Quentin Laporte-Fauret
email: quentin.laporte-fauret@u-bordeaux.fr

Bruno Castelle

Vincent Marieu

Alexandre Nicolae Lerma

David Rosebery

Title: Foredune blowout formation and subsequent evolution along a chronically eroding high-energy coast

Keywords: Coastal dune, Remote sensing, Marine erosion, Blowout, Parabolic dune. 1. Introduction

Coastal dune systems provide important ecosystem services, while being vulnerable to marine erosion. In these environments, blowouts can develop and promote sand transport from the beach to the back of the dune, but are generally fought by coastal dune managers. There are only few quantitative studies on the 3D evolution of blowouts and how they can develop into parabolic dune. We investigate the morphological evolution of a 2-km long freely evolving dune system in southwest France from 1947 to 2021 using historical aerial photos and digital surface models from Lidar and UAV photogrammetry. The combination of these remote sensing methods shows an alongshore non-uniform erosion with a mean of 1.26 m/yr, and with erosion rates in the north of the study area four times larger than in the south. Over the study period, three large blowouts developed in the northern, (more rapidly eroding) sector and subsequently evolved into parabolic dunes, with a depositional lobe migrating landward into the forest. Two parabolic dunes naturally stabilized by vegetation colonization, without any reactivation phase, with the third one still migrating landward with an average migration of 7.2 m/yr. A highfrequency and high-resolution analysis of the active blowout was performed between 2014 and 2021. Compared to the adjacent areas, this blowout promoted dune landward migration. Since 2014, the high erosion scarp in the adjacent southern sector prevented the transport of sand resulting in a loss of dune volume due to marine erosion. In contrast, in the adjacent northern sector and in front of the blowout, the presence of vegetation and paleosols at the dune toe favored sand deposition and limited marine erosion. In chronically eroding sectors, promoting blowouts and thus landward dune migration may be considered as an efficient management approach to maintain the dune system.

With increased anthropic pressure (e.g., demographic expansion of coastal areas, [START_REF] Neumann | Future coastal population growth and exposure to sea-level rise and coastal flooding -A global assessment[END_REF][START_REF] Merkens | Regionalization of population growth projections in coastal exposure analysis[END_REF] and climate change (e.g., sea level rise and increase in storminess, [START_REF] Zappa | A multi model assessment of future projections of North Atlantic and european extratropical cyclones in the CMIP5 climate models[END_REF][START_REF] Cazenave | The rate of sea-level rise[END_REF], sandy coasts (31% of the world coastlines, [START_REF] Luijendijk | The State of the World's Beaches[END_REF] are among the most vulnerable environments. In the backshore and landwards of sandy beaches with sufficient sand supply, coastal dunes can develop through many interactions between marine and aeolian processes on the one hand [START_REF] Nordstrom | Beaches and Dunes of Developed Coasts[END_REF][START_REF] Hesp | Foredunes and blowouts: Initiation, geomorphology and dynamics[END_REF][START_REF] Cohn | New insights on coastal foredune growth: The relative contributions of marine and aeolian processes[END_REF], and vegetation growth and burial on the other hand [START_REF] Maun | Adaptations of plants to burial in coastal sand dunes[END_REF][START_REF] Maun | Zonation of vegetation on lacustrine coastal dunes: Effects of burial by sand[END_REF][START_REF] Zarnetske | Biophysical feedback mediates effects of invasive grasses on coastal dune shape[END_REF]. Dune vegetation traps windblown sand and promotes sand deposition, which, in turn, can promote plant growth. This positive feedback is responsible for the growth of coastal dunes along many coasts worldwide [START_REF] Maun | Adaptations of plants to burial in coastal sand dunes[END_REF][START_REF] Emery | Biotic and abiotic predictors of ecosystem engineering traits of the dune building grass, Ammophila breviligulata[END_REF][START_REF] Charbonneau | Vegetation effects on coastal foredune initiation: Wind tunnel experiments and field validation for three dune-building plants[END_REF]. As coastal dunes develop, they provide new habitats that promote plant succession, eventually stabilizing the dune system (Barbour et al., 1985;[START_REF] Provoost | Changes in landscape and vegetation of coastal dunes in northwest Europe: A review[END_REF][START_REF] Pye | Coastal dune stabilization in Wales and requirements for rejuvenation[END_REF][START_REF] Charbonneau | Above vs. belowground plant biomass along a barrier island: Implications for dune stabilization[END_REF]. Coastal dunes also provide ecosystem services such as natural and effective barrier against flooding and storm waves, the formation of ecological niches for plants, the filtration of pollutants, fresh water supply or the expansion of nesting sites [START_REF] Martinez | Coastal dunes: Human impact and need for restoration[END_REF].

Foredune blowouts are erosional features typical of active dune systems (Black, 1951;Adamson et al., 1988). These depressions, formed by wind erosion, are characterized by a deflation basin, erosion walls, and a deposition lobe [START_REF] Glenn | Glossary[END_REF][START_REF] Carter | Erosional landforms in coastal dunes[END_REF][START_REF] Hesp | Foredunes and blowouts: Initiation, geomorphology and dynamics[END_REF]. Foredune blowouts can be classified into two broad categories according to their morphologies: saucer blowouts and trough blowouts [START_REF] Cooper | Coastal Sand Dunes of Oregon and Washington[END_REF][START_REF] Cooper | Coastal Sand Dunes of California[END_REF]. The formers are shallow with a circular shape while the latter are more elongated, deeper and with a well-defined deposition lobe. Blowouts are an important component of dune functioning. Under strong onshore wind, the airflow is topographically accelerated through the deflation basin, it erodes the base of the deflation basin and transports the sand into the deposition lobe or beyond to the back dune [START_REF] Hesp | Flow dynamics and geomorphology of a trough blowout[END_REF][START_REF] Fraser | Wind flow circulation patterns in a coastal dune blowout, south coast of Lake Michigan[END_REF][START_REF] Pease | The influence of topography and approach angles on local deflections of airflow within a coastal blowout[END_REF]. These new deposits generate a disturbance for plants species that can lead to a local vegetation rejuvenation (Nordstorm et al., 2007, Laporte-Fauret et al., 2021a;[START_REF] Laporte-Fauret | Morphological and ecological responses of a managed coastal sand dune to experimental notches[END_REF] and a vertical accretion potentially increasing the back dune resilience to sea-level rise and coastal erosion [START_REF] Clemmensen | Large-scale aeolian sand movement on the west coast of Jutland, Denmark in late Subboreal to early Subatlantic time-a record of climate change or cultural impact?[END_REF][START_REF] Hesp | Foredunes and blowouts: Initiation, geomorphology and dynamics[END_REF][START_REF] Petersen | Beaches and Dunes[END_REF][START_REF] Rhind | A framework for the management of sand dune systems in Wales[END_REF]Arens et al., 2013;[START_REF] Hesp | Restoration of foredunes and transgressive dunefields: case studies from New Zealand[END_REF][START_REF] Martinez | Coastal dunes: Human impact and need for restoration[END_REF][START_REF] Kuipers | The daring Dutch: restoring the dynamic dunes[END_REF][START_REF] Jewell | Phases of blowout initiation and stabilization on Padre Island revealed through ground-penetrating radar and remotely sensed imagery[END_REF][START_REF] Pye | Evolution of a sediment-starved, over-stabilized dune field: Kenfig Burrows, South Wales, UK[END_REF]. For this reason, in recent years (natural or artificial) blowouts have been progressively considered as a potential means for efficient management approach in some regions of the world [START_REF] Van Boxel | Ecological effects of reactivation of artificially stabilized blowouts in coastal dunes[END_REF]Arens et al., 2013;[START_REF] Kuipers | The daring Dutch: restoring the dynamic dunes[END_REF][START_REF] Pye | Dune Rejuvenation Trials: Overview Report[END_REF][START_REF] Ruessink | Coastal dune dynamics in response to excavated foredune notches[END_REF][START_REF] Laporte-Fauret | Morphological and ecological responses of a managed coastal sand dune to experimental notches[END_REF]. In particular, naturally (or artificially) stabilized dune systems threatened by chronic marine erosion can be remobilized and migrate landward through blowout development which, in sector under severe shoreline erosion, may maintain the dune system [START_REF] Castelle | Nature-based solution along high-energy eroding sandy coasts: Preliminary tests on the reinstatement of natural dynamics in reprofiled coastal dunes[END_REF].

