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Fifteen years of hydrodynamic forcing and morphological changes leading to breaching of a gravel spit, Sillon de Talbert (Brittany)

Quinze ans de forçage hydrodynamique et de changements morphologiques aboutissant à la rupture d’une flèche de galets, Sillon de Talbert (Bretagne)
Serge Suanez, Pierre Stéphan, France Floc’h, Ronan Autret, Bernard Fichaut, Emmanuel Blaise, Julien Houron, Jérôme Ammann, Philippe Grandjean, Mickaël Accensi, Gaël André et Fabrice Ardhuin
p. 403-428

Résumés

Le Sillon de Talbert est situé sur la côte nord de Bretagne ; c’est une flèche littorale de galets faisant face à la houle, qui s’étend sur 3,5 km et dont le volume de sédiment atteint 1,23 106 m³. Depuis 2002 un suivi morphodynamique a été entrepris. Il est basé sur des levés annuels permettant de calculer des MNT, et sur des mesures mensuelles de profils de plage. Le calcul du bilan sédimentaire sur les 15 ans (2002-2017) montre que les transits transversaux sont dominants et représentent un volume total de 370 000 m³, alors que le transit longitudinal est estimé à environ 50 000 m³. Toutefois, considérant le volume de plus de ‑411 000 m³ de sédiment érodé à l’avant plage, et le volume des transferts transversaux et longitudinaux atteignant +420 000 m³, le bilan sédimentaire global sur l’ensemble de la période de suivi est équilibré. Le déplacement vers la terre lié au phénomène de rollover a doublé durant les quinze dernières années avec un taux de recul de 2 m/an entre 2002 et 2017, contre 1,2 m/an entre 1930 et 2010. Les évènements tempétueux contrôlent plus de 95 % de ce recul lors des processus catastrophiques de submersion/inondation. Le recul de la flèche a conduit, au début du mois de mars 2018, à l’ouverture d’une brèche dans la partie proximale qui s’est rapidement élargie et approfondie, passant respectivement de 15 à 35 m, et de 1,25 à 3,4 m, au cours des 7 derniers mois. L’évolution morphologique du Sillon de Talbert est commandée par des forçages anthropiques (i.e., l’impact des ouvrages de défense côtière qui bloquent le transit sédimentaire longitudinal), et des forçages naturels, comme la pénurie de sédiments accumulés sur la plateforme pour l’alimentation de la flèche, ou l’augmentation des évènements extrêmes durant les dernières années.

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Notes de la rédaction

Article soumis le 15 octobre 2018, reçu sous sa forme révisée le 20 décembre 2018 et définitivement accepté le 22 décembre 2018.

Texte intégral

This work was supported by the municipality of Pleubian and the Conservatoire du Littoral. We are grateful to the staff of these two partners who provided technical assistance in the field and many observations during the survey period. This work was also supported by the French “Institut National des Sciences de l’Univers” (INSU) under-program, SNO-DYNALIT. We also thank the French meteorological service Météo-France for providing atmospheric pressure data of Ploumanac’h station. We finally thank very much the two reviewers, especially Donald L. Forbes, for their relevant comments that greatly improved the manuscript.

1. Introduction

1In the early 1980s the monitoring of beach morphology changes related to forcing conditions, notably in the USA and Australia, has considerably improved the knowledge of beach morphodynamics (Wright and Short, 1984; Birkemeier, 1985; Howd and Birkemeir, 1987; Thom and Hall, 1991; Lee and Birkemeir, 1993; Larson and Krauss, 1994). These surveys were facilitated by the abundant development, particularly from the 1990s onwards, of (i) tools and techniques of topo-morphological measurement, i.e., GPS (Morton et al., 1993; O’Regan, 1996; Rebêlo et al., 2002; Huang et al. 2002; Wheaton et al., 2009; Suanez et al., 2010; St-Hilaire-Gravel et al., 2012), i.e., Unmanned Aerial Vehicles (UAV) (Delacourt et al., 2009; Mancini et al., 2013; Bryson et al., 2013; Autret et al., 2018), i.e., terrestrial and airborne LiDAR (Jason and Woolard, 2002; Sallenger et al., 2003; Zhou and Xie, 2009; Fabris et al., 2010; Xhardé et al., 2011; Montreuil et al., 2013; Crapoulet et al., 2015; Letortu et al., 2015), and (ii) 3D digital processing (Westoby et al., 2012; Fonstad et al. 2013; Jaud et al., 2016, 2017; Letortu et al., 2018). The same may be said about the surveying of hydrodynamic conditions on shallow coastal zones. Although the measurement of waves using pressure transducers has been ongoing since the late 1940s, the improvement of the pressure sensors during the last few decades has considerably facilitated the measurement of gravity–infragravity waves and water levels (Cavaleri, 1980; Bishop and Donelan, 1987; Ruessink, 1998; Dodet et al., 2013). Nowadays, coastal monitoring is considered to be a major challenge in anticipating the response to coastal hazards (Ruggiero et al. 2000; Rieb and Walker 2001). In many countries, it leads to useful recommendations in terms of management policies regarding coastal defence, land use, and planning (Hamm et al. 2002; Meur-Férec et al. 2008). Furthermore, the success of Integrated Coastal Management (ICM) plans in terms of shoreline management requires that they explicitly incorporate a realistic range of coastal processes and responses based on an understanding of the physical environment by means of surveys (Evans, 1992; Solomon and Forbes, 1999).

2In 2001, after management of the Sillon de Talbert was transferred to the public French office “Conservatoire du Littoral”, a new strategy was adopted in terms of coastal management of this gravel spit barrier (Stéphan et al., 2018a). One of the options consisted of morphological monitoring, as the spit is strongly governed by both cross-shore and longshore dynamics (Pinot, 1994; Stéphan et al., 2012). As described by several authors, gravel spit barriers presenting a single crest are highly sensitive to landward migration due to rollover processes operating over both short term and long term time scales (Carter and Orford, 1984; Orford et al. 1991, 1995; Orford and Carter, 1995). Significant rollover processes occur during extreme events when wave runup, overtop, overwash, or strongly inundate the crest of the barrier (Donnelly et al., 2006; Matias et al., 2012; Masselink and van Heteren, 2014). These morphodynamic processes have been explained by storm-impact scaling models describing the response of the spit barriers to storms. The most effective model was proposed by Orford and Carter (1982), which recognizes four types of storm-impacts driving the gravel-dominated barrier deposits (fig. 1A). The first one deals with the “overtopping” process. In this case, the infiltration of the uprush reaching the crest diminishes the intensity of the backwash and the crest is therefore accreted. The next type is described as a “discrete overwash” when wave runup passes over the crest inducing a slight erosion of the top of the crest. The third type is described as a complete removal of the crest caused by a “sluicing overwash” process. In that case, extreme water levels generate a competent and unidirectional flow –largely unaffected by percolation– inducing a lowering of the crest. In some cases, the deposition of small-scale back-barrier washover fans is observed. The fourth type takes place during intense storm events, as the height of wave runup increases during the storm, and the “sluicing overwash” evolves into “overwashing” processes. In this case, the beachface and the crest are eroded in the form of a breach or throat, and washover fans and splays are deposited on the back-barrier. Generally, this latter extreme storm-impact type leads to barrier retreat due to rollover processes. Following the same approach, Sallenger (2000) and Stockon et al. (2007) identified four impact levels of storms on sandy barrier islands –as opposed to gravel-dominated barriers– that include an “overwash” regime (impact level 3), and culminate in an “inundation” regime (impact level 4) describing hydrodynamic conditions where extreme water levels are sufficient to completely and continuously submerge the barrier island. Therefore, overwash is the fundamental cross-shore mechanism forcing barrier retreat through rollover process, especially during storm events (Orford et al., 1995; Forbes et al., 1991, 1995; Tillmann and Wunderlich, 2013). Erosion processes may also be driven by longshore dynamics; when the sediment supply is depleted the wave energy reworks existing beach deposits through cannibalization (Carter and Orford, 1993). This process principally concerns drift-aligned spit barriers, where longshore process dynamics depends on the balance between the potential longshore transport (Qy) rate, as an energy term dependent on the angle of breaker approach (α-alpha), and the availability of sediment to be transported along the shore by this energy (Orford et al., 2002). Therefore, a drift-aligned barrier is associated with the sediment transport rate Qy > 0, while the barriers in a swash-aligned status are associated with Qy  0.

3Cross-shore (i.e., overwashing/rollover) and longshore (i.e., drifting) processes act together to weaken the spit barriers, whose final stage can culminate in the opening of a breach during storm events (SánchezArcilla and Jiménez, 1994; Orford et al., 1996; Thomas and Diaw, 1997; Kraus et al., 2002; Durand et al., 2010). In a recent paper, Orford and Anthony (2011) defined several elements related to barrier form, size, and constituent sediments that act as inertia to storm erosion. The authors detailed an approach based on measuring these components in order to estimate the resistance of the barrier to forcing, and to define the barrier resilience after extreme events (fig. 1B). The first component deals with the size/form of the crest sediment acting as a determinant to sediment transport during overflow. The width of the crest (Bw) measured at the reference elevation of “Mean High Spring Tide” (MHST) determines the sediment volume required to be eroded in order for barrier retreat (Br) to take place; it also acts as a brake to over-crest flow (Od,q and Oe). Finally, the relative steepness of the overall crest using barrier height (Bh) to barrier width is measured at the reference elevation of “Mean High Spring Tide” (MHST). This last component indexes the overall stability of the crest. However, the authors warned that this morphodynamic evaluation of storm-impact may be experimented only on gravel-dominated barriers evolving under conditions of limited longshore sediment supply, such as swash-aligned single beach-ridge barrier systems (Orford and Anthony, 2011).

Fig. 1 – Conceptual morphodynamic schemes of gravel-dominated spit barrier.
Fig. 1 – Schémas conceptuels de la morphodynamique des flèches de galets.

Fig. 1 – Conceptual morphodynamic schemes of gravel-dominated spit barrier.   Fig. 1 – Schémas conceptuels de la morphodynamique des flèches de galets.

A. Four types of storm-impacts driving the gravel-dominated barrier deposits proposed by Orford and Carter (1982), adapted for spit barriers where longshore drifting also controls morphodynamic processes. B. Domains of overflow relative to morphometric components of the gravel barrier. The boundaries of “overwash” and “sluicing overwash” domains are schematic, as they are variable across the back-barrier, being dependent upon the magnitude of the overflow (after Orford and Anthony (2011), modified and adapted to the median section beach profile of the Sillon de Talbert). Bw. Barrier width at the back-barrier ground level reference elevation; Bh. Barrier height at the back-barrier ground level reference elevation; Bw*. Width of the crest at MHST reference elevation; Bh*. Crest height at MHST reference elevation; Od,q. Flow depth (d), and volume discharge (q) overtopping the crest barrier; OePeak overflow elevation; Br. Barrier retreat.
A. Quatre types d’impact de tempête modifiant les cordons de galets, proposé par Orford and Carter (1982), adapté aux flèches de galets où les tansferts longitudinaux contrôlent également les processus dynamiques. B. Domaines de franchissement en fonctions des composants morphométriques du cordon de galets. Les limites des domaines de l’« overwash » et du « sluicing overwash » sont schématiques car elles varient transversalement par rapport au cordon en fonction de l'ampleur du franchissement (d’après Orford and Anthony (2011), modifié et adapté au profil de plage de la section médiane du Sillon de Talbert). Bw. Largeur du cordon à la hauteur de référence du sol de l’arrière cordon ; Bh. hauteur du cordon à la hauteur de référence du sol de l’arrière cordon ; Bw*. largeur du cordon à la hauteur de référence du niveau des PMVE ; Bh*. hauteur du cordon à la hauteur de référence du niveau des PMVE ; Od,q. hauteur du flot (d), et volume du flot (q) franchissant la crête du cordon ; Oehauteur maximum du franchissement ; Br. recul de la flèche.

4In this paper we examine both cross-shore and longshore dynamics through the morphosedimentary survey carried out since 2002 on the gravel spit of the Sillon de Talbert. This monitoring consists of yearly to monthly topo-morphology measurements (i) of the entire spit 3D-surface, (ii) and along two beach profiles situated on the most retreating zones. This survey is also based on hydrodynamic measurements (wave and water levels) using (i) WW3 wave modelling and tide records taken since 2002, and (ii) pressure sensor field-records taken since 2012. The quantification of cross-shore and longshore sediment transport has been underway for the past sixteen years. At the same time, the analysis of the hydrodynamic conditions is achieved by identifying storm events related to erosion phases vs calm periods suitable for barrier resilience. Finally, the morphological functioning of the gravel spit –especially related to cross-shore dynamics– is addressed using the storm-impact model of Orford and Carter (1982), for the entire survey period. The spatio-temporal sequence of beach-crest rebuilding after a storm is also analyzed through the changing position of crest elevation (vertical) and crest movement (horizontal).

