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International audience; Wind waves breaking at an angle with the shoreline force the drifting of littoral sediments, which is known for contributing to the formation and growth of barrier spits. Intriguingly, increased rates of longshore wave power have also been associated with the erosion of some barrier spits on the updrift margin of tidal inlets. Therefore, a numerical experiment was designed and is presented here, which investigates the possible links between the longshore wave power and the shortening of these elongated coastal barriers. Based on a process-based model, the experiment provides new insights into the forces at play in the redistribution of sediments between a sandspit and its adjacent inlet, respectively the Cap Ferret and the Bay of Arcachon's tidal inlet, in SW France. More particularly, model scenarios were defined that show how combined waves and tide create gradients of residual sediment transport responsible for a sediment deficit at the spit – inlet boundary. The deficit was also found to deepen with increasing longshore wave energy, as if the transfer of sediment from the spit to inlet shoals was accelerated. This physically explains the previously observed retreat of the spit's distal end during periods dominated by the positive phase of North Atlantic Oscillation (NAO) in winter. Indeed, according to model results, higher and/or more oblique waves associated with the positive phase of the NAO are expected to increase the transfer and storage of the drifting sediments to and by the inlet shoals, and this at the expense of the spit. While these conclusions remain valid, we noticed that the sensitivity of model results to the bottom friction enhanced the importance of accurately representing the spatio-temporal distribution of bed roughness when investigating the morphodynamic interactions between real-world tidal inlets and their margins.

This paper examines the potential of an optical flow video-based technique to estimate wave-filtered surface currents in the nearshore where wave-breaking induced foam is present. This approach uses the drifting foam, left after the passage of breaking waves, as a quasi-passive tracer and tracks it to estimate the surface water flow. The optical signature associated with sea-swell waves is first removed from the image sequence to avoid capturing propagating waves instead of the desired foam motion. Waves are removed by applying a temporal Fourier low-pass filter to each pixel of the image. The low-pass filtered images are then fed into an optical flow algorithm to estimate the foam displacement and to produce mean velocity fields (i.e., wave-filtered surface currents). We use one week of consecutive 1-Hz sampled frames collected during daylight hours from a single fixed camera located at La Petite Chambre d’Amour beach (Anglet, SW France) under high-energy conditions with significant wave height ranging from 0.8 to 3.3 m. Optical flow-computed velocities are compared against time-averaged in situ measurements retrieved from one current profiler installed on a submerged reef. The computed circulation patterns are also compared against surf-zone drifter trajectories under different field conditions. Optical flow time-averaged velocities show a good agreement with current profiler measurements: coefficient of determination (r2)= 0.5–0.8; root mean square error (RMSE) = 0.12–0.24 m/s; mean error (bias) =−0.09 to −0.17 m/s; regression slope =1±0.15; coherence2 = 0.4–0.6. Despite an underestimation of offshore-directed velocities under persistent wave breaking across the reef, the optical flow was able to correctly reproduce the mean flow patterns depicted by drifter trajectories. Such patterns include rip-cell circulation, dominant onshore-directed surface flow and energetic longshore current. Our study suggests that open-source optical flow algorithms are a promising technique for coastal imaging applications, particularly under high-energy wave conditions when in situ instrument deployment can be challenging.

International audience; Foredunes are efficient natural coastal defenses acting as protective barriers during storm events. They also have the capacity to be an ecosystem hosting significant biodiversity. The economic development and/or recreational use of the foredune commonly results in a modification of natural functioning and the concomitant mixing of natural and anthropogenic processes. While the impact of human interventions on the short term evolution of coastal dunes is reasonably well understood, relatively less is known on their imprint at a scale of several decades. The Truc Vert beach-dune system (SW France), which has been exposed to various dune management strategies for more than a century, provides a relevant site to explore the respective contributions of natural and anthropogenic processes on coastal foredune evolution and the current coastal dune landscape. For this purpose, the coastal dune system was investigated using several approaches that combine ground penetrating radar (GPR), topographic data, aerial photographs and historical maps.A 20-m thick GPR sequence provides a stratigraphic record from which we detail ~150-year period of coastal dune change, including the initiation of the foredune. Results show a mixture of radar facies typical of natural aeolian erosion or deposition and radar facies that are the signature of human actions. These anthropogenic works include a large fence emplaced in 1860 to build and fix the foredune, and intense mechanical reshaping of the dune profile by bulldozers in 1972 followed by an intensive planting of vegetation. These various management strategies had a profound influence on coastal dune changes and, in turn, on the current coastal dune landscape. Historic archives documenting coastal dune works were critical to discriminate some of the radar facies, which could be wrongfully interpreted as natural erosion or deposition facies. Therefore, these results demonstrate the importance of coupling GPR and historical documentation wherever possible to determine, in part or fully, the contributions of human interventions and actions in modern dune evolution and morphological development.