Many studies on blowouts have focused on the airflow modification, including acceleration and steering, using two-and three-dimensional anemometers [START_REF] Hesp | Flow dynamics and geomorphology of a trough blowout[END_REF][START_REF] Hugenholtz | Form-flow interactions of an aeolian saucer blowout Earth Surf[END_REF][START_REF] Hesp | Three-dimensional aeolian dynamics within a bowl blowout during offshore winds, Greenwich Dunes, Prince Edwards Island, Canada[END_REF][START_REF] Pease | The influence of topography and approach angles on local deflections of airflow within a coastal blowout[END_REF]) and computational fluid dynamics [START_REF] Jackson | Investigation of three-dimensional wind flow behaviour over a coastal dune morphology under offshore winds using computational fluid dynamics (CFD) and ultrasonic anemometry Earth Surface Processes and Landforms[END_REF][START_REF] Smyth | High resolution measured and modeled threedimensional airflow over a coastal bowl blowout[END_REF] by predominantly focusing on the short-term (event) scale. Studies on morphological changes on medium (months) to long (annual/decadal) temporal scales, are scarcer and focus mainly on planview (horizontal) changes using erosion pins, aerial photography or ground penetrating radar [START_REF] Jungerius | The development of dune blowouts, as measured with erosion pins and sequential air photos[END_REF][START_REF] Gares | A cyclic model of foredune blowout evolution for a leeward coast, Island Beach, New Jersey Ann[END_REF][START_REF] Hugenholtz | Morphodynamics and climate controls of two aeolian blowouts on the northern Great Plains, Canada Earth Surf[END_REF][START_REF] González-Villanueva | Blowout evolution in a coastal dune: using GPR, aerial imagery and core records[END_REF]Abhar et al., 2015;[START_REF] Jewell | Phases of blowout initiation and stabilization on Padre Island revealed through ground-penetrating radar and remotely sensed imagery[END_REF].

Blowout evolution depends on complex interactions between abiotic (e.g., windblown sand, marine erosion) and biotic (e.g., vegetation colonization and growth) processes [START_REF] Gares | A cyclic model of foredune blowout evolution for a leeward coast, Island Beach, New Jersey Ann[END_REF][START_REF] Hesp | Foredunes and blowouts: Initiation, geomorphology and dynamics[END_REF][START_REF] Pye | Evolution of a sediment-starved, over-stabilized dune field: Kenfig Burrows, South Wales, UK[END_REF]. [START_REF] Schwarz | Feedbacks between Biotic and Abiotic Processes Governing the Development of Foredune Blowouts: A Review[END_REF] developed a conceptual model of blowout evolution highlighting these bio-geomorphological interactions according to three stages. Blowout initiation (Stage 1) is dominated by abiotic (aeolian) processes that increase the size and depth of the blowout. Blowout development (Stage 2) is governed by bio-geomorphological interactions between vegetation development, which inhibits blowout extension, and physical disturbance, which inhibits vegetation development. Finally, closure (Stage 3) is related to biotic processes where vegetation development prevents erosion and promotes deposition in the blowout. However, in stormy environments the constant erosion of the erosion walls, by avalanches of sediment from the crest to the deflation basin, generates significant sedimentary supplies that further feed the deposition lobe. If the disturbance remains too large for vegetation colonization, the lobe may continue to migrate downwind leading to the development of hairpin or long-walled parabolic dunes with trailing arms [START_REF] Melton | A tentative classification of sand dunes: its application to dune history in the southern High Plains[END_REF][START_REF] Cooper | Coastal Sand Dunes of Oregon and Washington[END_REF][START_REF] Wiedemann | The coastal parabola dune system at Sandlake, Tillamook County, Oregon[END_REF][START_REF] Hesp | The Manawatu dune field: environmental change and human impacts[END_REF][START_REF] Girardi | Parabolic dune reactivation and migration at Napeague, NY, USA: insights from aerial and GPR imagery[END_REF]. The size of the parabolic dune may continue to increase if enough sand supply through the deflation basin is sustained. In contrast, the growth may cease if the deflation basin becomes too large, preventing airflow acceleration and sand transport; if it reaches a non-erodible layer (e.g., ground water table, limestone bed, or paleosol); or if it becomes colonized by vegetation [START_REF] Melton | A tentative classification of sand dunes: its application to dune history in the southern High Plains[END_REF][START_REF] Cooper | Coastal Sand Dunes of Oregon and Washington[END_REF][START_REF] Livingstone | Aeolian Geomorphology: An Introduction Longman[END_REF]Warren, 1996, Hesp, 2002). [START_REF] Yan | Parabolic dunes and their transformations under environmental and climatic changes: Towards a conceptual framework for understanding and prediction[END_REF] highlighted approximatively 20 studies documenting the formation of parabolic dunes from blowout. Most of these studies were based on field surveys, aerial photo interpretation on multi-year to decadal time scales (see Table 4 in [START_REF] Yan | Parabolic dunes and their transformations under environmental and climatic changes: Towards a conceptual framework for understanding and prediction[END_REF], or numerical modeling (Baas and Nield, 2007;[START_REF] Nield | Investigating parabolic and nebkha dune formation using a cellular automaton modelling approach[END_REF]. However, as shown by [START_REF] Bosq | Event-scale dynamics of a parabolic dune and its relevance for mesoscale evolution[END_REF], large morphological evolutions can take place on very short time scales (e.g., storm event). Therefore, there is a real lack of quantitative studies at high spatial resolution and on time scales ranging from event to decade on blowouts transformation into parabolic dune.

The objective of this study is to investigate the morphological evolution of a freely evolving dune system dissected by many natural blowouts in a chronically eroding sector in southwest France. These investigations are carried out using historical aerial photos to understand these evolutions since 1947 and Digital Surface Models (DSMs) performed by Lidar and UAV photogrammetry on event to seasonal timescales in order to provide fresh and quantitative insight into the 3D dune evolution, focusing on an active blowout that recently developed into a parabolic dune.

Material and methods

Study site

The study site, l'Anse du Gurp, is located in the northern part of an almost uninterrupted 110km dune system along the Gironde coast, southwest of France (Figure 1.a). This coast is mainly characterized by open sandy beaches facing W-WNW, turning to WNW-facing to the north at the Pointe de la Négade. The coast is meso-to macro-tidal with an average tidal range of 3.7 m, reaching a maximum of 5 m during spring tides [START_REF] Castelle | Impact of the winter 2013-2014 series of severe Western Europe storms on a double barred sandy coast: beach and dune erosion and megacusp embayments[END_REF], measured at the Arcachon-Eyrac tide gauge, Figure 1.a). The beaches are exposed to an energetic wave climate with waves generated in the North Atlantic Ocean arriving with a dominant W to NW incidence. Incident waves show strong seasonal variability with monthly averages of significant wave heights (Hs) and period between peaks (Tp) ranging from 1.1 m and 8.5 s in July with a dominant W-NW direction, to 2.4 m and 13 s in January with a dominant W direction [START_REF] Castelle | A new climate index controlling winter wave activity along the Atlantic coast of Europe: The West Europe Pressure Anomaly[END_REF], measured at the wave buoy, Figure 1.a). This strong seasonal variability is also found in the wind conditions with an average hourly mean wind speed and direction of 4.5 m/s and 311° (NW) in summer and 5.2 m/s 263° (W) in winter, potentially exceeding 36 m/s during extreme storms [START_REF] Laporte-Fauret | Morphological and ecological responses of a managed coastal sand dune to experimental notches[END_REF], measured at the weather station, Figure 1.a). Winter (wind and waves) conditions also show a large interannual variability due to large-scale climate patterns of atmospheric variability. The West Europe Pressure Anomaly (WEPA) was found to be a dominant mode explaining the winter interannual variability in the Bay of Biscay of e.g., westerly winds, wave height, precipitations, river discharge [START_REF] Castelle | A new climate index controlling winter wave activity along the Atlantic coast of Europe: The West Europe Pressure Anomaly[END_REF][START_REF] Jalón-Rojas | Climate Control of Multidecadal Variability in River Discharge and Precipitation in Western Europe[END_REF]. The positive phase of WEPA reflects a southward-shifted and intensified Icelandic Low / Azores funneling high-energy waves and storm winds towards the Bay of Biscay [START_REF] Castelle | A new climate index controlling winter wave activity along the Atlantic coast of Europe: The West Europe Pressure Anomaly[END_REF].