2. Study site

5The gravel spit barrier of the Sillon de Talbert is located on the northern Brittany coast (France) in the department of Côtes d’Armor (fig. 2). According to the classifications (Zenkovitch, 1967; Buscombe and Masselink, 2006), it may be described as a single-ridge drift-aligned barrier that stretches over 3.5 km long. The sediment volume is estimated at 1.23 106 m³ (Stéphan, 2011). The barrier can be classed as a “composite gravel beach” type (Carter and Orford, 1984; Jennings and Shulmeister, 2002). The beach face is characterised by a break slope point at the mean water level which delimitates the gravel spit accumulation from the large flat rocky platform (slope = 0.01%) covered by thin and patchy periglacial deposits and/or some rare recent sandy sheets. The upper part of the beach face shows steeper slopes of between 5% and 15%. As already described by Stéphan et al. (2012), the barrier can be subdivided into four distinct morphosedimentary units (fig. 3A). Unit 1 corresponds to the proximal sandy section mainly composed by fine to medium sand material (pebbles fraction < 30%). The slope gradient is between 5% and 8% (fig. 3C). The crest height exceeds 8.5 m above sea level (a.s.l.) in places due to the formation of dunes on the top of the barrier (fig. 3D). This section is sheltered by many reefs located in front on the rocky platform. The upper-beach/dune zone is artificially protected by a riprap over a distance of 120 m, and a groin, “Chouck”, has been installed at the end of the cell to prevent loss of sediments due to longshore drift oriented to the NE (fig. 2B). Due to its mainly sandy nature, the existence of the dunes and the presence of the coastal defence structures, this section will not be studied in this paper. The proximal gravel section (Unit 2) is composed of a mixed sand and pebble material (pebble fraction < 40%). The barrier presents a low slope gradient of between 5% and 7% (fig. 3C). The crest has a small embryonic sand dune with an elevation around 6 m a.s.l. (fig. 3D). In Unit 3 (median section), the sediment material is mainly composed of pebbles (pebble fraction > 70%). The beach slopes are steeper and the crest is about 7 m a.s.l. (fig. 3C). Unit 4 forms the distal section of the Sillon de Talbert. It corresponds to the most accreted zone of the spit due to the fact that the longshore sediment transport accumulates on this tip zone. The net positive sediment supply explains the enlargement of the back-barrier tip of the spit, whose morphology is characterized by accreted ridges due to wave diffraction (fig. 3B). The pebble fraction exceeds 80%. The beachface slope increases to 15% while the elevation of the crest reaches 7.5 m a.s.l. Due to this morphology, this section is therefore the most reflective part of the spit as shown by rip-current circulation during high tide levels (fig. 3B). These hydrodynamic conditions are related to the formation of beach cusps, which are to this section.

Fig. 2 – Location map.
Fig. 2 – Carte de localization.

Fig. 2 – Location map.   Fig. 2 – Carte de localization.

A. Regional scale. B. Local scale. C. Oblique aerial photo of the Sillon de Talbert taken on the 23 September 2009 (source: D. Halleux) showing the coastal defense structures on the sandy proximal section. D. Wave characteristics. Wave rose and histogram of Hs established from the data obtained by WW3 numerical model over the period 01/01/1994-31/03/2018 at the calculation point 3.047°W, 48.927°N.
A. Échelle régionale. B. Échelle locale. C. Photo aérienne oblique du Sillon de Talbert prise le 23 septembre 2009 (source : D. Halleux) montrant les ouvrages défense côtière sur la section proximale sableuse. D. Caractéristiques de houle. Rose des houles et histogramme des HS établis à partir des données du modèle WW3 sur la période 01/01/1994-31/03/2018 au point de calcul 3,047°W, 48,927°N.

6This coastal area is located in a macrotidal to megatidal context with a maximum tidal range of 10.95 m (SHOM, 2016). The most frequent swells come from the WNW with a resultant vector of around 303° (fig. 2). Consequently, the waves break with a slight angle according to the coastline’s orientation ( 67°). This non-parallel swash alignment (α-alpha > 0) generates a longshore drift oriented to the NE. Modal heights (Hsig) of deepwater waves are between 1 and 1.5 m and modal periods (Tpic) are between 9 and 10 seconds. During storms wave heights can reach 9 m with periods of 20 seconds.

Fig. 3 – Morphological setting of the Sillon de Talbert.
Fig. 3 – Morphologie du Sillon de Talbert.

Fig. 3 – Morphological setting of the Sillon de Talbert.   Fig. 3 – Morphologie du Sillon de Talbert.

A. Different longshore morphological units of the spit and representative beach profiles of the four units; B. Oblique aerial photo of the distal section (Photo taken on 29 September 2011); C. Crest height (2017) ; D. Landward spit displacement in m.yr¹ between 1930 and 2010, after Stéphan et al. (2012), modified.
A. Différentes unités morphologiques de la flèche et profils de plage représentatifs des quatre sections ; B. Photographie aérienne oblique de la partie distale (photo prise le 29 septembre 2011) ; C. Hauteur de la crête (2017) ; D. Recul de la flèche exprimé en m/an entre 1930 et 2010, d’après Stéphan et al. (2012), modifié.

7The analysis of the long-term morphological evolution of the Sillon de Talbert indicates that up to the end of the 17th century it was connected to the islets of the Olone archipelago located to the NE (fig. 2). The detachment of the barrier occurred in the early 18th century and gave rise to a 3.2 km long gravel spit (Stéphan et al., 2012). This is probably due to the severe storm of November 26, 1703, which was one of the most violent events recorded along the south England and northwest French coasts over the past few centuries (Lamb and Frydendahl, 2005). The transformation from anchored barrier to free spit was associated to (i) a slight longshore cannibalization process, which increased throughout the 19th and 20th centuries due to sediment depletion, and (ii) a “cross-shore” landward displacement by rollover facilitated by the disconnection. Since 1770, the rate of spit retreat has been estimated at 1 m.yr¹ (Pinot, 1994). More recently, Stéphan et al. (2012) have shown that the average landward migration rate for the entire spit reached 1.1 m.yr¹ between 1930 and 2010 (fig. 3D). During the same time period, longshore sediment transport through cannibalization from the proximal to the distal section was evaluated at 1.4 m³.m¹.yr¹ (Stéphan et al., 2010). From the mid-1970s to the beginning of the 1980s, this caused several coastal defence structures to be installed in order to prevent the retreat of the spit barrier; the 200 m long riprap and the “Chouk” groin (fig. 2B, 3A) were installed on the proximal section, and a 1,400 m long riprap was installed on the median section (Pinot, 1994; Stéphan et al., 2012). A change in coastal management strategy took effect when the Sillon de Talbert became the property of the “Conservatoire du Littoral” in 2001, a major part of the riprap of the median section was removed with the aim of returning the spit barrier to its natural morphological functioning.

3. Data and methods

3.1. Topo-morphological survey

8The topo-morphological survey is based on both yearly DEM and monthly beach profile measurements.

3.1.1. 3D topo-morphological measurements (DEM)

9DEM measurements started in October 2002 and are ongoing. These measurements are carried out using various techniques (tab. 1). First, an Airborne LiDAR measurement of the entire spit area was performed in October 2002 with an altimetry accuracy of ± 10 cm (Boersma and Hoenderkamp, 2003). From the LiDAR raw records, a 3D digital elevation model (DEM) was computed using a kriging interpolation model to produce a regular 1-m grid. The Real-time Kinematic Differential Geographic Positioning System (RTK-DGPS) topographic measurement technique was also used between 2003 and 2018, at least one or twice a year (tab. 1). Each DGPS measurement was horizontally and vertically calibrated using the geodetic marker from the French datum and the geodesic network provided by the IGN (Institut Géographique National) located on the study area (fig. 3A). The ground control points (GCPs) were measured to calculate the x, y, and z, with the margin of error reaching ± 5 to 7 cm in x and y, and ± 2 cm in z. These values were taken into account in calculating the margin of error associated with the sediment budget calculation. Finally, several UAV (Unmanned Aerial Vehicle) flight campaigns and Kite Aerial Photography (KAP) surveys were conducted in 2016 and 2018 (tab. 1). During each UAV campaign, six to seven flights were needed to cover the whole spit area. The survey was performed using an electric hexacopter UAV based on a DS6 multi-rotor platform, equipped for nadir photography with a Nikon D800 reflex camera with a focal length of 35 mm. The flight took place at an altitude of around 115 m, which gave a spatial resolution of 1.7 cm. The camera was set up to acquire RAW images every 10 seconds, allowing a quasi–systematic image side lap higher than 60% for an optimized SfM photogrammetric process. During each aerial survey, ground control points (GCPs) were surveyed using rtk-DGPS measurements. The margin of error calculated from GCPs reached ± 0.1 m in x, y, and z. For the KAP surveys, aerial images were taken using a Ricoh® GR camera (16.2 MPix resolution, fixed focal length equivalent to 28 mm) connected to the kite line by a BROOXES® Gent–X picavet set. The use of a picavet greatly improved the quality of the kite–aerial photographs. The kite used to carry out the survey was a Delta Trooper by Dan Leigh®, supporting strong wind speeds from 7 to 14 m.s¹. The camera was set to acquire RAW images every 10 seconds, allowing a quasi–systematic image side lap higher than 60% for an optimized SfM photogrammetric process. During each aerial survey ground control points (GCPs) were also surveyed by RTK-GPS measurements giving a margin error of ± 0.02 m in x, y, and z (tab. 1).

Tab. 1 – Inventory of the topo-morphological surveys carried out between 2002 and 2018.
Tab. 1 – Inventaire des levés topo-morphologiques réalisés entre 2002 et 2018.

Tab. 1 – Inventory of the topo-morphological surveys carried out between 2002 and 2018.   Tab. 1 – Inventaire des levés topo-morphologiques réalisés entre 2002 et 2018.

3.1.2. DEM generation and morphological changes analysis

10From the RTK-DGPS field measurements, a digital elevation model (DEM) was computed using Surfer software with a kriging interpolation algorithm supporting breaklines to produce a regular 1-m grid. For the UAV and KAP surveys, the SfM photogrammetric process was performed using Agisoft® Photoscan Professional software. We chose this user–friendly commercial software for its ease of use and the quality of data produced (Jaud et al., 2016). For each aerial survey a set of about 250/300 images were processed separately. An orthorectified aerial image at a resolution of 1 cm and a DSM at a resolution of 1 m were produced for comparison with the LiDAR and DGPS DEMs (tab. 1).

11The spit contour lines and 3D visualization were generated and the sediment budget due to morphological changes was calculated for each period. Calculation of the sediment budget was accomplished for each survey from the interpolated surface by calculating the volumetric difference between it and the preceding surface based on grid subtraction. DEM of Differences (DoD) were produced following the method implemented by Wheaton et al. (2009). The net change (Δznet) and absolute change (Δzmax) were generated from the interpolated surface plots with the vertical change (m) presented for each 1 m² grid cell. The volume calculation focused on (i) the material deposited on the crest by overtopping, (ii) sediment accumulated on the back-barrier slope by overwash processes, and (iii) sediment deposited in the distal section due to the longshore sediment transfer (fig. 4). Each DEM was also sliced into 110 cross-shore transects along which two main morphological indicators, (i) crest lowering/accretion (Δzcrest) and (ii) landward spit migration (Δretreat), were measured. The quantification of spit retreat was based on the landward limit of the rear of the spit that represents the best shoreline demarcation between gravel sediments of the spit barrier and the mud of the back-barrier low-lying zone.

Fig. 4 – DEMs of the Sillon de Talbert gravel spit and sediment budget calculation between 2002 and 2018.
Fig. 4 – MNT de la flèche de galets du Sillon de Talbert et calcul des bilans sédimentaires entre 2002 et 2018.

Fig. 4 – DEMs of the Sillon de Talbert gravel spit and sediment budget calculation between 2002 and 2018.   Fig. 4 – MNT de la flèche de galets du Sillon de Talbert et calcul des bilans sédimentaires entre 2002 et 2018.

Fig. 4 (continued) – DEMs of the Sillon de Talbert gravel spit and the sediment budget calculation between 2002 and 2018.
Fig. 4 (suite) – MNT de la flèche de galets du Sillon de Talbert et calcul des bilans sédimentaires entre 2002 et 2018.

Fig. 4 (continued) – DEMs of the Sillon de Talbert gravel spit and the sediment budget calculation between 2002 and 2018.   Fig. 4 (suite) – MNT de la flèche de galets du Sillon de Talbert et calcul des bilans sédimentaires entre 2002 et 2018.

3.1.3. Beach profile measurements

12Since September 2012, two beach profile measurements have been regularly executed along two transects located on the proximal (Unit 2) and medium (Unit 3) spit sections where retreat rates are the most significant (fig. 5). This survey took place at a bi-weekly to monthly frequency using a Leica tacheometer TCR303®. Each measurement was referenced to the geodesic marker from the French datum and the geodesic network provided by the IGN (Institut Géographique National). From the position of the control points was measured to estimate the margin of error was estimated at ± 5 to 7 cm in x, y, and ± 2 cm in z.

Fig. 5 – Location of the two transects A and B for the monthly beach profile measurements.
Fig. 5 – Localisation des deux transects A et B pour le suivi mensuel de profil de plage.

Fig. 5 – Location of the two transects A and B for the monthly beach profile measurements.   Fig. 5 – Localisation des deux transects A et B pour le suivi mensuel de profil de plage.

A. Shoreline changes indicating the most retreating zone between 1930 and 2010 (after Stéphan et al., 2012). B. Setting of beach profiles A and B (topographic setting and aerial photo from 2015). C. Photo of pressure sensor installed on the top-head of the transect B. D. Envelope of beach profile A realized between the 18 of September, 2012 and the 17 of April, 2018. E. Envelope of beach profile B realized between the 18 of September, 2012 and the 17 of April, 2018.
A. Cinématique du trait de côte indiquant les zones les plus en recul entre 1930 et 2010 (d’après Stéphan et al., 2012). B. Installation des profils de plage A et B (la topographie et la photo aérienne datent de 2015). C. Photo du capteur de pression installé en tête de transect du profil B. D. Enveloppe de profils de plage A réalisés entre le 18 septembre 2012 et le 17 avril 2018. E. Enveloppe de profils de plage B réalisés entre le 18 septembre 2012 et le 17 avril 2018.