International audience; Publicly available satellite imagery can now provide multi-decadal time series of shoreline data from local to global scale, enabling analysis of sandy beach shoreline variability across a spectrum of temporal scales. Such data can, however, be associated with large uncertainties, particularly for beaches experiencing a large tidal range (>2 m) and energetic incident waves. We use a decade of bi-monthly topographic surveys at the high-energy meso-macrotidal beach of Truc Vert, southwest France, and concurrent wave and water-level hindcast to investigate the uncertainties associated with satellite-derived time series of the shoreline position. We show that consideration of the water level and wave runup elevation is critical for accurately estimating waterline position and, in turn, shoreline position. At Truc Vert, including non-tidal water level residuals (e.g. wind-driven surge) and accounting for time- and elevation-varying beach slope for horizontal correction did not improve satellite-derived shoreline position. A new total water level threshold is proposed to maximize the number of usable images while minimizing errors. Accounting for wave runup and the new water level threshold at Truc Vert, the number of usable satellite images is doubled and shoreline position errors are at least halved compared to previous work at this site. Using the 1984–2019 reconstructed shoreline, we also show that the satellite-derived shoreline trends and interannual variability are in better agreement with field measurements. Although the approach proposed here needs to be tested on other sites in different tidal/wave forcing environments with different morphological and sediment characteristics, we anticipate that it will improve the temporal and spatial description of shoreline change on most surf tidal beaches where accurate continuous water level and wave hindcasts and/or observations are available.

In northern Europe, coastal dune remobilization by restoring natural processes is considered by some to maintain the coastal dune in chronically eroding sectors by migrating landward and to restore dune ecology. In wet climatic contexts, this nature-based solution has been shown to induce an increase in both sand bare areas and vegetation diversity. However, it has never been tested in the coastal dunes of southern Europe with a drier climate and, thus, more stressful conditions, where disturbance may inversely decrease vegetation diversity. An original experiment was set up in 2018 on a 4-km stretch of coastal dune in southwest France where Experimental Notches (EN) were excavated in the incipient foredune, referred to as West Experimental Notch (WEN), and in the established foredune, referred as to East Experimental Notch (EEN). Morphological and ecological responses were monitored using UAV photogrammetry and vegetation sampling along transects during two years with contrasted winter storm conditions. During the first winter characterized by calm wind conditions, a rapid filling of the WENs and the initiation of deposition lobes landward of the EENs were observed. Stronger winds during the second winter led to the development of deposition lobes of the EENs, increasing both their volume, up to 6 times, and their cross-shore elongation. The increase in disturbance induced by the notches had a significant impact on vegetation. New sandy bares were colonized by pioneer species leading to an increase in species richness and rejuvenation, in particular landward of the EENs. Although longer-term monitoring is required to draw conclusions, these results suggest that the excavation of foredune notches are able to re-establish an ecomorphological dynamic in the dunes of southwest France on the time scales of years, promoting landward sand transport and, thus, the foredune landward translation, while not threatening diversity. Such approach may become a relevant adaptation strategy to sea level rise and increased erosion in this region of the world.