Figure 1. (a) Aerial photograph of l'Anse du Gurp (Ph. Quentin Laporte-Fauret), the red bubble, the green square, the blue triangle and the orange star show the location of the study site, the Arcachon-Eyrac tide gauge, the wave buoy and the weather station, respectively, (b) 2 km alongshore-averaged beach dune profile at l'Anse du Gurp, (c) 2 km curvilinear grid used at l'Anse du Gurp. At l'Anse du Gurp, the dune system, located between the beach and the forest, is composed of a foredune and a transition dune, both showing large alongshore variability, reaching a mean height of 16 m with a mean width of 85 m (Figure 1.b). The dune system is composed of medium grain sand from 300 µm to 415 µm [START_REF] Bosq | Stratigraphy and chronology of Pleistocene coastal deposits in Northern Aquitaine, France: a reinvestigation[END_REF][START_REF] Stéphan | Holocene coastal changes along the Gironde estuary (SW France): New insights from the North Médoc peninsula beach/dune system[END_REF]. The overall shape of the dune is largely inherited from the action of the coastal stakeholders who built the dune systems using fences and vegetation planting from the 19 th century (Buffault, 1942, Bossard andLerma, 2020;[START_REF] Robin | 150 years of Foredune initiation and evolution driven by human and natural processes[END_REF]. Unlike some other dune systems where soft engineering methods are still applied (e.g., Truc Vert, Laporte-Fauret et al., 2021b), the dune system at l'Anse du Gurp has not been managed since the early 2000s. This area is characterized by a chronic, quasi-steady, erosion since the 1950s [START_REF] Castelle | Spatial and temporal patterns of shoreline change of a 280-km long high-energy disrupted sandy coast from 1950 to 2014: SW France[END_REF], which prevents the formation of incipient foredune and generates high erosion cliffs. It is a remote system, with only an old and partially destroyed concrete road dating from the German occupation. However, despite access is prohibited, the area is often used as attested by paths formed by animals, hunters, dissident walkers and even motocross riders (see Figure 4.i). Finally, the dune system is perched on Pleistocene paleosols, which are more resistant to marine erosion than sand and locally outcrop at the dune toe and/or upper part of the beach, particularly after high-energy winters [START_REF] Pontee | Morphosedimentary evidence of Holocene coastal changes near the mouth of the Gironde and on the Medoc Peninsula, SW France[END_REF][START_REF] Castelle | Impact of the winter 2013-2014 series of severe Western Europe storms on a double barred sandy coast: beach and dune erosion and megacusp embayments[END_REF][START_REF] Stéphan | Holocene coastal changes along the Gironde estuary (SW France): New insights from the North Médoc peninsula beach/dune system[END_REF].

Data

A set of 6 B&W and 3 RGB historical aerial photographs covering the study area were collected from the French National Institute of Geography and Forest Information (IGN) (Table 1). These surveys cover the period from 1947 to 2012, with a quasi-decadal frequency during the 1900s and then approximately every four years in the 2000s, with a pixel size ranging from 0.33 m to 1.87 m. , all the photos were assembled and merged using Agisoft Metashape (v1.7.2) in order to build a 3D georeferenced model and an orthomosaic following a 4-step workflow, (1) the cameras were aligned by the detection of matching points on overlapping image parts, (2) the 3D sparse point cloud, generated by the camera alignment, was orthorectified using 13 fixed Ground Control Points (GCPs, e.g., crossroads) from the recent IGN referenced images and a camera optimization procedure was performed to compute the intrinsic camera calibration parameters by minimizing the error on the ground control points, (3) a 3D dense point cloud was generated and ( 4) interpolated with Matlab on a 0.2 x 0.2 m regular grid (Figure 1.c) in order to build a textured DSM that can be exploited in geographic, projected, or local (cross-shore/longshore) coordinates. The planimetric errors of DSMs from historical aerial photos are under 3 m.

Two topographic surveys performed by airborne Lidar on the entire Aquitaine coast were used, the first dated October 23-24, 2014 with a density of 4 points/m² and an altimetric error of 0.14 m [START_REF] Lerma | Pluriannual beach-dune evolutions at regional scale: Erosion and recovery sequences analysis along the aquitaine coast based on airborne LiDAR data[END_REF], and the second dated October 07, 2021, with a density of 15 points/m² and an altimetric error of 0.10 m. The second Lidar survey was mainly used to estimate the DSM error due to vegetation coverage, in particular at the parabolic dune toe. From January 2016 to January 2018, 17 surveys were performed using a DJI Phantom 2 (DP2) quadricopter UAV equipped with 12MPix camera (GoPro Hero 4 Black), and, from November 2018 to July 2021, four surveys were performed using a DJI Phantom 4 Pro (DP4P) quadricopter equipped with a 20MPix camera. All surveys were performed according to the monitoring strategy designed and validated in [START_REF] Laporte-Fauret | Low-Cost UAV for High-Resolution and Large-Scale Coastal Dune Change Monitoring Using Photogrammetry[END_REF]. In short, the surveys covered 2 km of beach/dune system with an automatized fly plan. In order to georectify the DSM, an array of 14 permanents GCPs were implemented by pairs (one in the foredune and one in the transition dune) every 250 m alongshore. Their positions were measured with GNSS before each winter. The DSM generation process follows the same workflow as the one applied to the historical aerial photos except that the rectification is done with the permanent GCPs instead of crossroads. [START_REF] Laporte-Fauret | Low-Cost UAV for High-Resolution and Large-Scale Coastal Dune Change Monitoring Using Photogrammetry[END_REF] showed that this protocol leads to altimetric RMSE around 0.13 m and 0.05 m for DP2 and DP4P, respectively. Given the curvature of the coast, here the DSMs were computed on a curvilinear, cross-shore/longshore, grid (Figure 1.c).

Coastal dune metrics

There is a large range of shoreline definitions [START_REF] Boak | Shoreline definition and detection: a review[END_REF]. In our study, in line with previous work in southwest France [START_REF] Castelle | A new climate index controlling winter wave activity along the Atlantic coast of Europe: The West Europe Pressure Anomaly[END_REF][START_REF] Lerma | Pluriannual beach-dune evolutions at regional scale: Erosion and recovery sequences analysis along the aquitaine coast based on airborne LiDAR data[END_REF], from the LiDAR and UAV DSMs we computed the shoreline position the iso-contour 𝑧𝑧 = 5.5 m, which correspond to the dune toe on almost the entire site. Given the poor altimetric accuracy of the historical DSMs, the shoreline position was digitized by an operator. Given the curvilinear grid used for DSM generation, the dune volume from 2014 to 2021 was computed as:

𝑉𝑉 = � � 𝑆𝑆 𝑖𝑖𝑖𝑖 (𝑧𝑧 𝑖𝑖𝑖𝑖 - 𝑖𝑖 𝐸𝐸 𝑖𝑖=𝑖𝑖 𝑊𝑊 𝑧𝑧 𝐷𝐷𝐷𝐷 ) 𝑖𝑖 𝑁𝑁 𝑖𝑖=𝑖𝑖 𝑆𝑆 (1)
where 𝑗𝑗 𝑆𝑆 and 𝑗𝑗 𝑁𝑁 are the grid indices of the southern and northern limit, respectively, 𝑖𝑖 𝑊𝑊 and 𝑖𝑖 𝐸𝐸 are the grid indices of the western and the eastern limit, respectively. 𝑖𝑖 𝑊𝑊 is defined as the shoreline position index of the dune toe and 𝑖𝑖 𝐸𝐸 is defined as the forest limit index, manually digitized on the last drone survey (2021-07-09). 𝑆𝑆 𝑖𝑖𝑖𝑖 and 𝑧𝑧 𝑖𝑖𝑖𝑖 are the grid cell surface and elevation at grid point (i,j), respectively and 𝑧𝑧 𝐷𝐷𝐷𝐷 is the elevation of the dune toe. The evolution of the alongshore-averaged dune volume through the survey was computed as:

𝐷𝐷𝑉𝑉 = 𝑉𝑉 𝑖𝑖 -𝑉𝑉 0 𝑑𝑑 𝑆𝑆𝑁𝑁 (2)
where 𝐷𝐷𝑉𝑉 is the dune volume difference per beach width (m 3 /m) between the dune volume of the date 𝑖𝑖 (i.e., 𝑉𝑉 𝑖𝑖 ) and the initial dune volume (i.e., 𝑉𝑉 0 ) computed from the Lidar 2014, and 𝑑𝑑 𝑆𝑆𝑁𝑁 is the alongshore distance between the northern (i.e., 𝑗𝑗 𝑁𝑁 ) and the southern (i.e., 𝑗𝑗 𝑆𝑆 ) limits. All calculations were performed using the curvilinear grid shown in Figure 1.c, which allows straightforward computations of alongshore-averaged quantities. The cross-shore position 𝑥𝑥 = 0 m corresponds to the shoreline position measured in the field along the 110-km Gironde coast using a DGPS-equipped ATV on 2014-11-14 [START_REF] Castelle | Impact of the winter 2013-2014 series of severe Western Europe storms on a double barred sandy coast: beach and dune erosion and megacusp embayments[END_REF]. The models generated by Lidar data and UAV photogrammetry are Digital Elevation Model (DEM) and DSM, respectively. In order to correct the DSM-DEM bias, the 2017-10-04 airborne Lidar (DEM) survey and the 2017-10-26 UAV photogrammetric (DSM) survey were compared, showing an average 0.1-m vertical difference in the vegetated areas. Hence the 2014 initial volume 𝑉𝑉 0 was corrected by rising the DEM elevation by 0.1 m in vegetated areas of the dune, in order to be consistent with subsequent photogrammetric DEMs. From the October 26, 2017 survey, a dense vegetation zone started to develop in front of the parabolic dune, which could alter the volume computation with the DSMs. By comparing the UAV-DSM of July 9, 2021 and the Lidar-DEM of October 07, 2021, the vegetation was outlined and its volume computed. Then its effect on the elevation was calculated as:

𝐷𝐷 𝑧𝑧 = 𝑉𝑉 𝑈𝑈𝑈𝑈𝑈𝑈 -𝑉𝑉 𝐿𝐿𝑖𝑖𝐿𝐿𝐿𝐿𝐿𝐿 𝑆𝑆 𝑣𝑣𝑣𝑣𝑣𝑣 (3)
where 𝑉𝑉 𝑈𝑈𝑈𝑈𝑈𝑈 and 𝑉𝑉 𝐿𝐿𝑖𝑖𝐿𝐿𝐿𝐿𝐿𝐿 are the volume of the outlined vegetation for the UAV-DSM and the Lidar-DEM, respectively and 𝑆𝑆 𝑣𝑣𝑣𝑣𝑣𝑣 is the surface of the outlined vegetation. Then, for each DEM from October 26, 2017, the vegetation was outlined and then a correction was applied by: 𝑉𝑉 𝑐𝑐𝑐𝑐𝐿𝐿𝐿𝐿𝑣𝑣𝑐𝑐𝑐𝑐𝑣𝑣𝐿𝐿 = 𝑉𝑉 -(𝑆𝑆 𝑣𝑣𝑣𝑣𝑣𝑣 𝐷𝐷 𝑧𝑧 ) (4) where 𝑉𝑉 and 𝑆𝑆 𝑣𝑣𝑣𝑣𝑣𝑣 are the initial volume and the vegetation surface for each survey, respectively.

Results

Long term spatial coastal dune evolution

Figure 2.a shows the time series of shoreline position from 1947 to 2021, highlighting chronic, quasi-steady, marine erosion. The shoreline erosion rate increases northwards, from 0.68 m/yr in the south to 2.72 m/yr in the north (Figure 2.b). Erosion rates are fairly uniform alongshore in the south of the study zone, before dramatically increasing northwards in the northern part (Figures 2 and3). Lidar surveys, respectively.

Zoom onto the parabolic dune

As revealed by the historical aerial photos (Figure 4), the blowout that formed after 2004 (Figure 4.g-k) has evolved into a parabolic dune (Figure 7). Figure 8 shows the high-frequency morphological evolution of this blowout from 2016 to 2021. In 2016, the distances between the base of the erosion walls and from the throat to the end of the deposition lobe were 40 m and 118 m, respectively. After 5.5 years of monitoring, the parabolic dune is still mostly unvegetated, which is characteristic of the first stage of the [START_REF] Schwarz | Feedbacks between Biotic and Abiotic Processes Governing the Development of Foredune Blowouts: A Review[END_REF] blowout development model, indicating that its current evolution is mainly related to physical forcing. Indeed, the deposition lobe continuously migrated landward with concurrent widening of the deflation basin, reaching distances between erosion walls of 54 m, and 159 m between the throat to the end of the deposition lobe. The latter has migrated landward by 39.6 m, i.e., by 7.2 m/yr on average. An average (maximum) vertical erosion within the deflation basin is 2.4 m (5.9 m), with an average (maximum) vertical accretion of 1.95 m (4.7 m) in the deposition lobe. Net elevation difference between 2016-01-08 and 2021-07-09. All these DSMs were performed by UAV photogrammetry. Figure 9 shows the 2014-2021 morphological evolution (i.e., cross-shore profile, volume difference and alongshore-averaged shoreline position) of the parabolic dune system (green area in Figure 9.a), compared to two adjacent control areas in the north (blue area in Figure 9.a) and in the south (orange area in Figure 9 The time series of volume differences show three periods with different evolution patterns (Figure 9.c). During the first period (i.e., from 2014 to the beginning of winter 2017-2018) the northern, the central and the southern areas all show a small decrease in dune volume with seasonal variation amplitude of approximatively 10 m 3 /m. At the end of the winter of 2017-2018, the northern, the central and the southern areas all show an increased volume loss by 13.8 m 3 /m, 14.5 m 3 /m and 21.3 m 3 /m, respectively. During the second period (i.e., from the end of the winter of 2017-2018 to July 2020), the dune volume of the southern area has remained relatively stable, while the northern and central areas, although located in slightly more chronically eroding sectors, largely recovered with the dune volume even exceeding that in 2014. The third period (i.e., from July 2020 to July 2021) shows an alongshore quasi-uniform dune volume decrease, with 19.5 m 3 /m for the southern area, 23.4 m 3 /m for the northern and 27 m 3 /m for the central areas. Thus, over the study period, the dune volume decreased by 23.4 m 3 /m (i.e., -1.75%), 33 m 3 /m (i.e., -2.03%) and 40.1 m 3 /m (i.e., -3.72%) for the northern, the central, and the southern areas, respectively.

The three distinct time periods described above are also found in the evolution of the alongshoreaveraged shoreline position (Figure 9.d). The first period (i.e., from 2014 to the beginning of winter of 2017-2018) shows relatively stable alongshore-averaged shoreline position for the three areas with some seasonal variations in the order of 1.5 m. The winter of 2017-2018 induced a 1.67 m retreat of the alongshore-averaged shoreline position of the southern area, whereas during the second period (i.e., from the end of the winter 2017-2018 to July 2020) the alongshore-averaged shoreline position of the southern area remained relatively stable. During the same period, the northern and central areas show a seaward shoreline migration by 1.20 m and 4.25 m, respectively. The alongshore-averaged shoreline position of the three areas migrated landward by about 2 m during the third period (i.e., from July 2020 to July 2021). Thus, over the study period, the alongshore-averaged shoreline position eroded by 2.51 m and 5.84 m, for the northern and southern areas respectively, and migrated seaward by 0.65 m for the central area where the parabolic dune is observed, which will be discussed in the next section.

Discussion

The main objective of this study was to investigate the morphological evolution of a freely evolving dune system dissected by blowouts at different stages of development, and the role of blowouts in sediment transfer to the back of the dune, in a chronical erosion context exceeded 2 m/yr since at least 1947.

