# Morphodynamics of two Mediterranean microtidal beaches presenting permanent megacusps under the influence of waves and strong offshore winds

Pierre Feyssat, Raphaël Certain, Nicolas Robin, Jean-Paul Barusseau, Antoine Lamy, Olivier Raynal, Bertil Hebert

► To cite this version:

Pierre Feyssat, Raphaël Certain, Nicolas Robin, Jean-Paul Barusseau, Antoine Lamy, et al. Morphodynamics of two Mediterranean microtidal beaches presenting permanent megacusps under the influence of waves and strong offshore winds. CONTINENTAL SHELF RESEARCH, 2024, 272, 10.1016/j.csr.2023.105160. insu-04729601

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# Morphodynamics of two Mediterranean microtidal beaches presenting permanent megacusps under the influence of waves and strong offshore winds

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Pierre FEYSSAT¹, Raphaël CERTAIN¹, Nicolas ROBIN¹, Jean-Paul BARUSSEAU¹, Antoine LAMY¹, Olivier RAYNAL¹ and Bertil HEBERT¹

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1. CEFREM, UMR 5110, Université de Perpignan Via Domitia, 52 Avenue Paul Alduy, 66000 Perpignan, France. pierre.feyssat@univ-perp.fr

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## Highlights:

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A conceptual morphodynamic model for two Mediterranean emerged beaches is proposed.

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Offshore wind induces erosion on the backshore and produces shoreline advancement.

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Beach evolutions are controlled by the balance of storms and offshore wind periods.

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Fair-weather morphologies on the beachface are fully expressed due to low tidal range.

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Slight differences exist depending on whether the beach is backed or not by a dune.

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## Abstract:

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This study focuses on four years of survey of two low-microtidal emerged beaches (average tidal range ≈ 0.3 m), presenting permanent megacusps on the French Mediterranean coast. These beaches are representative of the southern part of the Gulf of Lion (45 km of coast): the Leucate beach backed by a small dune and the La Franqui beach backed by a small lagoon. An analysis of the hydrodynamic and meteorological forcing along with morphological changes allows to define the key parameters controlling the system. These results are used to develop a conceptual model composed of five main situations and then compare it to the microtidal literature. Dominant offshore winds can reach speeds of up to 20 m.s⁻¹ (Gusts up to 30 m.s⁻¹) and have an erosive action on the backshore and produce advance of the shoreline of a few metres. This shoreline advance is mainly concentrated on the horns when it is concomitant with small fair-weather waves. Waves of moderate to high intensity (storms) lead to strong erosion and steepening of the beach face, as well as the migration or
construction of a new higher berm located landward and associated with sediment deposition
on the upper beach. The low tidal range may also allow the high run-up level to stabilise for a
longer period of time in a high elevation on the backshore. The alternation of storm episodes
and offshore wind periods could control the relative stability of the emerged beach.

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32 **Keywords:** Sandy beach, Barrier beach, Emerged beach, Offshore wind, Gulf of Lions

33 **1. Introduction**

Knowledge of beach dynamics in various and contrasting environments is essential to effectively adapt management strategies and improve assessments of their resilience (Fanini et al., 2020; Power et al., 2021). Beaches are known to evolve according to forcing conditions, and this close relationship induces a cyclicity closely associated with the seasonality of weather patterns. For temperate climates, winter periods are characterised by high-energy conditions that induce erosion, while calmer summer periods allow the reconstruction of the system (Castelle et al., 2015; Konstantinou et al., 2021; Masselink and Pattiaratchi, 2001; Senechal and Ruiz de Alegria-Arzaburu, 2020). Beaches can be classified according to their morphological characteristics. One of the most widely used classifications of beaches is based on the Australian model, which is drawn from the study of sandy beaches along a microtidal coastline with moderate to high wave energy and is mainly focused on the dynamics of nearshore bars (Wright and Short, 1984). This study has since been extensively applied to other environments, but always remaining focused on the dynamics of nearshore bars and little on the morphodynamics of the emerged beach (Aleman et al., 2015; Castelle et al., 2007; Lippmann and Holman, 1990; Masselink and Short, 1993; Scott et al., 2011). The model assumes that a system can evolve towards a dynamic steady state under stable forcing conditions. Thus, several main beach states are recognized: reflective, intermediate and dissipative. Intermediate beaches can display large-scale rhythmic crescentic features along the shoreline called megacusps. These megacusps are usually defined as a succession of horns extending seaward and embayments extending landward (Thornton et al., 2007). The patterns of horns and embayments are induced by hydrometeorological forcing and antecedent morphology (Benavente et al., 2011; Coco et al., 2000; Guza and Inman, 1975). The beach is narrower and lower in the embayment of the mega-cusps, which allows the swash of large storm waves to reach a maximum elevation on the beach (e.g. Castelle et al., 2015; Short and Hesp, 1982; Thornton et al., 2007; Wright and Short, 1984). The response of
these features is however complex and needs further investigation, although some studies have found that megacusps are forming during less energetic condition and disappearing during storms (Birrien et al., 2013), others have also observed megacusps persisting during storms (Aagaard et al., 2005; Castelle et al., 2019; Quartel, 2009) and disappearing during less energetic wave conditions (Segura et al., 2018).