3.2. Hydrodynamic analysis: data acquisition and methods

3.2.1. Acquisition of offshore wave conditions

13Offshore waves were hindcast using the WAVEWATCH III V4.18 spectral model (Tolman, 2014; Roland and Ardhuin, 2014) at coordinates 3.047°W, 48.927°N, at a depth of about 50 m, between 01/01/2002 and 31/03/2018. The system was implemented with unstructured and higher resolution grids on the coastal zone, thus enabling the reproduction of the wave climate in the shallow zone. For the Atlantic coast, the HOMERE hindcast data set (Boudière et al., 2013), extended to the English Channel, was used. It was forced by the wind fields from the CFSR reanalysis with a resolution ranging from 0.25° to 0.5° (Saha et al., 2010) and by the surface currents generated from an atlas of harmonic tidal constituents obtained from outputs of the MARS circulation model (Lazure and Dumas, 2008). Wave parameters such as direction (D), wave height (Hsig) and period (Tm0,-1) were extracted from this data set for the analysis of offshore wave conditions.

14Wave height thresholds corresponding to the maximum Hsig, exceeding the 2% exceedance (percentile 98%) wave height of 3.15 m, were used to identify the major storm wave events. The wave energy flux, F, of these major events was calculated using:

             F =

ρg² H²sig T

             [1]

     64 π

15where F is expressed per unit crest length of the wave, ρ = 1,025 kg.m-³ is the density of seawater, g = 9.81 m.s² is the acceleration of gravity, Hsig is the significant wave height, and T is the wave period (Tm0,-1).

3.2.2. Wave records in the shallow tidal zone

16As part of the survey undertaken on the Sillon de Talbert, the wave conditions in the shallow tidal zone have been recorded since September 2012 using an OSSI-010-003C pressure sensor (Ocean Sensor Systems Inc.®, accuracy ± 1.5 cm specification). It was deployed along transect B, at 0.461 m a.s.l., which approximately corresponds to the mean sea water level (fig. 5B). Therefore, for about half a day, depending on the tidal range, the sensor is out of the water. A recording frequency of 5 Hz was chosen to reproduce as accurately as possible the wave spectrum. The sensor was calibrated before and after each deployment by comparing the pressure measured at the low tide level (when the sensor is out of the water and thus measures atmospheric pressure) with the atmospheric pressure recorded in situ. Wave parameters such as wave height (Hsig) and period (Tm0,-1) were also extracted for the time periods corresponding to the high tide level in such a way as to eliminate the lowest values corresponding to records taken in very shallow water depths (when the rising tide starts to submerge the sensor). Extreme wave conditions are defined as events during which the significant wave height Hsig exceeds the 2% exceedance wave height of 1.28 m.

3.2.3. Tide gauge records

17Tide records were provided by the French hydrographic service (Service Hydrographique et Océanographique de la Marine - SHOM) for the referenced tide gauge station of Roscoff, located about 70 km west of the Sillon de Talbert (fig. 2A). From these data sets, surge levels were extracted for the entire survey period (2002-2018). These surge levels were then added to predicted tide levels obtained by modeling the Les Héaux-de-Bréhat site (fig. 2A) in order to find the extreme water level component. This was the most relevant methodological approach to estimate the measured tide levels on the Sillon de Talbert, considering there is no tide gauge station referenced in this zone. Significant extreme water levels are identified as an event during which the maximum water level exceeds the 2% exceedance water level of 5.5 m.

3.2.4. In situ water level measurements

18Analysis of in situ water levels was based on records taken in the tidal zone using the OSSI-010-003C wave gauge sensor (see section 3.2.2.). The height of the water level was computed by taking into account the atmospheric pressure with the following expression:

             

(water level) = (Psensor - Patmosphere) / ρ.g

             [2]

19where H is the height of the water column (in m), Psensor is the pressure measured by the sensor (in Pa), Patmosphere is the atmospheric pressure (in Pa), ρ is the density of water (= 1,025 kg.m³), and g is the acceleration of gravity (= 9.81 m.s²).

20Water levels were smoothed to a moving average of 10 min to filter out deformations of the water surface related to waves, and water levels corresponding to both daily high tides were extracted.

4. Morphodynamic results

4.1. Medium-term morphological analysis based on DEMs

4.1.1. Sediment budget changes

21Longshore and cross-shore sediment budget analyses were calculated from the DEMs and DoDs produced over the entire survey period 2002-2017 (fig. 4). Two significant rollover process events affecting the entire spit were clearly identified between 2007 and 2008, and between 2013 and 2014. The sediment budget of these two overwash events is characterized by strong erosion of the seaward beach face and a net accretion on the back-barrier leading to significant spit retreat. As indicated in previous papers, these two major events are related to the Johanna storm of March 10, 2008 (Stéphan et al., 2010), and the series of storms that occurred during the winter of 2013-2014 (Blaise et al., 2015). However, the periods of overwash are most often followed by recovery periods during which the morphology of the spit is stable and crest-overtopping processes are dominant. These recovery processes occur under fair climate conditions, following periods of intense storm activity, i.e., between 2010-2011, 2011-2012, and 2016-2017. Important morphological changes were also observed on the tip of the spit (i.e., ebb tide lobe) during the survey period. They are related to (i) longshore sediment transport from the proximal to the distal section, and (ii) the sediment removal caused by the interaction of incident waves and the ebb tide currents on this zone. Therefore, the tip of the spit is affected by sediment transfers either to the northwest or to the southeast. Most DoDs indicate an alternation of erosion and accretion sub-cells along the back-barrier beach face of the distal and median sections. This rhythmic morphology reflects the back-barrier longshore sediment transfer, oriented to the SW due to wave diffraction on the tip of the spit.

Fig. 6 – Longshore and cross-shore sediment budget of the Sillon de Talbert for the entire survey period 2002-2017 (after Stéphan et al., 2018a, modified).
Fig. 6 – Budget sédimentaire longitudinal et transversal du Sillon de Talbert pour l’ensemble de la période de suivi 2002-2017 (d’après Stéphan et al., 2018a, modifié).

Fig. 6 – Longshore and cross-shore sediment budget of the Sillon de Talbert for the entire survey period 2002-2017 (after Stéphan et al., 2018a, modified).   Fig. 6 – Budget sédimentaire longitudinal et transversal du Sillon de Talbert pour l’ensemble de la période de suivi 2002-2017 (d’après Stéphan et al., 2018a, modifié).

1. Overwash; 2. Main longshore drift; 3. Secondary longshore drift; 4. Weakened zone (future breach); (a). Location of the photos in the Figure 11; (b). Location of the photos in the Figures 10 and 13.
1. Submersion (overwash) ; 2. Dérive littorale principale ; 3. Dérive littorale secondaire ; 4. Zone faible (amorce brèche) ; (a). Localisation des photos de la Figure 11 ; (b). Localisation des photos des Figures 10 et 13.

22The sediment budget for the entire period of 2002-2017 remains relatively stable; the seaward beach face erosion is estimated at 411,000 ± 26,000 m³, while the back-barrier deposition is about +420,000 ± 20,000 m³ (fig. 6). The back-barrier deposition of +420,000 m³ is the sum of (i) the total overwashing inducing a net volume of about +370,000 m³, and (ii) the accretion of the back-barrier at the tip of the spit (i.e., distal section) of about +50,000 ± 4,400 m³ after waves are diffracted on the tip of the spit (fig. 6). The NE seaward longshore sediment transport is relatively constant over time and reaches about 3,200 m³.yr¹ on average. This is mainly due to a cannibalization process from the proximal to the distal section. The SW back-barrier longshore sediment transport acting on the distal and median sections is estimated at 100 to 500 m³.yr¹ (fig. 6).

4.1.2. Dynamics of spit retreat (Br) and crest evolution (Bh and Bw)

23The results concerning the spit migration between 2002 and 2017 show that the average of the net landward displacement of the spit reached about 19 m, 39 m, and 29 m, on the proximal gravel, median, and distal sections, respectively (fig. 7A-C). However, the maximum retreat of up to 66 m was recorded in the first part of the distal section along transect P091, while the proximal gravel and median sections retreated by a maximum of about 53 m (transect P034), and 63 m (transect P052) (fig. 8D). The cumulative frequency (Br) and the annual frequency (ΔBr) indicate that there were three years, 2008, 2014, and 2016, during which significant landward displacement took place.

Fig. 7 – Annual morphological changes along the three morphosedimentary units of the Sillon de Talbert from 2002 to 2017.
Fig. 7 – Changements morphologiques annuels le long des trois unités morphosédimentaires du Sillon de Talabert de 2002 à 2017.

Fig. 7 – Annual morphological changes along the three morphosedimentary units of the Sillon de Talbert from 2002 to 2017.   Fig. 7 – Changements morphologiques annuels le long des trois unités morphosédimentaires du Sillon de Talabert de 2002 à 2017.

Unit 2: proximal gravel section; Unit 3: median section; Unit 4: distal section (see Figure 3). A‑C: Barrier retreat in cumulative frequency Br (curve), and frequency ΔBr (bars), for the proximal gravel section – Unit 2 (A), for the median section – Unit 3 (B), and the distal section – Unit 4 (C). D‑F: Barrier height changes in cumulative frequency Bh (curve), and frequency ΔBh (bars) for the proximal gravel section – Unit 2 (D), for the median section – Unit 3 (E), and the distal section – Unit 4 (F). G‑I: Barrier width changes in cumulative frequency Bw (curve), and frequency ΔBw (bars) for the proximal gravel section – Unit 2 (G), for the median section – Unit 3 (H), and the distal section – Unit 4 (I). Grey bars correspond to main morphogenetic events (1. in 2008; 2. in 2014; 3. in 2016).
Unit 2 : section graveleuse proximale ; Unit 3 : section médiane ; Unit 4 : section distale (cf. Figure 3). A‑C : Recul du cordon en fréquences cumulées Br (courbe), et séparées ΔBr (barres) pour la section proximale graveleuse – Unit 2 (A), pour la section médiane – Unit 3 (B), et pour la section distale – Unit 4 (C). D‑F : Changements de hauteur de cordon en fréquences cumulées Bh (courbe) et séparées ΔBh (barres) pour la section proximale graveleuse – Unit 2 (D), pour la section médiane – Unit 3 (E), et pour la section distale – Unit 4 (F). G‑I : Changements de la largeur du cordon en fréquences cumulées Bw (courbe) et séparées ΔBw (barres) pour la section proximale graveleuse – Unit 2 (G), pour la section médiane – Unit 3 (H), et pour la section distale – Unit 4 (I). Barres grises correspondent aux évènements morphogènes principaux (1. en 2008 ; 2. en 2014 ; 3. en 2016).

24The analysis of the evolution of crest height (Bh) and crest width (Bw) between 2002 and 2017 shows significant variations (fig. 7D-I). In the proximal gravel and median sections, the evolution of the crest elevation is characterized by three major lowering events in 2008, 2014, and 2016. For these three years, the lowering of the crest reached an average of 0.2 to 0.3 m on the proximal gravel section (fig. 7D). It reached an average of 1 m on the median section, notably in 2008 and 2014 (fig. 7E), with a maximum value reaching 2.44 m and 1.79 m, respectively (fig. 8E). However, after each of these three episodes, recovery processes due to overtopping resulted in crest elevation reaching its pre-storm height, such as on the median section (fig. 7E), or higher than the pre-storm height, such as on the proximal gravel section (fig. 7D). Such is the case on the proximal gravel and median sections following the three years of 2008, 2014, and 2016, where the rise of the crest exceeded +1 m (fig. 7G-H). Conversely, parameter Bw, corresponding to the crest width, varies in the opposite direction to that of the crest height (Bh). In general, the width of the crest increases when the latter decreases in height, and vice versa. The results show different morphological behaviors concerning the evolution of the distal section (fig. 7F, 7I). This is mainly due to the morphological features of this zone, which are characterized by a massive accumulation of sediments inducing the highest elevation and largest width of the crest (see section 2.). Therefore, the major part of this distal section is rarely impacted by the catastrophic overwash/inundation events that generate crest lowering and crest width enlarging.

4.2. Medium-term hydrodynamic analysis

25The results show a clear seasonal variation of wave height (Hsig) with a maximum of up to 5 to 6 m during the winter periods, which corresponds to the major storm events that occurred during the survey period (fig. 8A), i.e., January 18, 2005 (5.37 m), February 12, 2007 (5.47 m), December 9, 2007 (6.35 m), December 5, 2008 (5.04 m), February 8, 2016 (5.29 m), and January 17, 2018 (5.17 m). The early spring period may also be affected by extreme significant wave heights, as was the case during the Johanna storm on March 10, 2008 (6 m), and the storm event of March 9, 2016 (5.21 m) (fig. 8A). Significant morphogenetic events combining extreme wave height and water levels were isolated using both wave height and water level thresholds. As indicated in the methodological section (see sections 3.2.1., and 3.2.3.), these thresholds correspond to significant wave events during which the maximum Hsig exceeds the 2% exceedance wave height of 3.15 m, and the highest water level exceeds the 2% exceedance water level of 5.5 m (fig. 8A-B). Major morphogenetic events were identified when both thresholds of significant wave height and water level were combined. Following this statistical analysis, 11 morphogenetic events were identified (tab. 2, fig. 8A). Some of the storm events identified previously, such as the Johanna storm on March 10, 2008, and the Ruzica/Imogen storm on February 8, 2016 were recognized as major morphogenetic events although they occurred during the high spring tide period (tab. 2). The winter of 2013‑2014, notably storm Anne on February 1‑2, 2014, and storm Eleanor on January 5, 2018, was also characterized by significant morphogenetic events. As mentioned earlier, other events occurring at the beginning of March were also identified, as was the case in 2007, 2010, and 2016 (tab. 2). However, they were mainly related to very high water levels generated by astronomical tides, while wave heights were not very high, except during the event of 31/03/2010, and the morphological response of the spit did not lead to significant erosion because in that context the wave energy was low (tab. 2).