Historical blowout evolution

The lifetime of blowouts is dictated by the interactions between physical and biological processes. A blowout can be considered no longer active when enough vegetation is present to overwhelm physical processes (e.g., erosion of the deflation basin) [START_REF] Schwarz | Feedbacks between Biotic and Abiotic Processes Governing the Development of Foredune Blowouts: A Review[END_REF]. [START_REF] Jewell | Phases of blowout initiation and stabilization on Padre Island revealed through ground-penetrating radar and remotely sensed imagery[END_REF] attempted to characterize the initiation and stabilization phases of foredune blowouts in two sites on the Texas coast (USA) between 1969 and 2010. Over 41 years of monitoring, they highlighted at least five phases of foredune recovery (i.e., defined as the expansion of vegetation in the deflation basin) and blowout reactivation, each of them lasting about five years at the first site, and four phases lasting about 10 years at the second site. The blowout reactivation can occur after severe storms, with strong and funneled winds rapidly locally eroding the dune where there is disruption of vegetation. At our study site, the oldest blowout developed in 1977 and was active until 2012 when vegetation began to colonize the deposition lobe and then the deflation basin. Its evolution is currently primarily controlled by biotic processes (i.e., ecological stage in the model of [START_REF] Schwarz | Feedbacks between Biotic and Abiotic Processes Governing the Development of Foredune Blowouts: A Review[END_REF]. The northern blowout developed between 2004 and 2009 and vegetation began to colonize the southern part of the deposition lobe in 2012 . It currently appears to be in the bio-morphological stage but if the current physical conditions persist over time, it is possible that this blowout will stabilize because of its limited development space with the presence of the forest to the east, and the chronic marine erosion of the dune to the west. It is also possible that the blowout will disappear due to coastal erosion, which is severe in the north of the study site. Indeed, from 2009 to 2021, the deposition lobe migrated landward by 2.15 m/yr while the shoreline erosion rate reached 3.65 m/yr. Finally, the third blowout that formed between 2004 and 2009 is still active as it developed into a parabolic dune. Its evolution is mainly controlled by physical processes (i.e., geomorphologic stage) preventing the establishment of vegetation. Contrary to [START_REF] Jewell | Phases of blowout initiation and stabilization on Padre Island revealed through ground-penetrating radar and remotely sensed imagery[END_REF], we therefore did not observe any reactivation phases in any of the three blowouts. It is possible that our time series is too short to capture reactivation phases as blowout lifetime at l'Anse de Gurp could be longer, i.e., decades. Indeed, [START_REF] Girardi | Parabolic dune reactivation and migration at Napeague, NY, USA: insights from aerial and GPR imagery[END_REF] have highlighted stabilization and reactivation phases over several decades (i.e., three episodes of reactivation between 1969 and 2001) through their GPR monitoring and aerial photography in the northeast coast of the USA. Another explanation is related to the local wave and wind climate. The low-energy conditions of the Texas coast favor stabilization, and occasionally isolated cyclonic events drive reactivation, while the southwest coast of France is a storm-dominated environment. Local climate drives wind, precipitation, waves which in turn influence sand transport (i.e., erosion/deposition) and vegetation dynamics (i.e., growth, colonization) which, in addition to e.g., dune size and shape or chronic marine erosion, are crucial elements to consider in the blowout life cycle (Gonzáles-Villanueva et al., 2011;[START_REF] Hesp | Foredunes and blowouts: Initiation, geomorphology and dynamics[END_REF][START_REF] Schwarz | Feedbacks between Biotic and Abiotic Processes Governing the Development of Foredune Blowouts: A Review[END_REF]. Finally, as described by [START_REF] Jackson | A global 'greening' of coastal dunes: An integrated consequence of climate change?[END_REF] and [START_REF] Gao | Coastal dune mobility over the past century: A global review[END_REF], worldwide, and particularly in temperate climates, coastal dunes tend to stabilize due to global increases in temperature, precipitation, and greenhouse gases that result in increased vegetation cover. [START_REF]As erosion walls remained vegetated and stable, this blowout transformed into a parabolic dune which will be analyzed in detail in Section 3[END_REF]

.2. Recent morphological evolution

The volumetric analyses of the dune system between 2014 and 2021 (Figure 6.b) are discussed in the frame of the 2013-2014 winter, which was a winter characterized by extreme storm clustering along the Atlantic coast of Europe driving large-scale coastal erosion [START_REF] Masselink | Extreme wave activity during 2013/2014 winter and morphological impacts along the Atlantic coast of Europe[END_REF][START_REF] Dodet | Beach recovery from extreme storm activity during the 2013-14 winter along the Atlantic coast of Europe[END_REF]. During the first years of monitoring, the dune volume showed some seasonal variations with a decrease during winters and a recovery the rest of the year. Volume evolution shows some interannual variability, although less pronounced than further south along the coast at Truc Vert (Laporte-Fauret et al., 2020) where the shoreline has been relatively stable over the last decades [START_REF] Castelle | Spatial and temporal patterns of shoreline change of a 280-km long high-energy disrupted sandy coast from 1950 to 2014: SW France[END_REF][START_REF] Robin | 150 years of Foredune initiation and evolution driven by human and natural processes[END_REF]. However, two winters, 2017-2018 and particularly 2020-2021, appear to have had a larger erosive impact in terms of dune volume (Figure 9.c), which is negatively correlated with shoreline erosion (Figure 9.d). These two winters are both associated with a positive (+1.0 and +0.59 for the winters of 2017-2018 and 2020-2021, respectively) WEPA index [START_REF] Castelle | A new climate index controlling winter wave activity along the Atlantic coast of Europe: The West Europe Pressure Anomaly[END_REF], which reflects a stormy winter in this region. However, the winter of 2019-2020 was also stormy (WEPA of +0.95), as evidenced in [START_REF] Laporte-Fauret | Morphological and ecological responses of a managed coastal sand dune to experimental notches[END_REF] by the largest integrated onshore windblown sand flux over at least the last 17 years and the large coastal dune changes observed at Truc Vert. At this stage it is unclear why this 2019-2020 winter had a limited impact on the coastal dune at l'Anse du Gurp, which will need further study.

Interestingly, although chronic shoreline erosion is less intense in the south, the area in the south of the parabolic dune is losing more sediment than the two other areas (Figure 9.b,c). Ignoring the last winter, the central (parabolic dune) and northern areas even show an almost full recovery to their 2014 dune volumes, mostly by either rising in elevation (i.e., northern area) or by migrating landward (i.e., central area -parabolic dune). In these two areas, the shoreline erosion is (more than) compensated by landward transport. On the contrary, the decrease in dune volume and the shoreline landward migration in the southern part seems to characterize an erosion of the dune according to the conceptual model of [START_REF] Sherman | Dynamics of beach-dune systems[END_REF]. An explanation can be that higher erosion scarp in the southern area (i.e., steep 17.3-m high from the shoreline to the crest) limits the transport of windblown sand from the beach to the dune, thus preventing its growth and landward migration [START_REF] Castelle | Nature-based solution along high-energy eroding sandy coasts: Preliminary tests on the reinstatement of natural dynamics in reprofiled coastal dunes[END_REF]. Accordingly, erosion scarp height appears to be an important parameter for the landward transport in sectors with roughly similar chronic erosion rate [START_REF] Davidson | Controls on dune scarping[END_REF]. Another factor controlling the dune response is the dense and high vegetation at the dune toe that has grown on the paleosols. These paleosols, which outcropped after the outstanding winter of 2013-2014, are more resistant to marine erosion than sand [START_REF] Stéphan | Holocene coastal changes along the Gironde estuary (SW France): New insights from the North Médoc peninsula beach/dune system[END_REF][START_REF] Verdin | Humans and their environment on the Médoc coastline from the Mesolithic to the roman period[END_REF], with fresh groundwater resurgence also favoring dense plant growth (i.e., reeds, Figure 10.c). The dense and high vegetation promotes windblown sand trapping and consolidates the underground by rhizomes system (see review of [START_REF] Mcguirk | The role of vegetation in incipient dune and foredune development and morphology: A review[END_REF]. This rapid growth since approximately 2017 is evidenced by the large accretion patterns facing the parabolic dune (central area) in the DSM difference plot (Figure 8). This vegetation is also present, although less dense, along the northern area (Figure 10.a, b). In contrast, such vegetation is restricted to an isolated patch along the southern, more eroding, area (Figure 10.d). By comparing the nearly concurrent UAV-DSM (July 9, 2021) and Lidar-DEM (October 07, 2021), a correction was applied to the UAV-DSM from October 26, 2017. The estimated vegetation volume facing the parabolic dune (central area) reaches 5.68 m 3 /m (Figure 11), which is minor compared to the total volume differences computed in Figure 9.c, and with vegetation volume in the other two areas an order of magnitude smaller (Figure 11). However, it is possible that vegetation height and density impacted the landward dune evolution and volume change by preventing the sand transport from the beach to the deposition lobe. Thus, without new sand input, the deposition lobe may only be fed by the deflation basin with dune volume either constant or slightly decreasing. The latter is especially true during winter storms causing diffuse deposition landward from the deposition lobe). In contrast, the effect of this vegetation on the alongshore-averaged shoreline position time series is more complex to estimate due to the lack of Lidar surveys between 2017 and 2021. Looking only at the differences between the latest drone and lidar surveys, the alongshore-averaged shoreline position difference for the central zone was 1.56 m, highlighting that there is not a seaward shoreline migration of 0.65 m (Figure 9.d), but a landward migration of 0.91 m. As the effect of vegetation is negligible in the northern and southern areas, the slight seaward migration is characteristic of light summer recovery periods related to less marine erosive action. Finally, the alongshore-averaged shoreline position is characterized by a landward migration for the three areas, the central area retreating 2.4 times less and 5.7 times less than the northern and southern areas, respectively. Accordingly, this vegetation, which location and density is strongly controlled by local paleosol characteristics, is critical to local multi-year shoreline erosion.