Sandy beaches are highly dynamic environments buffering shores from storm waves and providing outstanding recreational services. Long-term beach monitoring programs are critical to test and improve shoreline, beach morphodynamics and storm impact models. However, these programs are relatively rare and mostly restricted to microtidal alongshore-uniform beaches. The present 16-year dataset contains 326 digital elevation models and their over 1.635 × 106 individual sand level measurements at the high-energy meso-macrotidal rip-channelled Truc Vert beach, southwest France. Monthly to bimonthly topographic surveys, which coverage progressively extended from 300 m to over 2000 m to describe the alongshore-variable changes, are completed by daily topographic surveys acquired during a 5-week field campaign. The dataset captures daily beach response at the scale of a storm to three large cycles of interannual variability, through the impact of the most energetic winter since at least 75 years and prominent seasonal erosion/recovery cycles. The data set is supplemented with high-frequency time series of offshore wave and astronomical tide data to facilitate its future use in beach research., Measurement(s) Topography Technology Type(s) GPS navigation system Factor Type(s) temporal interval Sample Characteristic - Environment sandy beach Sample Characteristic - Location Aquitaine Region Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.13070153

International audience; Mapping coastal dune vegetation is critical to understand dune mobility and resilience in the context of climate change, sea level rise, and increased anthropogenic pressure. However, the identification of plant species from remotely sensed data is tedious and limited to broad vegetation communities, while such environments are dominated by fragmented and small-scale landscape patterns. In June 2019, a comprehensive multi-scale survey including unmanned aerial vehicle (UAV), hyperspectral ground, and airborne data was conducted along approximately 20 km of a coastal dune system in southwest France. The objective was to generate an accurate mapping of the main sediment and plant species ground cover types in order to characterize the spatial distribution of coastal dune stability patterns. Field and UAV data were used to assess the quality of airborne data and generate a robust end-member spectral library. Next, a two-step classification approach, based on the normalized difference vegetation index and Random Forest classifier, was developed. Results show high performances with an overall accuracy of 100% and 92.5% for sand and vegetation ground cover types, respectively. Finally, a coastal dune stability index was computed across the entire study site. Different stability patterns were clearly identified along the coast, highlighting for the first time the high potential of this methodology to support coastal dune management.

Robin, N.; Billy, J.; Castelle, B.; Hesp, P.; Laporte-Fauret, Q.; Nicolae-Lerma, A.; Marieu, V.; Rosebery, D.; Bujan, S.; Destribats, B., and Michalet., 2020. Beach-dune recovery from the extreme 2013-2014 storms erosion at Truc Vert Beach, Southwest France: New insights from ground-penetrating radar. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 588–592. Coconut Creek (Florida), ISSN 0749-0208.Foredunes are efficient natural coastal defenses acting as protective barriers during storm events. A striking example is the response of coastal dunes along the Aquitaine Coast (SW France) to the 2013-2014 winter, which was the most energetic winter since at least the mid-20th century in most of the NE Atlantic. The aim of this study is to analyze the (partial) foredune recovery from the 2013-2014 winter at Truc Vert beach using ground-penetrating radar (GPR) and through the coastal dune internal structure. Geophysical data was compared with semi-monthly DGPS topographic data of the beach, UAV-photogrammetry and Airborne LiDAR surveys of the coastal dune. Three main recovery phases were identified from the 2013-2014 winter: (i) aggradation of the upper-beach and dune front-crest during the post-storm summer; (ii) distinct phases of incipient dune-foot development with dune-crest stability during the following year, and (iii) since 2016, aggradation of both dune-foot and dune front-crest tending towards an equilibrium profile. This study shows that GPR surveys, combined with chronological topographic data, is an efficient tool to provide comprehensive understanding of the beach-dune system evolution at a decadal scale. This approach opens new perspectives to document foredune recovery timing and magnitude along sandy coasts.

