Delacourt, C.; Cocquempot, L., and Castelle, B., 2019. Introduction: Special Issue on Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 1–2. Coconut Creek (Florida), ISSN 0749-0208.

This paper presents how major French science-oriented public institutions and Ministry of Higher Education, Research, and Innovation (MESRI) have developed and structured coastal observatories along metropolitan and oversea coastlines in order to better understand the metropolitan and oversea French coastal evolution on a wide range of temporal and spatial scales. Scientific results presented in this special Issue have been obtained in the framework of the resulting DYNALIT / ILICO observatory network.

Castelle, B. and Chaumillon, E., 2019. Coastal change in tropical overseas and temperate metropolitan France inferred from a national monitoring network: A summary from the current special issue. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 3-9. Coconut Creek (Florida), ISSN 0749-0208.

This paper provides an overview of the papers published in this Special Issue « Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France» of Journal of Coastal Research. This special issue reflects the large diversity of mainland and tropical French coast and highlights the complex, site- and timescale-specific, combination of factors driving coastal evolution. Many contributions reveal a strong climate control on storm wave activity and, in turn, coastal response. In some contributions, the inherited geology and anthropogenic factors clearly appear affect coastal change. This special issue emphasises the need to monitor the coast combining different means to improve our understanding and predicting capacities of the natural variability of coastal response in a changing climate.

Dodet, G.; Bertin, X.; Bouchette, F.; Gravelle, M.; Testut, L., and Wöppelmann, G., 2019. Characterization of sea-level variations along the metropolitan coasts of France: Waves, tides, storm surges and long-term changes. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 10–24. Coconut Creek (Florida), ISSN 0749-0208.

With 5853 km of coastlines facing the North Sea, the English Channel, the Atlantic Ocean and the Mediterranean Sea, France displays littoral zones exposed to a wide spectrum of wave climates, tidal ranges and storm surges. This study aims at characterizing in a systematic way sea level variations along the coasts of France. Wave climates are first characterized using state-of-the-art high resolution hindcasts validated against available observations from the national network CANDHIS. This analysis reveals a contrasting situation between coastal zones located along the Bay of Biscay, exposed to energetic and moderate-to-long period swells, and coastal zones located in the English Channel and the Mediterranean Sea, exposed mostly to wind seas and shorter period swells. Tides are characterized using available tide gauge measurements retrieved from the national REFMAR database and display large contrasts, between microtidal regimes along the Mediterranean Sea and macrotidal regimes along the English Channel. The analysis of storm surges computed from the same data reveals that they are controlled not only by storm tracks but also by the width of the continental shelf. Thus, during the studied period 1998-2018, storm surges derived from tide gauges measurements hardly reach 1.0 m along the coastlines of the southern Bay of Biscay and the eastern Mediterranean Sea but can exceed 2.0 m in the English Channel. The analysis of long-term sea level trends reveals significant variabilities, with an accelerated sea level rise on average from 1.2 mm/yr over the 20th century to 2.4 mm/yr over the two last decades, and ranging locally from 1.23 mm/yr (Roscoff) to 4.25 mm/yr (Nice). Finally, these values are compared against vertical land motions, computed from a state-of-the-art GPS reanalysis. A moderate land subsidence (<1 mm/year) is found for the majority of the stations.

Héquette, A.; Ruz, M.H.; Zemmour, A.; Marin, D.; Cartier, A., and Sipka, V., 2019. Alongshore Variability in Coastal Dune Erosion and Post-Storm Recovery, Northern Coast of France. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 25–45. Coconut Creek (Florida), ISSN 0749-0208.

