortunato et al., 2014; Zippel and Thomson, 2015; Orescanin and Scooler, 2018) due to the relative ease of monitoring and modeling the hydrodynamics and morphological changes. Larger systems, however, typically influence much longer stretches of adjacent coasts (Hayes, 1980; FitzGerald, 1982; Burvingt et al., 2022; Castelle et al., 2022), spanning tens of kilometers of coastline, and exhibit generally more complex morphology, with channels of various sizes and dynamics (Styles et al., 2016).

The North-Médoc sandy coast, located in the southwest of France, lies downdrift of the Gironde estuary, Europe's largest estuary by size (Fig. 1a). It forms a dynamic interface between a macrotidal, tide-dominated estuarine environment (Castaing and Allen, 1981) and a wave-dominated, high-energy sandy coastline (Castelle et al., 2017). Strong geological constraints, such as rocky outcrops (Fig. 1a), within the inlet significantly complicates wave and flow patterns, which in turn affects sediment transport and shoreline change. This coast has experienced dramatic shoreline variability, with chronic erosion threatening infrastructure, interspersed with periods and areas of rapid accretion, leading to extensive coastal protection measures since the 19th century (Vandenhove et al., 2024). However, little is known about the hydrodynamics in this region (Howa, 1987), and field data is severely lacking to provide quantitative insight into the respective roles of waves and tides in nearshore currents. This study aims to document the spatio-temporal variability of hydrodynamics along the North-Médoc coast through a two-week intensive field experiment that includes wave and current measurements in both the inner shelf and nearshore regions. Ultimately, this dataset will serve as a valuable resource for local coastal managers for designing strategies and will support the development and validation of process-based and reduced-complexity models for such coastal environments.

# 2. Field experiment campaign

## 2.1. Field site description

The study site, located to the south of the Gironde estuary against one of its primary channels (South Pass), extends from Cape Grave to Cape Négade (Fig. 1a). The estimated tidal prism varies from $1.1 \times 10^9$ m³ during neap tide to $2 \times 10^9$ m³ during spring tide (Allen et al., 1974), with an average water discharge of 900 m³/s (Sottolichio and Castaing, 1999), with complex mud and sand transfers at the estuary mouth (Diaz et al., 2020, 2024). The site experiences semi-diurnal, macrotidal tides, ranging from 1.5 m (neap) to 5.5 m (spring) (Castaing and Allen, 1981), resulting in tide-driven currents with velocities of up to 1 m/s near the coast (Howa, 1987). The inner shelf area is characterized by prominent rocky outcrops alternating with dynamic

sandy shoals (Vandenhove et al., 2024). Wave buoy measurements and numerical wave hindcasts offshore of the study site show a prevailing W-NW (W) wave direction in July (January) and significant wave heights of 1.11 m (2.40 m) (Castelle et al., 2017). In addition, the sandy coastline facing north-northwest and the dominant wave incidence result in a net northeastward longshore drift along the North-Médoc coast (Fig. 1a), diverging at Cape Négade from the net southerly longshore drift along the coast further south (Idier et al., 2013; Howa, 1997). For a more comprehensive understanding of the field site and its morphological changes over the past century, the reader should refer to Vandenhove et al. (2024).

## 2.2. Field experiment

An intensive field measurement campaign was conducted from September 12 to 26, 2022, along three cross-shore transects distributed across the North-Médoc coast. The campaign involved detailed wave and current measurements in both the inner shelf and nearshore regions. In addition, offshore directional wave measurements and nearby tidal data were collected as part of various ongoing monitoring programs.

### 2.2.1. Boundary condition datasets

The 2016 bathymetric map and the location of rocky outcrops (Fig. 1a) were used to plan the deployment strategy, as it covers a much larger area than more recent bathymetric maps (e.g., 2019 and 2021), which typically focus only on the second main channel, the South Pass. Key geomorphological features identified in the 2016 map include rocky outcrops, the southern secondary channel, the South Pass, and large sandbanks. Verification using the more recent maps confirmed that these features remain present and broadly similar in shape and position, considering the spatial scales analyzed. However, subtle morphological changes were observed locally, primarily concerning the widening and deepening of the southern secondary channel. An orthophotograph collected in September 2022 by the Observatoire de la Côte de Nouvelle-Aquitaine (OCNA) provided insight into the beach morphological patterns and coastline position during the experiment. At the offshore boundary of the study area, bulk wave parameters, including significant wave height ($H_{m0}$), peak wave period ($T_p$), and peak wave direction ($\theta_p$), were provided by a directional wave buoy from the CANDHIS network (Centre d'Archivage National de Données de Houle In Situ), located approximately 30 km northwest of the study site in approximately 50-m depth (Fig. 1a). Wind and atmospheric condition datasets were obtained from a nearby weather station (Local Vensac station, MétéoFrance). In addition, tidal elevation data were acquired from a tide gauge situated north of the study area (Fig. 1a), complementing the hydrostatic reconstruction from our pressure sensors.

