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This paper describes the methodology followed to implement social distancing recommendations in the COVID-19 context along the beaches of the coast of Gipuzkoa (Basque Country, Northern Spain) by means of automated coastal videometry. The coastal videometry network of Gipuzkoa, based on the KostaSystem technology, covers 14 beaches, with 12 stations, along 50 km of coastline. A beach user detection algorithm based on a machine learning approach has been developed allowing for automatic assessment of beach attendance in real time at regional scale. For each beach, a simple classification of occupancy (low, medium, high, and full) was estimated as a function of the beach user density (BUD), obtained in real time from the images and the maximum beach carrying capacity (BCC), estimated based on the minimal social distance recommended by the authorities. This information was displayed in real time via a web/mobile app and was simultaneously sent to beach managers who controlled the beach access. The results showed a strong receptivity from beach users (more than 50.000 app downloads) and that real time information of beach occupation can help in short-term/daily beach management. In the longer term, the analysis of this information provides the necessary data for beach carrying capacity management and can help the authorities in controlling and in determining their maximum capacity. This work has been supported by the Gipuzkoa Provincial Council, the Zarautz Council, and the Donostia-San Sebastian Council. The au-thors would like to acknowledge the technical assistance and data pro-vided by the Bizkaia Provincial Council. Roland Garnier acknowledges funding from the Provincial Council of Gipuzkoa through the Fellows Gipuzkoa Programme (Ref: 2020-FELL-000007-01)

A field campaign was undertaken at Somo beach in northern Spain, with the aim of understanding the processes involved in the generation of low frequency swash. Taking the parameterization presented in earlier studies as a starting point, a novel empirical model was defined based on a database which included datasets from Somo beach and from 9 other experiments. This new parameterization was obtained by relating the horizontal cross-shore component of the infragravity swash to the foreshore slope and the morphodynamic beach state through the non-dimensional fall velocity parameter. The role of previous sea states when determining the morphodynamic beach state was also assessed. A strong correlation between low frequency oscillations and the morphology of the beach was verified, resulting in a substantial improvement over existing infragravity swash predictions proposed in the literature.

Laida beach, located at the Oka estuary mouth (Urdaibai Biosphere Reserve) in the southeastern region of the Bay of Biscay, suffered the impact of a severe succession of storms during the first months of 2014. As a result of the erosion induced by these events, the beach lost its supratidal zone almost completely. The absence of a supratidal beach generated an impact on the recreational use of the beach during the summer 2014, and represented a potential impact for the coming summer 2015. Furthermore, it resulted in an overexposure and damage of adjacent infrastructures due to impinging strong waves. Therefore, the competent authorities, in coordination, decided to take action in order to nourish the supratidal zone of this beach. The solution adopted combined two different actions. The first one accomplished in spring of 2015, consisted in the mobilization of 44,800 m3 of sand from an area of 35,200 m2 equal to the 7% of the intertidal zone of Laida beach interpreted as the existing surface between the average low and high tidal limits, to the zone next to the eastern rocky beach contour. This action successfully resulted in an increase of the supratidal beach for the entire summer 2015 without negatively perturbing the morphological system. The second action was somewhat experimental and consisted in the mechanical plough of the previously existing intertidal low-amplitude ridges with the aim of increasing the sand transport toward the supratidal beach. Although this action did not lead to the increase of the supratidal beach, it seems to have resulted in an acceleration of the natural onshore migration of the bars. The objective of this contribution is to describe the morphodynamical response of the estuarine mouth after the performed actions with special emphasis on the evolution of extracted sites and the supratidal Laida beach area. The information here presented represents an innovative step in the understanding of the complex mechanisms driving the supratidal beach formation at the mouth of Oka estuary and by extension of the majority of the estuaries of the southeastern Bay of Biscay.