Our results provide new insight into potential coastal dune management methods in chronically eroding areas. In our study zone, blowouts can help the dune to maintain by migrating landward. Such process is limited if the erosion scarp is high, thus acting as a natural barrier for landward sediment transport from the beach into the dunes. This strategy of coastal dune remobilization is debated because some authors see it as human intervention that alters the landscape and interferes with natural processes, and argue that nature should be left alone [START_REF] Delgado-Fernandez | Is 'remobilisation' nature restoration or nature destruction? A commentary[END_REF]. On the contrary, other authors defend this remobilization strategy and see it as a way to promote dynamic processes stopped by plant succession and artificial stabilization in order to create a mosaic of vegetated dunes [START_REF] Creer | Is 're-mobilisation' nature conservation or nature destruction? A commentary[END_REF][START_REF] Pye | Is 're-mobilisation' nature restoration or nature destruction? A commentary. Discussion[END_REF]Arens et al., 2020). Finally, according to Arens et al. ( 2020), dune management is not about making good or bad choices. Each choice is valid as long as it is based on a good understanding of the system. Through our study, we believe that in the chronically eroding sectors of southwest France, these blowouts can develop and thus promote landward dune migration, they will evolve through the ecological stage described by [START_REF] Schwarz | Feedbacks between Biotic and Abiotic Processes Governing the Development of Foredune Blowouts: A Review[END_REF] and eventually reach stabilization. Such dune migration should allow the dune system to survive on the long term, in contrast with some coastal dunes a couple of kilometers further north in more rapidly eroding sectors, which have now disappeared. However, such blowout systems should be carefully monitored and management action should be taken (e.g., wind fences) to stabilize the parabolic dunes that migrate too far landward. Such controlled dynamism strategy, which could also include lowering the dune scarp by mechanically smoothing the scarp crest, appears as an interesting coastal management avenue in chronically eroding sectors. However, it is necessary to conduct other quantitative studies in different settings worldwide to draw conclusions.

Conclusion

The 70-year evolution of a 2-km long high-energy, chronically eroding (average of 1.26 m/yr), coastal dune system has been studied by combining historical aerial photographs, Lidar and UAV surveys. During this period, three major blowouts have developed, all in the northern, more chronically eroding, sector. Two of these blowouts have been stabilized by vegetation colonization or are currently stabilizing, and the third one rapidly evolved into a parabolic dune, which is still migrating into the inland forest. No blowout reactivation phase was observed. Focusing on the 3D evolution of the blowout that recently developed into a parabolic and its adjacent sectors, we found that the blowout promoted dune inland migration. Despite the southern area (no blowout) was in a less chronically eroding sector, the dune volume in this area decreased by the largest amount during the last seven years of highfrequency surveys. This can be explained by the presence of the highest erosion scarp in this area, thus limiting the beach sand transport to the dune. Finally, paleosols outcropping at the dune toe and fresh groundwater resurgence favoring dense plant growth, can also limit marine erosion but also sand transfer from the beach to the dune, which could not be addressed here. Further research in other coastal dune settings is required to explore if and how management of blowout dynamics and potentially scarp lowering/incising can be a relevant coastal dune management strategy in chronically eroding sectors, in order to maintain the coastal dune systems and its ecosystem services.

  Figure 2. Orthophotos of l'Anse du Gurp in 2021-07-09 with superimposed (a) shorelines from 1947-05-29 to 2021-07-09 and (b) colored shoreline migration rate computed from 1947-05-29 (black line) to 2021-07-09 (colored line).

  Figure 5. DSM of l'Anse du Gurp in (a) 2016-01-08 and (b) 2021-07-09 and (c) and corresponding difference plot.

Figure 7 .

 7 Figure 7. Aerial photograph (a) from the front (taken on July 9, 2021, Ph. Quentin Laporte-Fauret), (b) from the back (taken on January 15, 2019, Ph. Bruno Castelle) and (c) drone-derived orthophotograph of the parabolic dune at l'Anse du Gurp (from the July 9, 2021 survey).

Figure 8 .

 8 Figure 8. (Left-hand panels): DSMs of l'Anse du Gurp zoomed onto the parabolic dune from 2016-01-08 to 2021-07-09. (Right-hand panels): Elevation difference between two consecutive surveys. (l)Net elevation difference between 2016-01-08 and 2021-07-09. All these DSMs were performed by UAV photogrammetry. Figure9shows the 2014-2021 morphological evolution (i.e., cross-shore profile, volume difference and alongshore-averaged shoreline position) of the parabolic dune system (green area in Figure9.a), compared to two adjacent control areas in the north (blue area in Figure9.a) and in the south (orange area in Figure9.a). The cross-shore profiles in the central area (green curves in Figure 9.b) highlight the landward migration (by approximately 40 m) of the deposition lobe with crest lowering (up to 3.3 m). The northern profiles (blue curves in Figure 9.b) are characterized by a dune toe retreat by 4.9 m. The dune crest migrated landward by 3.2 m and raised by 1.14 m. The northern and central profiles are characterized by an approximately 5-m high erosion scarp with a slope break around elevation 𝑧𝑧 = 9.5 m. The southern profiles (orange curves in Figure 9.b) show a retreat of the dune toe by 5.5 m, with the dune crest migrating landward by 7 m and lowering by 1.14 m. Contrary to the two other zones, the profile of the southern zone does not show any break in slope around 𝑧𝑧 = 10 m, with a steep 17.3-m high face from the shoreline to the crest.The time series of volume differences show three periods with different evolution patterns (Figure9.c). During the first period (i.e., from 2014 to the beginning of winter 2017-2018) the northern, the central and the southern areas all show a small decrease in dune volume with seasonal variation amplitude of approximatively 10 m 3 /m. At the end of the winter of 2017-2018, the northern, the central and the southern areas all show an increased volume loss by 13.8 m 3 /m, 14.5 m 3 /m and 21.3 m 3 /m, respectively. During the second period (i.e., from the end of the winter of 2017-2018 to July 2020), the dune volume of the southern area has remained relatively stable, while the northern and central areas, although located in slightly more chronically eroding sectors, largely recovered with the dune volume even exceeding that in 2014. The third period (i.e., from July 2020 to July 2021) shows an alongshore quasi-uniform dune volume decrease, with 19.5 m 3 /m for the southern area, 23.4 m 3 /m for the northern and 27 m 3 /m for the central areas. Thus, over the study period, the dune volume decreased by 23.4 m 3 /m (i.e., -1.75%), 33 m 3 /m (i.e., -2.03%) and 40.1 m 3 /m (i.e., -3.72%) for the northern, the central, and the southern areas, respectively.The three distinct time periods described above are also found in the evolution of the alongshoreaveraged shoreline position (Figure9.d). The first period (i.e., from 2014 to the beginning of winter of 2017-2018) shows relatively stable alongshore-averaged shoreline position for the three areas with some seasonal variations in the order of 1.5 m. The winter of 2017-2018 induced a 1.67 m retreat of the alongshore-averaged shoreline position of the southern area, whereas during the second period (i.e., from the end of the winter 2017-2018 to July 2020) the alongshore-averaged shoreline position of the southern area remained relatively stable. During the same period, the northern and central areas show a seaward shoreline migration by 1.20 m and 4.25 m, respectively. The alongshore-averaged shoreline position of the three areas migrated landward by about 2 m during the third period (i.e., from July 2020 to July 2021). Thus, over the study period, the alongshore-averaged shoreline position eroded by 2.51 m and 5.84 m, for the northern and southern areas respectively, and migrated seaward by 0.65 m for the central area where the parabolic dune is observed, which will be discussed in the next section.