As previously stated, most researches had focused primarily on coastline and nearshore morphodynamics, but the study of emerged beach (backshore) dynamics, which is the part of the beach between the beach face and the dune foot is particularly relevant since it addresses many scientific concerns (Eichentopf et al., 2019a; Fanini et al., 2020) and socioeconomic issues (Fanini et al., 2020). Most of the studies on emerged beaches refer to moderate to high-energy and/or meso to macrotidal coasts. Only limited research has been undertaken for example on emerged beaches subject to long period of fair-weather interspersed with short and energetic storm event intermittent forcing (like the intermittent forcing of the Mediterranean, e.g. Amores et al., 2020; Buosi et al., 2019; Gervais et al., 2012; Sabatier et al., 2008) or low energy, which have been shown to respond very differently to changing wave conditions (Costas et al., 2005; Gómez-Pujol et al., 2007; Hegge et al., 1996; Jackson et al., 2002). Moreover, in microtidal environments, attention has been mainly given to storm effects and beach recovery processes (Davidson et al., 2013; Eichentopf et al., 2020; Gallop et al., 2011; Phillips et al., 2017) as well as shoreline dynamics (Davidson et al., 2017; Van Gaalen et al., 2011; Yates et al., 2009), inlet dynamics (Kennedy et al., 2020; McSweeney et al., 2020; Meslard et al., 2022), aeolian processes (Lynch et al., 2013; Schwarz et al., 2021) or beach-dune interaction (Medellín and Torres-Freyermuth, 2021; Short and Hesp, 1982; Thornton et al., 2007). These studies are mainly concentrated on sites where the tidal range is close to the upper class limit (2 m) and the impact of certain processes is not fully investigated. This applies to aeolian processes, which can give rise to considerable sediment transport and significant morphological evolution on the backshore (Bauer et al., 2012; Gares et al., 1996; Nordstrom et al., 1996; Nordstrom and Jackson, 2017), or the impact of long periods of fair-weather (Buosi et al., 2019; Gómez-Pujol et al., 2011; Longhitano, 2015). Environments with a low tidal variation (Tidal Range < 0.5 m) are common in large lakes (Law and Davidson-Arnott, 1990a) or enclosed basins such as the Azof, Baltic (Käärd et al., 2016; Soomere et al., 2020), the Mediterranean or the Black Seas. In these environments, wave energy is primarily distributed at the same elevation due to the low tidal range (Cooper, 1994) inducing a time action of hydrodynamic processes different from higher tidal environments. The northwestern Mediterranean is an example of a singular low microtidal
environment with a very narrow intertidal zone (less than a few meters). This area is also characterized by long fair-weather periods of weak wave energy interspersed with short high-energy storms (Aleman et al., 2015; de Swart et al., 2022) where the significant wave height (Hs) can reach 6 m, inducing a surge level close to 1-1.5m (Aleman et al., 2015; Guizien, 2009; Mendoza et al., 2011). In these singular environments, the backshore is only subject to the influence of marine processes during these short storm periods (Armaroli et al., 2013, 2012; Balouin et al., 2013; Gervais et al., 2012b; Romagnoli et al., 2021; Sabatier et al., 2008; Sedrati et al., 2011). Such episodes can also lead to an impact on coastal infrastructures (De Muro et al., 2017; Porta et al., 2020; Trogu et al., 2020). Despite the fact that low-microtidal environments can be potentially affected by long fair weather periods, it should be distinguished from the low-energy environments defined by Jackson et al. (2002). These latter are commonly observed on beaches in estuaries and bays or on sheltered coasts where non-storm significant wave heights are minimal (<0.25 m) and waves are of small amplitude, despite strong onshore wind forcing (<0.50 m). During the long periods of weak waves in the northwestern Mediterranean, changes in beach-face morphology can be relatively intense (Bertoni et al., 2013; Sedrati et al., 2009) and the wind regime is also to be considered in the evolution of the system as the beach exposure time is sufficient to cause onshore and offshore transport and induce morphological changes on the backshore or the beach face (Feyssat et al., 2022; Law and Davidson-Arnott, 1990b; Sabatier et al., 2007). As a result, the coastal Mediterranean environment is complex, with processes (offshore/onshore) acting in very different ways in both time and space.

The aim of this study is to complete the conceptual beach models, using simple semi-quantitative parameters and only focusing on the emerged beach. This involves discriminating the influence of each control parameter on the emerged beach morphodynamics, with particular emphasis on fair-weather and offshore wind conditions which are less discussed in the literature. Two sandy beaches evolving in a low-microtidal environment were surveyed over four years. The methodological approach is based on frequent topographic surveys as well as hydrodynamic and meteorological data from nearby stations. The results are then summarised in a conceptual beach model that combines a set of factors (wave and wind) that are not usually considered simultaneously and compared to the microtidal literature.
## 2. Study area

The Gulf of Lions is a low-microtidal, wave-dominated environment with an average tidal range <0.3 m at mean spring tides. Under stormy conditions, a surge of about 1 m may occur due to the combined action of onshore wind stress, the reduction in atmospheric pressure and/or breaking waves (Certain, 2002).

Weather patterns can be divided into two main regimes as follows. The prevailing offshore winds (NW) frequently reach daily average speeds of more than 10 m.s⁻¹ with speed gusts exceeding 30 m.s⁻¹ for a few hours (Fig. 1a). This strong offshore wind generates waves that propagate away from the study sites (Grey area Fig. 1b). Onshore winds (SE) are less frequent (Fig. 1a) and are accompanied by waves (mean annual Hs = 0.66 m) or winter storms (Aleman et al., 2015). The S/SE swell (Fig. 1b) is characterised by higher wave heights and long periods (5≤Ts≤10 s), due to the more extensive fetch (Certain, 2002). In the present study, a storm is defined as a wave event in which Hs exceeds the 2 m threshold value commonly used in the region (Aleman et al., 2011; Mendoza and Jiménez, 2006). According to this definition, on the Leucate site, about 7 to 8 storm events are reported per year (Cerema and Dreal LR, 2018). Hs can reach up to more than 6.5 m during severe storms (Cerema and Dreal LR, 2018; Certain, 2002). A seasonality pattern can be observed: in summer, sea breezes are frequent, whereas, in late autumn, winter and spring, there are stronger offshore wind episodes interspersed with storms induced by onshore winds (Certain, 2002).
<figure><img src="image_2.png" /></figure>

Figure 1 Location of study sites (outlined in red). (a) Wind conditions (measured at the tip of Cap Leucate). (b) Wave conditions (measured at 40 m depths, about 2.1 nautical miles in front of the weather station). Meteorological data are given for the period 2007-2020.

The study sites (Leucate and La Franqui) are both located in the south of the Gulf of Lions, on both sides of Cape Leucate (French Mediterranean coast) (Fig. 1). These two sites are representative of the beaches in this coastal area, which are described as intermediate, with a permanent double crescentic nearshore sandbar system giving an undulating pattern to the coastline (Aleman et al., 2015, 2011; Ferrer, 2010). These beaches show an alternating pattern of horns and embayments (Fig. 2a, b), with varying depths and a wavelength of about 80 to 100 m at La Franqui (Fig. 2a & 3c) and 100 m at Leucate (Fig. 2b & 3b). These megacusps are relatively stable over time (even on an annual scale) and show very little longshore migration during a single storm, which is not common, due to the stability of the nearshore bars (Ferrer, 2010). To the south, the Leucate site (Fig. 2b & 3b) corresponds to a barrier beach made up of mostly coarse-grained sand (D50= 0.75 mm). The beach width is about 70 m and is backed by a small vegetated dune, mostly artificial and stabilised (fencing and vegetation), with a dune foot located at 2 m NGF (NGF, French national topographic reference) (Fig. 4b) and its elevation reaching up to 4.5 m NGF. Similar sites are shown in yellow on the map (Fig. 3a), representing 42% of the depicted coastline. To the north, the La
Franqui barrier beach (Fig. 2a & 3c) is also mostly coarse-grained sand (D50= 0.5 mm). The beach width is about 90 m and is directly backed by a small shallow lagoon, displaying an intermittent inlet on the southern part (Fig. 3c). It shows a flatter profile only reaching around 1.6 m NGF, without a dune system (Fig. 4a). Similar sites are shown in green on the map (Fig. 3a), representing 26% of the depicted coastline, mainly located in the northern part. The longshore drift is oriented northward at both sites (Kulling, 2017).