This study investigates short-crested waves in the surf zone by using the mesh-free Smoothed Particle Hydrodynamics model, GPUSPH. The short-crested waves are created by generating intersecting wave trains in a numerical wave basin with a beach. We first validate the numerical model for short-crested waves by comparison with large-scale laboratory measurements. Then short-crested wave breaking over a planar beach is studied comprehensively. We observe rip currents as discussed in Dalrymple (1975) and undertow created by synchronous intersecting waves. The wave breaking of the short-crested wavefield created by the nonlinear superposition of intersecting waves and wave-current interaction result in the formation of isolated breakers at the ends of breaking wave crests. Wave amplitude diffraction at these isolated breakers gives rise to an increase in the alongshore wave number in the inner surf zone. Moreover, 3-D vortices and multiple circulation cells with a rotation frequency much lower than the incident wave frequency are observed across the outer surf zone to the beach. Finally, we investigate vertical vorticity generation under short-crested wave breaking and find that breaking of short-crested waves generates vorticity as pointed out by Peregrine (1998). Vorticity generation is not only observed under short-crested waves with a limited number of wave components but also under directional wave spectra.

A user-friendly system designed to understand local littoral processes and design/evaluate coastal interventions, named the Coastal Modeling System (SMC e ) is presented. The system, which comprises a set of numerical models, state-of-the-art methodologies and numerical databases, is prepared to provide a method for coastal practitioners, researchers and decision-makers to address coastal issues, such as erosion and flooding or to evaluate coastal defense structures. The system incorporates a method to transfer numerically generated, calibrated and validated wave series to the surf zone; to estimate the sediment littoral drift by means of up-to-date formulations; and to estimate the flooding level and impacts such as those produced by climate change. In this paper, these skills are detailed. The system, which is adaptable to any coastal region, was implemented for the Brazilian coast (SMC-Brasil). The implementation includes databases and methodological adaptations to local characteristics, a dissemination plan and the development of several study cases.

This review highlights the important role of the depth-averaged sediment concentration (DASC) to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern: beach cusps, surf zone transverse and crescentic bars, and shoreface-connected sand ridges. We present a formulation and methodology, based on the knowledge of the DASC (which equals the sediment load divided by the water depth), that has been successfully used to understand the characteristics of these features. These sand bodies, relevant for coastal engineering and other disciplines, are located in different parts of the coastal zone and are characterized by different spatial and temporal scales, but the same technique can be used to understand them. Since the sand bodies occur in the presence of depth-averaged currents, the sediment transport approximately equals a sediment load times the current. Moreover, it is assumed that waves essentially mobilize the sediment, and the current increases this mobilization and advects the sediment. In such conditions, knowing the spatial distribution of the DASC and the depth-averaged currents induced by the forcing (waves, wind, and pressure gradients) over the patterns allows inferring the convergence/divergence of sediment transport. Deposition (erosion) occurs where the current flows from areas of high to low (low to high) values of DASC. The formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features, but the physical mechanisms for their migration, their finite-amplitude behavior and their decay can also be explored.

A system of 15 small-scale finger bars has been observed, by using video imagery, between 23 June 2008 and 2 June 2010. The bar system is located in the intertidal zone of the swell-protected beaches of El Puntal Spit, in the Bay of Santander (northern coast of Spain). The bars appear on a planar beach (slope = 0.015) with fine, uniform sand (D50 = 0.27 mm) and extend 600 m alongshore. The cross-shore span of the bars is determined by the tidal horizontal excursion (between 70 and 130 m). They have an oblique orientation with respect to the low-tide shoreline; specifically, they are down-current-oriented with respect to the dominant sand transport computed (mean angle of 26° from the shore normal). Their mean wavelength is 26 m and their amplitude varies between 10 and 20 cm. The full system slowly migrates to the east (sand transport direction) with a mean speed of 0.06 m day-1, a maximum speed in winter (up to 0.15 m day-1) and a minimum speed in summer. An episode of merging has been identified as bars with larger wavelength seem to migrate more slowly than shorter bars. The wind blows predominantly from the west, generating waves that transport sediment across the bars during high-tide periods. This is the main candidate to explain the eastward migration of the system. In particular, the wind can generate waves of up to 20 cm (root-mean-squared wave height) over a fetch that can reach 4.5 km at high tide. The astronomical tide seems to be important in the bar dynamics, as the tidal level changes the fetch and also determines the time of exposure of the bars to the surf-zone waves and currents. Furthermore, the river discharge could act as input of suspended sediment in the bar system and play a role in the bar dynamics.