Ndour, A.; Ba, K.; Almar, A.; Almeida, P.; Sall, M.; Diedhiou; P.M., Floc'h, F.; Daly, C.; Grandjean, P.; Boivin, J-P; Castelle, B.; Marieu, V.; Biausque, M.; Detandt, G.; Tomety Folly, S.; Bonou, F.; Capet, X.; Garlan, T., Marchesiello, P.; Benshila, R.; Diaz, H.; Bergsma, E.; Sadio, M.; Sakho, I., and Sy, B.A., 2020. On the natural and anthropogenic drivers of the Senegalese (West Africa) low coast evolution: Saint Louis Beach 2016 COASTVAR experiment and 3D modeling of short term coastal protection measures. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 583-587. Coconut Creek (Florida), ISSN 0749-0208.West Africa's low and densely populated coasts crystallize most of the environmental and societal problems and resulting vulnerability. It is becoming urgent to document this coast and the natural and anthropogenic forces to understand its evolution. Saint Louis is a historic (World Heritage) city located on the Langue de Barbarie, a 10 km sandspit at the mouth of the Senegal River. Because of its location, it is vulnerable (erosion, flooding) to river and ocean variability. This intermediate barred microtidal beach is located in a storm-free intertropical environment, but is exposed to distant oblique energetic waves from high latitudes, causing one of the highest coastal drifts in the world (∼800,000 m3/year). As part of the COASTVAR project, an intensive international field experiment was conducted in Saint Louis from 4 to 13 December 2016 to quantify the natural protective role played by the sandbar, coastal currents and transient exchanges with the inner shelf. Many instruments have been deployed to measure waves, currents, bathymetry and topography. This article provides an overview on the objectives of the experiment, the deployment and the first results of the modeling of coastal protection strategy.

International audience; Most of rip-current field experiments have focused on persistent rips along rip-channeled sandy beaches or transient rips along reasonably alongshore-uniform surf-zone morphology, while experiments on rip flowing against structures are scarce. In October 2018, a 3-week field experiment was performed at Anglet beach, SW France, aiming at examining the dynamics of high-energy rip currents in complex settings. The beach is barred with prominent inherited geology, characterized by the presence of a 500-m headland and a natural submerged reef. A large array of in-situ instruments was deployed to capture the temporal and spatial variability of rip flow circulations, including ADCPs, surf-zone drifters and video monitoring. The latter allowed to identify a wide range of rip-flow patterns. Among these patterns, a high-intensity rip current flowing against the headland was a dominant feature for obliquely incident waves. Such a boundary rip current was driven by the deflection of the longshore current against the headland, peaking at 0.7 m/s (5-min time-and depth-averaged) 800-m offshore in 12-m depth for a moderate storm event with 4-m obliquely incident waves. Very-lowfrequency (O(1h) and O(30min)) fluctuations of this rip current were observed around low tide. Measurements of the vertical structure of the rip reveal that the deflection rip was more vertically-sheared as the water depth increases, with higher velocities near the surface, which is typical of a theoretical rip head structure.

Tidal sand dune dynamics is observed for two tidal cycles in the Arcachon tidal inlet, southwest France. An array of instruments is deployed to measure bathymetric and current variations along dune profiles. Based on the measurements, dune crest horizontal and vertical displacements are quantified and show important dynamics in phase with tidal currents. We observed superimposed ripples on the dune stoss side and front, migrating and changing polarity as tidal currents reverse. A 2D RANS numerical model is used to simulate the morphodynamic evolution of a flat non-cohesive sand bed submitted to a tidal current. The model reproduces the bed evolution until a field of sand bedforms is obtained that are comparable with observed superimposed ripples in terms of geometrical dimensions and dynamics. The model is then applied to simulate the dynamics of a field of large sand dunes of similar size as the dunes observed in situ. In both cases, simulation results compare well with measurements qualitatively and quantitatively. This research allows for a better understanding of tidal sand dune and superimposed ripple morphodynamics and opens new perspectives for the use of numerical models to predict their evolution.

Accretionary down-state beach sequence is a key element of observed nearshore morphological variability along sandy coasts. We present and analyze the first numerical simulation of such a sequence using a process-based morphodynamic model that solves the coupling between waves, depth-integrated currents, and sediment transport. The simulation evolves from an alongshore-uniform barred beach (storm profile) to an almost featureless shore-welded terrace (summer profile) through the highly alongshore-variable detached crescentic bar and transverse bar/rip system states. A global analysis of the full sequence allows determining the varying contributions of the different hydro-sedimentary processes. Sediment transport driven by orbital velocity skewness is critical to the overall onshore sandbar migration, while gravitational down-slope sediment transport acts as a damping term inhibiting further channel growth enforced by rip-flow circulation. Accurate morphological diffusivity and inclusion of orbital velocity skewness opens new perspectives in terms of morphodynamic modeling of real beaches.