### 2.2.2. Experimental setup

During the field campaign, an Acoustic Doppler Current Profiler (ADCP) was deployed near the seabed for each transect, located approximately 0.5 to 2 km from the coast, at a mean depth of 9–10 m relative to mean sea level (Fig. 2a). Along each transect, a second current profiler (ADCP Aquadopp) was positioned in the intertidal zone, accompanied by three pressure sensors. A fourth pressure sensor was placed at the central profile (Fig. 1b–d). All sensors were oriented upward near the seabed, with the distance $\delta_s$ between the sensors and the seabed (Fig. 2a). For nearshore sensors, this distance was measured daily at low tide. However, for the inner shelf ADCPs, $\delta_s$ was set constant at 0.8 m throughout the field campaign, assuming no change in the seabed, as regular measurements of $\delta_s$ at this depth (9–10 m) were not feasible. The sampling modes for all instruments are detailed in Table 1. Note that differences in instrument settings are primarily due to technical specifications, as well as memory and battery limitations.
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** (a) Map of the study site with the 2016 bathymetry (TANDEM project, source: SHOM) colored. Hatched areas indicate rocky outcrops (source: Communauté De Communes Medoc Atlantique (CDCMA)). The tide gauge and directional wave buoy locations are indicated by the yellow circle (red dot). The three instrumented cross-shore transects are aligned with an inner shelf ADCP (Acoustic Doppler Current Profiler, colored triangles), with the north profile in orange, central profile in green, and the south profile in blue. (b, c, d) Zoom onto the nearshore instruments across the three transects with ADCP Aquadopp (colored rectangles) and pressure sensors (colored circles) superimposed onto an orthophotograph taken in September 2022 (source: PIGMA network), with $\theta_n$ (10°N), $\theta_c$ (23°N), and $\theta_s$ (4°N), the respective coastline orientation angles.</figcaption></figure>

<table><caption>**Table 1**
Field measurement instrument specifications.</caption><thead><tr><th>Name</th><th>Model</th><th>Height (m)<sup>a</sup></th><th>Sampling mode</th><th>Sampling frequency (Hz)</th></tr></thead><tbody><tr><td colspan="5"><strong>Inner shelf ADCPs (3)</strong></td></tr><tr><td>A<sub>n1</sub></td><td>Sentinel V20</td><td>-9.3</td><td>Burst: 30 min</td><td>2 Hz (20 min); 1 Hz (10 min)</td></tr><tr><td>A<sub>c1</sub></td><td>Signature 1000 kHz</td><td>-9.2</td><td>Continuous</td><td>2 Hz</td></tr><tr><td>A<sub>s1</sub></td><td>Signature 500 kHz</td><td>-10.4</td><td>Continuous</td><td>2 Hz</td></tr><tr><td colspan="5"><strong>Nearshore ADCPs Aquadopp (3)</strong></td></tr><tr><td>A<sub>n2</sub></td><td></td><td>-1.2<sup>b</sup></td><td></td><td>1 Hz</td></tr><tr><td>A<sub>c2</sub></td><td>Profiler 2 MHz</td><td>-0.9<sup>b</sup></td><td>Averaged: 10 min</td><td>1 Hz</td></tr><tr><td>A<sub>s2</sub></td><td></td><td>-1.2<sup>b</sup></td><td></td><td>1 Hz</td></tr><tr><td colspan="5"><strong>Pressure sensors (10)</strong></td></tr><tr><td>P<sub>n1</sub></td><td>RBR</td><td>-1.5<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>n2</sub></td><td>RBR</td><td>-1.3<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>n3</sub></td><td>RBR</td><td>-0.4<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>c1</sub></td><td>RBR</td><td>-1.2<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>c2</sub></td><td>RBR</td><td>-1.0<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>c3</sub></td><td>OSS</td><td>-0.4<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>c4</sub></td><td>OSS</td><td>0.3<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>s1</sub></td><td>RBR</td><td>-1.3<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>s2</sub></td><td>OSS</td><td>-0.4<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr><tr><td>P<sub>s3</sub></td><td>OSS</td><td>0.4<sup>b</sup></td><td>Continuous</td><td>2 Hz</td></tr></tbody></table>

<footnote><sup>a</sup> Sensors' height at deployment relative to mean sea level.</footnote>

<footnote><sup>b</sup> Intertidal zone, averaged only when underwater.</footnote>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** (a) 3D schematics of a seabed-mounted up-looking current profiler measuring velocities at each cell $\delta_c$ (i.e., cell size), at a $\delta_i$ distance from the seabed along its range (i.e., number of cells $n \times$ cell size $\delta_i/2$), within a water column depth of $h$. Current profile velocities cleaning steps consisted of removing altered data depending on (b) tilt (i.e., pitch and roll exceeding 30°) and correlation values; (c) data above the mean free surface elevation $\eta$ depicted as the black line, obtained by hydrostatic reconstruction from the relative measured pressure; and (d) side lobe interference, $R_{max}$.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Nearshore instrument positions and altitude at deployment, with beach profile changes throughout the study period at the (a) northern, (b) central, and (c) southern profiles, relative to mean sea level.</figcaption></figure>

### 2.2.3. Topography

Beach elevation measurements were conducted at low tide using centimeter-level GNSS technology along the three instrumented cross-shore transects, from the waterline to the dune foot. Due to logistical limitations, only one transect was surveyed each day. In total, 18 beach elevation profiles, interpolated along these reference transects,

were collected over the study period. The evolution of the beach elevation profiles is shown in Fig. 3, along with the positions of all nearshore instruments. The surveys revealed relatively small morphological changes, except for a nearly 1-m-high sandbar that migrated onshore across the central profile (Fig. 3a).
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Tidal, wave, wind, and atmospheric conditions during the field experiment from September 12 to 26, 2022. Time series of (a) tidal elevation from (Ac1), (b) significant wave height $H_{m0}$, (c) peak wave period $T_p$, (d) peak wave incidence from the CANDHIS directional wave buoy, (e) wind direction and magnitude (averaged over half-day intervals) from the Météo-France weather station in Vensac located approximately 15 km from the field site, and (f) atmospheric pressure at the same station. Wind and wave directions are reported as directions of origin in nautical convention, with 0° as True North.</figcaption></figure>

### 2.2.4. Currents

Eulerian total currents were obtained from the ADCP current profilers. Following the manufacturer's instructions, a cleaning procedure was applied. As shown in Fig. 2, velocity data were post-processed to exclude inconsistent measurements, such as those above the mean free surface elevation and/or those affected by interference (e.g., tilt, correlation values, and side lobe interference). Velocities were initially measured in a North-East (ENU) coordinate system and were then converted into a local coordinate system (u, v) for each instrument. Specifically, the longshore component v was oriented orthogonally to the direction minimizing cross-shore flow at inner shelf stations (An1, Ac1, and As1) and aligned with the coastline orientation at the nearshore stations (An2, Ac2, and As2). This enabled a local analysis of longshore (v: directed northward when $V > 0$ and southward when $V < 0$) and cross-shore (u: directed offshore when $U > 0$ and onshore when $U < 0$) velocities. A 10-min moving average was applied to

the inner shelf ADCP measurements to estimate mean total currents, ensuring consistency with the nearshore ADCP Aquadopp data, which were already averaged over 10-min bursts. Finally, all ADCP current profiles were depth-averaged over the entire water column.