  Figure 8. (Left-hand panels): DSMs of l'Anse du Gurp zoomed onto the parabolic dune from 2016-01-08 to 2021-07-09. (Right-hand panels): Elevation difference between two consecutive surveys. (l)Net elevation difference between 2016-01-08 and 2021-07-09. All these DSMs were performed by UAV photogrammetry. Figure9shows the 2014-2021 morphological evolution (i.e., cross-shore profile, volume difference and alongshore-averaged shoreline position) of the parabolic dune system (green area in Figure9.a), compared to two adjacent control areas in the north (blue area in Figure9.a) and in the south (orange area in Figure9.a). The cross-shore profiles in the central area (green curves in Figure 9.b) highlight the landward migration (by approximately 40 m) of the deposition lobe with crest lowering (up to 3.3 m). The northern profiles (blue curves in Figure 9.b) are characterized by a dune toe retreat by 4.9 m. The dune crest migrated landward by 3.2 m and raised by 1.14 m. The northern and central profiles are characterized by an approximately 5-m high erosion scarp with a slope break around elevation 𝑧𝑧 = 9.5 m. The southern profiles (orange curves in Figure 9.b) show a retreat of the dune toe by 5.5 m, with the dune crest migrating landward by 7 m and lowering by 1.14 m. Contrary to the two other zones, the profile of the southern zone does not show any break in slope around 𝑧𝑧 = 10 m, with a steep 17.3-m high face from the shoreline to the crest.The time series of volume differences show three periods with different evolution patterns (Figure9.c). During the first period (i.e., from 2014 to the beginning of winter 2017-2018) the northern, the central and the southern areas all show a small decrease in dune volume with seasonal variation amplitude of approximatively 10 m 3 /m. At the end of the winter of 2017-2018, the northern, the central and the southern areas all show an increased volume loss by 13.8 m 3 /m, 14.5 m 3 /m and 21.3 m 3 /m, respectively. During the second period (i.e., from the end of the winter of 2017-2018 to July 2020), the dune volume of the southern area has remained relatively stable, while the northern and central areas, although located in slightly more chronically eroding sectors, largely recovered with the dune volume even exceeding that in 2014. The third period (i.e., from July 2020 to July 2021) shows an alongshore quasi-uniform dune volume decrease, with 19.5 m 3 /m for the southern area, 23.4 m 3 /m for the northern and 27 m 3 /m for the central areas. Thus, over the study period, the dune volume decreased by 23.4 m 3 /m (i.e., -1.75%), 33 m 3 /m (i.e., -2.03%) and 40.1 m 3 /m (i.e., -3.72%) for the northern, the central, and the southern areas, respectively.The three distinct time periods described above are also found in the evolution of the alongshoreaveraged shoreline position (Figure9.d). The first period (i.e., from 2014 to the beginning of winter of 2017-2018) shows relatively stable alongshore-averaged shoreline position for the three areas with some seasonal variations in the order of 1.5 m. The winter of 2017-2018 induced a 1.67 m retreat of the alongshore-averaged shoreline position of the southern area, whereas during the second period (i.e., from the end of the winter 2017-2018 to July 2020) the alongshore-averaged shoreline position of the southern area remained relatively stable. During the same period, the northern and central areas show a seaward shoreline migration by 1.20 m and 4.25 m, respectively. The alongshore-averaged shoreline position of the three areas migrated landward by about 2 m during the third period (i.e., from July 2020 to July 2021). Thus, over the study period, the alongshore-averaged shoreline position eroded by 2.51 m and 5.84 m, for the northern and southern areas respectively, and migrated seaward by 0.65 m for the central area where the parabolic dune is observed, which will be discussed in the next section.

  Figure 9. (a) Orthophoto of l'Anse du Gurp zoomed onto the parabolic dune in 2021-07-09. The green area includes the parabolic dune whereas the blue and orange areas are adjacent control areas at the north and south of the parabolic dune, respectively, (b) Representative cross-shore profile of each area

  Figure 10. (a) Aerial photograph of the parabolic dune and of the adjacent areas in l'Anse du Gurp, photography taken from the beach highlighting the presence (or absence) of vegetation and paleosols near the dune toe at the (b) northern, (c) central and (d) southern zone. Photos a-b were taken on July 9, 2021 (Ph. Quentin Laporte-Fauret) and photos b-c were taken on November 10, 2021 (Ph. Bruno Castelle).

Figure 11 .

 11 Figure 11. Time series of the volume difference of the vegetation in front of the northern (blue line), central (green line) and southern (orange line) areas from 2017-04-20 to 2021-07-09. Grey rectangles represent the winter periods.5. ConclusionThe 70-year evolution of a 2-km long high-energy, chronically eroding (average of 1.26 m/yr), coastal dune system has been studied by combining historical aerial photographs, Lidar and UAV surveys. During this period, three major blowouts have developed, all in the northern, more chronically eroding, sector. Two of these blowouts have been stabilized by vegetation colonization or are currently stabilizing, and the third one rapidly evolved into a parabolic dune, which is still migrating into the inland forest. No blowout reactivation phase was observed. Focusing on the 3D evolution of the blowout that recently developed into a parabolic and its adjacent sectors, we found that the blowout promoted dune inland migration. Despite the southern area (no blowout) was in a less chronically eroding sector,

Table 1 .

 1 Characteristics of the aerial photo survey dataset from 1947 to 2012.

	Date	Number of photos	Color data	Pixel size (m)
	1947-05-29	6	B&W	1.12
	1957-06-18	9	B&W	1.87
	1969-01-01	9	B&W	0.84
	1977-09-10	7	B&W	0.87
	1986-07-18	6	B&W	1.33
	1998-06-24	22	B&W	0.33
	2004-06-14	5	RGB	1.62
	2009-06-23	8	RGB	0.79
	2012-08-08	14	RGB	0.46
	For each survey


The camp and the city: Insights from a multi-stakeholder community consultation in the port of Lavrio in Attica (Greece) | TR… https://trafig.eu/blog/the-camp-and-the-city-insights-from-a-multi-stakeholder-community-consultation-in-the-port-of-lavrio-in-attica-greece 1/7

Start / Blog / The camp and the city: Insights from a multi-stakeholder community consultation in the port of Lavrio in Attica (Greece)

The camp and the city: Insights from a multistakeholder community consultation in the port of Lavrio in Attica (Greece)

by Filyra Vlastou Aristotle University of Thessaloniki 27/10/2022, 12:35

The camp and the city: Insights from a multi-stakeholder community consultation in the port of Lavrio in Attica (Greece) | TR… https://trafig.eu/blog/the-camp-and-the-city-insights-from-a-multi-stakeholder-community-consultation-in-the-port-of-lavrio-in-attica-greece 7/7

needed now is to connect these initiatives so that more displaced persons can use their skills for their benefit and that of receiving countries.

Introduction and context

During June and July 2021, the Greek team carried out a set of multi-stakeholder community consultations (MSCC) in the port of Lavrio. The small town of more than 7,000 inhabitants in southern Attica had hosted three refugee camps until very recently: The former state-, now self-organised old Lavrio camp located in the very centre of the town since 1947, a makeshift campsite created in 2010 in Neraki, an area about one kilometre away from the town, and the official camp of Lavrio (located at neaby Sounio), founded in 2017 and closed in June 2021, located approximately five kilometres outside the town. The old Lavrio camp, located right in the heart of the town-around the corner from the central square, next to the police station and just five minutes' walk from the Town Ηall-is the first state camp for political refugees in Greece. It was founded in 1947 by the national authorities to shelter refugees fleeing from the communist regimes of that time. To this day, the camp accommodates displaced people. For the last 35 years, its population has mostly consisted of (initially) Turkish and (lately) Kurdish political exiles, many of whom are members or/and sympathisers of the Kurdistan Workers' Party (PKK). Almost since its foundation and until recently, the camp was run by the Red Cross. After 2010, disputes emerged between the Red Cross and the government, among others, over funding. In 2016, following the refusal of residents to abide with the authorities' demand to 'de-politicise' the camp by taking down all symbols, posters and images defining it as a political space and move to other facilities, the Greek government abandoned the site. From then on, the camp became self-organised and hosted approximately 400 asylum seekers, migrants and refugees-the majority of whom were Kurds from Syria, but also from Turkey, Iraq and Iran, as well as a small group of Afghans. The length of time people stay there is between a couple of months and a few years. The campsite in the area of Neraki was created by the authorities in 2010. A number of containers were put in an empty delimited field on the outskirts of the town to shelter some of the residents of the old camp while some of its buildings were being repaired. However, following the refusal of residents to be transferred, Neraki camp stayed empty. That changed in 2015, when following the increase of displaced people's arrivals in the country, the old Lavrio camp exceeded its capacity and some of its residents moved there voluntarily. Today, the campsite is an "extension" of the old Lavrio camp, hosting some more than 100 persons, in the https://trafig.eu/blog/the-camp-and-the-city-insights-from-a-multi-stakeholder-community-consultation-in-the-port-of-lavrio-in-attica-greece 2/7 majority Kurds.

The official camp of Lavrio, operated from 2017 to June 2021 in a forest area at facilities previously used as a summer camp for children, was founded in the region following the abandonment of the old camp by the state authorities. Before its closure, it hosted 235 people (43% children)-mainly from Afghanistan (33.2%) from Syria (29.8%) and from Congo (18.3%). In the absence of any facilities in the area, despite the distance, the camp residents visited the city of Lavrio for everyday needs (e.g. health issues, shopping groceries etc).

It is important to mention that the town of Lavrio itself is widely known as a city of migrants, not only due to the presence of the camps but mainly because of its own past. Lavrio is a relatively new town. It was created in the 1870s, after an Italian entrepreneur established a mining company on an empty plot and and Greek and foreign migrants moved there to work in the mines. Soon, the city expanded, becoming an important industrial centre, which throughout the 20th century was a focal point for migrants and refugees seeking employment. Lavrio remained an industrial town until the early 1990s, when it experienced harsh de-industrialisation, causing an unemployment rate of 70 per cent. Today, the town is reliant on tourism and other services to survive.