As along many parts of the world's shoreline, the coastal dunes extending along the macrotidal coast of northern France represent important defenses against marine flooding. The impacts of storms on the upper beach and foredunes and their post-storm recovery were analyzed using nearly 10 years of offshore wave measurements, water level records, wind measurements, and in situ and airborne LiDAR topographic surveys of the beach and foredunes. Our results show that coastal dunes located at a relatively short distance apart along a coastal stretch with the same wave exposure can have significantly different responses to storms. Not only the impacts of storm events were greater on some dunes, but post-storm recovery also varied from one foredune to another. A strong alongshore variability in dune erosion and recovery was observed with a positive eastward gradient in dune volume change, probably related to longshore and onshore-directed sediment supply. Our measurements revealed that even where the foredune underwent significant erosion during the first years of the survey period, progressive full dune recovery took place through the development of a sand ramp at the dune toe that favored landward sediment transport from the upper beach to the foredune. This period was followed by an unusual series of closely spaced storms during fall-winter 2013-2014 that had major impacts on the coasts of Western Europe. Several of these storms were responsible for extreme water levels, which resulted in significant retreat of the dune front and massive volume loss in places. Our analyses show that the maximum water levels reached during storms represent a major factor explaining dune erosion compared to wave energy that is of secondary importance along this macrotidal coast. Our results also suggest that dune volume change during storms and subsequent recovery were largely controlled by the initial dune and upper beach morphology. A strong correspondence was found between dune front volume change and initial upper beach width and with dune toe elevation, but a somewhat weaker relationship was observed between dune volume change and initial dune height.

Costa, S.; Maquaire, O.; Letortu, P.; Thirard, G.; Compain, V.; Roulland, T.; Medjkane, M.; Davidson, R.; Graff, K.; Lissak, C.; Delacourt, C.; Duguet, T.; Fauchard, C., and Antoine, R., 2019. Sedimentary coastal cliffs of Normandy: Modalities and quantification of retreat. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 46–60. Coconut Creek (Florida), ISSN 0749-0208.

This paper examines the spatial and temporal variations of cliff retreat rates over multi-temporal data of the Normandy cliffs. Data are derived from historical maps and aerial photographs and from recent lasergrammetry and photogrammetry monitoring. The diachronic analysis of all these data gives retreat rates of -0.1 to -0.5 m/yr. in line with the international literature. The spatial variations of the cliff retreat rates, at the Normandy scale, can be explained by geological structure, especially at the cliff foot, but also by the influence of cliff collapses or anthropogenic obstacles that disrupt the longshore drift. Multi-temporal data shows that the evolution of the cliffs occurs on scales from 10 to 70 years according to the lithology. The high resolution and frequency monitoring also provide information about the factors responsible for triggering gravitational landslides (rockfalls, slides, debris falls). The study proposes a regional warning threshold for the cliff characterized by landslides, under the dominating influence of rainfall and groundwater level evolution. In this respect the monitoring is inconclusive for chalk and limestone cliffs, because the origin of evolutions is more multifactorial (combination of more continental and marine processes).

Levoy, F.; Anthony, E.J.; Dronkers, J.; Monfort, O., and Montreuil, A.-L., 2019. Short-term to decadal-scale sand flat morphodynamics and sediment balance of a megatidal bay: Insight from multiple LiDAR datasets. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 61–76. Coconut Creek (Florida), ISSN 0749-0208.

This study presents the results of LiDAR surveys, spanning 15 years (2002-2017), of the shallow inner sector of Mont-Saint-Michel Bay, a megatidal embayment in Normandy, France, and a UNESCO World Heritage site famous for its monastery. The objective of these surveys was to obtain a better understanding of the morphodynamic processes in this megatidal environment. The campaign has highlighted the circumstances and very short timescales at which changes occur in the channel configuration of the inner bay. It has also been demonstrated the high variability of sediment import and export, which involves not only the tide, but also wind speed and direction. Part of the variability is also related to fluctuations in river discharges into the bay. Since 2009, engineering works have been carried out in the bay. These are aimed at re-establishing the maritime character of the bay around the granite outcrop (Mont-Saint-Michel) on which was built the monastery, and which was increasingly subject to accretion. The main channel of the inner bay shifted recently to a more southward position than observed during the past decades. Yet, from the topographic LiDAR data up to May 2017, it is presently not possible to identify the influence of the engineering works on sedimentation processes beyond a limited zone around the Mont. However, the pursuit of LiDAR surveys is necessary in order to determine whether, in the course of the next decade, the North-South migrations of the Sée-Sélune channel induced by the 18.6 y nodal cycle are affected or not by these works.

Tessier, B.; Poirier, C.; Weill, P.; Dezileau, L.; Rieux, A.; Mouazé, D.; Fournier, J., and Bonnot-Courtois, C., 2019. Evolution of a shelly beach ridge system over the last decades in a hypertidal open-coast embayment (western Mont-Saint-Michel Bay, NW France). In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 77–88. Coconut Creek (Florida), ISSN 0749-0208.