<figure><img src="image_3.png" /><figcaption>Figure 2. Oblique aerial views of the La Franqui (a) and Leucate (b) study sites with the location of horns and embayments.</figcaption></figure>

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<figure><img src="image_4.png" /></figure>

Figure 3 (a) Regional classification of beaches presenting permanent megacusps, differentiating those that are backed by a dune (yellow), not backed by a dune (green) or urbanised (black); red square corresponding to study sites. LiDAR topo-bathymetry of Leucate (b) and La Franqui (c), thick black line is 0 m NGF corresponding to the mean sea level; dashed red squares on the aerial photos correspond with the topobathymetric coverage area. See Figure 4 for beach elevation profiles.

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<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Figure 4 Topographical transects sections at the Horns (dotted lines) and embayment (solid lines); for the beach of a) La Franqui and b) Leucate. See the position of the profiles on Figure 3 b and c</figcaption></figure>

## 3. Methods

### 3.1 Morphological Surveys

The beach morphological survey was carried out using a DGPS-RTK mounted on a load carrier following transverse transects spaced at 10 m and supplemented by longitudinal transects for the coastline and the berm crest. The shoreline is defined as the static position of the water body on the field by the operator for this no-tidal environment, and the berm crest is defined as the highest point of the berm. Surveys were performed using an Ashtech Proflex 500 and a Trimble R8s. Digital elevation models (DEM) were generated on QGIS using the natural neighbour interpolation method (cell size : 0.5m, Z precision ≈ 2.5 cm). Using benchmarks located on the beach, the operational accuracy of individual surveys was estimated at +/- 3.5 cm including instrument, field measurement and interpolation uncertainties. Thus resulting in a +/- 5 cm uncertainty margin on differentials between two surveys. The survey is event-based, with measurements taken before and after morphogenetic periods. A total of 44 DEMs were generated and used for this study over a survey period ranging mostly from November 2017 to May 2019 for the La Franqui beach and from July 2019 to December 2020 for the Leucate beach (Fig. 5).
## 3.2 Hydrodynamic and meteorological data

Offshore wave conditions are recorded every 30 minutes by the CANDHIS network at the Leucate buoy moored 2.1 nautical miles off the study area (in front of the weather station, Fig. 1) at a depth of 40 m (Cerema et al., 2019).

The wave energy density E (in J/m²) is calculated, according to the linear wave theory (1):

$$E = \frac{1}{8} \rho g H_{m0}^2 (1)$$

Where $H_{m0}$ is the spectral significant wave height.

The hourly averaged wind data come from the Météo France weather station of Cape Leucate semaphore (Fig. 1) located about 2.5 km from the study sites at a height of 42 m above sea level.

Hourly water level (WL) measurements are taken from the tide gauge located in Port-Vendres (45 km south of the study site). To evaluate the extent of the marine influence on the beach, the 2 % exceedance value of run-up on the beach face ($R_{2\%}$) is computed using Stockdon et al.'s (2006) empirical formula (2):

$$R_{2\%} = WL + \left[ 1.1 + \left( 0.35\beta_f(H_0L_0)^{1/2} + \frac{[H_0L_0(0.563\beta_f^2 + 0.004)]^{1/2}}{2} \right) \right] \quad (2)$$

where WL is the water level, $H_0$ is the deep water wave-height, $L_0$ the deep-water wave length and $\beta_f$ the beach slope from Aleman et al., 2015.

# 4. Results

## 4.1 Forcing conditions

Weather pattern statistics during the survey period (2017-2020, Fig. 5), are consistent with long-term statistics (2007-2020, Fig. 1). Globally, the summer period is the least energetic, with an $H_s$ value around 1 m ($\approx$ 50% of the time) and mean wind velocity lower than $10 m^{-1}$ most of the time (Fig. 5). The winter period is more energetic, with possible waves higher than 2 m (2% of the time) and mean wind velocity over $10 m s^{-1}$ and up to $27 m s^{-1}$ (Fig. 5).

Two main types of weather pattern can be distinguished: 1) dominant offshore conditions with an NW wind (73% of the time) and 2) less common onshore wind conditions (27% of the time) but rather energetic because accompanied by waves that can exceed 2.5m. These two main forcing conditions can be subdivided as following (Tab. 1 and Fig. 6):
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Most of the time (41%), strong offshore winds generate offshore-directed waves (Tab. 1 and Fig. 6b), that does not impact the study sites and escape the Gulf of Lions. In some cases (29% of the time), the offshore wind can be paired with onshore waves of less than 1 m. Offshore conditions occur throughout the year, but are more energetic in winter (Fig. 5), when the mean wind can reach speeds of around 15 m.s⁻¹ for several days, i.e. from 04/01/2019 to 12/01/2019, the average daily offshore wind speed was 12 m.s⁻¹ (gusts reaching over 30 m.s⁻¹). Offshore wind conditions accompanied by onshore waves higher than 1 m are not discussed further here, as these forcing conditions are developed in a minority of cases and correspond to only short transitional states.

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The most common onshore forcing conditions are low-amplitude waves (Hs<1 m) accompanied by sea breezes (25% of the time), while storm conditions with Hs greater than 2 m, usually accompanied by onshore winds, are the least common (2% of the time) (Tab. 1 and Fig. 6c). Storms are common during the winter period (Fig. 5), and can reach Hs > 6 m, Hmax> 8 m (for a wave energy flux over 30*10³ J/m) with wind speeds > 20 m.s⁻¹ on daily average (i.e. 06/03/2018). Water-level measurements show large variations in winter linked to storm surges (that can reach 0.2 m to 0.6 m).

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Finally, 97% of the forcing conditions can be summarised according to 5 forcing situations: (1) offshore wind alone were sea-breezes and waves are cancelled, 41% (Light blue, Tab. 1); (2) offshore wind and small waves < 1m, 29% (Dark blue, Tab. 1); (3) onshore fair-weather waves < 1 m, 20% (Green, Tab. 1); (4) onshore waves between 1 and 2 m, 5% (Orange, Tab. 1) and (5) onshore storm waves > 2 m, 2% (Red, Tab. 1).