26The comparison of these morphogenetic events with the morphological changes of the Sillon de Talbert spit (i.e., barrier retreat ΔBr and barrier height ΔBh) was then analysed (fig. 8C-D). The results show a significant association between these extreme events and the morphological response of the barrier. For example, this is the case for the morphogenetic events related to storm Johanna on March 10, 2008, where a maximum retreat of up to 22 m was recorded on the median and distal sections, and during the winter of 2013-2014 where the cluster of storms generated a maximum retreat reaching 30 m, while the major part of the median section retreated from 20 to 22 m (fig. 8C). The extreme event related to storm Ruzica/Imogen on February 8, 2016 also produced a maximum landward spit migration of up to 20 m on the proximal gravel section, and between 5 to 10 m on the median section. The results obtained for the crest lowering/elevation ΔBh showed the same behavior with significant crest erosion of up to 2.5 to 2 m in association with these three morphogenetic events (fig. 8D).

Fig. 8 – Hydrodynamic conditions and morphological changes of Sillon de Talbert from 2002 to 2018.
Fig. 8 – Conditions hydrodynamiques et changements morphologiques du Sillon de Talbert de 2002 à 2018.

Fig. 8 – Hydrodynamic conditions and morphological changes of Sillon de Talbert from 2002 to 2018.   Fig. 8 – Conditions hydrodynamiques et changements morphologiques du Sillon de Talbert de 2002 à 2018.

A. Time series of significant wave heights (Hsig) from WW3 modeling data (see the location of the calculation point on Figure 2) where a morphogenetic event (orange dot) is defined as a significant wave event during which the maximum Hsig exceeds the 2% exceedance wave height of 3.15 m (threshold red line), combined with water levels exceeding the 2% exceedance water level. B. Time series of water levels (tide and surge calculated from data collected at Les Héaux lighthouse by the SHOM 2016) (see location on Figure 2) for both daily high tides. The threshold red line corresponds to the 2% exceedance water level of 5.5 m a.s.l. C. Barrier retreat along the 110 cross-shore profiles. D. Barrier height variations along the cross-shore profiles.
A. Série temporelle des hauteurs significatives de houle (Hsig) à partir des données WW3 modélisées (voir la localisation du point de calcul sur la Figure 2), où un évènement morphogène (point orange) est défini comme un épisode de forte houle durant lequel la hauteur significative (Hsig) maximum supérieure au percentile 2 % de 3,15 m (ligne seuil en rouge) a été combiné à un niveau d’eau supérieur au percentile 2 %. B. Série temporelle des niveaux d’eau (marée + surcote) calculés à partir des données acquises au phare Les Héaux par le SHOM, 2016 (localisation sur la Figure 2) pour les deux pleines mers journalières. La ligne seuil en rouge correspond au niveau d’eau maximum supérieure au percentile 2 % de 5.5 m a.s.l. C. Recul du cordon le long des 110 radiales de mesures transversales. D. Variations de la hauteur du cordon le long des 110 radiales de mesures transversales.

Tab. 2 – Inventory of the major morphogenetic events combining extreme storm wave heights and water levels.
Tab. 2 – Inventaire des évènements morphogènes majeurs combinant des hauteurs de houle de tempête et des niveaux d’eau extrêmes.

Tab. 2 – Inventory of the major morphogenetic events combining extreme storm wave heights and water levels.   Tab. 2 – Inventaire des évènements morphogènes majeurs combinant des hauteurs de houle de tempête et des niveaux d’eau extrêmes.

The wave energy flux allows to threshold the effectiveness of the storms between them.
Le flux énergétique des vagues permet de seuiller l’efficacité des tempêtes entre elles.

4.3. Short-term morphological analysis based on beach profiles

4.3.1. Morphological changes

27The high-frequency monitoring of the Sillon de Talbert makes it possible to analyze short-term morphological changes (at the scale of events). The results show very similar evolution along both profiles A and B, characterized by alternating erosive and recovery phases (fig. 9). A first phase, from September 18, 2012 to December 18, 2013, indicated no significant changes. The position of the crest of the spit remained stable (fig. 9A, 9G), and variations in the height of the crest never exceeded ± 20 cm (fig. 9B, 9H). In detail, the envelopes of profiles show the migration of step beachface berms (fig. 9C, 9I).

28A second phase characterized by severe erosion took place between the beginning of January 2014 and April 10, 2014. During this period, the crest of the spit lowered by 0.43 m and 1.65 m along profiles A and B, respectively (fig. 9A, 9G), and retreated by about 20 m to 23 m (fig. 9B, 9H). The envelope of profiles extended from December 18, 2013 to April 14, 2014 illustrated the process of rollover leading to a great landward displacement of the spit (fig. 9D, 9J), for both profiles A and B. This dynamic resulted in an intense erosion of the seaward beach face, and similarly significant accretion on the back-barrier.

29From April 2014 to December 2017, a third phase was mainly characterized by the elevation of the crest of the spit reaching about +0.44 m at profile A, and +1.1 m at profile B. However, this phase of recovery was interrupted by a significant episode of crest lowering on February 12, 2016 (fig. 9B, 9H). The envelope of profiles shows that the raising of the crest is related to the retreat of the seaward beach face (fig. 9E, 9K). Therefore, part of the sediment eroded to the beach face contributed to the elevation of the crest. In terms of spit migration, a significant landward displacement of the crest of about 7 m was experienced on profile B as shown by the February 12, 2016 measurement (fig. 9G). The envelope of profiles indicated a net back-barrier landward migration for both profiles A and B during the same event (fig. 9E, 9K).

30Between December 4, 2017 and January 5, 2018, a major episode of crest lowering and retreat was experienced on both profiles A and B. Crest lowering reached 0.42 m and 1.28 m on profiles A and B, respectively (fig. 9B, 9H), while the retreat of the crest was about 6.25 m and 8 m, respectively (fig. 9A, 9G). This fourth phase, strongly erosive, indicated that the part of the sediment that eroded from the top of the spit was moved and accreted to the back-barrier (fig. 9F, 9L). A fifth and last phase began at the end of January 2018, and is still ongoing. It is characterized by the crest elevation reaching +0.25 m and +0.64 m on the profiles A and B, respectively (fig. 9B, 9H), while the position of the crest experienced no significant changes (fig. 9A, 9G).

31Regarding the crest migration over the entire survey period between September 2012 and April 2018, the proximal gravel section (i.e., profile A) and the median section (i.e., profile B) of the Sillon de Talbert have retreated by about 23 m and 32 m, respectively. This landward displacement due to rollover processes represents a migration rate of about 3.8 to 5.3 m.yr¹ for the last 6 years.

Fig 9 – Short-term morphological changes of profiles A and B between September 2012 and April 2018.
Fig. 9 – Changements morphologiques à court terme des profils A et B entre septembre 2012 et avril 2018.

Fig 9 – Short-term morphological changes of profiles A and B between September 2012 and April 2018.   Fig. 9 – Changements morphologiques à court terme des profils A et B entre septembre 2012 et avril 2018.

A and G: Crest migration on profiles A and B, respectively, showing 5 distinct phases. B and H: Crest elevation/lowering on profiles A and B, respectively, showing 5 distinct phases. C and I: Envelope of profiles between 18/09/2012 and 02/12/2013, respectively, for profiles A and B describing the first phase characterized by no significant changes. D and J: Envelope of profiles between 02/12/2013 and 10/04/2014, respectively, for profiles A and B illustrating the second phase characterized by severe erosion. E and K: Envelope of profiles between 10/04/2014 and 04/12/2017, respectively, for profiles A and B illustrating the third phase regeneration processes including the short erosive event measured the 12/02/2016. F and L: Envelope of profiles between 04/12/2017 and 17/04/2018, respectively, for profiles A and B describing the erosive event measured the 05/01/2017 (fourth phase) followed by the regeneration processes (fifth phase).
A et G : Déplacement de la crête respectivement des profils A et B montrant 5 phases distinctes. B et H : Élévation/abaissement de la crête respectivement des profils A et B montrant 5 phases distinctes. C et I : Enveloppes de profils entre le 18/09/2012 et le 02/12/2013, respectivement pour les profils A et B, décrivant la première phase caractérisée par aucun changement. D et J : Enveloppes de profils entre le 02/12/2013 et le 10/04/2014, respectivement pour les profils A et B, illustrant la seconde phase caractérisée par une sévère érosion. E et K : Enveloppes de profils entre le 10/04/2014 et le 04/12/2017, respectivement pour les profils A et B, décrivant la troisième phase incluant le court épisode érosif mesuré le 12/02/2016. F et L : Enveloppes de profils entre le 04/12/2017 et le 17/04/2018, respectivement pour les profils A et B, décrivant la phase érosive mesurée le 05/01/2017 (quatrième phase), suivi par une cinquième phase régénératrice.

4.3.2. Hydrodynamic forcing and overwashing processes

32The Sillon de Talbert experienced more than 30 major storms from September 2012 to January 2018, where, for the present analysis, a storm is defined as an event during which the significant OSSI wave height Hsig exceeds the 2% exceedance wave height of 1.28 m (fig. 10). However, taking into account the gaps in the data set when the OSSI was not working, such as the case of the Ruzica/Imogen storm on February 8, 2016, well identified by WW3 offshore waves, this figure is an underestimation. Less than three days after storm Ruzica/Imogen, the waves were still energetic and, combined with a very high spring tide, generated a complete inundation of the proximal section of the spit by catastrophic overwashing (fig. 10E).

Fig. 10 – Hydrodynamic conditions recorded by the OSSI pressure sensor between September 2012 and January 2018.
Fig. 10 – Conditions hydrodynamiques enregistrées par le capteur de pression OSSI entre les mois de septembre 2012 et janvier 2018.

Fig. 10 – Hydrodynamic conditions recorded by the OSSI pressure sensor between September 2012 and January 2018.   Fig. 10 – Conditions hydrodynamiques enregistrées par le capteur de pression OSSI entre les mois de septembre 2012 et janvier 2018.

A. Water level corresponding to both daily high tide levels. B. Time series of significant wave height Hsig with maximum Hsig exceeds the 2% exceedance wave height of 1.28 m (red line). Grey bars + red circles and black stars identify the most significant morphogenetic events, where Hsig exceeded the threshold of 2% exceedance of 1.28 m combines with spring tide conditions. C and D: sluicing to catastrophic overwash of the Chouck beach (proximal dune section) during the both severe storm events of January 3-4, 2014, and February 2, 2014, respectively. E and F: sluicing to catastrophic overwash of the proximal gravel section (down drift of Chouck groin) during both high energetic periods of February 11, 2016 (less than 3 days after the Ruzica/Imogen storm of February 8, 2016), and January 5, 2018 (less than two days after the Eleanor storm of January 3-4, 2018). Photo credit: 04/01/2014 (Jacky Laveaud); 02/02/2014 (Gabriel Le Boubennec); 11/02/2014 and 05/01/2018 (Julien Houron). See location of the photos in Figure 6.
A. Niveaux d’eau correspondant au deux pleines mers journalières. B. Série temporelle des hauteurs significatives de houle (Hsig) avec la hauteur Hsig seuil correspondant au percentile 2 % de 1,28 m (ligne rouge). Les barres grises + les cercles rouges et les étoiles noires inventorient les évènements les plus morphogènes durant lesquels Hsig supérieur au seuil de 2 % de 1,28 m ont été combinés à une marée de vive-eau. C et D : processus de “lavage” à “catastrophique submersion” (inondation) de la plage du Chouck (section dunaire proximale), respectivement durant les deux tempêtes majeures du 3-4 janvier 2014, et du 2 février 2014. E et F : processus de “lavage” à “catastrophique submersion” (inondation) de la section proximale graveuleuse (en aval dérive de l’épi du Chouck), respectivement durant les deux periodes morphogènes du 11 février 2016 (moins de trois jours après la tempête Ruzica/Imogen du 8 février 2016), et du 5 janvier 2018 (moins de deux jours après la tempête Eleanor du 3-4 janvier 2018). Sources photographiques : 04/01/2014 (Jacky Laveaud) ; 02/02/2014 (Gabriel Le Boubennec) ; 11/02/2014 and 05/01/2018 (Julien Houron). Voir la localisation des photos sur la Figure 6.

33Among the 30 storm events inventoried from wave data, seven major episodes were identified as potentially the most morphogenetic because they occurred during the spring tide period (tab. 3, fig. 10). More than half of them occurred during the winter of 2013-2014, from the end of November 2013 until March 2014. As shown by Blaise et al. (2015), the three storms of January 3-4, February 2, and March 3, 2014 combined with spring tide levels, and had a large impact on the Sillon de Talbert. They induced a significant retreat of the spit as shown by beach profile measurements (fig. 9D, 9J), and the overwash processes affecting the proximal dune section of the spit (fig. 11A-B). Another major event was storm Eleanor on January 3-4, 2018, during which the significant wave height reached more than 2 m during the spring tide period (tab. 3). Here again, the beach profile measurements showed that this storm event caused significant retreat of the spit (fig. 9F, 9L) due to a severe inundation regime as shown on the proximal dune section (fig. 11D). As we indicated earlier, the Ruzica/Imogen storm of February 8, 2016 is missing from this inventory because the pressure sensor was not running at that time. However, as shown by the beach profile measurements, this event was particularly morphogenetic in terms of landward spit migration (fig. 9E, 9K) and overwash processes (fig. 11C).

Tab. 3 – Inventory of the 32 storm events where Hsig exceeded the 2% exceedance of 1.28 m, associated to water level.
Tab. 3 – Inventaire des 32 évènements tempétueux où Hsig est supérieur à percentile 2 % de 1,28 m, associés aux niveaux d’eau.

Tab. 3 – Inventory of the 32 storm events where Hsig exceeded the 2% exceedance of 1.28 m, associated to water level.   Tab. 3 – Inventaire des 32 évènements tempétueux où Hsig est supérieur à percentile 2 % de 1,28 m, associés aux niveaux d’eau.