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Location of Lavrio in Attica, the town centre (left) and the camp (right)

Our previous TRAFIG-related research in Lavrio already revealed the complexity of relations between camp residents and the local society. The location of each camp, their presence over time, the profile of their residents in terms of ethnicity and legal status and the specific social and economic relations developed over time shape the attitudes of members of the local community towards these camps and the displaced people inhabiting them. For instance, residents' associations saw the newly founded (albeit shortlived) official camp of Lavrio, located in the heart of a forest area, as a threat to the surrounding area.

Several testimonies from such associations (whose members in their majority are owners of summer residences in the area), revealed that they feared that camp residents might cause fires or/and destroy the green environment on purpose. On the other hand, the old Lavrio camp, located in the very centre for more than 50 years, has been part of the city's identity as many locals have noted. Yet, its very location in a period when local authorities' vision is to turn the former industrial town into a seaside tourist stopover for yachts makes it highly disputed and contested.

Aims, process, participants and key outcomes

The overall aims of the multi-stakeholder community consultations in the town of Lavrio was to better understand the relations between the members of the local community and those living in the camps and to discuss possible solutions to the problems identified by the participants. All our participants referred to the old Lavrio camp and the Neraki campsite, as in reality, both camps formed one large entity being, in fact, the same community with strong bonds and relations. We decided, therefore, to disregard the particular aspects of the Neraki locality in the present MSCC.

COVID-19 pandemic-related precautions and restrictions did not allow us to have one large event. So, despite our original intentions and the project goals, we decided to host several subsequent meetings/interviews. Another reason for this decision was the potential danger of bringing people with conflicting interests together, exacerbating already existing tensions in the current sensitive period, characterised by increasingly restrictive policies against displaced people in the country. We, therefore, sought instead to give a voice to various stakeholders living in the town of Lavrio who represented the local authorities, civil society and the camp community, as well to speak with members of the local society in key positions (school, health center), grouping some of them around the same table. To this end, the team held Questions to be asked concerned the relation of each stakeholder group with the camps and their residents, the particular characteristics of the two camp localities, the social relationships between locals and camp residents, the economic "footprint" of the camps but also of their people and whether this is related to a more general vision for the economic development of the town. The guide was adapted according to participating stakeholders. The methodology was inspired by the timeline focus group method guidelines (also used in the TRAFIG fieldwork), according to which the interviewers asked the participants to emphasise the changes and mutations they observed over time. In what follows, a summary of the three key issues that were raised during the interviews and the meetings is presented below:

1. Long-term residents stressed structural factors affecting their relationship with displaced people, including the state's deficiency or unwillingness to appropriately staff key public institutions such as the school and the one and only health centre of the town to adequately address the increasing workload caused by the presence of displaced people. For instance, the lack of interpreters in these key public institutions complicates and undermines communication and creates unnecessary tensions and fights. Problems of communication because of the lack of interpreters, it was mentioned, may be a factor why displaced people are unwilling to send their children to school. Also, the replacement of https://trafig.eu/blog/the-camp-and-the-city-insights-from-a-multi-stakeholder-community-consultation-in-the-port-of-lavrio-in-attica-greece 5/7

AMKA (National Insurance Number) with PAAYPA (Foreigner's Temporary Insurance and Health

Coverage Number), which added more obstacles to already exisiting bureaucratic procedures, deprived many displaced people of the chance to have themselves vaccinated in the context of the pandemic, generating perceptions about the camps as a health threat for the town. In general, contacts mentioned that the situation was easier in the official camp of Lavrio, due to the mediation of humanitarian actors active in the camp.

2. The local authorities' vision (mayor, port authorities) is to turn the town of Lavrio into a hub for "upstream" high added value economic activities, as well as a tourist destination. "In this vision, refugees are not included" stressed the mayor. Local authorities explicitly said that they want the old Lavrio camp to be closed "as we closed the official camp of Lavrio". In their view, its central location increases the visibility of its residents and hinders the town from capitalising on its privileged location.

They argued that, first, tourists do not want to see refugees moving around the city. Second, since the old camp ceased to be official and, therefore, locals stopped working in it, the local society would be more receptive to its closure as there is no direct profit. 3. In response to these arguments, the Coordinator of the Kurds' Committee in Lavrio camp noted that tourists "always stop before the camp and want to learn about us, asking how they can be helpful". As far as the vision is concerned, he said that this is only the local authority's "capitalist vision", which does not reflect the perceptions of the local society. This is supported by a number of testimonies from long-term residents that show this cleavage between the local authorities and the local society. The local society is "tolerant, welcoming displaced people", as one of our participants said (the general practicionner), what in his opinion proves why "the camp was never an issue at stake in local elections". An initiative called Lavrio Refugee Support Committee has been actively supporting the refugees for years, publicising the problems of camp residents, demanding solutions from the local authority, facilitating communication with the local community (Dirakis, 2019) and networking with local, translocal and transnational organisations.

Concluding remarks

Currently, we observe a proliferation and multiplication of spaces of encampment in Europe and globally.

Over half of the migrants stranded in Greece since 2015 are confined in camps and camp-like spaces, usually located in isolated and segregated areas at the peripheries of cities or even far beyond. The case of the old Lavrio camp is a positive exception.

1. Its central location allows many daily micro-economies and transactions between the camp and the city. Every day many camp residents wait outside its main entrance to be picked by locals for casual employment around the area. Some others work in the nearby shops, restaurants, or the marina.

Besides, as the Coordinator of the Kurds' Committee in the old Lavrio camp put it: "the residents of the camp live just like all the other locals: We have our micro-jobs, we shop, we drink our coffee, we walk along the seaside".

2. Although the temporary stay of many of the camp residents does not leave room for many meaningful contacts, the long-term presence of the camp itself seems to allow locals to familiarise themselves with the "other", an important aspect of intergroup relations. 3. The fact that the camp is self-managed gives it a community character that allows its inhabitants autonomy and the development of strong networks. Simultaneously, as the MSCC has also shown, this

  two separate meetings: a first one with the respective mayors in office at the time (from 2002 to 2010 and from 2014 to present), a former mayor (from 1995 to 2002) and two members of the administration of the L trafig.eu/blog/the-camp-and-the-city-insights-from-a-multi-stakeholder-community-consultation-in-the-port-of-lavrio-in-attica-greece 4/7Lavrio Port Authority; and a second with the headmaster of a local public primary school and a general practitioner employed in Lavrio's Health Centre-the only public health facility in the town. In addition to these meetings, we also conducted two interviews: one with the NGO Homespot based in Lavrio since 2016, which provides humanitarian assistance to displaced people in both the informal and official camps; and one with the Coordinator of the Kurds' Committee in the old Lavrio camp and responsible for the Cultural Association of Kurdistan in Greece. These interviews and meetings were scheduled for between late June and early July 2021.Michalis Kastanidis, July 2021, Outside view of the old Lavrio camp. In the wall a graffiti states: Kurd loveGreekBefore the discussions took place, the interviewers had developed a guide to support the discussions.

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant No 822453.

The content reflects only the authors' views, and the European Commission is not responsible for any use that may be made of the information it contains. (coordinating partner) → Datenschutzerklärung -BICC (koordinierender Partner)

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autonomy and the development of strong networks. Simultaneously, as the MSCC has also shown, this community character adds to the "tensions" with the local community, which are related to the camp's location in the centre of the town. The camp residents and their networks are aware of the "tensions" and the importance of addressing them. In our discussion, the Coordinator of the Kurds' Committee in Lavrio concluded: "Some years ago, the Caravan of Solidarity from France brought to the town some medical equipment that cost close to 15,000 euros. We donated them all to the medical centre of Lavrio; we also do things for the town".

Reference: Dirakis, Y. ( 2019) "Claiming the right to the camp - In the shoes of a Congolese refugee:

Online game helps to understand urban refugees' legal struggles

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12 July 2022

The missing link

Promoting refugees' skills-based mobility within Europe

The Common European Asylum System prohibits the mobility of persons entitled to international protection within the European Union, making it more difficult for displaced persons to rebuild their lives even after arriving in Europe and receiving protection status. Recent developments soften this strict policy of immobility for some. In this context, intra-EU mobility based on refugees' skills could become a game-changer. The tools are there. What is needed now is to connect these initiatives so that more displaced persons can use their skills for their benefit and