### 2.2.5. Waves

Wave parameters ($H_{m0}$, $T_p$) were computed from the time series of sea-surface elevation, which was reconstructed from bottom pressure measurements. First, the measured pressure was corrected for atmospheric pressure (see Section 2.2.1) to enable a hydrostatic reconstruction of the free surface elevation. At the inner shelf stations (An1, Ac1, and As1), non-hydrostatic effects were estimated using the transfer function from linear wave theory, with a cut-off frequency set at 0.3 Hz (Mouragues et al., 2019; Martins et al., 2020) to minimize the effects of noise and non-linearities (Bonneton and Lannes, 2017). The elevation of the inner shelf sensor above the seabed was added ($\delta s = 0.8$ m).
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Wave and total current roses at the measurement stations in the inner shelf ($A_{n1}$, $A_{c1}$, and $A_{s1}$) and nearshore ($A_{n2}$, $A_{c2}$, and $A_{s2}$) regions during the field experiment. The segment length shows the percentage of occurrence of this direction in the distribution. (a) Wave roses with significant height ($H_{m0}$) colored. Wave characteristics at the CANDHIS offshore directional wave buoy are represented in the upper left corner. (b) Current roses, with velocity colored and the residual total current velocities represented by the black arrow with their respective values in bold italic.</figcaption></figure>

In contrast, nearshore measurements face challenges due to highly nonlinear wave dynamics near the breaking point, where the accuracy of linear methods in reconstructing wave-induced surface elevation is limited because of the weakly dispersive nature of the waves (Bonneton and Lannes, 2017; Martins et al., 2017). Consequently, nonlinear approaches were favored in this region (Bonneton et al., 2018; Mouragues et al., 2019; Martins et al., 2020). A weakly dispersive reconstruction method based on nonlinear theory was employed (Bonneton and Lannes, 2017), with a 1-Hz cut-off frequency (Bonneton et al., 2018). Additionally, the time-varying sensor elevation during the measurements was incorporated into the reconstruction. Solely periods during which those pressure sensors were completely submerged were kept.

The spectral density of variance was calculated from the reconstructed sea-surface elevation for all pressure sensors (inner shelf and nearshore) using a fast Fourier transform (FFT) approach, applied over 20-min Hanning windows with 75% overlap (degrees of freedom = 7). Bulk wave parameters ($H_{m0}$, $T_p$) were subsequently derived from the energy spectrum within the frequency range of 0.04 Hz to 1 Hz, except for the inner shelf ADCP $A_{n1}$, which was limited to 0.04 Hz to 0.5 Hz due to its sampling frequency. Additionally, for the inner shelf stations, depth-averaged velocities were used to compute the angle of wave incidence and estimate the mean wave direction, following the method described by Herbers et al. (1999).

## 3. Results

The first sub-section presents the wave, tide and flow conditions averaged over the entire field experiment. We then analyze the temporal

evolution of these parameters at the inner shelf stations ($A_{n1}$, $A_{c1}$, and $A_{s1}$), and subsequently at the nearshore stations ($A_{n2}$, $A_{c2}$, and $A_{s2}$, as well as $P_{n1-3}$, $P_{c1-4}$, and $P_{s1-3}$).

### 3.1. Synoptic insight into waves and currents

During the two-week field experiment, the mean wind speed was 0.67 m/s, predominantly from the North with a mean atmospheric pressure of 1016 hPa (Fig. 4e and f). At the offshore boundary of the domain, the wave climate was characterized by waves coming from the W-NW (mean $\theta_p$ ≈ 297°) with $H_{m0}$ ($T_p$) ranging from 0.49 m to 1.70 m (3.1 s to 18.7 s) with a mean of 1.00 m (8.9 s) (Fig. 4). The period with $\theta_p$ ≈ 50° (Fig. 4d) corresponds to an approximately 5-day low-energy swell event, with $H_{m0}$ < 1.32 m and dominant local wind sea. Tidal regime ranged from neap tide (minimum tide range of 1.16 m) to spring tide (maximum tide range 4.75 m) with a mean range of 2.71 m.

Fig. 5 shows the spatial distribution of the bulk wave statistics (Fig. 5a) and mean total currents (Fig. 5b) during the field experiment. Fig. 5a shows that waves experienced significant transformation from offshore (CANDHIS buoy) to the inner shelf stations as evidenced by the substantial differences in wave height and direction patterns between the three inner shelf instruments ($A_{n1}$, $A_{c1}$, and $A_{s1}$) (Table 2). Even if the largest wave conditions are observed at $A_{n1}$ (1.55 m, Table 2), on average the most and less energetic conditions are observed at the central station ($A_{c1}$) and the southern station ($A_{s1}$), respectively. The complex wave transformation is also evidenced by the latitudinal variability in wave incidence with waves coming from the W-SW (mean
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Time series of significant wave height $H_{m0}$ (m) with corresponding tidal elevation (blue-gray-red colorbar) at the three inner shelf stations: (a) $A_{n1}$, (b) $A_{c1}$ and (c) $A_{s1}$. The black line represents $H_{m0}$ at the offshore boundary of the domain (CANDHIS buoy).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Ratio of the $H_{m0}$ the inner shelf stations to that at the offshore boundary of the domain (CANDHIS buoy) against the barotropic gradient between the nearshore station and the estuary entrance (Tide$_{gauge}$, location in Fig. 1), for nearshore locations (a) $P_{n1}$, (b) $P_{c1}$, and (c) $P_{s1}$. In all panels the color scale indicates the rate of tidal elevation change ($dh/dt$ in m/h), and the solid black line shows the linear regression with $m$ the regression slope.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Time series of tidal elevation overlaid with total current intensity (jet colorbar) and velocity components decomposed into local coordinates (see Section 2.2.4): alongshore *V* (parallel to the coastline) and cross-shore *U* (perpendicular to the coastline) at the three inner-shelf stations: (a) *A*<sub>n1</sub>, (b) *A*<sub>c1</sub>, and (c) *A*<sub>s1</sub>.</figcaption></figure>