The behavior of the shelly beach ridge system forming a more or less continuous barrier lying along the southern coast of the hypertidal Mont-Saint-Michel bay (NW France) is examined using a set of various data including aerial photographs since 1947, sediment cores collected in back-barrier salt marshes, ground penetrating radar profiles, and differential digital elevation models (DEMs). The latter shows that since the middle of the 20th century, the tidal flats extending in front of the ridge system experience erosion and accretion that alternate spatially according to cross-shore corridors. The shelly ridges are better developed on line with the erosional bands. The reasons for this erosion/accretion compartment pattern are not totally elucidated. It probably results from the combined influence of shellfish farms, tidal channels and reef present on the tidal flats on wave and current dynamics. At the decadal to pluri-decadal scale, the shelly beach ridge system behavior as well as its internal organization can partly be related to the fluctuations of hydrodynamic conditions controlled by the 18.6 years nodal tidal cycle. Periods of intense landward migration alternate with periods of relative stability during the peak and the trough of the cycle respectively. This general scheme is complicated due to variations in storm wave dynamics. The most morphogenic events happen potentially during coinciding nodal cycle peaks and enhanced storminess. Such conditions actually occurred around 1995-2000, resulting in the most prominent washover fan deposit preserved within the back-barrier infilling successions. These sediment successions evidence as well that no beach ridge system older than the 19th century is preserved in front of the Medieval dike built during the 11th century on a relict beach ridge, and on which the present-day system is backed. We suggest that the enhanced storminess conditions of the Little Ice Age provoked the reworking of beach ridges formed between the 11th and 19th centuries. Changes in quantity and type of mollusk shells, related to the shellfish farming development during the 20th century are also thought to have had positive consequences on the barrier building and stability.

Stéphan, P.; Blaise, E.; Suanez, S.; Fichaut, B.; Autret, R.; Floc'h, F.; Cuq, V.; Le Dantec, N.; Ammann, J.; David, L.; Jaud, M., and Delacourt, C., 2019. Long, medium, and short-term shoreline dynamics of the Brittany Coast (western France). In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 89–109. Coconut Creek (Florida), ISSN 0749-0208.

This paper aims to analyze the shoreline changes of coastal accumulations (sandy and gravel beaches/barriers) of Brittany (Western France). Three long, medium, and short term spatio-temporal scale observations are taken into consideration for the assessment of shoreline dynamics at this regional scale. Firstly, the long-term shoreline position evolution is based on a comparison of two sets of aerial orthophotos (1949-1952 and 2006-2009). A total of 652 beaches were analyzed in order to map and quantify erosion (35% of the total studied coastline), stability (38%), and accretion (27%) over the last 60 years. In detail, these percentages vary significantly according to the beach/barrier morphologies (spits vs pocket beaches), sediment composition (sandy vs gravelly), and hydrodynamic context (exposed vs sheltered). Secondly, a pluri-annual (i.e., medium-term) shoreline change analysis based on five representative beaches was conducted. This analysis was also based on image processing using sets of aerial photos taken every five years over the last 60 years (1948-2013). Results show an alternation of significant erosion- and accretion-dominated periods (respectively EDP and ADP), with six main EDP (i.e., periods 1962-1968, 1977-1978, 1980-1985, 1987-1990, 1993-1997, and 2013-2014) related to an increase in the frequency of extreme water levels associated with storm events. Finally, the short-term change analysis based on high-frequency monitoring of 11 sites was carried out over the period 1998-2017. These surveys, based on field topo-morphological measurements, highlight the impact of five morphogenetic events associated with significant storm events: 1998-2000 (storms Lothar and Martin in December 1999), 2008 (storm Johanna on March 10, 2008), the winter of 2013-2014 (a cluster of storms in January, February, and March 2014), 2016 (storm Ruzica/Imogen on February 8, 2016) and 2018 (storm Eleanor on January 2, 2018). A relevant recovery phase, which took place between 2008 and 2012 due to the calm and cold winters, was also recognized. The identification of parameters involved in shoreline variations at these three timescales is important for future management options of the Brittany coast.

Menier, D.; Mathew, M.; Cherfils J.-B.; Ramkumar, M.; Estournès, G.; Koch, M.; Guillocheau F.; Sedrati, M.; Goubert, E.; Gensac, E.; Le-Gall, R., and Novico, F., 2019. Holocene sediment mobilization in the inner continental shelf of the Bay of Biscay: Implications for regional sediment budget offshore to onshore. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp.110-121. Coconut Creek (Florida), ISSN 0749-0208.