In this work we present an analysis of the effect of the processes related to beach morphodynamic state on high frequency (incident band) swash oscillations. The effect of wave reflection and saturation on swash oscillations was assessed based on new field data measured under different morphodynamic conditions on the northern coast of Spain. Data measured under more reflective conditions indicated the occurrence of a cross-shore partial standing structure, a process which was observed earlier using laboratory and numerical data. Part of the dataset data showed saturation in accordance to limits between reflection and saturation established in previous works. The new measured data was added to a large database compiled from previous works, and, assuming the hypothesis that the reflection and saturation processes are related to the beach state, a new parameterization was established to calculate the incident swash under different morphodynamic conditions. Our results indicate that the role of the foreshore slope and wave steepness varies according to the morphodynamic beach state. The proposed formula showed a reasonable correlation with measured data ( ρ 2 =0.56 and RMSE = 0.41 m), and improvements in S i n c predictions were observed for all beach types when compared to existing formulas.

Gainza, J.; Garnier, R.; Nunez, P.; Jaramillo, C.; Gonzalez, E.M.; Medina, R.; Liria, P.; Epelde, I.; Uriarte, A., and Monge-Ganuzas, M., 2019. Accelerating beach recovery by plowing the i...

This research has been funded by the Spanish government through the research projects CTM2012-35398 (cofunded by FEDER, U.E.) and CTM2015-66225-C2-1-P (MINECO/FEDER). This review summarizes the morphological characteristics and dynamics of nearshore sand bars observed in the surf zone of sandy beaches worldwide, with length scales ranging from tens to hundreds of meters and time scales ranging from hours to weeks. They include shore-parallel bars (straight and crescentic) and transverse bars of different types. Furthermore, the present knowledge on the physical processes behind their formation and development is discussed.

This contribution reviews the existing observations of nearshore sand bars (including shore-parallel straight and crescentic bars, and transverse bars) on western Mediterranean beaches. In general, their morphological characteristics and the dynamic processes they experience do not differ qualitatively from those measured on other coasts. A peculiarity of the bars in western Mediterranean is their slower dynamics due to the milder climate, characterized by short-period waves and long episodes of small waves. During these calm periods, the morphology is arrested because there is no significant sediment transport.

The wave runup, defined as the vertical oscillation of the water edge at the coast, is commonly used as criterion for coastal design projects and flooding/erosion risk analysis. Due to the complexity of nearshore wave processes, most runup studies are based on empirical approaches which directly relate these oscillations to the beach and offshore wave characteristics. However, there is still considerable debate about just how runup is related to these environmental parameters, as well as about the range of application of empirical models due to site specific conditions. Recent works emphasized the importance of including site specific conditions to reduce the scatter in available parameterizations. Parameters related to beach characteristics like the sediment size, the amount of reflection, wave spectral shape and morphodynamic beach state may improve runup predictions (Poate et al., 2016, Guza and Feddersen, 2012).

A beach sedimentation problem is presented. This problem involves assessing the impact and importance of land-based sediment sources (erosion from gullies during intense rainfall, and directly from cliffs by wave action) on morphodynamic changes at a beach. The site motivating the study is La Barrosa beach, Spain. The location is described and a discussion of possible approaches presented. A schematized representation of a gully and beach is used. A hybrid approach is adopted, in which sediment deposition by run-off (and to a lesser extent cliff erosion) is modelled using wave-resolving hydrodynamic models. It is found that significant amounts of sediment can be supplied by gully run-off during storms. For small run-offs the swash can be involved in determining the region of deposition within the gully area, with longer period motions favouring increased sediment deposition at gully sides. Simulations also show that swash motions can erode gully sides, and the presence of a cross-shore jet associated with the swash and gullies can also impact on sediment movement. Longer term, wave-averaged morphodynamic modelling (without a swash zone) indicates that run-off deposits subsequently diffuse laterally, whether under normal or oblique incidence. These simulations also reveal the possibility of underlying crescentic bar-shedding dynamics that may pertain during changes of wave conditions.