$\theta$ of 265°) at $A_{s1}$ to W-NW (mean $\theta$ of 287°) at $A_{n1}$, with a W incidence in between (mean $\theta$ of 272°) at $A_{c1}$ (Fig. 5a).

Fig. 5b shows the mean currents at both inner shelf and nearshore stations, revealing a pronounced latitudinal gradient in the total currents. Stations located further south recorded the highest velocities, both at the inner shelf and nearshore locations. At the southern station (As1) on the inner shelf, and at the nearshore station (As2), the average mean total current velocities were 0.35 m/s and 0.26 m/s, respectively. Further north, at the central stations, inner shelf (0.32 m/s) and nearshore (0.20 m/s) currents were slightly weaker, with velocities averaged over the entire field experiment. At the northern station, the mean nearshore current velocity ($A_{n2}$, 0.15 m/s) slightly exceeded the inner shelf velocity ($A_{n1}$, 0.14 m/s), though both were substantially weaker than those recorded at the central and southern stations.

Overall, total currents exhibit a predominantly alongshore orientation, with mean longshore currents from 2.5 to 18 times greater than cross-shore currents (see Table 3). However, the net longshore current velocity direction varies spatially. The inner shelf stations $A_{s1}$ and $A_{c1}$, located in a channel (Fig. 1a), exhibit strong alongshore currents (see Table 3, $V_{min}$ and $V_{max}$) with ebb dominance ($V_{mean}$ < 0, in Table 3). In contrast, all nearshore stations, and the inner shelf northern station which is the closest to the estuary mouth, are flood-dominated.

## 3.2. Tidal modulation of inner shelf hydrodynamics

### 3.2.1. Waves

Fig. 6 clearly shows the tidal modulation of $H_{m0}$ at the three inner shelf stations ($A_{n1}$, $A_{c1}$, and $A_{s1}$) throughout the study period. During

<caption>**Table 2**</caption>

Bulk wave statistics of significant wave height $H_{m0}$ (m) at the inner shelf and nearshore stations with averaged, maxima and minima over the field experiment.

<table><thead><tr><th>Name</th><th>Mean H<sub>m0</sub> (m)</th><th>Max H<sub>m0</sub> (m)</th><th>Min H<sub>m0</sub> (m)</th><th>θ<sub>mean</sub> (°N)</th></tr></thead><tbody><tr><td>A<sub>n1</sub></td><td>0.53</td><td><strong>1.55</strong></td><td>0.12</td><td>287°</td></tr><tr><td>A<sub>c1</sub></td><td><strong>0.63</strong></td><td>1.35</td><td><strong>0.18</strong></td><td>272°</td></tr><tr><td>A<sub>s1</sub></td><td>0.49</td><td>1.15</td><td><strong>0.18</strong></td><td>265°</td></tr><tr><td>P<sub>n1</sub></td><td>0.54</td><td><strong>1.55</strong></td><td>0.16</td><td>—</td></tr><tr><td>P<sub>c1</sub></td><td><strong>0.62</strong></td><td>1.48</td><td>0.19</td><td>—</td></tr><tr><td>P<sub>s1</sub></td><td>0.51</td><td>1.04</td><td><strong>0.20</strong></td><td>—</td></tr></tbody></table>

spring tides (September 12–17 and September 21–26), the significant wave height at the inner shelf stations was, on average, 38% lower than at the offshore CANDHIS buoy (Fig. 4). The southern station (As1) experienced the greatest wave height attenuation, with a decrease of 45%, while the central station (Ac1) experienced the smallest reduction at 31% (see Table 4). During neap tides (September 17–21), the tidal range decreased, and $H_{m0}$ attenuation increased, reaching up to 60% on average, resulting in the smallest wave conditions of the study period (Table 4). This was also influenced by the presence of a side-onshore wind sea. Additionally, the $H_{m0}$ spatial gradient observed in Section 3.1 was not present during this period. The central station (Ac1) consistently recorded the largest average and maximum wave heights, while the northern station (An1) experienced the least energetic conditions.

Fig. 6 further shows that wave height attenuation was, on average, 23% greater at low tide than at high tide across all nearshore stations.
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Zoomed time series of tidal elevation overlaid with total current intensity (jet colorbar) during two spring tide cycles, along with velocity components decomposed into local coordinates (see Section 2.2.4): alongshore *V* (parallel to the coastline) and cross-shore *U* (perpendicular to the coastline) at the three inner-shelf stations: (a) *A*<sub>nl</sub>, (b) *A*<sub>c1</sub>, and (c) *A*<sub>s1</sub>.</figcaption></figure>

<caption>**Table 3**</caption>

Alongshore *V* and cross-shore *U* currents velocities variations (m/s) during the field experiment at inner shelf (*A*<sub>nl</sub>, *A*<sub>c1</sub> and *A*<sub>s1</sub>) and nearshore (*A*<sub>n2</sub>, *A*<sub>c2</sub> and *A*<sub>s2</sub>) stations, with: *V*<sub>mean</sub> the residual of alongshore currents, *V*<sub>max</sub> the maximum flood current, *V*<sub>min</sub> the maximum ebb current, *U*<sub>mean</sub> the residual of cross-shore currents, *U*<sub>max</sub> the maximum inner shelf currents and *U*<sub>min</sub> the maximum onshore currents.