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<figure><img src="image_7.png" /></figure>
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Figure 5. Time series of recorded daily: a) wave heights; b) wave direction; c) wave period; d) wave energy density (periods of offshore waves are not indicated for sake of clarity); e) hourly mean wind speed and gusts; f) hourly mean wind direction; g) water levels. The red line is the Port-Vendres tide gauge and the blue bars are the calculated run-up height (all levels are in metre NGF, French official topographic reference). The surveys are shown at the top of the first graph (a) with circles for La Franqui beach and triangles for Leucate beach. The colour scale indicates the dominant forcing condition prior to the survey: black refers to surveys influenced by offshore winds, blue to offshore wind conditions concomitant with waves < 1 m, white to waves < 1m with onshore wind or no wind, red to waves > 1m and brown to storm events (onshore influence).
<figure><img src="image_8.png" /><figcaption>Figure 6. (a) Wind conditions during the study period 2018-2020; (b) wave directions and heights during offshore wind events; (c) wave conditions during onshore wind events.</figcaption></figure>

Tab 1. Forcing conditions and their occurrence (percentage of time), computed from the Leucate buoy and weather station data
<table><thead><tr><th colspan="2">Wind</th><th colspan="4">Waves</th></tr><tr><th colspan="2">Dir.</th><th>Dir.</th><th>Hs&lt;1m</th><th>1m&lt;Hs&lt;2m</th><th>2m&gt;Hs</th></tr></thead><tbody><tr><td rowspan="2">Offshore</td><td rowspan="2">73%</td><th>Offshore</th><td colspan="3">41%</td></tr><tr><th>Onshore</th><td>29%</td><td colspan="2">3%</td></tr><tr><td>Onshore</td><td>27%</td><th>Onshore</th><td>20%</td><td>5%</td><td>2%</td></tr></tbody></table>

## 4.2 Morphological evolution

The established scale of forcing conditions (Tab. 1) is used to describe the observed backshore morphology response as a function of the 5 forcing conditions: (situation 1) offshore wind alone were sea-breezes and waves are cancelled; (situation 2) offshore wind and small waves < 1m; (situation 3) onshore waves < 1 m (fair weather); (situation 4) onshore waves between 1 and 2 m; and (situation 5) onshore waves > 2 m (storm conditions). In each subsection below, one example for Leucate beach and one for La Franqui beach are presented for each situation, in the form of two differential maps per site, accompanied by two topographic sections showing the evolution of the beach at a horn and an embayment (Fig. 7, 8, 9). These examples are representative of the main morphological changes observed under similar forcing conditions for each case (Tab. 2, 3, 4). Finally, tables summarise all the observations made on each of the two sites for each situation (Tab. 2, 3, 4), based on the 44 DEM measured. This allows us to reach a conclusion on the main morphological evolutions observed for each situation.

### 4.2.1 Offshore wind conditions (situation 1 & 2)

For situation 1 the offshore wind is the only forcing agent, sea-breezes and waves are not present, as in Leucate from 17/09/2020 to 01/10/2020 (see differential map Fig. 7 I), with an average hourly speed of 6.3 m.s⁻¹ (maximum hourly average 17.2 m.s⁻¹, maximum gust 34 m.s⁻¹) (Fig. 5). At La Franqui (Fig. 7 III), from 09/11 to 16/11/2017 the average hourly speed was 9.6 m.s⁻¹ (maximum hourly average of 17.5 m.s⁻¹, maximum gust 32 m.s⁻¹) (Fig. 5). The sediment is transported towards the beach face creating an accretion (respectively for each site +0.2 / +0.4 m), and leading to a strong uniform advance of the coastline by up to +10 m for the two beaches (see profiles Fig. 7 a, c, e, g). While the backshore is eroded up to -0.3m and losing around -0.2m³/m² (Fig. 7 I, III), a zone of slight accretion is observed at the foredune foot on Leucate beach, up to +0.1 m³/m² (Fig. 7 I) and very locally in some depression zones of the beach of La Franqui (+0.2 m³/m²). The same observations are made
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for all the periods in situation 1 (41% of the time **Tab.1**), namely 5 times at Leucate and 4 times at La Franqui (**Tab. 2**).

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In the situation 2 the main forcing is the offshore wind paired with a small onshore waves ($H_s < 1$ m), as in Leucate, from 18 to 23/11/2020 (**Fig. 7 II**), with an average hourly speed of 5.3 m.s⁻¹ (maximum: 14.2 m.s⁻¹, maximum gust 27 m.s⁻¹) paired with a small onshore waves $H_s < 1$ m (total wave energy flux over the entire period: 3.9*10³ J/m; with a peak flux of 0.7*10³ J/m per hour) (**Fig. 5**) where maximum run-up height is $R_{2\%} = 0.68$ m, which corresponds to the berm crest level. At La Franqui, from 07/02 to 20/02/2019 (**Fig. 7 IV**), offshore wind is an average hourly speed of 5.3 m.s⁻¹ (maximum: 12 m.s⁻¹, maximum gust 25 m.s⁻¹) combined with a small onshore waves $H_s < 1$ m (total wave energy flux over the entire period: 13.5*10³ J/m; with a peak flux of 4*10³ J/m per hour) (**Fig. 5**) with maximum run-up height $R_{2\%} = 0.73$ m, which corresponds to the berm crest. The backshore is also eroded for the two beaches, up to -0.3m corresponding to -0.3 m³/m² on the whole backshore (**Fig. 7 II, IV**). The beach face morphologies show a clear longshore pattern, horns positions shows a seaward advance of the shoreline of a few metres, associated with beach-face accretion that can reach + 0.4 m³/m² (**Fig. 7 d, h**). On the other hand, the position of the embayments shows a landward retreat of a few metres and the beach face is in erosion, losing around -0.2m³/m² (**Fig. 7 b, f**). Similar observations are made, for all the periods of situation 2 (29% of the time **Tab.1**), namely 6 times at Leucate and 8 times at La Franqui (**Tab. 2**).

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<figure><img src="image_9.png" /><figcaption>Figure.7. Morphological evolutions on Leucate beach (top) and La Franqui (bottom), for offshore wind periods (I, a, c & III, e, g) and for periods combining offshore wind and onshore waves Hs<1 m</figcaption></figure>

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(II, b, d & IV, f, h). Dotted profiles correspond to the initial profile and solid profiles to the final situation.