Sediment mobilization, especially from offshore bay to the coast during the Holocene, along the inner continental shelf of Bay of Biscay is less understood. Acoustic and sediment sampling and surveys were conducted offshore of the Bay of Étel to define the shallow geologic framework and the sedimentology. Results of these campaigns are used to identify and map Holocene deposits, discuss sediment transport pathways and depth of closure using Hallermeier's approach in order to define the active sedimentary prism using the depth of closure on a meso-tidal and wave dominated coast. In the Bay of Étel, Holocene sediments are concentrated between rocky shoals and emerged rocks, passing from external zone by sand lobe, 20 m thick to the medium zone, 15 m thick, to the beaches of the Gâvres-Penthièvre beach dune system (modern coastal prism). This case study found that the thickest deposits of Quaternary sediments observed along the inner shelf have a strong dependence on coastal topography and structural heritage. The presence of offshore basement shoal has been determined to have exercised control over sediment transfers from offshore to landward regions during the last marine inundation. Due to the lack of any significant modern fluvial input of sand in the region, the Holocene deposits are inferred to have been derived by reworking of relict Pleistocene and older inner-shelf deposits. Sediment textural trends and seafloor morphology from the medium zone to the beach dune system indicate an absence of relationships (linking) between the modern sedimentary prism and the potential source of sediment to the shoreline.

Robin M.; Juigner M.; Luquet F., and Audère M., 2019. Assessing surface changes between shorelines from 1950 to 2011: The case of a 169-km sandy coast, Pays de la Loire (W France). In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 122–134. Coconut Creek (Florida), ISSN 0749-0208.

This paper aims to highlight the evolution of the Pays de la Loire sandy coast. No global synthesis has been done on the evolution of this coast and its rates of change at various periods of time. Furthermore, to our knowledge, no synthetic research has been carried out using this methodology, which enables the release of surface statistics per unit of time. The shoreline's evolution has been measured over various periods since 1950: from 1950 to 2010, the intermediate periods being 1950-1970, 1970-1990, 1990-2010 and finally 1990-2000 and 2000-2010. These dates are among the only ones to allow complete air coverage by aerial photographs. The photographs are used to finely analyze changes in 60 to 10-year periods of time, in order to target non-linear rates of change. Our methodology is rigorously applied as follow: (1) we create of a database of exhaustive aerial photographs, (2) we define and digitize the shoreline, (3) we estimate the error margins, (4) we calculate a regular segmentation of the shoreline per surface unit (which are regularly spaced at 25m along the coast), (5) we intersect the shorelines and these spatial units to measure the surfaces between each shoreline inside each spatial unit. Various linear and surface statistics expressing erosion or accretion rates are then calculated for each of the 6652 spatial units. Global statistics are presented at the regional level. They are then presented in more detail at the level of the sedimentary cells, and finally at the level of the spatial units constituting the sedimentary cells. This approach permits us to present a global sedimentary balance at various scales through (i) the rates of evolution converted in linear meter (ii) and especially the emphasis of the evolution of surfaces between these shorelines. This leads us to estimate precisely the accreting, eroding and stable or insignificant sectors per period. It appears that the Pays de la Loire region has a sandy coast which is usually in sedimentary accretion even though some local sectors have been in chronic erosion over the last 60 years.

Chaumillon, E.; Cange, V.; Gaudefroy, J.; Mercle, T.; Bertin, X., and Pignon, C., 2019. Controls on shoreline changes at pluri-annual to secular timescale in mixed-energy rocky and sedimentary estuarine systems. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 135-156. Coconut Creek (Florida), ISSN 0749-0208.