<table><thead><tr><th>Name</th><th>V<sub>mean</sub> (m/s)</th><th>V<sub>max</sub> (m/s)</th><th>V<sub>min</sub> (m/s)</th><th>U<sub>mean</sub> (m/s)</th><th>U<sub>max</sub> (m/s)</th><th>U<sub>min</sub> (m/s)</th></tr></thead><tbody><tr><td>A<sub>n1</sub></td><td><strong>0.02</strong></td><td>0.33</td><td>-0.26</td><td><strong>0.02</strong></td><td><strong>0.33</strong></td><td>-0.06</td></tr><tr><td>A<sub>c1</sub></td><td>-0.07</td><td>0.60</td><td>-0.86</td><td><strong>-0.03</strong></td><td>0.11</td><td><strong>-0.11</strong></td></tr><tr><td>A<sub>s1</sub></td><td><strong>-0.09</strong></td><td><strong>0.79</strong></td><td><strong>-1.05</strong></td><td>-0.01</td><td>0.10</td><td>-0.06</td></tr><tr><td>A<sub>n2</sub></td><td><strong>0.07</strong></td><td>0.54</td><td>-0.29</td><td>0.00</td><td>0.25</td><td>-0.12</td></tr><tr><td>A<sub>c2</sub></td><td>0.05</td><td>0.67</td><td>-0.41</td><td><strong>0.04</strong></td><td>0.35</td><td>-0.24</td></tr><tr><td>A<sub>s2</sub></td><td>0.01</td><td><strong>0.96</strong></td><td><strong>-0.58</strong></td><td>0.00</td><td><strong>0.46</strong></td><td><strong>-0.45</strong></td></tr></tbody></table>

Moreover, *H*<sub>m0</sub> was, on average, 14% (7%) higher during the rising tide than during the falling tide at the northern (central) stations, while no significant difference was observed at the southern station. This is further illustrated in Fig. 7 showing the ratio of *H*<sub>m0</sub> to the offshore significant wave height (Candhis buoy) versus the barotropic gradient between inside and outside of the estuary. It shows that the ratio significantly increases (p-value < 0.05 at all stations) with decreasing barotropic gradient, i.e. that wave height is increased during rising tide compared to falling tide. In addition, the influence of the barotropic gradient decreases with increasing distance from the estuary (regression slope *m* = -0.26 m⁻¹ in the north and -0.09 m⁻¹ in the south, Fig. 7).

### 3.2.2. Tide-driven currents

Fig. 8 shows the time series of total currents, which, as described in the previous section, indicate overall weaker currents at the northern station compared to the two southern stations. Alongshore currents, which are the predominant component of total currents, decrease in intensity with the decreasing tidal range (from approximately 4.50 m to approximately 1.25 m) at all inner shelf stations (*A*<sub>nl</sub>, *A*<sub>c1</sub>, and *A*<sub>s1</sub>), showing stronger currents during spring tides than during neap tides (see Table 5). During neap tides, the north-south gradient described in Section 3.1 changes slightly, with the central station (*A*<sub>c1</sub>) exhibiting larger mean alongshore currents compared to the southern station (*A*<sub>s1</sub>), though maximum current values remain higher at the latter station (see Table 5). Over the course of a tidal cycle (Fig. 9), near-zero currents are observed just after both low and high tides. This
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Relationship between alongshore current velocities (V), tidal elevation, and rate of tidal elevation change (dh/dt in m/h) at the three inner shelf stations: (a) A<sub>n1</sub>, (b) A<sub>c1</sub> and (c) A<sub>s1</sub>. The color scale represents dh/dt (red for rising tide, blue for falling tide). Green sectors indicate the relative intensity of alongshore currents during the highest and lowest tides, with larger sectors reflecting stronger currents at these tidal phases. The obliquity of the area with grayish points (near high-tide or low-tide alongshore velocities) gives a measure of the alongshore current asymmetry between high and low tides for all tide ranges. The bottom panels further depict how (a–c) can be interpreted.</figcaption></figure>

<caption>**Table 4**</caption>

Bulk wave statistics H<sub>m0</sub> variations (m) at inner shelf ADCPs with averaged, maxima and minima of H<sub>m0</sub> calculated over spring and neap tides throughout the field experiment.

<table><caption>**Table 5**</caption><thead><tr><th rowspan="3">Name</th><th colspan="3">Spring tides</th><th colspan="3">Neap tides</th><th colspan="3">Spring tides</th></tr><tr><th colspan="3">12–17 sept</th><th colspan="3">17–21 sept</th><th colspan="3">21–26 sept</th></tr><tr><th>Mean</th><th>Max</th><th>Min</th><th>Mean</th><th>Max</th><th>Min</th><th>Mean</th><th>Max</th><th>Min</th></tr><tr><th></th><th>H<sub>m0</sub></th><th>H<sub>m0</sub></th><th>H<sub>m0</sub></th><th>H<sub>m0</sub></th><th>H<sub>m0</sub></th><th>H<sub>m0</sub></th><th>H<sub>m0</sub></th><th>H<sub>m0</sub></th><th>H<sub>m0</sub></th></tr></thead><tbody><tr><td>A<sub>n1</sub></td><td>0.71</td><td><strong>1.55</strong></td><td>0.19</td><td>0.27</td><td>0.62</td><td>0.12</td><td>0.59</td><td><strong>1.30</strong></td><td>0.16</td></tr><tr><td>A<sub>c1</sub></td><td><strong>0.79</strong></td><td>1.35</td><td><strong>0.28</strong></td><td><strong>0.36</strong></td><td><strong>0.73</strong></td><td><strong>0.18</strong></td><td><strong>0.71</strong></td><td>1.11</td><td><strong>0.29</strong></td></tr><tr><td>A<sub>s1</sub></td><td>0.63</td><td>1.15</td><td>0.21</td><td>0.30</td><td>0.61</td><td><strong>0.18</strong></td><td>0.52</td><td>1.11</td><td>0.25</td></tr></tbody></table>

Alongshore V currents variations (m/s) during spring tides (ST) and neap tides (NT) at inner shelf ADCPs, with: V<sub>mean</sub> the residual of alongshore currents, V<sub>max</sub> the maximum flood current, V<sub>min</sub> the maximum ebb current.