<table><caption>Tab.2. Summary of all the morphological evolutions observed following episodes of offshore wind (11 observations/44) and offshore wind combined with waves $H_s<1 m$ (13 observations/44) at Leucate and La Franqui beaches. Shl. Stands for shoreline.</caption><thead><tr><th>Forcing condition</th><th>Site</th><th>Date</th><th>Backshore</th><th>Embayment</th><th>Horn</th></tr></thead><tbody><tr><td rowspan="2">Situation 1<br>Offshore<br>wind</td><td>Leucate</td><td>11/03/20,<br>21/05/20,<br>07/07/20,<br>01/10/20</td><td>Back berm<br>eroded, up to<br>-0.2 m. Dune<br>foot accretion<br>up to +0.1 m</td><td>-Shl. advance up to +5 m<br>-beach face accretion up<br>to +0.3m<br>-stable berm</td><td>-Shl. advance up to +8 m<br>-beach face accretion up to<br>+0.4m<br>-slight berm accretion</td></tr><tr><td>La<br>Franqui</td><td>09/11/17,<br>16/11/17,<br>12/07/18,<br>31/01/19,<br>27/01/22,<br>31/01/22,<br>03/02/22</td><td>Eroded up to<br>-0.3 m</td><td>-Shl. advance up to +8 m<br>-beach face accretion up<br>to +0.4m<br>-well marked berm</td><td>-Shl. advance up to +10 m<br>-beach face accretion up to<br>+0.3m<br>-well marked berm</td></tr><tr><td rowspan="2">Situation 2<br>Offshore<br>wind +<br>$H_s<1m$</td><td>Leucate</td><td>11/12/19,<br>10/01/20,<br>31/01/20,<br>06/02/20,<br>14/02/20,<br>01/07/20,<br>23/11/20</td><td>Back berm<br>eroded, up to<br>-0.2 m. Dune<br>foot accretion<br>up to +0.1 m</td><td>-Shl. retreat up to -2 m<br>-steeper beach face,<br>collision slope erosion up<br>to -0.3 m<br>-raised berm +0.3 m</td><td>-Shl. advance of the<br>shoreline up to +2 m<br>-beach face<br>accretion/advance up to<br>+0.3 m<br>-eroded top berm top -<br>0.2m</td></tr><tr><td>La<br>Franqui</td><td>29/11/17,<br>08/02/19,<br>20/02/19,<br>27/03/19,<br>22/05/19,<br>16/06/20</td><td>Eroded up to<br>-0.3 m</td><td>-Shl. retreat up to -5 m<br>-beach face erosion up to<br>-0.4m<br>-stable berm</td><td>-Shl. advance up to +6 m<br>-stable beach face slope<br>-slight berm accretion</td></tr></tbody></table>

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The following results are presented in two main parts, drawing a distinction between fair-weather conditions ($H_s<1 m$, situation 3 and $H_s=1-2 m$, situation 4) and high-energy storm conditions ($H_s>2 m$, situation 5).
Hs< 1 m (situation 3) and Hs =1-2 m (situation 4)

The situation 3 is characterised by significant waves height under 1 m of frontal incidence (fair weather waves). For example, at Leucate from 23/07/2019 to 03/09/2019 (Fig. 8 I), where the period is characterised by average Hs around 0.4 m (total wave energy flux over the entire period: 70*10³ J/m; with a peak flux of 3*10³ J/m per hour) (Fig. 5). The run-up height was R₂%= 0.76 m, which corresponds to the berm crest. During this period, the average hourly wind speed is around 5.5 m.s⁻¹ coming from the W/NW sector (maximum gust 15 m.s⁻¹) (Fig. 5). At La Franqui, from 20/02/2019 to 27/02/2019 (Fig. 8 III), with average Hs around 0.5 m (total wave energy flux over the entire period: 4.2*10³ J/m; with a peak flux of 0.5*10² J/m per hour) (Fig. 5). The run-up height was R₂%= 0.73 m, which corresponds to the berm crest. During this period, the average hourly wind speed is around 3 m.s⁻¹ coming from the SW sector (maximum gust 13 m.s⁻¹) (Fig. 5). The morphological responses for horns and embayments on both locations are similar to the situation 2 (Fig. 8 I, III). The embayments show signs of erosion, with a retreat of up to -5 m and erosion of the beach face and berm (-0.3m³/m²) (Fig. 8 a, e). By contrast, the horns are accreting, associated with an advance of the shoreline of up to +5 m, while the beach face shows an accretion of + 0.3 m (+ 0.3m³/m²) and the berm remains relatively stable (Fig. 8 d, g). Similar morphological changes are observed for all periods in situation 3 (20% of the time Tab.1), namely 4 times at Leucate and 4 times at La Franqui (Tab. 3).

The situation 4 is characterised by Hs over 1 m. For example, at Leucate, from 01/10/2020 to 18/11/2020 (Fig. 8 II), where the period is characterised by two events with Hs remained around 2 m (total wave energy flux over the entire period: 47.2*10⁴ J/m; with a peak flux of 9*10³ J/m per hour). The first event lasted for more than 12 h, while the second one was shorter (Fig. 5). The run-up height R₂% for these two events is 0.90 m, which allows only overtopping of the berm. During these two events, the maximum hourly wind speed is around 16 m.s⁻¹ coming from the S/SE sector (maximum gust 22 m.s⁻¹) (Fig. 5). At La Franqui, from 27/02/2019 to 13/03/2019 (Fig. 8 IV), with Hs of around 2 m for more than 10 h (total wave energy flux over the entire period: 14*10⁴ J/m; with a peak flux of 5.7*10³ J/m per hour) (Fig. 5) and run-up height R₂% = 0.84 m, which allows also overtopping of the berm. During this period, the maximum hourly wind speed is around 11 m.s⁻¹ coming from the S sector (maximum gust 15 m.s⁻¹) (Fig. 5). The shorelines record respectively a retreat of between -5 and -15 m (Fig. 8 II, IV), and the berms become raised and well-marked (up to + 0.6 m³/m² of accretion) (Fig. 8 b, d, f, h). This can lead to a steeper beach face (Fig. 8 b, d). The
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backshore is slightly affected by these changes due to the low surge level. The same
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observations are made for all the periods in situation 4 (5% of the time **Tab.1**), namely both 3
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<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>(a) Embayment period I</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>(b) Embayment period II</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>(c) Horn period I</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>(d) Horn period II</figcaption></figure>

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

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

<figure><img src="image_17.png" /><figcaption>(e) Embayment period III</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>(f) Embayment period IV</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>(g) Horn period III</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>(h) Horn period IV</figcaption></figure>

<figure><img src="image_21.png" /></figure>
Figure.8. Morphological evolutions on the beaches of Leucate (top) and La Franqui (bottom), for onshore-wave periods with $H_s<1 m$ (I, a, c & III, e, g) and for onshore-wave periods with $H_s = 1-2 m$ (II, b, d & IV, f, h). Note that (I) the backshore is actively cleaned by the passage of a Beach Cleaning machines towed by tractors, and is not affected by the morphological evolution of the shoreline. Dotted profiles correspond to the initial profile and solid profiles to the final situation.