International audience; The main objective of the VULSACO (VULnerability of SAndy COasts to climate change and anthropic pressure) project was to investigate present day and potential future vulnerability of sandy coasts at the 2030 horizon, i.e. on a time scale related to climate variability. The method, based on a multidisciplinary approach bringing together geologists, geographers, physicists , social psychologists, engineers and stakeholders, was structured around 4 axes: field data analysis; numerical modelling; analysis of governance and stakeholder perceptions; and development of vulnerability indexes. This approach was designed to investigate vulnerability at a local scale and was applied to 4 contrasting beaches located in France: Sète Lido (Mediterranean Sea), Truc Vert and La Tresson beaches (Atlantic Ocean), and Dewulf (English Channel). The results focus on decadal and multi-annual beach trends at the Truc Vert beach site. There is almost no trend in beach volume at Truc Vert beach, although there is a variation in this parameter on a cycle of 2 to 3 yr, with variations related to wave energy and probably to indexes of climate variability. Numerical modelling identified the sensitivity of beach responses to changes in wave height and direction, especially in terms of subtidal morphology and the potential development of shoreline instability. Together with the observed offshore wave angle at the Biscay Buoy, these model results suggest that a potential change in wave angle due to climate variability could significantly modify the bars' morphology. The combination of data analysis and numerical modelling contributed to the development of vulnerability indexes designed for sandy coasts, which take into account climate-dependant variables such as waves. This allowed the differentiation of the sites in terms of vulnerability to erosion: Sète Lido and Truc Vert beach were the most and least vulnerable sites, respectively. These indexes help in identifying the dominant components of beach vulnerability, and provide potential for the study of how anthropogenic factors affect vulnerability. The study of stakeholder perceptions and decision-making with regard to climate-related risk also highlighted potential anthropogenic effects on beach vulnerability, and identified possible site-specific outcomes.

We indentify the presence of an intertidal finger bar system in the swell-protected beaches of El Puntal Spit (Bay of Santander, Spain). The Horus video monitoring system allows us to perform a 2 year survey (June 2008- June 2010). We found that the bar system persists during the survey period with a wavelength of about 25 m and an oblique orientation with respect to the shore. Moreover, the analysis of the position of a particular bar (Bar 10) shows a movement characterized by a net migration to the west and a faster oscillation to the east or the west. Preliminary climate data analysis and morphological model results show that tide seems to be a forcing in the motion of the bars but also wind and waves generated by the wind probably affects the bar dynamics.

International audience; Les modèles morphodynamiques de plages permettent de simuler l’apparition de figuressédimentaires rythmiques par auto-organisation. Les approches les plus classiquesconsistent à simuler la formation de barres en croissant par auto-organisation à partird’une barre rectiligne. En nature, ce type d’évolutions morphodynamiques se produitpendant les séquences down-state selon la classification de WRIGHT & SHORT(1984). Les mécanismes contrôlant la disparition des figures rythmiques pendant lesséquences up-state sont mal compris aujourd’hui et les modèles actuels n’arrivent pas àles reproduire correctement. Dans cet article, nous étudions l’influence du courantlongshore sur la disparition des figures rythmiques. Le modèle utilisé est appeléMORFO55. C’est un modèle 2DH non linéaire. Une configuration de plage idéaliséeavec une barre parallèle à la côte est considérée. La phase préliminaire des simulationsconsiste à former des barres en croissant par auto-organisation (i.e. évolution observéependant les séquences down-state). Une fois que les barres croissant sont formées etqu’un équilibre dynamique est atteint, la direction des vagues au large est modifiée pourengendrer un courant parallèle à la plage. Dans les configurations testées, l’apparitiond’un courant longshore est rapidement suivie par la disparition des figures rythmiques.Il semble donc que les courants parallèles à la plage peuvent jouer un rôle significatifsur la disparition des barres en croissants pendant les séquences up-state. La réponse dusystème est analysée afin de préciser les mécanismes contrôlant ces évolutions.