<table><thead><tr><th rowspan="2">Name</th><th colspan="2">V<sub>mean</sub> (m/s)</th><th colspan="2">V<sub>max</sub> (m/s)</th><th colspan="2">V<sub>min</sub> (m/s)</th></tr><tr><th>ST</th><th>NT</th><th>ST</th><th>NT</th><th>ST</th><th>NT</th></tr></thead><tbody><tr><td>A<sub>n1</sub></td><td><strong>0.02</strong></td><td><strong>0.01</strong></td><td>0.33</td><td>0.14</td><td>-0.26</td><td>-0.14</td></tr><tr><td>A<sub>c1</sub></td><td>-0.09</td><td>-0.05</td><td>0.60</td><td>0.29</td><td>-0.86</td><td>-0.39</td></tr><tr><td>A<sub>s1</sub></td><td>-0.09</td><td>-0.03</td><td><strong>0.79</strong></td><td><strong>0.31</strong></td><td>-1.05</td><td>-0.41</td></tr></tbody></table>

ST: Spring tide ; NT: Neap tide.

phase lag decreases southwards (Fig. 9c), while alongshore currents tend to be more intense at low tide, as shown in Fig. 10. However, this lag is not symmetrical (Table 6), with the ebb current duration being longer than the flood current duration at the central and southern stations (Ac1 and As1), in contrast to the northern station (An1), where the opposite is true. Additionally, falling tides are shorter than rising tides at all inner shelf stations (Table 6). This tidal asymmetry, with an approximately 20-min difference between rising and falling tides, was only about 8 min on average at the tide gauge located further north at the inlet mouth (location shown in Fig. 1). Tidal elevation amplitude differences between this tide gauge and the inner shelf stations were approximately -10 cm during rising tides and +10 cm during falling tides.

## 3.3. Nearshore hydrodynamics

### 3.3.1. Waves

Figs. 11 and 12 illustrate the tidal modulation of H<sub>m0</sub> at all nearshore stations throughout the study period. Consistent with observations at the inner shelf stations (An1, Ac1, and As1) discussed in Section 3.1, wave heights showed greater attenuation during neap tides (and low tides) compared to spring tides (and high tides) (see Table 7). However, beyond the breaking zone, wave heights were 2 to 11% higher than at the corresponding inner shelf stations (An1, Ac1, and As1) (refer to Table 2) due to wave shoaling. Additionally, as shown in Fig. 11d, e, H<sub>m0</sub> was generally higher during rising tides than during falling tides, with differences of 8% observed at the northern and central stations, respectively. However, this trend diminished shoreward as wave heights became increasingly depth-limited in the surf zone.

### 3.3.2. Currents

Fig. 13 illustrates the tidal modulation of total current intensity, showing larger flow velocities during spring tides compared to neap tides (Table 8). It also highlights the contribution of breaking-wave-induced currents to the total current (Fig. 13a, b, c) and its alongshore component (Fig. 13d, e). Although the average wave height (period) changed only slightly from 0.81 m (13.7 s) to 0.71 m (10.5 s) between the two spring tide periods (September 12–17 and September 21–26), the alongshore current patterns differed substantially. This variation
<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Time series of significant wave height $H_{m0}$ (m) with corresponding tidal elevation (blue-gray colorbar) at the three nearshore stations: (a) $A_{n2}-P_{n1}$, (b) $A_{c2}-P_{c1}$ and (c) $A_{s2}-P_{s1}$. The black line represents $H_{m0}$ at the outer boundary of the domain (CANDHIS buoy). In (d,e) the same time series is zoomed onto two tidal cycles from (b) $A_{s2}-P_{s1}$. Vertical gray bands indicate when the sensor is emerged or located in the surf zone ($H_{m0}/h > 0.4$).</figcaption></figure>

<table><caption>**Table 6**

Currents and tide temporal asymmetries.</caption><thead><tr><th>Name</th><th>RT (h)</th><th>FT (h)</th><th>Flood (h)</th><th>Ebb (h)</th><th>HT-SW lag (min)</th><th>LT-SW lag (min)</th></tr></thead><tbody><tr><td>A<sub>n1</sub></td><td>6 h22</td><td>6 h02</td><td>6 h47</td><td>5 h38</td><td>34</td><td>22</td></tr><tr><td>A<sub>c1</sub></td><td>6 h22</td><td>6 h02</td><td>5 h47</td><td>6 h38</td><td>10</td><td>45</td></tr><tr><td>A<sub>s1</sub></td><td>6 h24</td><td>6 h00</td><td>5 h43</td><td>6 h42</td><td>0</td><td>40</td></tr></tbody></table>

RT: Rising tide ; FT: Falling tide ; HT: High tide ; SW: Slack waters ; LT: Low tide.

can be explained by the 15–30° difference in wave incidence between the two periods. During the first spring tide period (September 12–17), a reverse alongshore current was observed at the three nearshore stations (An2, Ac2, and As2) during falling tide (Fig. 13d), resulting

in flood dominance ($V_{mean} > 0$), which was most pronounced at the northern station An2 (Table 8). In the last spring tide period (September 21–26), with wave incidence from the north being 15–30° greater, the reverse alongshore current was less intense than during the first spring
<figure><img src="image_16.png" /><figcaption>Fig. 12. Time series of significant wave height $H_{m0}$ at all nearshore pressure gauges of the (a) northern, (b) central and (c) southern profiles.</figcaption></figure>

<caption>Table 7</caption>

Bulk wave statistics of significant wave height $H_{m0}$ (m), outside of the breaking zone, at three nearshore stations with averaged, maximum and minimum $H_{m0}$ calculated over spring and neap tides throughout the field experiment.