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<table><caption>Tab.3. Summary of all the morphological evolutions observed following wave episodes with $H_s<1 m$ (8 observations/44) and with $H_s = 1-2 m$ (6 observations/44) observed on Leucate and La Franqui beaches. Shl. Stands for shoreline.</caption><thead><tr><th>Forcing condition</th><th>Site</th><th>Date</th><th>Backshore</th><th>Embayment</th><th>Horn</th></tr></thead><tbody><tr><td rowspan="2">Situation 3<br>$H_s < 1 m$</td><td>Leucate</td><td>23/07/19,<br>03/09/19,<br>26/09/19,<br>17/09/20</td><td>Stable</td><td>-Shl. retreat up to -3 m<br>-beach face erosion up to<br>-0.3m<br>-berm less marked</td><td>-Shl. advance up to +5 m<br>-beach face accretion up to<br>+0.3m<br>-stable berm</td></tr><tr><td>La<br>Franqui</td><td>27/02/19,<br>27/09/18,<br>22/03/19,<br>21/11/19</td><td>Slight<br>accretion</td><td>-Shl. retreat up to -5 m<br>-beach face less steep<br>-berm erosion up to -0.3<br>m</td><td>-Shl. advance up to +4 m<br>-beach face accretion up to<br>+0.3m<br>-stable berm</td></tr><tr><td rowspan="2">Situation 4<br>$H_s = 1-2 m$</td><td>Leucate</td><td>16/10/19,<br>24/04/20,<br>18/11/20</td><td>Stable dune<br>foot</td><td>-Shl. retreat up to -10 m<br>-steeper and raised beach<br>face up to +0.3 m<br>-massive retreat of the<br>shoreline</td><td>-Shl. retreat up to -15 m<br>-steeper and raised beach<br>face up to +0.9 m<br>-massive retreat of the<br>shoreline</td></tr><tr><td>La<br>Franqui</td><td>12/03/19,<br>25/10/18,<br>06/03/19</td><td>Stable</td><td>-Shl. retreat up to -5 m<br>-stable beach face slope-<br>berm retreat up to -6 m,<br>raised up to +0.6 m</td><td>-Shl. retreat up to -5 m<br>-stable beach face<br>-berm retreat up to -6 m,<br>raised up to +0.6 m and<br>beach accretion along the<br>profile</td></tr></tbody></table>

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## Storm conditions ($H_s> 2 m$) (situation 5)

In Leucate for situation 5 where the main forcing agent is an easterly storm, such as between 10/01/2020 and 27/01/2020 (Fig. 9 I), with $H_s> 4 m$ during 30 h (total wave energy flux over the entire period: $12.96*10^5 J/m$; with a peak flux of $34*10^3 J/m$ per hour) and a maximum around 8 m (the main peak on 23/01/2020, Fig. 5) and corresponding run-up height is $R_{2%} = 1.80 m$, which corresponds to the foot of the dune, the beach is submerged. During this storm event, the maximum hourly wind speed is around $15 m.s^{-1}$ coming from the S/SE sector (maximum gust $22 m.s^{-1}$) (Fig. 5). Another example occurs in Leucate from
23/11/2020 to 01/12/2020 (Fig. 9 II) when an easterly storm, with an Hs> 2 m during 3 days and a maximum of around 6 m (total wave energy flux over the entire period: 30.8*10⁴ J/m; with a peak flux of 10*10³ J/m per hour) (Fig. 5) with the Run-Up height of R₂% = 1.47 m, which corresponds to the back of the pre-storm berm. During this storm event the maximum hourly wind speed is around 15 m.s⁻¹ coming from the SE sector (maximum gust 19 m.s⁻¹) (Fig. 5). Following these two major storm events at Leucate, large quantities of sediment are deposited on the backshore (+ 0.5 m³/m²) around the maximum submergence limit which reaches the foot of the dune (Fig. 9 I, II). This boundary shows a longshore variability in relation to the position of the horns (Fig. 9 I, II) which are topographically lower and less sloping than the embayments (Fig. 9 a,c & b,d). Sediment deposition is therefore slightly more important on the backshore behind the horns. A new berm is formed in a landward position with a higher altitude along the horns, whereas the berm is just over-raised along the embayments (+ 0.3m). This result in a straighter and higher berm. The position of the shoreline is relatively constant given the forcing conditions. The same observations are made for all periods in situation 5 (5% of the time Tab.1), that is, a total of 3 times in Leucate (Tab. 4).

At La Franqui for situation 5, a major easterly storm of decadal recurrence, occurs between the 24/01/2018 and 07/03/2018 (Fig. 8 III), (storm peak : 01/03/2018 to 02/03/2018, reaching 34*10³ J/m in a day Fig. 5), with an Hs> 6 m during 40 h and maxima of around 10 m (total wave energy flux over the entire period: 82.4*10⁴ J/m; with a peak flux of 34.10³ J/m per hour); the run-up height is R₂%=2.4 m (Fig. 5), the beach is fully submerged. During this storm event the maximum hourly wind speed is around 20 m.s⁻¹ coming from the SE sector (maximum gust 27 m.s⁻¹) (Fig. 5). From 27/09/2018 to 28/11/2018 (Fig. 9 IV), La Franqui beach experiments also an easterly storm of lesser intensity (peak on 17/11/2018, 10*10³ J/m), with Hs>2 m during 36 h and maxima of around 6 m (total wave energy flux over the entire period: 91*10³ J/m; with a peak flux of 10*10³ J/m per hour) with the maximum run-up height of R₂%= 1.4 m, which indicates that the beach becomes completely submerged. During these storm events the maximum hourly wind speed is around 15 m.s⁻¹ coming from the SE sector (maximum gust 20 m.s⁻¹) (Fig. 5). Deposition occurs over the entire backshore (up to +0.8 m, corresponding to + 0.8 m³/m²) (Fig. 9 III, IV) and extends into the lagoon. The formation of the new well-marked berm in retreat on the beach concentrates most of the sediment input (up to +0.7m, corresponding to + 0.8 m³/m²) (Fig. 9 III, IV). In some cases, washover cones are formed (Fig. 9 III). The coastline, on the other hand, shows major retreat
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of the shoreline and linearization. Similar observations are made for all periods in situation 5 (5% of the time **Tab.1**), that is, a total of 3 times in La Franqui (**Tab. 4**).

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

<figure><img src="image_23.png" /><figcaption>(a) Embayment period I</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>(b) Embayment period II</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>(c) Horn period I</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>(d) Horn period II</figcaption></figure>

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

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

<figure><img src="image_29.png" /><figcaption>(e) Embayment period III</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>(f) Embayment period IV</figcaption></figure>

<figure><img src="image_31.png" /><figcaption>(g) Horn period III</figcaption></figure>

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

<figure><img src="image_33.png" /></figure>
Figure.9 Morphological evolutions on the beaches of Leucate (top) and La Franqui (bottom). For storm periods of annual recurrence (I, a, c; II, b, d & IV, f, h) and for a storm of decadal recurrence at the La Franqui (III, e, g). Dotted profiles correspond to the initial profile and solid profiles to the final situation. *The beach is completely submerged during storms.