The highlighting in gray indicates the seven major morphogenetic events when storms occurred during the spring tide period.
Le surlignage en gris indique les 7 évènements morphogéniques majeurs survenus durant une période de vive-eau.

Fig. 11 – Photos illustrating overwashing processes under a sluicing to inundation regime on the proximal dune section (Chouck beach).
Fig. 11 – Photos illustrant les processus de submersion (overwash) de la section proximale dunaire (plage du Chouck) par régime de « lavage » à « inondation ».

Fig. 11 – Photos illustrating overwashing processes under a sluicing to inundation regime on the proximal dune section (Chouck beach).   Fig. 11 – Photos illustrant les processus de submersion (overwash) de la section proximale dunaire (plage du Chouck) par régime de « lavage » à « inondation ».

The coarser sediment deposits and the removed vegetation give information about flow velocity and direction. Photo credit: A and B (Serge Suanez); C and D (Julien Houron). See location of the photos in Figure 6.
Les dépôts de sédiments grossiers et la végétation couchée donnent des informations sur la puissance et la direction des flux. Sources photographiques : A et B (Serge Suanez) ; C et D (Julien Houron). Voir la localisation des photos sur la Figure 6.

4.4. Opening of a breach

34The retreat of the Sillon de Talbert gravel spit over the entire survey period has led to the weakening of a small section located downdrift of the Chouck groin, at the contact between the proximal sandy and gravel sections (fig. 12). In a previous study, this section, characterized by significant lowering and narrowing of the barrier, was called a "wasp waist" (i.e., taille de guêpe in French) (Stéphan et al., 2018a). In 2005, it consisted of a 30 m wide low-elevated sandy dune without vegetation and was connected to the rip-rap at its base (fig. 13A). During the last decade, this rip-rap was totally disconnected from the base of the barrier due to the retreat of the shoreline and was no longer offering protection against erosion (fig. 13B). The most significant erosion occurred during the winter of 2013-2014 with a shoreline retreat of about –10 m to –15 m (fig. 13C). During that winter the dune was totally flooded by wave runup. Several hundred cubic meters of sediments were overwashed from the seaward beachface to the back-barrier, reducing the width of the vegetated dune to a few meters (fig. 13C). As described earlier (see section 4.1), this topo-morphological evolution is the result of both the longshore (i.e., cannibalization) and cross-shore (i.e., rollover) processes, exacerbated by the interruption of the updrift sediment inputs by the Chouck groin. According to observations made by the warden of the Natural Reserve of Sillon de Talbert, the breach opened during the spring tide at the beginning of March 2018, certainly by the 3rd or 4th when tide levels were highest (fig. 13D-G). The opening of the breach did not occur in a storm context; therefore, the breach was generated by strong tide currents, not by waves.

Fig. 12 – Shoreline changes of the proximal section (on the breach zone) between 2002 and 2017.
Fig. 12 – Cinématique du trait de côte de la section proximale (dans le secteur de la brèche) entre 2002 et 2017.

Fig. 12 – Shoreline changes of the proximal section (on the breach zone) between 2002 and 2017.  Fig. 12 – Cinématique du trait de côte de la section proximale (dans le secteur de la brèche) entre 2002 et 2017.

The limit of the shoreline is defined by the highest astronomical tide level (HAT) extracted from DEMs produced from 2002 to 2017 (after Stéphan et al., 2018a, modified).
La limite du trait de côte est définie par le niveau des plus hautes mers astronomiques (PHMA) extrait des MNT produits entre 2002 et 2017 (d’après Stéphan et al., 2018a, modifié).

Fig. 13 – Morphological changes of the proximal section situated downdrift of the Chouck groin between 2005 and 2018.
Fig. 13 – Changements morphologique de la section proximale située en aval dérive de l’épi du Chouck entre 2005 et 2018.

Fig. 13 – Morphological changes of the proximal section situated downdrift of the Chouck groin between 2005 and 2018.   Fig. 13 – Changements morphologique de la section proximale située en aval dérive de l’épi du Chouck entre 2005 et 2018.

A. Photo taken on 03/08/2005 showing the dune section –at that time without vegetation– directly fronted by the rip-rap (Photo credit: Julien Houron). B. Photo taken on 01/02/2012 showing the dune section completely covered by vegetation –after the dune restauration was undertaken by the Conservatoire du Littoral– disconnected from the rip-rap due to shoreline erosion (Photo credit: Julien Houron). C. Photo taken on 13/02/2014 after the major storm at the beginning of February 2014, which generated severe shoreline erosion and the destruction of dune fences (Photo credit: Serge Suanez). D. Photo taken on 07/03/2018, a few days after the opening of the breach (Photo credit: Serge Suanez). E. Aerial orthophoto of the breach produced by kite aerial photography (KAP) on March 7, 2018 (Photo credit: Pierre Stéphan). F. Photo taken on March 7, 2018 showing of the depth of the breach (Photo credit: Serge Suanez). G. Photo taken on April 17, 2018 showing the width of the breach after dynamic enlargement (Photo credit: Serge Suanez). H. Oblique aerial photo of the breach taken on September 13, 2018 during high spring tide level (Photo credit: Pierre Stéphan). See location of the photos in Figure 6.
A. Photo prise le 03/08/2005 montrant la section dunaire –à cette date non végétalisée– connectée à l’enrochement (crédit photo : Julien Houron). B. Photo prise le 01/02/2012 montrant la section dunaire complètement végétalisée –après que la restauration des dunes ait été entreprise par le Conservatoire du littoral–, déconnectée de l’enrochement par l’érosion du trait de côte (crédit photo : Julien Houron). C. Photo prise le 13/02/2014 après la grosse tempête du début du mois de février 2014 qui a généré une importante érosion du trait de côte et une destruction des ganivelles (crédit photo : Serge Suanez). D. Photo prise le 07/03/2018, quelques jours après l’ouverture de la brèche (crédit photo : Serge Suanez). E. Ortho-photographie aérienne de la brèche produite par un levé au cerf-volant le 7 mars 2018 (crédit photo : Pierre Stéphan). F. Photo prise le 7 mars 2018 montrant la profondeur de la brèche (crédit photo : Serge Suanez). G. Photo prise le 17 avril 2018 montrant la largeur de la brèche après une phase dynamique d’élargissement (crédit photo : Serge Suanez). H. Photo aérienne oblique de la brèche prise le 13 septembre 2018 durant une pleine mer de vive-eau (crédit photo : Pierre Stéphan). Voir la localisation des photos sur la Figure 6.

35Since the breach was opened, a specific survey of this zone has been undertaken using KAP (Kite Aerial Photography) and UAVs (Unmanned Aerial Vehicle), such as drones (see section 3.1.1.) (fig. 14). Figure 14 shows that during March 2018 no significant changes occurred (fig. 14B). The depth of the breach reached 1.25 m, and the width was 15 m (fig. 14F). The survey undertaken between March and September 2018 indicated very significant deepening and widening of the breach, reaching 3.4 m depth and 35 m width (fig. 14F). The present situation shows that in the course of the flooding of the breach during the high tides, whether in neap-tide or spring-tide contexts, the Sillon de Talbert spit is now cut from its median and distal sections (fig. 13H).

Fig. 14 – Morphological changes of the breach between September 2017 and September 2018.
Fig. 14 – Changements morphologiques de la brèche entre septembre 2017 et septembre 2018.

Fig. 14 – Morphological changes of the breach between September 2017 and September 2018.   Fig. 14 – Changements morphologiques de la brèche entre septembre 2017 et septembre 2018.

A. Elevation changes between September 4, 2017 and March 7, 2018. B. Elevation changes between March 7, 2018 and March 22, 2018. C. Elevation changes between March 22, 2018 and April 17, 2018. D. Elevation changes between April 17, 2018 and July 17, 2018. E. Elevation changes between July 17, 2018 and September 13, 2018. F. Morphological changes of the breach along the cross-shore profile a-b.
A. Changements topo-morphologiques entre le 4 septembre 2017 et le 7 mars 2018. B. Changements topo-morphologiques entre le 7 mars 2018 et le 22 mars 2018. C. Changements morphologiques entre le 22 mars 2018 et le 17 avril 2018. D. Changements morphologiques entre le 17 avril 2018 et le 17 juillet 2018. E. Changements morphologiques entre le 17 juillet 2018 et le 13 septembre 2018. F. Changements morphologiques de la brèche le long du profil transversal a-b.

5. Discussion

5.1. Sediment transfers and global sediment budget

36The sediment budget analysis of the Sillon de Talbert between 2002 and 2017 has confirmed that both longshore and cross-shore sediment transfers have driven the morphological evolution of the spit. Cross-shore sediment transfer, reaching 370,000 m³, is significantly dominant. It corresponds to about 30% of the whole spit sediment volume and it is driven by overwash processes only during storm events. Considering the entire survey period, three main storm events and/or storm periods are responsible for these transfers: storm Johanna on March 10, 2008, the 2013-2014 winter period, and storm Ruzica/Imogen on February 8, 2016, which generated 111,540 m³, 175,435 m³, and 40,980 m³, respectively. Based on the sediment accumulation on the distal section (i.e., ebb lobe and tip of the spit), the northeastward longshore sediment transfer was estimated at about 50,000 m³ (i.e., 3,200 m³.yr¹ between 2002 and 2017). It occurs by cannibalization processes that increased over the survey period, and induced the breach opening on the proximal gravel section. Finally, a secondary southwest back-barrier longshore sediment transport, characterized by migration of rhythmic forms of low amplitude, also drives the morphological changes of the spit. It has been estimated between 100 to 500 m³.yr¹. If we consider the –411,000 m³ volume of sediment eroded from the beachface due to both cross-shore and longshore transfers, and the volume of sediment accumulation reaching +420,000 m³ (i.e., 370,000 + 50,000 m³), it is important to note that the global sediment budget is balanced for the whole survey period. Considering the total volume of sediment in the Sillon de Talbert, up to 1.2 106 m³, 43 years are required for a complete remobilization of this volume by cross-shore and longshore transfers.

5.2. Storminess and acceleration of the barrier retreat

37The Sillon de Talbert experienced important landward migration rates during the entire survey period (2002-2017), with maximum values reaching –3 to –4 m.yr¹ on proximal and median sections (fig. 15A). By comparison, the migration rates calculated by Stéphan et al. (2012) over the last decades (1930-2010) indicate maximum values of around –1.5 to –2 m.yr¹. Figure 15B shows the mean migration rates for the entire spit (i.e., averaged value calculated for all 110 transects) calculated by linear regression over the first period of 80 years 1930-2010 (Stéphan et al., 2012), and over the second period of 15 years corresponding to the survey period 2002-2017. Even if we have to be cautious about interpreting these statistical results that have been calculated for two different lengths of time (80 years vs 15 years), the results indicate that the rate of the barrier retreat has increased twofold for the last period beyond 2002 when the monitoring began (i.e., –2 m.yr¹ vs –1.2 m.yr¹) (fig. 15B). However, note that the period between 1961 and 1966 also was characterized by an acceleration of barrier retreat just as significant. As indicated by several authors (Cariolet, 2011, Stéphan et al., 2012, Stéphan et al., 2018b), this period was characterized by two severe storm episodes combined with high spring tide: that of April 5, 1962 and January 17 to 20, 1965. This suggests that variations in the migration rates over the multi-decade time scale may be simply due to the impact of some severe storm events over a short time, without significant change of the long term tendency.

38Since 1930, the maximum spit landward migration reached –143 to –160 m, which corresponds to migration rates of –1.6 to –1.8 m.yr¹ (fig. 15A). The short term survey based on monthly beach profile measurements between September 2012 and April 2018, indicated that the proximal gravel section (i.e., profile A) and the median section (i.e., profile B) have retreated about –23 m and –32 m, respectively. This landward displacement due to rollover processes represents a migration rate of about –3.8 to –5.3 m.yr¹ for the last six years. The acceleration of spit landward migration rates during the past fifteen years raises the question of enhanced morphogenetic events in Northern Brittany (due to a more frequent combination between storm waves and high spring tide levels) during recent years as suggested by the analysis of hydrodynamic conditions (fig. 8). However, there is no evidence of an increase in storminess regarding the wave data series.

39The morphodynamic analysis indicates that the acceleration of the barrier retreat is mainly related to the impact of four significant storm events. As previously shown by Stéphan et al., 2010, storm Johanna on March 10, 2008 generated a retreat of the entire median section reaching –10 to –20 m, with a maximum retreat up to –22 m on the first part of the distal section. During the winter of 2013-2014, which was the most dramatic in terms of shoreline erosion (Blaise et al., 2015; Masselink et al., 2016), the cluster of storms generated a maximum retreat reaching –30 m, while the major part of the median section retreated between –20 to –22 m. The third storm event, Ruzica/Imogen, occurring on February 8, 2016, produced a maximum landward spit migration reaching –10 to –20 m on the proximal gravel section, and –5 to –10 m on the median section. Finally, the short-term monitoring based on beach profile measurements showed the severe impact of storm Eleanor on January 3, 2018 in terms of crest lowering, reaching –0.42 m and –1.28 m on profiles A and B (fig. 9B, 9H), while the retreat of the crest was about –6.25 m and –8 m, respectively (fig. 9A, 9G).

40Between 2002 and 2017, the entire spit retreat of the proximal gravel section and the median section was up to –63 m and –66 m, respectively. Storm events controlled more than 95% of the retreat due to catastrophic overwash/inundation processes.

Fig. 15 – Statistical analysis of the spit retreat since 1930.
Fig. 15 – Analyse statistique du recul de la flèche depuis 1930.

Fig. 15 – Statistical analysis of the spit retreat since 1930.  Fig. 15 – Analyse statistique du recul de la flèche depuis 1930.