<table><caption>Table 8</caption><thead><tr><th rowspan="3">Name</th><th colspan="3">Spring tide</th><th colspan="3">Neap tide</th><th colspan="3">Spring tide</th></tr><tr><th colspan="3">12–17 sept</th><th colspan="3">17–21 sept</th><th colspan="3">21–26 sept</th></tr><tr><th>Mean<br>H<sub>m0</sub></th><th>Max<br>H<sub>m0</sub></th><th>Min<br>H<sub>m0</sub></th><th>Mean<br>H<sub>m0</sub></th><th>Max<br>H<sub>m0</sub></th><th>Min<br>H<sub>m0</sub></th><th>Mean<br>H<sub>m0</sub></th><th>Max<br>H<sub>m0</sub></th><th>Min<br>H<sub>m0</sub></th></tr></thead><tbody><tr><td>P<sub>n1</sub></td><td>0.80</td><td><strong>1.55</strong></td><td>0.38</td><td>0.31</td><td>0.73</td><td>0.16</td><td>0.65</td><td>1.03</td><td>0.24</td></tr><tr><td>P<sub>c1</sub></td><td><strong>0.94</strong></td><td>1.48</td><td><strong>0.64</strong></td><td><strong>0.37</strong></td><td><strong>0.80</strong></td><td>0.19</td><td><strong>0.79</strong></td><td><strong>1.20</strong></td><td><strong>0.32</strong></td></tr><tr><td>P<sub>a1</sub></td><td>0.67</td><td>1.02</td><td>0.39</td><td>0.31</td><td>0.63</td><td><strong>0.20</strong></td><td>0.65</td><td>1.04</td><td>0.30</td></tr></tbody></table>

Alongshore current velocity $V$ (m/s) variations at the nearshore stations calculated over spring and neap tides throughout the field experiment, with: $V_{mean}$ the residual alongshore current, $V_{max}$ the maximum flood current, $V_{min}$ the maximum ebb current.

<table><thead><tr><th rowspan="3">Name</th><th colspan="3">Spring tide</th><th colspan="3">Neap tide</th><th colspan="3">Spring tide</th></tr><tr><th colspan="3">12–17 sept</th><th colspan="3">17–21 sept</th><th colspan="3">21–26 sept</th></tr><tr><th>V<sub>mean</sub></th><th>V<sub>max</sub></th><th>V<sub>min</sub></th><th>V<sub>mean</sub></th><th>V<sub>max</sub></th><th>V<sub>min</sub></th><th>V<sub>mean</sub></th><th>V<sub>max</sub></th><th>V<sub>min</sub></th></tr></thead><tbody><tr><td>An</td><td><strong>0.16</strong></td><td>0.54</td><td>-0.24</td><td>-0.03</td><td>0.24</td><td>-0.25</td><td><strong>0.09</strong></td><td>0.46</td><td>-0.29</td></tr><tr><td>Ac</td><td>0.12</td><td>0.67</td><td>-0.41</td><td>-0.03</td><td><strong>0.29</strong></td><td>-0.30</td><td>0.07</td><td>0.49</td><td>-0.38</td></tr><tr><td>As</td><td>0.12</td><td><strong>0.96</strong></td><td>-0.58</td><td>-0.06</td><td>0.24</td><td>-0.31</td><td>-0.06</td><td><strong>0.50</strong></td><td>-0.54</td></tr></tbody></table>

tide (not shown) at the northern and central stations (An2 and Ac2), and it was no longer observed at station As2 (Fig. 13e), resulting in ebb dominance at this station (Table 8).

## 4. Discussion and conclusions

Nearshore and inner shelf hydrodynamics are typically dominated by tide-driven currents in tide-dominated estuaries (Wells, 1995), with flow velocities increasing as the tidal range expands (e.g. Mirfenderesk and Tomlinson, 2008; Li et al., 2020). Similar trends were observed along the North-Médoc coast, which lies adjacent to the tide-dominated Gironde estuary. However, in the surf zone, wave-driven longshore

currents superimpose on tidal flows, and under the relatively low- to moderate-energy wave conditions of the field experiment, these longshore currents can even balance tidal currents. Given that the inner shelf's significant wave height can exceed 8 m during severe storms (Castelle et al., 2017), with complex wave-breaking patterns over the sandy shoals and rocky outcrops several kilometers offshore, wave-induced currents are expected even at the inner shelf stations under such storm conditions. Therefore, the results presented here are relevant only for low- to moderate-energy conditions. Hydrodynamics under storm conditions should be further investigated using process-based models or through winter field campaigns.
<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Time series of tidal elevation overlaid with total current intensity (jet colorbar) and corresponding alongshore *V* and cross-shore *U* velocity components at the nearshore stations: (a) *A*<sub>n2</sub>, (b) *A*<sub>c2</sub> and (c) *A*<sub>s2</sub>. For *As2*, panels (d, e) show tidal elevation overlaid with alongshore *V* velocity components (blue-gray-red colorbar). Vertical gray bands indicate when the sensor was either emerged or located in the surf zone (*H*<sub>m0</sub>/h > 0.4).</figcaption></figure>

Tide can also affect wave transformation and, in turn, breaking
wave conditions, wave-driven currents and sediment transport. Dif-
ferent processes can affect wave height modulation by tide, whose
respective contributions are essentially site specific. These processes
include e.g. depth- or current-induced wave refraction, bottom friction,
wave steepening or dissipation by opposing tidal flows (Davidson et al.,
2008). In the case of opposing currents, wave blocking and current-
induced wave breaking dissipation can result in an attenuation of up
to 40% of the wave height through the inlet (Dodet et al., 2013).
Along the North-Médoc coast the primary factor affecting nearshore
wave height is the water level, for the same inner shelf wave con-
ditions, with nearshore wave height increasing with increasing tidal
elevation. This is due to increased bottom friction and potentially
depth-induced breaking (across the shoals and rocky outcrop further
offshore) wave energy dissipation for lower tide levels (Kim, 2003).