Fixed video camera systems are consistently prone to importune motions over time due to either thermal effects or mechanical factors. Even subtle displacements are mostly overlooked or ignored, although they can lead to large geo-rectification errors. This paper describes a simple and efficient method to stabilize an either continuous or sub-sampled image sequence based on feature matching and sub-pixel cross-correlation techniques. The method requires the presence and identification of different land-sub-image regions containing static recognizable features, such as corners or salient points, referred to as keypoints. A Canny edge detector ( C E D ) is used to locate and extract the boundaries of the features. Keypoints are matched against themselves after computing their two-dimensional displacement with respect to a reference frame. Pairs of keypoints are subsequently used as control points to fit a geometric transformation in order to align the whole frame with the reference image. The stabilization method is applied to five years of daily images collected from a three-camera permanent video system located at Anglet Beach in southwestern France. Azimuth, tilt, and roll deviations are computed for each camera. The three cameras showed motions on a wide range of time scales, with a prominent annual signal in azimuth and tilt deviation. Camera movement amplitude reached up to 10 pixels in azimuth, 30 pixels in tilt, and 0.4° in roll, together with a quasi-steady counter-clockwise trend over the five-year time series. Moreover, camera viewing angle deviations were found to induce large rectification errors of up to 400 m at a distance of 2.5 km from the camera. The mean shoreline apparent position was also affected by an approximately 10–20 m bias during the 2013/2014 outstanding winter period. The stabilization semi-automatic method successfully corrects camera geometry for fixed video monitoring systems and is able to process at least 90% of the frames without user assistance. The use of the C E D greatly improves the performance of the cross-correlation algorithm by making it more robust against contrast and brightness variations between frames. The method appears as a promising tool for other coastal imaging applications such as removal of undesired high-frequency movements of cameras equipped in unmanned aerial vehicles (UAVs).

International audience; 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.

The present study documents the interactions between Southern Hemisphere high-latitude (Antarctica & Southern Ocean), southern Indian Ocean subtropics (Agulhas leakage) and Asian summer monsoon. The study uses SST and sea-ice reconstructions along with diatom absolute abundances and diatom biometry from two sediment cores located at the Subantarctic Front (SAF) and Antarctic Polar Front (APF) in the southwest Indian sector of the Southern Ocean. Sea-ice records suggest the presence of the mean winter sea ice limit at around the modern APF location during MIS 2 and MIS 4 and episodic and unconsolidated winter sea ice far north as ∼43°S during LGM, when the SSTs were lowest. Higher diatom productivity and larger mean sizes of F. kerguelensis and T. lentiginosa recorded at the northern core site during the glacial stages suggest a northward shift of the APF. A decrease in diatom productivity and sizes at the southern core site highlights stratified Permanent Open Ocean Zone (POOZ) surface waters in response to longer sea-ice presence during the glacial stages. The comparative study between the records of Southern Hemisphere high-latitude and Asian summer monsoon climate variability revealed that the Asian summer monsoon variability could have been more likely forced by low latitude insolation gradient changes and supported by Antarctic climate changes via meridional shifts of the fronts and sea ice. The past changes in the intensity of Asian summer monsoon along with the Southern Ocean frontal variation might have influenced the Southern Indian Ocean surface circulation by changing the Agulhas leakage intensity.

In this paper, coastal dune data are collected at Truc Vert, SW France, using photogrammetry via Unmanned Aerial Vehicles (UAVs). A low-cost GoPro-equipped DJI Phantom 2 quadcopter and a 20 MPix camera-equipped DJI Phantom 4 Pro quadcopter UAVs were used to remotely sense the coastal dune morphology over large spatial scales (4 km alongshore, i.e., approximately 1 km2 of beach-dune system), within a short time (less than 2 h of flight). The primary objective of this paper is to propose a low-cost and replicable approach which, combined with simple and efficient permanent Ground Control Point (GCP) set-up, can be applied to routinely survey upper beach and coastal dune morphological changes at high frequency (after each storm) and high resolution (0.1 m). Results show that a high-resolution and accurate Digital Surface Model (DSM) can be inferred with both UAVs if enough permanent GCPs are implemented. The more recent DJI Phantom 4 gives substantially more accurate DSM with a root-mean-square vertical error and bias of 0.05 m and &minus, 0.03 m, respectively, while the DSM inferred from the DJI Phantom 2 still largely meets the standard for coastal monitoring. The automatic flight plan procedure allows replicable surveys to address large-scale morphological evolution at high temporal resolution (e.g., weeks, months), providing unprecedented insight into the coastal dune evolution driven by marine and aeolian processes. The detailed morphological evolution of a 4-km section of beach-dune system is analyzed over a 6-month winter period, showing highly alongshore variable beach and incipient foredune wave-driven erosion, together with wind-driven inland migration of the established foredune by a few meters, and alongshore-variable sand deposition on the grey dune. In a context of widespread erosion, this photogrammetry approach via low-cost flexible and lightweight UAVs is well adapted for coastal research groups and coastal dune management stakeholders, including in developing countries where data are lacking.