International audience; Nearshore sandbars are often characterized by three-dimensional bed patterns. To analyze the influence of wave direction on the morphological response of a double sandbar system, this paper uses the 2DH nonlinear surf zone model MORFO55. Depending on wave direction, different morphologies emerge as free instabilities. These morphological responses differ in terms of geometry (shape of the alongshore rhythmic patterns) and temporal evolution. Nearly shore-normal waves favor the emergence of crescentic patterns along both the inner and outer bars. These instabilities arise from ''bedsurf'' coupling, which is the positive feedback between the bed perturbations and the wave-breaking patterns resulting in rip-cell circulations. The systemis stable for intermediate wave incidence angles because the longshore current shifts the circulation cells alongshore, inhibiting the growth of initial perturbations. For larger angles, the system is again unstable. Two types of oblique bars emerge. The first type forms between the crest of the inner bar and the shoreline and is oriented down-current. The second type is superimposed on the two bars and is oriented up-current. Although previous studies attribute the formation of oblique bars to the deflection of the longshore current (that is a cross-shore process), we show here that this mechanism contributes to the bar formation but the growth rate is mainly governed by a longshore process. Specifically, it is the positive feedback between the bathymetry and the longshore gradient of the sediment concentration. Finally, interactions between the patterns in the two shore-parallel bars are analyzed. In the observed configurations, the influence is always one way as the inner-bar dynamics never influence those of the outer bar. The outer-bar instabilities cause undulations in the inner bar when the outer-bar instabilities grow faster than those of the inner bar.

Large beach cusps (LBC, wavelength of ~ 30 m) are intertidal features that can alternately exist in the swash and in the inner surf zone due to tidal sea level changes. They have a larger cross-shore extent (up to 50 m) than traditional cusps. This extent has been explained by a shift of the swash zone during falling tide. The cusps immerse at rising tide and previous studies indicate that surf zone processes are exclusively destructive. Here, the behaviour of large beach cusps in the inner surf zone is investigated by using a 2DH morphological numerical model applied to Trafalgar Beach (Cádiz, Spain). The model results indicate that the inner surf zone processes do not always destroy the cusps but can in fact reinforce them by considering neither the swash processes nor the tidal changes. More generally, in conditions favouring the presence of the LBC the surf zone of a beach can be unstable, leading to the formation of transverse/oblique sand bars that can have characteristics similar to the LBC. Thus, in principle, the LBC could emerge not only due to swash zone morphodynamics but also due to surf zone morphodynamics or a combination of both. Las formas cuspidales de grandes dimensiones (LBC, longitudes de onda ~30 m) constituyen un sistema morfológico rítmico a lo largo de la playa que tiene una parte que se encuentra alternativamente en la zona de swash (flujo/reflujo) y en la zona de rompientes interna debido a los cambios del nivel del mar. Tienen una distancia de penetración de hasta 50 m, superior por tanto a la de las cúspides ordinarias. Esta elongación parece debido a la traslación de la zona de swash durante la marea descendente. En marea ascendente estas estructuras están sumergidas y los estudios previos consideran que los procesos de la zona de rompientes las destruyen. En este trabajo se analiza el comportamiento de estas formas en la zona de rompientes de la playa de Trafalgar (Cádiz) usando un modelo numérico morfológico 2DH. Los resultados muestran que, sin considerar ni los procesos de swash ni el cambio de marea, los procesos de la zona de rompientes no necesariamente destruyen LBC, sino que pueden reforzarlas. De forma más general, en condiciones favorables a la presencia de LBC, se pueden formar barras de arena con características similares a LBC debido a procesos de auto-organización en la zona de rompientes. The work of R. Garnier was supported by the University of Nottingham and is part of the Spanish Government project under contract CTM2006-08875. The Spanish Ministry of Science and Education (Project BORRASCAS CTM2005-06583) and the Junta de Andalucía (Projects P05-RNM-968 and P06-RNM-1573) funded part of this research.

In this contribution a discussion is presented on the development of self-organized coastal morphodynamic patterns which are due to the joint action of gradients in the depth-integrated concentration and the flow. This is done in the context of a depth-averaged shallow water model. Two physical mechanisms produce deposition-erosion patterns. Deposition either occurs where the current flows from high to low depth-averaged concentrations (1) or where the flow diverges (2). If flow conditions are quasi steady (i.e., the time scale on which bedforms evolve is much larger than the hydrodynamic time scales) only the former mechanism contributes to the formation of bottom patterns.