<table><caption>Tab. 4. Summary of all the morphological evolutions observed following storm events (6 observations/44) observed on Leucate and La Franqui beaches (Fig. 7). Shl. Stands for shoreline.</caption><thead><tr><th>Forcing condition</th><th>Site</th><th>Date</th><th>Backshore</th><th>Embayment</th><th>Horn</th></tr></thead><tbody><tr><td rowspan="2">Situation 5<br>Hs> 2 m<br>(Storms)</td><td>Leucate</td><td>07/11/19,<br>27/01/20,<br>01/12/20</td><td>Sedimentation<br>on the<br>backshore<br>around max.<br>submergence<br>limit up to<br>+0.5 m</td><td>-globally stable Shl.<br>respecting high<br>forcing<br>-steeper beach face<br>-berm raised up to<br>+0.4 m</td><td>-Shl. retreat<br>-beach face less steep or<br>stable<br>-berm retreat up to -15 m,<br>raised up to +0.5 m</td></tr><tr><td>La<br>Franqui</td><td>24/01/18,<br>28/11/18,<br>07/03/18</td><td>Completely<br>submerged<br>with accretion<br>and washover<br>deposits</td><td>-Shl. retreat, up to -20<br>m<br>-steeper beach face<br>-berm raised up to<br>+0.7 m</td><td>-Shl. retreat, up to -25 m<br>-steeper beach face<br>-berm raised up to +0.7 m</td></tr></tbody></table>

## 5. Discussion

### 5.1. Morphodynamics evolutions of two Mediterranean beaches presenting permanent megacusps

Several conceptual models of beach evolution can provide a rough classification of beach states (e.g. Castelle et al., 2007; Lippmann and Holman, 1990; Masselink and Short, 1993; Short and Woodroffe, 2009; Wright and Short, 1984), but they mostly focus on the dynamics of the nearshore area and little on the emerged beach. They do not provide a detailed understanding of the morphological changes generated by forcing conditions. Here, a conceptual morphodynamic model is proposed for the two studied Mediterranean low-microtidal beaches presenting permanent megacusps. It summarises the different forcing parameters and their morphological impacts on the emerged beach, based on the observations of 44 DEM during four years of survey:
- The first situation is dominated only by offshore wind, sea breezes, tides and waves are cancelled (Fig. 10 a). The result is a significant aeolian transport inducing an erosion on the upper beach down to -0.3 m (see Fig. 7 f,h), as well as an accretion on the shoreline (about + 0.4 m for the beach face) leading to a uniform seaward migration of the shoreline (Fig. 10 I a, b). This first situation purely driven by offshore wind occurs preferentially in autumn and winter, when such winds are frequent (Fig. 11) with high intensity. Although this situation is common, several obstacles can modulate the imprint of this process: 1) The presence of a dune that can slow down or disturb the flow (Bauer et al., 2012; Delgado-Fernandez et al., 2013; Jackson et al., 2013; Lynch et al., 2013; Walker and Shugar, 2013). This latter effect is particularly visible when comparing the 2 sites, at Leucate a light erosion zone is visible at the dune foot (Fig. 10 II a, b) whereas at La Franqui in the absence of a dune the erosion on the upper beach is general (Fig. 10 I a, b); 2) Foredunes are also often stabilised by vegetation or a fence preventing the free exchange of sediments with the rest of the beach (Itzkin et al., 2020; Nordstrom, 2021; Robin et al., 2020; Walker et al., 2022); 3) In addition, surface moisture due to the proximity of the water table or climatic condition can also slow down or prevent the aeolian sediment transport by aggregating the grains of sand together (Davidson-Arnott et al., 2008; He et al., 2022; Jackson and Nordstrom, 1997; Li et al., 1997; Namikas et al., 2010). This latter effect should be taken in consideration, especially on low-lying beaches such as La Franqui (Fig. 10 I a, b) where the presence of moisture at a depth of a few centimetre (derived from the capillary fringe of the water table) acts as a constraint limiting beach erosion.

- The second situation (Fig. 10 b) is quite common throughout the year (Fig. 11), it arises from the coupled action of offshore winds and very low amplitude waves (Hs<1 m). There is sediment accretion on the horns while the embayments are only slightly eroded. In this situation, the seaward migration of the horns can reach its maximum (up to 6 m). Field observations suggest that this dynamic is due to the refraction of waves according to the shape of the coastline, the seaward extension of the horns leading to the convergence of the waves and their breaking on the horns (e.g. Falqués et al., 2021), thus attenuating their energy, while it is maximal in the bays. The morphodynamic of the emerged beach is similar to that described for the first situation (Fig. 10 a).

- The third situation (Fig. 10 c) corresponds to low-intensity waves (Hs<1 m) without wind. This situation is frequent, particularly in summer (Fig. 11). Morphological changes on the beach face are similar to those described in situation 2 (Fig. 10 b), with accretion
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inducing advance of the horns, due to refraction of small waves on the salient and slight
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- The fourth situation (Fig. 10 d) shows the effect of moderate-intensity waves (Hs> 1 m) that mostly occurs in autumn and spring (Fig. 11). When the run-up limit overtops the berm, it results in a homogeneous landward migration of the beach face, the narrowing of the beach and the formation of a higher berm with a steeper beach face.

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- The fifth situation (Fig. 10 e) is associated with morphological changes induced by storm conditions (Hs> 2 m) inducing a partial or total submersion of the emerged beach. The beach face is eroded, the coastline migrates landward and a new higher berm with a steeper beach face is established in a retreated position. In the case of the low-lying beach of La Franqui where the backshore is not backed by a dune (Fig. 10 Ie), the beach is easily submerged due to its low elevation and sediments are deposited on the backshore or on the lagoon in the form of washover fans. In cases where the beach is backed by a dune like Leucate (Fig. 10 Ile), sediments can be deposited at the foot of the dune, and erosion notches are observed only locally. Similar dynamics are commonly observed on other sites (Eichentopf et al., 2019b; Karunarathna et al., 2014; Sallenger, 2000). This fifth situation predominantly occurs during the winter months (Fig. 11).

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At a yearly scale, the balance between periods of erosion and accretion is generally attributed to seasonal variability, with winter being rather energetic and erosive (Casamayor et al., 2022; Dolique et al., 2019; Quartel et al., 2008; Wright and Short, 1984). During the summer period, there is an alternation between situations 1, 2, 3 (Fig. 11 a, b, c). Winter is marked by periods of strong offshore winds (situation 1; Fig. 11 a, b) punctuated by brief energetic events (situations 4; Fig. 11 d) or storms (situation 5; Fig. 11 e). The constructive small waves (situation 2 and 3; Fig. 11 b, c) can participate in a partial reconstruction of the beach face (Bugajny and Furmańczyk, 2020). The sediment supply on the emerged beach by storm episodes can be remobilized by offshore wind events (situation 1 and 2; Fig. 11 a, b) and contribute to induce a new seaward migration of the shoreline.
<figure><img src="image_34.png" /><figcaption>Figure.10 Morphological model of beach evolution for a beach backed by a lagoon with a planar upperbeach profile, La franqui (I) and with a dune, Leucate (II). R₂% marks the maximum limit of the Run-Up. Dotted profiles correspond to embayments and solid profiles to the horns. (*) the beach is totally submerged by the storm.</figcaption></figure>
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<figure><img src="image_35.png" /></figure>

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Figure.11 Monthly occurrences for each of the five forcing conditions, over the period ranging from 2006 to 2021

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## 5.2 Morphodynamic specificities of the two Mediterranean beaches

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The conceptual model proposed in **Fig.10** for both study sites fits globally with the observations made for other microtidal environments worldwide. However, some significant particularities can be highlighted: 1) the low-microtidal context has a significant impact on the morphodynamic of the beach face during fair-weather or storm conditions, and 2) The role of the offshore wind is major.