A. Net landward spit migration and retreating rates (m.yr¹) calculated for the three proximal, median and distal gravel sections between 1930 and 2017. B. Mean retreating rates (m.yr¹) for the entire spit (i.e., averaged value calculated for all 110 transects using linear regression).
A. Migration nette de la flèche vers la terre et taux de recul (m/an) calculés pour les trois sections proximale de galets, médiane et distale, entre 1930 et 2017. B. Moyenne des taux de recul (m/an) pour la flèche entière (i.e., valeur moyenne calculée sur les 110 transects par régression linéaire).

5.3. Post-storm resilience processes

41The survey indicated that despite severe crest erosion and landward migration of the spit during significant storm events, beach profile regeneration processes take place following these events, so much so as to rebuild the barrier near to its initial state. These processes have been described by Orford (2011) and Orford and Anthony (2011) as the result of negative feedback mechanisms, and they compare the post-storm morpho-sedimentary adjustment of the barrier to a resilience phenomenon. Barrier resilience can be considered in terms of the temporal evolution (regeneration) of barrier height (Bh), and width of the crest (Bw) from pre-event to some point post-event when the crest is re-built and has some form of geomorphological stability (Orford and Anthony, 2011). However, it is important to note that in this case study, the resilience phenomenon do not concern the entire spit barrier which is experiencing long-term retreat trend. The regeneration processes only concern the beachface profile which is changing according to the meteo-oceanic conditions. Therefore, the resilience can be seen as an adaptation of the seaward beach to the wave energy also dependent to the height of the water level (i.e., tide condition). As indicated by Forbes et al. (1995, 2004), long intervals of slow evolution of gravel-dominated barriers are punctuated by episodes of rapid reorganisation, involving breakdown of stable barrier structures and facies patterns, remixing of sediment, and accelerated migration of transgressive systems. As shown by this study, in depleted sediment supply setting, the probability of rebuilding to the original condition is very low. Therefore, the potential for instability may be enhanced by gradual sediment supply depletion and also by storm clustering that may result in sequential impacts without the time for post-storm recovery that lead to breaching.

42The results of the survey indicate two different resilience trajectories depending on the section of the spit. (i) On the proximal gravel section (i.e., Unit 2), the reconstruction of the beach crest after storm events resulted in the beach crest being rebuilt to its original height, or higher, as indicated by profile A on Figure 9B, or the average value of Bh on Figure 7D. The rise of the crest due to post-storm rebuilding activity can reach +0.6 m, as was the case after storm Johanna on March 10, 2008, while the net evolution of the crest height for the whole survey period indicates a slight gain from 6.2 to 6.5 m a.s.l. (fig. 7D). (ii) On the median section, the storm-lowering of the crest is always more significant, reaching –1.5 to –1 m (with a maximum value between –1.7 to –2.4 m), as was the case during storm Johanna on March 10, 2008, and during the stormy 2013-2014 winter (fig. 7E, 9H). However, rebuilding the barrier post-storm is also an efficient recovery process, inducing an elevation of the crest reaching its initial height up to 7 m a.s.l. The difference of crest elevation rebuilding between the proximal gravel and the median section (6.2 to 6.5 m vs 7 to 7.2 m, respectively) is one factor explaining the more rapid landward migration of the barrier for the latter (fig. 15A). Therefore, post-storm resilience processes are more efficient on the median section. This efficiency is also controlled by longshore sediment transport, which induces a significant accretion that increases towards the distal part of the spit, while the proximal section is eroding. This cannibalization process, which has already been described by many authors for many sites (Aubrey and Gains, 1982; Orford et al., 1996, 2002; Jolicoeur et al., 2010) explains the dislocation of the Sillon de Talbert through the breach opening in its proximal part.

5.4. The breach opening as a response to natural and anthropogenic forcing

43The opening of the breach in the beginning of March 2018 is a result of both natural and anthropogenic forcing. The survey revealed significant longshore sediment transport from the proximal to the distal section of the Sillon de Talbert, reaching about 3,200 m³.yr¹. As shown by the analysis of the shoreline changes of the proximal section, this transfer of sediments occurred through cannibalization processes that have increased over the last centuries/decades (fig. 12). The opening of the breach was clearly predictable, as the final stage of the cannibalization process is the dislocation of the barrier through the breach opening in its proximal part (Kidson, 1964; Aubrey and Gains, 1982; Carter and Orford, 1991; Orford et al., 1991, 1996, 2002; Jolicoeur et al., 2010; Bujalesky and Bonorino, 2015; Sabatier and Anthony, 2015). The cannibalization process is firstly a consequence of the cut off of sediment supply due to the depletion of sediments accumulated on the platform. It is also due to the inefficiency of soft cliff erosion, located updrift from the Sillon de Talbert, in delivering significant volumes of coarse sediments into the littoral cell. As indicated by Stéphan, 2011, and Stéphan et al., 2015, these soft-cliffs, mainly composed of periglacial deposits (head), are now stabilized by the vegetation and/or high perched high on the bedrock at their base, reducing the erosion processes. Secondly, the cannibalization process is also due to anthropogenic forcing caused by the setup of the Chouck groin. This hard coastal structure has led to the cut off of longshore sediment transport from the proximal sandy section to the proximal gravel section, inducing an acceleration of the shoreline erosion in this zone. Therefore, one of the options for plugging the breach would be the removal of the Chouck groin and the small portion of riprap situated just downstream, which is also a contributing factor to the erosion of the barrier (Stéphan et al., 2018a). The topo-morphological survey of the breach accomplished between March and September 2018 indicated a rapid enlargement (35 vs 15 m) and deepening (3.4 vs 1.25 m).

5.5. Morphodynamic monitoring and variation of meteo-oceanic conditions

44Medium to short-term morphodynamic monitoring is very efficient in identifying and quantifying the variations in meteo-oceanic conditions. Between 2002 and 2007, a long phase was identified as a “calm” period in terms of morphogenetic conditions. The wave conditions showed no significant storm activity, with Hsig < 5 m, except for the event of January 18, 2005 (5.37 m), which occurred during a neap tide period. During this 5-year period no significant morphological changes were recorded (fig. 8).

45This “calm” phase was followed by a short period of two years, 2007-2008, characterized by intense morphogenetic events, as shown by both the storm of February 12, 2007 (Hsig: 5.47 m), and storm Johanna on March 10, 2008 (Hsig: 6 m); both of which occurred during spring tide conditions. During this “erosion” phase, significant spit retreat reaching a maximum of –22 m was recorded, and more than 10% of the total volume of sediment of the barrier (120.000 m³ vs 1,200.000 m³) was transferred from the beachface to the back-barrier (Stéphan et al., 2010). This “erosion” phase was followed by a long 5-year “calm” period that ended in December 2013. Wave heights never exceeded 5 m, and no extreme morphogenetic events were identified except the one of 31/03/2010 (tab. 2). However, despite the severe meteo-oceanic conditions that characterized this event, the morphological changes experienced by the spit were limited to a small part of the barrier (fig. 8).

46The end of the survey period, from the winter of 2013-2014 to 2018, was characterized by intense morphogenetic activity. As indicated by Blaise et al. (2015), the winter of 2013-2014 (from December 2013 to March 2014) was certainly the most erosive one since the winter of 1989-1990 on the entire Brittany coast. The maximum retreat of the spit, up to –30 m, was generated by three major storms that occurred in the beginning of January, February, and March, and which were combined with high spring tide levels (tab. 3). Finally, the event related to the Ruzica/Imogen storm of February 8, 2016 (Hsig: 5.29 m) that also produced a maximum spit retreat up to –20 m, and the Eleanor storm of January 17, 2018 (Hsig: 5.17 m), have both been identified as severe morphogenetic events leading to significant morphological changes.

47Based on monitoring carried out on other study sites situated along the Brittany coast, this alternation of “erosion” and “calm” phases -in terms of morphodynamic conditions- has already been identified and discussed by many authors. This is the case for north Brittany with the survey of shoreline changes of the Saint-Michel-en-Grève Bay (Suanez and Stéphan, 2011), and Vougot beach (Suanez et al., 2015). It is also the case for western Brittany with the monitoring of gravel beaches and barriers of the archipelago of Molène (Suanez et al., 2011), and the Rade de Brest (Stéphan, 2011). Finally, it is also the case for south Brittany with the coastal survey of the “Pays Bigouden” including the Penmarc’h, Treffiagat, Lesconil, Ile-Tudy/Combrit, and Mousterlin beaches (Hallégouët and Hénaff, 2006; Blaise et al., 2015).

6. Conclusion

48The topo-morphological monitoring of the Sillon de Talbert undertaken between 2002 and 2018 highlighted particular issues and challenges associated with the morphological and sedimentary functioning of gravel spits. The conceptual approaches used to the longshore and cross-shore dynamics are fully illustrated by the annual DEM measurements, and the monthly beach profile measurements. The main result of this survey is the rapid landward migration of the gravel proximal and median sections between 2002 and 2017, with a maximum average rate of –3 to –4 m.yr¹ (reaching –5.3 m.yr¹ between 2012 and 2018). Regarding the average rate for the whole spit barrier, this landward displacement increased during the last fifteen years to almost twice the rate observed during the 20th century (2 m.yr¹ vs 1.2 m.yr¹). As shown by the hydrodynamic analysis, the storm events control more than 95% of this retreat due to catastrophic overwash/inundation processes (e.g., extreme storm waves combined with high spring tide levels during the Johanna storm of March 10, 2008 and the cluster of storms during the winter 2013-2014 generated a maximum spit retreat of –22 m and –30 m, respectively). Therefore, the possible increase of storm frequency and/or intensity during the next decades would further weaken the Sillon de Talbert. In terms of morphosedimentary behavior, the cross-shore sediment transfers are dominant and represented a total volume of 370,000 m³, while the longshore sediment transfer was estimated at about 50,000 m³. Considering this total volume of 420,000 m³, 43 years would be required to remobilize the total volume of the Sillon de Talbert (i.e., 1.2 106 m³) by cross-shore and longshore transfers. However, considering the volume of sediment eroded from the beachface up to –411,000 m³ due to both cross-shore and longshore transfers, and the sediment accumulation reaching +420,000 m³, the global sediment budget is balanced for the entire survey period. Therefore, these results clearly illustrate the effectiveness of the rollover process. Finally, the longshore sediment transfer through cannibalization processes due to the cut-off of the longshore sediment transport by coastal defence structures, and the depletion of sediments accumulated on the platform, has led to the opening of a breach in March 2018. The rapid enlargement (35 vs 15 m) and deepening (3.4 vs 1.25 m) of this breach between March and September 2018 raise the question of the future morphological evolution of the spit barrier towards an “island” of gravel deposits.

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Annexe

Version française abrégée

Le Sillon de Talbert est une flèche littorale à pointe libre constituée essentiellement de galets, située sur la côte nord de la Bretagne dans le département des Côtes d’Armor (fig. 2). Le volume sédimentaire qui la compose est estimé à 1,23 106 m³. Elle s’étend sur 3,5 km de longueur et s’organise autour de quatre unités morphosédimentaires bien distinctes (fig. 3). Si les deux sections, proximale sableuse et distale, restent assez stables, les deux sections, proximale et médiane graveleuses, observent un recul important depuis plusieurs décennies (fig. 3). Depuis 2002 un suivi morphosédimentaire et hydrodynamique a été entrepris, notamment par le laboratoire LETG-Brest à la demande du conservatoire du littoral. Ce suivi, financé par la mairie de Pleubian, s’inscrit dans les actions scientifiques de la Réserve naturel du Sillon de Talbert. Il est basé sur des levés topo-morpologiques annuels permettant de calculer des MNT (fig. 4), auxquels s’ajoutent depuis le mois de septembre 2012 des mesures de profils de plage et des conditions hydrodynamiques (houle et marée) (fig. 5). Le calcul du bilan sédimentaire sur les 15 ans (2002-2017) montre que les transits transversaux sont dominants et représentent un volume total de 370 000 m³ (i.e., 30 % du volume sédimentaire de la flèche) (fig. 6). Ces transferts ont été réalisés par overwash, principalement durant les trois épisodes et/ou périodes morphogènes les plus significatifs enregistrés durant la période d’observation. Il s’agit de la tempête Johanna du 10 mars 2008, des tempêtes de l’hiver 2013-2014, et de la tempête Ruzica/Imogen du 8 février 2016, qui ont généré des transferts sédimentaires atteignant respectivement 111 540 m³, 175 435 m³, and 40 980 m³ (fig. 7-8). L’analyse des conditions hydrodynamiques montre que ces trois épisodes et/ou périodes morphogènes ont été caractérisés par la combinaison d’une tempête et d’une marée de vive-eau (fig. 8). Le transit longitudinal lié à la dérive littoral est estimé à environ 50 000 m³, soit 3 200 m³/an entre 2002 et 2017 (fig. 6). Ces transferts longitudinaux ont principalement été réalisés suivant un processus de cannibalisation qui s’est accru au cours du temps, conduisant à une fragilisation de la flèche dans sa partie proximale (fig. 12). Toutefois, considérant le volume de plus de –411 000 m³ de sédiment érodé à l’avant plage, et le volume des transferts transversaux et longitudinaux cumulés atteignant +420 000 m³, le bilan sédimentaire global sur l’ensemble de la période de suivi est équilibré (fig. 6). De même, en tenant compte du volume global de sédiment qui constitue le Sillon de Talbert (i.e., 1,23 106 m³), 43 années sont nécessaires pour une complète remobilisation de ce stock par les transferts transversaux et longitudinaux. Depuis 1930, le déplacement maximum de la flèche vers la terre, lié au phénomène de rollover, a atteint –1,6 à –1,8 m/an (fig. 15). Le suivi à court terme réalisé entre les mois de septembre 2012 et avril 2018 (basé sur la mesure de profils de plage) indique que la section proximale graveleuse (i.e., profil A), et la section médiane (i.e., profil B), ont respectivement reculé d’environ –23 m and –32 m (fig. 9). Ces déplacements représentent –3,8 à –5,3 m/an pour les 6 dernières années. Ainsi, le recul du Sillon de Talbert a doublé durant les quinze dernières années avec un taux de recul de 2 m/an entre 2002 et 2017, contre 1,2 m/an entre 1930 et 2010 (fig. 15). Les évènements tempétueux contrôlent plus de 95 % de ce recul, notamment durant les processus catastrophiques de submersion/inondation (fig. 10-11). Le recul de la flèche s’est traduit par l’ouverture d’une brèche dans sa partie proximale au début du mois de mars 2018 (fig. 13). Le processus de cannibalisation est le principal responsable de cette rupture. Il s’explique par l’absence d’alimentation en sédiment de la flèche, notamment dans sa partie proximale, dû au tarissement des sources sédimentaires accumulées sur la plateforme. C’est aussi la conséquence d’une très faible alimentation à partir de l’érosion très réduite des falaises meubles (head périglaciaire) situées en amont dérive du Sillon de Talbert qui sont aujourd’hui stabilisées par la végétation et/ou hautement perchées sur un substrat rocheux à leur base. Ce processus de cannibalisation est enfin lié à l’impact de l’épi du Chouck qui bloque le transit sédimentaire longitudinal de la section proximale sableuse vers les sections proximale et médiane graveleuse (fig. 2, 12). Le suivi de cette brèche réalisé entre les mois de mars et septembre 2018 montre qu’elle s’est rapidement élargie et approfondie, passant respectivement de 15 à 35 m, et de 1,25 à 3,4 m, au cours des 7 derniers mois (fig. 14). Pour autant, la politique de gestion adoptée par le conservatoire du littoral, également approuvée par le conseil scientifique de la Réserve naturelle et les responsables de la Région Bretagne, est de « laisser faire la nature ». Une option supplémentaire serait de retirer les derniers ouvrages de défense côtière situés à la racine du Sillon de Talbert (i.e., l’épi du Chouck et l’enrochement frontal) afin de réamorcer le transit sédimentaire longitudinal dans les deux sections proximale et médiane de la flèche.