The second factor affecting nearshore wave height along the North-
Médoc coast was the stage (rising versus falling) of the tide with, for the
same offshore wave conditions, larger waves during rising tides. This
phenomenon, often referred to as “tidal push” by surfers, can be due
to wave dissipation by opposing tidal flows (Davidson et al., 2008),
but also current-induced wave refraction. This agrees with the fact
that the importance of this modulation increases northwards between
the three transects, i.e. closer to the estuary mouth. This was also
illustrated at the inner shelf stations by addressing by comparing the
change in inner shelf wave height (with respect to offshore) against the
barotropic gradient (Fig. 7). A similar analysis was performed for the
nearshore stations (not shown), showing similar but noisier relationship
due to nearshore wave transformation and induced circulation (set-
up/set-down, shoaling, depth-induced breaking). The influence of these
processes and of other factors, and how their respective contributions
vary with different offshore wave conditions should be addressed with a process-based model adapted to environments with strong wave and tide interactions (e.g. Mengual et al., 2022). The model should rely on an unstructured grid to capture the complex shape of the coastline and to finely resolve channels, sandbanks, and coastal defenses in the area.

The inner shelf and nearshore flows are affected by the inner shelf morphology. The flow at the northern inner shelf station is flood-dominated, while at the two other inner shelf stations located within a well-defined channel further south it is ebb-dominated. As a result of this channelized flow (e.g. Mullarney et al., 2008; Héquette et al., 2009; de Schipper et al., 2016), tidal currents are intensified at these stations. Such convergence also results in a shorter falling tide and an increase of ebb current with respect to flood current (Allen et al., 1980; Toublanc et al., 2015, e.g.), leading to an ebb-dominance of the central and southern inner shelf stations located in the channel. This channel is also constrained close to the coast because of the rocky outcrops in the west, which explains the strong tide-driven longshore currents observed along the central and southern nearshore stations. This is particularly evident for the southern station $A_{s1}$, showing the strongest alongshore flows, located in a channel section located only a few hundreds of meters from the shoreline. This is also within this southern sector that the largest long-term erosion rate (≈5 m/yr) is observed. Importantly, as shown by Vandenhove et al. (2024), such chronic erosion is mostly due the long-term diffusion of a large-scale estuary-mouth shoal attachment completed around the 1920s. We anticipate that the increasing tidal current velocities, as the channel has progressively developed in recent years, explain the sustained large shoreline erosion rates, which have even slightly increased in recent years.

Wave-driven currents were measured only at the nearshore stations when the current meter was located within the surf zone. The complex wave transformation further offshore, combined with the alongshore non-uniform coastline orientation, typically results in intricate wave-driven current patterns (Hayes et al., 1970; Keshtpoor et al., 2015). Using a simple approach that combines empirical formulations for offshore wave transformation and longshore drift quantities, Idier et al. (2013) demonstrated that the longshore drift reverses direction to the north along the North-Médoc coast, although no specific values were provided due to the strong assumptions involved in the study. Our findings confirm that a fully coupled process-based model is necessary to accurately compute the alongshore distribution of breaking wave conditions along this coast. Moreover, despite the relatively low-to-moderate-energy wave conditions, nearshore wave-driven currents sometimes counterbalance tide-driven currents, a phenomenon also observed along the sandy coast adjacent to the large inlet of San Francisco Bay (Shi et al., 2011), although under higher-energy wave conditions.

The field campaign presented here offers the first quantitative, in situ insights into the wave, tide, and flow patterns along the rapidly evolving North-Médoc coast. It explores the complex processes influencing wave transformation, including tidal modulation, and the characteristics of tidal currents. While the dataset is limited in spatial coverage due to the large scale of the system, it provides valuable data to validate process-based models and to parameterize longshore drift quantities, which can be used to inform reduced-complexity shoreline models for this dynamic coastline.

### CRediT authorship contribution statement

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

### 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.

### Acknowledgments

This work is part of the project ESTOC, co-funded by the Region Nouvelle-Aquitaine, France, the Communauté de Communes Médoc Atlantique (CdCMA), France, the BRGM, France and EPOC (University of Bordeaux - CNRS), France. BC received funding from Agence Nationale de la Recherche (ANR), France grant number ANR-21-CE01-0015 (SHORMOSAT). The authors would like to thank the SHOM for providing bathymetric charts and open access to tidal elevation measurements (REFMAR network), the Observatoire de la Côte Nouvelle Aquitaine (OCNA) for orthophotography (PIGMA network) and Centre for Studies and Expertise on Risks, the Environment, Mobility and Urban Planning (CEREMA) for open access to data from directional wave buoys (CANDHIS network). Atmospheric data was provided by Météo-France through the Radome network. We would also like to thank Energie de la lune, the consulting firm that facilitated the deployment and retrieval of the offshore stations. Finally, we would like to express our gratitude to Olivier Burvingt, Elsa Durand, Emilie Woussen, Carla Labarthe, Sophie Defontaine, Marie Branellec and Valentin Auger for their help during the field experiment.

### Data availability

Data will be made available on request.

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