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1- At the study sites, the sea level is relatively constant over time and thus the intertidal zone is very narrow. The maximal tidal range of 0.3 m induces an intertidal zone width of around 3 m at Leucate. However, a tidal range of 2 m on an identical beach face slope would produce an intertidal zone at least 26 m wide. The relative static position of the sea level has important consequences on the morphodynamics of the beaches. Firstly, the morphological changes induced by fair-weather conditions are clearly visible and concentrated on the beach face. The coastline can migrate several metres in a few days, either uniformly under the effect of offshore wind sediments inputs (situation 1 **Fig.10**), or only on the horns under the action of fair-weather waves while embayments may show minor erosion or stability (situation 2, 3 **Fig.10**). Fair-weather conditions are common and persist for long periods, nearly 90% of the
time (Tab. 1), this explains dominance of these situations in the conceptual beach model (situation 1, 2, 3 Fig.11). The sea level remaining practically static allows even the weakest hydrodynamic processes to take place on the beach faces without being disturbed by the tidal sweep as in higher tidal range environment (Bertoni et al., 2013; Cooper, 1994; Sedrati et al., 2009; Short and Aagaard, 2020). Secondly, during moderate-intensity waves episodes (situation 4, Fig.10 d), the low tidal range may also allow the high run-up level to stabilise for a longer period of time in a high elevation on the backshore. This can lead to the retreat and elevation of the berm, creating generally a massive berm. During more energetic storm episodes (situation 5, Fig.10 e), a new wider berm with higher height and relatively uniform crest elevation along the crescentic coastline may be established in a remote position on the backshore (Armaroli et al., 2013; Durán et al., 2016; Gervais, 2012; Gervais et al., 2010). At microtidal sites with a larger tidal range (≈ 2m), berm seems to be of smaller size even after storms episodes (see for comparisons: Karunarathna et al., 2014; Ruiz de Alegría-Arzaburu et al., 2022). At our sites, there is also some sediment accretion on the upper beach during storm-induced flooding events, either along the toe of the dune (situation 5, Fig.10 Ile) or behind the dune in extreme cases. At microtidal sites with a larger tidal sweep, these storm deposits along the dune foot are thinner compared to what is observed at the study sites (see for comparisons: Eichentopf et al., 2019b; Karunarathna et al., 2014; Leatherman, 1979). In the case of a low-lying beach (e.g. La Franqui), the overwash regime is reached even for low-intensity storms (Sallenger, 2000). This can result in washover deposits (situation 5, Fig.10 Ie) that are accumulated in poorly vegetated areas or in lagoons (Balouin et al., 2005; Durán et al., 2016; Sabatier et al., 2008; Sedrati et al., 2011). This process can potentially provide a stockpile of sand available to feed the beach face and the nearshore during future offshore wind events.

2- The study sites are particularly suitable for aeolian processes, except in cases of episodic storms, because the backshore is almost always above the recurrent sea level position. This situation allows the beach to remain dry and is very favourable for wind sediment transport (Cooper, 1994; Sabatier et al., 2007, 2004). This is particularly important in the study area, where the offshore wind is dominant (73% of the time) which can allow, depending on the sediment availability, a scouring of the emerged beach that can reach -0.3 m and a shoreline advance over several metres (Fig.7). Thus, on an annual scale, the emerged beach is not only affected by onshore sediment transport and superficial deposits during marine storm events but also by offshore sediment transport during offshore wind events. During which, some of this sediment is redistributed onto the beach and foreshore by the
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offshore wind (Dingler et al., 1992; Gares et al., 1996; Jackson and Nordstrom, 1997; Leatherman, 1979; Nordstrom et al., 1996; Sabatier et al., 2007, 2004). The opposition of onshore and offshore forcing agents highlights the particular balance of sediment transport between the backshore/dune system and the shoreline (Dingler et al., 1992; Leatherman, 1979; Sabatier et al., 2007; Wright et al., 1985). However, the specific effect of offshore wind on the dynamics of the emerged beach and its sediment budget is sparsely addressed in the literature and deserve to be studied in more detail given its importance at certain locations.

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Surveys were carried out on the beaches of Leucate and La Franqui over a period of 4 years; both sites are characterised by dominant offshore winds, contrasting wave regimes and low microtidal range. These forcing conditions lead to the development of a model with five situations driven by a strong seasonality describing the emerged beach response. This model can be applied to most of the southern sandy coastline of the Gulf of Lions (45 km of coast) and more generally, the dynamics described here may possibly concern some beaches of the Mediterranean and other closed seas. The offshore wind, due to its transport capacity and predominance (73% of the time), can have an erosive action up to - 0.3 m on the top of the backshore (situation 1). The consequent transport of sand to the coastline leads to an accretion on the beach face (up to + 0.4 m) which generates a uniform advance of the coastline (of up to several metres). The action of low-intensity waves (Hs< 1 m), either combined with offshore wind and sediment transport from the backshore to the coastline (situation 2) or associated with a stable backshore, without offshore wind forcing (situation 3), leads to accretion and advance of the megacusps horns due to wave refraction on the horns accompanied by a slight erosion and potential retreat of the embayments. The morphogenetic impacts of these fair-weather conditions are clearly visible, notably at the foot of the beach face, due to the constant sea level position over time characteristic of this low-microtidal environment. More dynamic onshore wave conditions (Hs> 1 m and higher) either without wind or only with onshore winds, lead to erosion of the lower beach by wave attack paired with a large swash creating a higher berm with a steeper beach face in a retreated position (situation 4). Following the short but violent storm episodes that occur in the Mediterranean (situation 5), the retreat of the berm is maximal and can become established on the central part of the pre-storm beach. This can also result in sediment deposition on the upper beach along the dune toe or in the form of washover deposits. The construction of this
596

massive sedimentary feature, not necessarily observed for high microtidal regimes and then
stable during months until the next storm, is possible because of the relatively stable and high
water level during the storm event permitting thus the construction of the berm during all the
storm duration. The alternation of storm episodes and offshore wind periods could control the
relative stability of the emerged beach.

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Acknowledgements

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The authors would like to thank the Region Occitanie for the grant thesis support, ObsCat,
the PNM du Golfe du Lion and the PNR de la Narbonnaise en Méditerranée for their
financial support. Leucate is a monitoring site of Service National d'Observation (SNO)
Dynalit. Dr M.S.N. Carpenter post-edited the English style and grammar.

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