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Table des illustrations

Titre Fig. 1 – Conceptual morphodynamic schemes of gravel-dominated spit barrier. Fig. 1 – Schémas conceptuels de la morphodynamique des flèches de galets.
Légende A. Four types of storm-impacts driving the gravel-dominated barrier deposits proposed by Orford and Carter (1982), adapted for spit barriers where longshore drifting also controls morphodynamic processes. B. Domains of overflow relative to morphometric components of the gravel barrier. The boundaries of “overwash” and “sluicing overwash” domains are schematic, as they are variable across the back-barrier, being dependent upon the magnitude of the overflow (after Orford and Anthony (2011), modified and adapted to the median section beach profile of the Sillon de Talbert). Bw. Barrier width at the back-barrier ground level reference elevation; Bh. Barrier height at the back-barrier ground level reference elevation; Bw*. Width of the crest at MHST reference elevation; Bh*. Crest height at MHST reference elevation; Od,q. Flow depth (d), and volume discharge (q) overtopping the crest barrier; Oe. Peak overflow elevation; Br. Barrier retreat. A. Quatre types d’impact de tempête modifiant les cordons de galets, proposé par Orford and Carter (1982), adapté aux flèches de galets où les tansferts longitudinaux contrôlent également les processus dynamiques. B. Domaines de franchissement en fonctions des composants morphométriques du cordon de galets. Les limites des domaines de l’« overwash » et du « sluicing overwash » sont schématiques car elles varient transversalement par rapport au cordon en fonction de l'ampleur du franchissement (d’après Orford and Anthony (2011), modifié et adapté au profil de plage de la section médiane du Sillon de Talbert). Bw. Largeur du cordon à la hauteur de référence du sol de l’arrière cordon ; Bh. hauteur du cordon à la hauteur de référence du sol de l’arrière cordon ; Bw*. largeur du cordon à la hauteur de référence du niveau des PMVE ; Bh*. hauteur du cordon à la hauteur de référence du niveau des PMVE ; Od,q. hauteur du flot (d), et volume du flot (q) franchissant la crête du cordon ; Oehauteur maximum du franchissement ; Br. recul de la flèche.
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Titre Fig. 2 – Location map. Fig. 2 – Carte de localization.
Légende A. Regional scale. B. Local scale. C. Oblique aerial photo of the Sillon de Talbert taken on the 23 September 2009 (source: D. Halleux) showing the coastal defense structures on the sandy proximal section. D. Wave characteristics. Wave rose and histogram of Hs established from the data obtained by WW3 numerical model over the period 01/01/1994-31/03/2018 at the calculation point 3.047°W, 48.927°N. A. Échelle régionale. B. Échelle locale. C. Photo aérienne oblique du Sillon de Talbert prise le 23 septembre 2009 (source : D. Halleux) montrant les ouvrages défense côtière sur la section proximale sableuse. D. Caractéristiques de houle. Rose des houles et histogramme des HS établis à partir des données du modèle WW3 sur la période 01/01/1994-31/03/2018 au point de calcul 3,047°W, 48,927°N.
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Titre Fig. 3 – Morphological setting of the Sillon de Talbert. Fig. 3 – Morphologie du Sillon de Talbert.
Légende A. Different longshore morphological units of the spit and representative beach profiles of the four units; B. Oblique aerial photo of the distal section (Photo taken on 29 September 2011); C. Crest height (2017) ; D. Landward spit displacement in m.yr¹ between 1930 and 2010, after Stéphan et al. (2012), modified. A. Différentes unités morphologiques de la flèche et profils de plage représentatifs des quatre sections ; B. Photographie aérienne oblique de la partie distale (photo prise le 29 septembre 2011) ; C. Hauteur de la crête (2017) ; D. Recul de la flèche exprimé en m/an entre 1930 et 2010, d’après Stéphan et al. (2012), modifié.
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Titre Tab. 1 – Inventory of the topo-morphological surveys carried out between 2002 and 2018. Tab. 1 – Inventaire des levés topo-morphologiques réalisés entre 2002 et 2018.
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Titre Fig. 4 – DEMs of the Sillon de Talbert gravel spit and sediment budget calculation between 2002 and 2018. Fig. 4 – MNT de la flèche de galets du Sillon de Talbert et calcul des bilans sédimentaires entre 2002 et 2018.
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Titre Fig. 4 (continued) – DEMs of the Sillon de Talbert gravel spit and the sediment budget calculation between 2002 and 2018. Fig. 4 (suite) – MNT de la flèche de galets du Sillon de Talbert et calcul des bilans sédimentaires entre 2002 et 2018.
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Titre Fig. 5 – Location of the two transects A and B for the monthly beach profile measurements. Fig. 5 – Localisation des deux transects A et B pour le suivi mensuel de profil de plage.
Légende A. Shoreline changes indicating the most retreating zone between 1930 and 2010 (after Stéphan et al., 2012). B. Setting of beach profiles A and B (topographic setting and aerial photo from 2015). C. Photo of pressure sensor installed on the top-head of the transect B. D. Envelope of beach profile A realized between the 18 of September, 2012 and the 17 of April, 2018. E. Envelope of beach profile B realized between the 18 of September, 2012 and the 17 of April, 2018. A. Cinématique du trait de côte indiquant les zones les plus en recul entre 1930 et 2010 (d’après Stéphan et al., 2012). B. Installation des profils de plage A et B (la topographie et la photo aérienne datent de 2015). C. Photo du capteur de pression installé en tête de transect du profil B. D. Enveloppe de profils de plage A réalisés entre le 18 septembre 2012 et le 17 avril 2018. E. Enveloppe de profils de plage B réalisés entre le 18 septembre 2012 et le 17 avril 2018.
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Titre Fig. 6 – Longshore and cross-shore sediment budget of the Sillon de Talbert for the entire survey period 2002-2017 (after Stéphan et al., 2018a, modified). Fig. 6 – Budget sédimentaire longitudinal et transversal du Sillon de Talbert pour l’ensemble de la période de suivi 2002-2017 (d’après Stéphan et al., 2018a, modifié).
Légende 1. Overwash; 2. Main longshore drift; 3. Secondary longshore drift; 4. Weakened zone (future breach); (a). Location of the photos in the Figure 11; (b). Location of the photos in the Figures 10 and 13. 1. Submersion (overwash) ; 2. Dérive littorale principale ; 3. Dérive littorale secondaire ; 4. Zone faible (amorce brèche) ; (a). Localisation des photos de la Figure 11 ; (b). Localisation des photos des Figures 10 et 13.
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Titre Fig. 7 – Annual morphological changes along the three morphosedimentary units of the Sillon de Talbert from 2002 to 2017. Fig. 7 – Changements morphologiques annuels le long des trois unités morphosédimentaires du Sillon de Talabert de 2002 à 2017.
Légende Unit 2: proximal gravel section; Unit 3: median section; Unit 4: distal section (see Figure 3). A‑C: Barrier retreat in cumulative frequency Br (curve), and frequency ΔBr (bars), for the proximal gravel section – Unit 2 (A), for the median section – Unit 3 (B), and the distal section – Unit 4 (C). D‑F: Barrier height changes in cumulative frequency Bh (curve), and frequency ΔBh (bars) for the proximal gravel section – Unit 2 (D), for the median section – Unit 3 (E), and the distal section – Unit 4 (F). G‑I: Barrier width changes in cumulative frequency Bw (curve), and frequency ΔBw (bars) for the proximal gravel section – Unit 2 (G), for the median section – Unit 3 (H), and the distal section – Unit 4 (I). Grey bars correspond to main morphogenetic events (1. in 2008; 2. in 2014; 3. in 2016). Unit 2 : section graveleuse proximale ; Unit 3 : section médiane ; Unit 4 : section distale (cf. Figure 3). A‑C : Recul du cordon en fréquences cumulées Br (courbe), et séparées ΔBr (barres) pour la section proximale graveleuse – Unit 2 (A), pour la section médiane – Unit 3 (B), et pour la section distale – Unit 4 (C). D‑F : Changements de hauteur de cordon en fréquences cumulées Bh (courbe) et séparées ΔBh (barres) pour la section proximale graveleuse – Unit 2 (D), pour la section médiane – Unit 3 (E), et pour la section distale – Unit 4 (F). G‑I : Changements de la largeur du cordon en fréquences cumulées Bw (courbe) et séparées ΔBw (barres) pour la section proximale graveleuse – Unit 2 (G), pour la section médiane – Unit 3 (H), et pour la section distale – Unit 4 (I). Barres grises correspondent aux évènements morphogènes principaux (1. en 2008 ; 2. en 2014 ; 3. en 2016).
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Titre Fig. 8 – Hydrodynamic conditions and morphological changes of Sillon de Talbert from 2002 to 2018. Fig. 8 – Conditions hydrodynamiques et changements morphologiques du Sillon de Talbert de 2002 à 2018.
Légende A. Time series of significant wave heights (Hsig) from WW3 modeling data (see the location of the calculation point on Figure 2) where a morphogenetic event (orange dot) is defined as a significant wave event during which the maximum Hsig exceeds the 2% exceedance wave height of 3.15 m (threshold red line), combined with water levels exceeding the 2% exceedance water level. B. Time series of water levels (tide and surge calculated from data collected at Les Héaux lighthouse by the SHOM 2016) (see location on Figure 2) for both daily high tides. The threshold red line corresponds to the 2% exceedance water level of 5.5 m a.s.l. C. Barrier retreat along the 110 cross-shore profiles. D. Barrier height variations along the cross-shore profiles. A. Série temporelle des hauteurs significatives de houle (Hsig) à partir des données WW3 modélisées (voir la localisation du point de calcul sur la Figure 2), où un évènement morphogène (point orange) est défini comme un épisode de forte houle durant lequel la hauteur significative (Hsig) maximum supérieure au percentile 2 % de 3,15 m (ligne seuil en rouge) a été combiné à un niveau d’eau supérieur au percentile 2 %. B. Série temporelle des niveaux d’eau (marée + surcote) calculés à partir des données acquises au phare Les Héaux par le SHOM, 2016 (localisation sur la Figure 2) pour les deux pleines mers journalières. La ligne seuil en rouge correspond au niveau d’eau maximum supérieure au percentile 2 % de 5.5 m a.s.l. C. Recul du cordon le long des 110 radiales de mesures transversales. D. Variations de la hauteur du cordon le long des 110 radiales de mesures transversales.
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Titre Tab. 2 – Inventory of the major morphogenetic events combining extreme storm wave heights and water levels. Tab. 2 – Inventaire des évènements morphogènes majeurs combinant des hauteurs de houle de tempête et des niveaux d’eau extrêmes.
Légende The wave energy flux allows to threshold the effectiveness of the storms between them. Le flux énergétique des vagues permet de seuiller l’efficacité des tempêtes entre elles.
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Titre Fig 9 – Short-term morphological changes of profiles A and B between September 2012 and April 2018. Fig. 9 – Changements morphologiques à court terme des profils A et B entre septembre 2012 et avril 2018.
Légende A and G: Crest migration on profiles A and B, respectively, showing 5 distinct phases. B and H: Crest elevation/lowering on profiles A and B, respectively, showing 5 distinct phases. C and I: Envelope of profiles between 18/09/2012 and 02/12/2013, respectively, for profiles A and B describing the first phase characterized by no significant changes. D and J: Envelope of profiles between 02/12/2013 and 10/04/2014, respectively, for profiles A and B illustrating the second phase characterized by severe erosion. E and K: Envelope of profiles between 10/04/2014 and 04/12/2017, respectively, for profiles A and B illustrating the third phase regeneration processes including the short erosive event measured the 12/02/2016. F and L: Envelope of profiles between 04/12/2017 and 17/04/2018, respectively, for profiles A and B describing the erosive event measured the 05/01/2017 (fourth phase) followed by the regeneration processes (fifth phase). A et G : Déplacement de la crête respectivement des profils A et B montrant 5 phases distinctes. B et H : Élévation/abaissement de la crête respectivement des profils A et B montrant 5 phases distinctes. C et I : Enveloppes de profils entre le 18/09/2012 et le 02/12/2013, respectivement pour les profils A et B, décrivant la première phase caractérisée par aucun changement. D et J : Enveloppes de profils entre le 02/12/2013 et le 10/04/2014, respectivement pour les profils A et B, illustrant la seconde phase caractérisée par une sévère érosion. E et K : Enveloppes de profils entre le 10/04/2014 et le 04/12/2017, respectivement pour les profils A et B, décrivant la troisième phase incluant le court épisode érosif mesuré le 12/02/2016. F et L : Enveloppes de profils entre le 04/12/2017 et le 17/04/2018, respectivement pour les profils A et B, décrivant la phase érosive mesurée le 05/01/2017 (quatrième phase), suivi par une cinquième phase régénératrice.
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Titre Fig. 10 – Hydrodynamic conditions recorded by the OSSI pressure sensor between September 2012 and January 2018. Fig. 10 – Conditions hydrodynamiques enregistrées par le capteur de pression OSSI entre les mois de septembre 2012 et janvier 2018.
Légende A. Water level corresponding to both daily high tide levels. B. Time series of significant wave height Hsig with maximum Hsig exceeds the 2% exceedance wave height of 1.28 m (red line). Grey bars + red circles and black stars identify the most significant morphogenetic events, where Hsig exceeded the threshold of 2% exceedance of 1.28 m combines with spring tide conditions. C and D: sluicing to catastrophic overwash of the Chouck beach (proximal dune section) during the both severe storm events of January 3-4, 2014, and February 2, 2014, respectively. E and F: sluicing to catastrophic overwash of the proximal gravel section (down drift of Chouck groin) during both high energetic periods of February 11, 2016 (less than 3 days after the Ruzica/Imogen storm of February 8, 2016), and January 5, 2018 (less than two days after the Eleanor storm of January 3-4, 2018). Photo credit: 04/01/2014 (Jacky Laveaud); 02/02/2014 (Gabriel Le Boubennec); 11/02/2014 and 05/01/2018 (Julien Houron). See location of the photos in Figure 6. A. Niveaux d’eau correspondant au deux pleines mers journalières. B. Série temporelle des hauteurs significatives de houle (Hsig) avec la hauteur Hsig seuil correspondant au percentile 2 % de 1,28 m (ligne rouge). Les barres grises + les cercles rouges et les étoiles noires inventorient les évènements les plus morphogènes durant lesquels Hsig supérieur au seuil de 2 % de 1,28 m ont été combinés à une marée de vive-eau. C et D : processus de “lavage” à “catastrophique submersion” (inondation) de la plage du Chouck (section dunaire proximale), respectivement durant les deux tempêtes majeures du 3-4 janvier 2014, et du 2 février 2014. E et F : processus de “lavage” à “catastrophique submersion” (inondation) de la section proximale graveuleuse (en aval dérive de l’épi du Chouck), respectivement durant les deux periodes morphogènes du 11 février 2016 (moins de trois jours après la tempête Ruzica/Imogen du 8 février 2016), et du 5 janvier 2018 (moins de deux jours après la tempête Eleanor du 3-4 janvier 2018). Sources photographiques : 04/01/2014 (Jacky Laveaud) ; 02/02/2014 (Gabriel Le Boubennec) ; 11/02/2014 and 05/01/2018 (Julien Houron). Voir la localisation des photos sur la Figure 6.
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Titre Tab. 3 – Inventory of the 32 storm events where Hsig exceeded the 2% exceedance of 1.28 m, associated to water level. Tab. 3 – Inventaire des 32 évènements tempétueux où Hsig est supérieur à percentile 2 % de 1,28 m, associés aux niveaux d’eau.
Légende The highlighting in gray indicates the seven major morphogenetic events when storms occurred during the spring tide period. Le surlignage en gris indique les 7 évènements morphogéniques majeurs survenus durant une période de vive-eau.
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Titre Fig. 11 – Photos illustrating overwashing processes under a sluicing to inundation regime on the proximal dune section (Chouck beach). Fig. 11 – Photos illustrant les processus de submersion (overwash) de la section proximale dunaire (plage du Chouck) par régime de « lavage » à « inondation ».
Légende The coarser sediment deposits and the removed vegetation give information about flow velocity and direction. Photo credit: A and B (Serge Suanez); C and D (Julien Houron). See location of the photos in Figure 6. Les dépôts de sédiments grossiers et la végétation couchée donnent des informations sur la puissance et la direction des flux. Sources photographiques : A et B (Serge Suanez) ; C et D (Julien Houron). Voir la localisation des photos sur la Figure 6.
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Titre Fig. 12 – Shoreline changes of the proximal section (on the breach zone) between 2002 and 2017. Fig. 12 – Cinématique du trait de côte de la section proximale (dans le secteur de la brèche) entre 2002 et 2017.
Légende The limit of the shoreline is defined by the highest astronomical tide level (HAT) extracted from DEMs produced from 2002 to 2017 (after Stéphan et al., 2018a, modified). La limite du trait de côte est définie par le niveau des plus hautes mers astronomiques (PHMA) extrait des MNT produits entre 2002 et 2017 (d’après Stéphan et al., 2018a, modifié).
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Titre Fig. 13 – Morphological changes of the proximal section situated downdrift of the Chouck groin between 2005 and 2018. Fig. 13 – Changements morphologique de la section proximale située en aval dérive de l’épi du Chouck entre 2005 et 2018.
Légende A. Photo taken on 03/08/2005 showing the dune section –at that time without vegetation– directly fronted by the rip-rap (Photo credit: Julien Houron). B. Photo taken on 01/02/2012 showing the dune section completely covered by vegetation –after the dune restauration was undertaken by the Conservatoire du Littoral– disconnected from the rip-rap due to shoreline erosion (Photo credit: Julien Houron). C. Photo taken on 13/02/2014 after the major storm at the beginning of February 2014, which generated severe shoreline erosion and the destruction of dune fences (Photo credit: Serge Suanez). D. Photo taken on 07/03/2018, a few days after the opening of the breach (Photo credit: Serge Suanez). E. Aerial orthophoto of the breach produced by kite aerial photography (KAP) on March 7, 2018 (Photo credit: Pierre Stéphan). F. Photo taken on March 7, 2018 showing of the depth of the breach (Photo credit: Serge Suanez). G. Photo taken on April 17, 2018 showing the width of the breach after dynamic enlargement (Photo credit: Serge Suanez). H. Oblique aerial photo of the breach taken on September 13, 2018 during high spring tide level (Photo credit: Pierre Stéphan). See location of the photos in Figure 6. A. Photo prise le 03/08/2005 montrant la section dunaire –à cette date non végétalisée– connectée à l’enrochement (crédit photo : Julien Houron). B. Photo prise le 01/02/2012 montrant la section dunaire complètement végétalisée –après que la restauration des dunes ait été entreprise par le Conservatoire du littoral–, déconnectée de l’enrochement par l’érosion du trait de côte (crédit photo : Julien Houron). C. Photo prise le 13/02/2014 après la grosse tempête du début du mois de février 2014 qui a généré une importante érosion du trait de côte et une destruction des ganivelles (crédit photo : Serge Suanez). D. Photo prise le 07/03/2018, quelques jours après l’ouverture de la brèche (crédit photo : Serge Suanez). E. Ortho-photographie aérienne de la brèche produite par un levé au cerf-volant le 7 mars 2018 (crédit photo : Pierre Stéphan). F. Photo prise le 7 mars 2018 montrant la profondeur de la brèche (crédit photo : Serge Suanez). G. Photo prise le 17 avril 2018 montrant la largeur de la brèche après une phase dynamique d’élargissement (crédit photo : Serge Suanez). H. Photo aérienne oblique de la brèche prise le 13 septembre 2018 durant une pleine mer de vive-eau (crédit photo : Pierre Stéphan). Voir la localisation des photos sur la Figure 6.
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Titre Fig. 14 – Morphological changes of the breach between September 2017 and September 2018. Fig. 14 – Changements morphologiques de la brèche entre septembre 2017 et septembre 2018.
Légende A. Elevation changes between September 4, 2017 and March 7, 2018. B. Elevation changes between March 7, 2018 and March 22, 2018. C. Elevation changes between March 22, 2018 and April 17, 2018. D. Elevation changes between April 17, 2018 and July 17, 2018. E. Elevation changes between July 17, 2018 and September 13, 2018. F. Morphological changes of the breach along the cross-shore profile a-b. A. Changements topo-morphologiques entre le 4 septembre 2017 et le 7 mars 2018. B. Changements topo-morphologiques entre le 7 mars 2018 et le 22 mars 2018. C. Changements morphologiques entre le 22 mars 2018 et le 17 avril 2018. D. Changements morphologiques entre le 17 avril 2018 et le 17 juillet 2018. E. Changements morphologiques entre le 17 juillet 2018 et le 13 septembre 2018. F. Changements morphologiques de la brèche le long du profil transversal a-b.
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Titre Fig. 15 – Statistical analysis of the spit retreat since 1930. Fig. 15 – Analyse statistique du recul de la flèche depuis 1930.
Légende A. Net landward spit migration and retreating rates (m.yr¹) calculated for the three proximal, median and distal gravel sections between 1930 and 2017. B. Mean retreating rates (m.yr¹) for the entire spit (i.e., averaged value calculated for all 110 transects using linear regression). A. Migration nette de la flèche vers la terre et taux de recul (m/an) calculés pour les trois sections proximale de galets, médiane et distale, entre 1930 et 2017. B. Moyenne des taux de recul (m/an) pour la flèche entière (i.e., valeur moyenne calculée sur les 110 transects par régression linéaire).
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Serge Suanez, Pierre Stéphan, France Floc’h, Ronan Autret, Bernard Fichaut, Emmanuel Blaise, Julien Houron, Jérôme Ammann, Philippe Grandjean, Mickaël Accensi, Gaël André et Fabrice Ardhuin, « Fifteen years of hydrodynamic forcing and morphological changes leading to breaching of a gravel spit, Sillon de Talbert (Brittany) », Géomorphologie : relief, processus, environnement, vol. 24 - n° 4 | 2018, 403-428.