This article examines the morphological evolution of 27 beaches, including sandy and mixed sandy and rocky beaches, located along a 460 km-long complex shoreline, indented by four incised-valleys. The wave climate, simulated numerically from 1999 to 2017 in front of each beach, allows distinguishing between exposed and sheltered beaches. The morphological changes are quantified at two time scales: the last two centuries, from old maps and aerial photos, and the last two decades, from topographic profiles, aerial photos and satellite images. The first-order parameters explaining most of the spatial variations in the shoreline evolutions are the presence or absence of bedrock outcrops in the foreshore and/or shoreface and the presence or absence of tidal inlets close to the beach. The fastest shoreline changes are observed along exposed sandy beaches close to tidal inlets, whereas moderate to slow evolutions are mainly observed along exposed beaches with rocky foreshore and sheltered beaches. Huge shoreline changes located close to tidal inlets are related to changes in littoral drift orientation and intensity, themselves related to changes in beach orientation. Maximum erosion rates at a decadal time scale are not associated with isolated exceptional storms, but rather with clusters of storms. Beaches evolutions on this indented shoreline are controlled by geological factors (coastal orientation and the basement topography) and display complex patterns that cannot be clearly related to past changes in wave climate and sea level rise, complicating the prediction of their future evolutions.

Castelle, B.; Marieu, V., and Bujan S., 2019. Alongshore-variable beach and dune changes on the timescales from days (storms) to decades along the rip-dominated beaches of the Gironde Coast, SW France. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 157–171. Coconut Creek (Florida), ISSN 0749-0208.

The high-energy meso-macrotidal 110-km long Gironde coast, SW France, is primary composed of quasi-straight sandy beaches bordered by high and wide coastal dunes. Beaches are intermediate double-barred and are essentially morphologically variable alongshore with ubiquitous rip channels incising both bars. These rip channels enforce a strong alongshore variability in the morphology of the dry beach and/or of the dune, morphological patterns referred to as megacusp embayments. In this study, we use 70-year diachronic shoreline data, 3.5-year semi-annual in situ shoreline surveys since 2014, combined with 12.5-year monthly to semi-monthly topographic surveys collected since 2005 at Truc Vert beach. Results show that 2 types of megacusp can be identified: (1) accretive megacusps on the upper beach, forming through a sequence of accretionary beach states following a storm event, are enforced by inner-bar rip channels with a spacing of O(100 m) and a typical lifetime of a few months and (2) erosive megacusps cutting the dune, forming during severe-storm driven erosive events, which are primarily enforced by the outer-bar morphology with a spacing of O(1000 m). These erosive megacusps do not migrate alongshore and can persist for years to decades. The outstanding winter of 2013/2014 drove the formation of erosive megacusps all along the coast, dramatically altered the coastal landscape and also impacted the behaviour and mean spacing of the accretive megacusps during the subsequent years. Overall, the study demonstrates the complex interplay between the nearshore morphology and the alongshore-variable changes of the foreshore/backshore from the timescales of days to decades, with accessional outstanding winters having the potential to deeply affect beach morphology and rhythmicity on the time scale of a few years, at least.

Dolique, F.; Sedrati, M.; Charpentier, J.; Jeanson, M.; Cohen, M.; Dupuy, L., and Alami, S., 2019. Beaches seasonal and paroxysmal morphosedimentary dynamics: Results of 10 years Martinique coastal observation network. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 172–184. Coconut Creek (Florida), ISSN 0749-0208.

Coastlines are vulnerable to the effects of climate change and sea-level rise as well as modifications in the wave climate and, in particular, increased storminess. The coastal areas of islands form part of this scenario and the Martinique shoreline illustrates this intensification of shoreline retreat and coastal erosion. In the context of this critical situation, a coastal observation network was set up in 2010 to monitor the evolution of the coast of Martinique. This network includes 28 beaches along the coast of Martinique and is based on studies of shoreline evolution using aerial photographs and topographic surveys of 71 cross-shore transects. The main aim of this network is to characterize the modal and seasonal dynamics of these vulnerable beaches and to estimate their resilience capacity and trends in the context of the increasing frequency of hurricanes. Analysis of the measurements over the last ten years highlights a general acceleration of the shoreline retreat. Furthermore, in terms of seasonal coastal evolution processes, the results clearly show the weak effect of the longshore sediment transport component on the open beaches of Martinique Island. However, a predominant cross-shore sediment transport component is observed on smaler bay beaches, which represent the most numerous beach type on the island (including several pocket beaches). On the short-term scale, the results of hydrodynamic and morphodynamic measurements highlight variable and intensive beach dynamic responses with a determining role played by shoreface parameters and coral-lagoon complexes. While cyclones have a significant impact on reducing the slope of beach profiles, our measurements reveal a significant resilience of eroded beaches owing to constructive swell. We also demonstrate that the hurricane season is not the season with the strongest morphosedimentary impacts. This almost ten-year-old observation network, provides an overview of shoreline evolution on Martinique and is an efficient management tool for the Lesser Antilles coastal environments.