We compare different methods to reconstruct the surface elevation of irregular waves propagating outside the surf zone from pressure measurements at the bottom. The traditional transfer function method (TFM), based on the linear wave theory, predicts reasonably well the significant wave height but cannot describe the highest frequencies of the wave spectrum. This is why the TFM cannot reproduce the skewed shape of nonlinear waves and strongly underestimates their crest elevation. The surface elevation reconstructed from the TFM is very sensitive to the value of the cutoff frequency. At the individual wave scale, high-frequency tail correction strategies associated with this method do not significantly improve the prediction of the highest waves. Unlike the TFM, the recently developed weakly-dispersive nonlinear reconstruction method correctly reproduces the wave energy over a large number of harmonics leading to an accurate estimation of the peaked and skewed shape of the highest waves. This method is able to recover the most nonlinear waves within wave groups which some can be characterized as extreme waves. It is anticipated that using relevant reconstruction method will improve the description of individual wave transformation close to breaking.

International audience; Disturbance increases ecosystem functioning in productive habitats but its effect in stressful conditions is less documented, although this is crucial for understanding the resilience of disturbed systems to natural and anthropogenic disturbances. Our goal is to assess the influence of physical disturbance for ecosystem functioning in coastal sand dunes. We set up an experimental design, including two treatments in four blocks, in a four km-long dune site from South West France. We simulated physical disturbance from marine and wind origin, digging Experimental Notches (EN), in the incipient (West EN treatment) and established foredunes (East EN treatment), respectively and compared the effects of EN to controls along transects including 13 positions from the beach to the transition dune behind ENs. We sampled litter decomposition rate, elevation variation, wind abrasion, sand grain size and vegetation composition. We also used drones to quantify sand deposition sheets during severe winter storms. Litter decomposition rate was the highest where sand accumulated the most, at the ecotone between the established foredune and transition dune and in the East EN treatment. This increase of ecosystem functioning was correlated to wind patterns. However, there was also a strong alongshore variability, with important sand deposition sheets occurring in some blocks depending on dune geomorphology. Vegetation composition was mainly influenced by shoreline distance, but also by the block and EN treatment, with a strong interaction between these three effects. We conclude that physical disturbance increase ecosystem functioning in the stressful conditions of the Atlantic sand dunes, only when sand accumulates, whereas excessive disturbances enhancing sand erosion are not favorable for ecosystem functioning.

Accurate methods to predict wave runup are of critical importance to coastal engineers. Extreme runup during storm conditions can present a danger to coastal infrastructure and lead to dune overtopping and erosion, overwash. of barrier beaches and is an essential design parameter for certain coastal protection works. This paper uses data obtained during the unique BARDEX II prototype-scale laboratory experiment on a reflective sand barrier to investigate wave runup and overwash. Runup measurements were acquired using ultrasonic altimeters, 2D scanning Lidar and camera systems under controlled wave and water level conditions. It was found that extreme runup (R-2%) scaled well with the deepwater Irribarren number where the beach slope was taken to be the beach face gradient. The lower limit of the swash zone, defined by the 2% wave rundown limit was also found to be a function of the Irribarren number and was consistently below the SWL in all tests. The high resolution free-surface measurements obtained using the ultrasonic altimeters and Lidar enabled an analysis of the dynamics of individual swash events. It was found that the vertical runup excursion of each swash correlated strongly with the height of the bore at collapse, and could be predicted based on the assumption of a conversion of potential to kinetic energy at bore collapse. (C) 2015 Elsevier B.V. All rights reserved. HYDRALAB IV grant, contract no. 261520; Australian Research Council Discovery Grant (DP110101176)