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Serge Suanez, Pierre Stéphan, France Floc’h, Ronan Autret, Bernard Fichaut, Emmanuel Blaise, Julien Houron, Jérôme Ammann, Philippe Grandjean, Mickaël Accensi, Gaël André et Fabrice Ardhuin, « Fifteen years of hydrodynamic forcing and morphological changes leading to breaching of a gravel spit, Sillon de Talbert (Brittany) », Géomorphologie : relief, processus, environnement [En ligne], vol. 24 - n° 4 | 2018, mis en ligne le 18 janvier 2019, consulté le 16 octobre 2019. URL : http://journals.openedition.org/geomorphologie/12677 ; DOI : 10.4000/geomorphologie.12677

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Auteurs

Serge Suanez

Université de Bretagne Occidentale, CNRS, UMR LETG 6554, Institut Universitaire Européen de la Mer, 29280 Plouzané, France (serge.suanez@univ-brest.fr). Tel : +33 (0)2 98 49 86 10.

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Pierre Stéphan

CNRS, Université de Bretagne Occidentale, UMR LETG 6554, Institut Universitaire Européen de la Mer, 29280 Plouzané, France (pierre.stephan@univ-brest.fr).

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France Floc’h

Université de Bretagne Occidentale, CNRS, UMR LGO 6538, Institut Universitaire Européen de la Mer, 29280 Plouzané, France (france.floch@univ-brest.fr).

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Ronan Autret

Université de Bretagne Occidentale, CNRS, UMR LETG 6554, Institut Universitaire Européen de la Mer, 29280 Plouzané, France (ronan.autret@univ-brest.fr).

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Bernard Fichaut

Université de Bretagne Occidentale, CNRS, UMR LETG 6554, Institut Universitaire Européen de la Mer, 29280 Plouzané, France (bernard.fichaut@univ-brest.fr).

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Emmanuel Blaise

Université de La Rochelle, UMR 7266 CNRS – LIENSs (AGÎLE), Bâtiment ILE, 2, rue Olympe de Gouges, 17000 La Rochelle, France | Université de Bretagne Occidentale, CNRS, UMR LETG 6554, Institut Universitaire Européen de la Mer, 29280 Plouzané, France (emmanuel.blaise@univ-lr.fr).

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Julien Houron

Réserve naturelle régionale du Sillon de Talbert, 22610 Pleubian, France (maison-littoral-pleubian@orange.fr).

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Jérôme Ammann

CNRS, Université de Bretagne Occidentale, UMR LGO 6538, Institut Universitaire Européen de la Mer, 29280 Plouzané, France (jerome.ammann@univ-brest.fr).

Philippe Grandjean

Université de Lyon 1 et ENS-Lyon, CNRS, UMR 5570, 69662 Villeurbanne, France (Philippe.Grandjean@univ-lyon1.fr).

Mickaël Accensi

Laboratoire d'Océanographie Physique et Spatiale, UMR 6523, Ifremer/CNRS/UBO/IRD, Z.I. Pointe du Diable, CS 10070, 29280 Plouzané, France (mickael.accensi@ifremer.fr).

Gaël André

Service hydrographique et Océanographique de la Marine, 13 rue du Chatelier, CS 92803, 29228 Brest CEDEX 2, France (gael.andre@shom.fr).

Fabrice Ardhuin

Laboratoire d'Océanographie Physique et Spatiale, UMR 6523, Ifremer/CNRS/UBO/IRD, Z.I. Pointe du Diable, CS 10070, 29280 Plouzané, France (fabrice.ardhuin@ifremer.fr).

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