Jolivet, M.; Gardel, A., and Anthony, E.J., 2019. Multi-decadal changes on the mud-dominated coast of western French Guiana: Implications for mesoscale shoreline mobility, river-mouth deflection, and sediment sorting. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 185–194. Coconut Creek (Florida), ISSN 0749-0208.

Mud supplied by the Amazon River forms large banks that migrate along the Amazon-Orinoco Guianas coast under the influence of waves and currents, separated by ‘inter-bank’ zones (phases). Bank-welding onto the coast creates new land, followed by rapid mangrove colonization, and westward deflection of the mouths of the smaller rivers. A fine example is Pointe Isère, a mud cape that deflected the mouth of the Mana River in French Guiana. During inter-bank phases, higher wave energy can result in rapid and massive shoreline erosion, except where sandy-shelly cheniers develop from winnowing of coarse-grained bedload from the surrounding muddy matrix. In order to contribute to a better understanding of the rates and scales of shoreline change on the French Guiana coast, we conducted a GIS analysis on Pointe Isère using a >60-year record of aerial photographs and satellite images. The results show significant and quasi-continuous erosion of this mud cape at the multi-decadal timescale. A phase of massive retreat of the shoreline (>1 km between 1955 and 1972), was followed by much slower erosion up to 2015, probably due to: (1) the alongshore passage of several mud banks, and (2) the formation of a sandy chenier through concentration, by waves, of contemporary sand of fluvial origin stored in the inner mud-dominated shoreface, and of sand derived from erosion of an old inland chenier exposed as a result of shoreline retreat. The new chenier has also been characterized by a westward-elongating spit. The progressive demise of Pointe Isère finally resulted, in 2001, in a breach that created a new direct outlet for the Mana River, and sealing of the former outlet through downdrift distal spit welding onshore. Over the study period (1955–2017), the demise of Pointe Isère has involved a loss of 41.8 km², much of it mangrove wetlands. This corresponds to about 8% of the total area of mangroves in French Guiana in 2015. Since 2015, a new phase of large-scale muddy accretion has been associated with the on-going isolation of the active sandy chenier. This probably heralds a new cycle of mud-cape formation in the vicinity of the mouth of the Mana that will lead to a new westward diversion of this river. During the continuous erosion of Pointe Isère, the longshore passage of several mud banks did not mitigate or counter erosion. This has important implications regarding the unpredictability of shoreline change on the Amazon-Orinoco coast, and the need to go beyond the common vision of such change in terms of mudbank (shoreline accretion) and inter-bank (shoreline erosion) phases that cover timescales of the order of years to a decade.

Jeansson, M.; Dolique, F.; Anthony, E.J., and Aubry, A., 2019. Decadal-scale dynamics and morphological evolution of mangroves and beaches in a reef-lagoon complex, Mayotte Island. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 195–208. Coconut Creek (Florida), ISSN 0749-0208.

Mayotte Island is characterized by a vast coral reef-lagoon complex comprising significant mangrove development and numerous pocket beaches nested between volcanic headlands. Since 2005, field experiments involving topographic surveys, observations and hydrodynamic measurements have been coupled with the analysis of aerial photographs (1950-2016) in order to improve understanding of the morphodynamic interactions between mangroves, beaches and the coral reefs. The results, integrated in a coastal observatory of Mayotte Island, highlight a remarkably variable mangrove system subject to advance or stability in the north and east of the island but exhibiting a clearly regressive pattern along the southern and western shores. This variability largely reflects the impact of humans on mangrove stability. The hydrodynamic data acquired during the field experiments clearly bring out the spatial and temporal variations in wave patterns involved in these differences. These data also shed light on the short-term morphodynamics of small pocket beaches associated with these mangroves. Patterns of beach morphological change driven by residual wave energy following reef attenuation are strongly affected by the importance of beach embayment. These patterns affect, in turn, mangrove resilience, which is weak on the more exposed south and west coasts of the island, where all the mangrove stands fronting the lagoon have retreated, compared to the more stable stands on the north coast. These results highlight the complex time and space-varying morphodynamic interactions involved in a reef shoreline environment, and how these can also reflect the consequences of humans on mangrove stability. The results should, within the framework of the Mayotte coastal observatory, contribute to the management and conservation of the coast.