The morphodynamic evolution of subaqueous sand dunes is investigated, using a 2-D Reynolds-averaged Navier-Stokes numerical model. A laboratory experiment where dunes are generated under stationary unidirectional flow conditions is used as a reference case. The model reproduces the evolution of the erodible bed until a state of equilibrium is reached. In particular, the simulation exhibits the different stages of the bed evolution, e.g., the incipient ripple generation, the nonlinear bed form growing phase, and the dune field equilibrium phase. The results show good agreement in terms of dune geometrical dimensions and time to equilibrium. After the emergence of the first ripple field, the bed growth is driven by cascading merging sequences between bed forms of different heights. A sequence extracted from the simulation shows how the downstream bed form is first eroded before merging with the upstream bed form. Superimposed bed forms emerge on the dune stoss sides during the simulation. An analysis of the results shows that they emerge downstream of a slight deflection on the dune profile. The deflection arises due to a modification of the sediment flux gradient consecutive to a reduction in the turbulence relaxation length while the upstream bed form height decreases. As they migrate, superimposed bed forms grow on the dune stoss side and eventually provoke the degeneration of the dune crest. Cascading merging sequences and superimposed bed forms dynamics both influence the dune field evolution and size and therefore play a fundamental role in the dune field self-organization process.

The use of shore-based video systems has become a very popular and accessible low-cost tool for coastal monitoring given their capability to deliver continuous and high-resolution temporal data over large enough spatial scales. However, the reliability of the final image products can be compromised by external factors, sometimes overlooked, that can alter the image geometry over time. In particular, unwanted camera movement, produced either by thermal or mechanical effects, can lead to significant geo-rectification errors if not properly corrected. This study addresses an alternative straightforward method to stabilize an either continuous or subsampled image sequence based on state-of-the-art techniques and available routines.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/xX1CrvPQpK8

We present field measurements of nearshore currents at a high-energy mesotidal beach with the presence of a 500-m headland and a submerged reef. Small changes in wave forcing and tide elevation were found to largely impact circulation patterns. In particular, under 4-m oblique wave conditions, our measurements indicate the presence of an intense low-frequency fluctuating deflection rip flowing against the headland and extending well beyond the surf zone. An XBeach model is further set up to hindcast such flow patterns.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/EiqnjBIkWJE

Laporte-Fauret, Q.; Castelle, B.; Marieu, V.; Bujan, S.; Michalet, R., and Rosebery. D., 2020. Coastal dune morphology evolution combining Lidar and UAV surveys, Truc Vert beach 2011-2019. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 163–167. Coconut Creek (Florida), ISSN 0749-0208.In the context of climate change, coastal dunes, which provide significant ecosystem services, are one of the most vulnerable coastal environments. Moreover, the lack of high resolution and large spatial scale data limits our understanding of coastal dunes, which are subject to important morphological variations over a wide range of spatial and temporal scales. This study is focused on combining non-intrusive remote sensing methods including a series of historical aerial photographs, airborne Lidar and UAV surveys in order to better understand the coastal dune morphodynamic on a wide range of spatial and temporal scales. On the time scale of decades, aerial photos indicate a reasonably stable coastal dune position, with large anthropogenic reprofiling and vegetation planting resulting in a reasonably alongshore-uniform morphology. On shorter time scales, the combination of Lidar and UAV 4-km surveys between 2011 and 2019 shows a quasi-steady dune volume increase by approximately 64 m3/m, associated with a foredune crest growth and a landward migration reaching 1.1 m and 0.54 m in average, respectively. On the other hand, the dune foot shows a more complex dynamics, dominated by a large landward migration (shoreline erosion) of approximately 20 m during the outstanding winter of 2013/2014 followed by a slow seaward migration (recovery). This study provides new insight into coastal dune morphological changes driven by both Aeolian and marine processes from the time scale of storm to approximately a decade, highlighting the relevance of UAV surveys for such applications.