[1] The instability leading to the formation of rip currents in the nearshore for normal waves on a nonbarred, nonerodible beach is examined with a comprehensive linear stability numerical model. In contrast to previous studies, the hypothesis of regular waves has been relaxed. The results obtained here point to the existence of a purely hydrodynamical positive feedback mechanism that can drive rip cells, which is consistent with previous studies. This mechanism is physically interpreted and is due to refraction and shoaling. However, this mechanism does not exist when the surf zone is not saturated because negative feedback provided by increased (decreased) breaking for positive (negative) wave energy perturbations overwhelms the shoaling/refraction mechanism. Moreover, turbulent Reynolds stress and bottom friction also cause damping of the rip current growth. All the nonregular wave dissipations examined give rise to these hydrodynamical instabilities when feedback onto dissipation is neglected. When this feedback is included, the dominant effect that destroys these hydrodynamical instabilities is the feedback of the wave energy onto the dissipation. It turns out that this effect is strong and does not allow hydrodynamical instabilities on a planar beach to grow for random seas.

International audience; Work undertaken in the EU HUMOR project on morphodynamical modelling, particularly with regard to simulating and understanding rhythmic surf zone bars and related morphodynamic self-organization, is presented. These features are reviewed and their engineering context stated. Hydrodynamical and morphodynamical models developed and/or applied within the HUMOR project in order to address these issues are briefly presented. The linear stability modelling concept and stability studies using fully nonlinear models are contrasted. The stability of a shoreparallel bar under normal or oblique wave incidence is chosen as a test case for the different models. The results are compared and discussed. Lastly, modelling efforts and main results from the project are summarized. Recommendations for further work are made.

Crescentic bars emerge as free instabilities of the coupling between topography and water motion. Their long term behaviour will be studied in the real case of the French Aquitaine beaches by using the 2DH numerical model MORFO55. For the first time, the equilibrium state of a crescentic bar system is obtained, in the case of steady incident wave conditions. Due to non linear interactions, the final wave length of the system does not inevitably increase with the incident wave height. Finally, the study of variable incident wave conditions suggests that the equilibrium state may depend on the initial conditions.

International audience; The growth of megacusps as shoreline instabilities is investigated by examining the coupling between wave transformation in the shoaling zone, longshore transport in the surf zone, cross-shore transport, and morphological evolution. This coupling is known to drive a potential positive feedback in case of very oblique wave incidence, leading to an unstable shoreline and the consequent formation of shoreline sandwaves. Here, using a linear stability model based on the one-line concept, we demonstrate that such instabilities can also develop in case of low-angle or shore-normal incidence, under certain conditions (small enough wave height and/or large enough beach slope). The wavelength and growth time scales are much smaller than those of high-angle wave instabilities and are nearly in the range of those of surf zone rhythmic bars, O(10^2-10^3 m) and O(1-10 days), respectively. The feedback mechanism is based on: (1) wave refraction by a shoal (defined as a cross-shore extension of the shoreline perturbation) leading to wave convergence shoreward of it, (2) longshore sediment flux convergence between the shoal and the shoreline, resulting in megacusp formation, and (3) cross-shore sediment flux from the surf to the shoaling zone, feeding the shoal. Even though the present model is based on a crude representation of nearshore dynamics, a comparison of model results with existing 2DH model output and laboratory experiments suggests that the instability mechanism is plausible. Additional work is required to fully assess whether and under which conditions this mechanism exists in nature.

International audience; Breaker bars in the surf zone of sandy beaches generally evolve between straight bars parallel to the shore and meandering crescentic bars associated with intense (dangerous) currents flowing seaward through rip channels. Understanding the behavior of such systems is fundamental as they control the entire surf zone dynamics, the shape of the coastline, and the exchange of floating material with the shoreface. Although the mechanisms behind the meandering of an originally straight bar have been studied extensively, a clear physical explanation on the crescentic bar straightening was missing. Recent field observations have highlighted that this morphological reset can be due to wave obliquity. By using a two‐dimensional horizontal morphological model, we show that the bar straightening by oblique waves occurs because the rip current is both weakened in intensity and shifted downdrift from the channel deepest section. The technique employed is useful for the study of other types of bed forms.