Spatial and temporal variability of shorefaces: A morpho-hydrodynamic controlled system

Shorefaces are transitional zones between the shelf and surfzone/beach systems. They are subdivided into ‘upper’ and ‘lower’ shoreface sectors which display particular morphodynamic behaviour, with the upper shoreface morphologically active over short (annual) time scales, and the lower shoreface evolving over much longer time scales (decadal and beyond). We examine morphodynamics using three distinct examples from the high-energy coast of Ireland. Numerical wave modelling (SWAN) is used to characterize wave energy dissipation across the shoreface. Our results show that zonation patterns and the physical extent of shorefaces display distinct spatial as well as temporal variations. The shoreface’s physical limits and internal zonation patterns are driven by the combination of local wave forcing and pre-existing shoreface morphology. Shoreface behaviour therefore, falls within a spectum between gradual and intensive wave energy dissipation. We show that particular seabed morphology controls the extent of, as well as the bed shear stress distribution across the shoreface. This can have significant implications for any adjoining beach systems as waves arriving at the surf zone acquire specific characteristics depending on the shoreface configuration, even under similar offshore wave conditions. Additionally, the variation of shoreface extent impacts the volume of sediment available for transport, both to the nearshore zone as well as local beach systems. The influence of shoreface morphology is therefore an important, but largely overlooked element in the study of mesoscale coastal behaviour as well as coastal response to global climate change and secular sea-level rise.

Shoreface mesoscale morphodynamics: a review

International audienc

A sustainable alternative for coastal dune restoration by sand-trapping fences and algae wrack: AlgoBox ®

International audienceShorelines are the locus of significant and diverse economic activity as well as ecosystem services and conservation interest. At a time of global climate change and sea level rise, the type of management being implemented has direct influence on the future of the shoreline and its resources. Several approaches can be taken, but in practical terms, the overwhelming response has been, and continues to be, to armour or nourish the shoreline and prevent it moving. This removes the shoreline's natural resilience, damages the coastal ecosystem and the resources it provides, and has a direct economic cost (in design, construction and maintenance) and indirect costs (through its impact on recreation, scenic quality, tourism potential and several other economic activities). The coast of south Brittany in France is subject to severe erosion. This coast is characterized mainly by seacliffs and headland embayed and pocket beaches with a "low-tide-terrace" active morphology. Wrack deposits (macroalgae: mainly Soleiria Chordalis) also form significant seasonal accumulations on the beaches, thus impacting their morphology and ecosystem. Because of the absence of sediment stocks that could be used for beach nourishment, shoreline management has had recourse, thus far, to "hard" engineering techniques to hold the coastline. The use of "soft" engineering techniques to protect dunes and beaches in south Brittany has been limited to the implementation of sand fences on the dunes to limit human access to these fragile systems. The aeolian sediment transport potential, with a relative abundance of onshore winds on this coast, has unfortunately been under-exploited. In order to gain maximum profit from the aeolian transport potential and from the nutrient release generated by wrack decomposition (which may facilitate plant growth and sediment fixing), we proposed an experimental tool, the "AlgoBox", which consists in sand-trapping fence cases filled by wrack after partial aeolian sand deposition. The fixing of sand in the "AlgoBox" by vegetation colonization is facilitated by the nutrients released by wrack decomposition during spring and summer seasons (Sedrati and Cochet, 2015). This paper presents the design and evaluation of the "AlgoBox" system which has now been successfully experimented at too sites in South Brittany coast (Penvins beach since July 2014 and Kerjouano beach since June 2016) including the regular topographic dune-beach profile surveys, algae wrack decomposition and nutrient generation, vegetation colonization processes and future considerations for the optimal design and improvement of the system as a new tool for ecological engineering of beach-dune systems

Montée des eaux : sur les littoraux, l’urgence de développer les solutions fondées sur la nature

International audienc

Montée des eaux : sur les littoraux, l’urgence de développer les solutions fondées sur la nature

International audienc

TI'FOND (dynamique hydro-sédimentaire des petits-fonds aux abords du GPM de Guadeloupe)

National audienc

: Effect of transversal groins on dissipative barred beach dynamics: a study case in Matalascañas beach, Huelva.

International audienceSeveral studies on barred beaches in settings with mesotidal to macrotidal regimes have focused on crossshoreand alongshore bar mobility. However, the role of coastal defense structures (breakwaters, groins, seawalls) onbar-trough systems morphodynamics have received more much less attention. The main aim of this paper is to highlightthe contrasting behavior of a natural and protected barred beach under several hydrodynamics conditions. This paperpresents detailed hydrodynamic and morphological data from a field experiment undertaken in Matalascañas beach, amesotidal protected vs natural barred beach in the Southern Spanish coast. The non-protected beach shows a lesspronounced bar-trough system than the protected zone. Under low energy conditions, onshore bar migration rate in theprotected area was more important than the non-protected area. This migration was associated with an onshoresediment transport, resulting from the erosion of the bar’s seaward slope. In the same moment, a clear longshore barmigration was observed in the non-protected zone. During few energetic tides, the protected and non-protected zonesshow a flattening bars processes. The findings of the present study suggest that cross-shore vs longshore bar mobilitymay even be mitigated by the presence of the groins, which favor onshore than longshore bar migration.Key words: Dissipative beach, bar dynamics, groins, Matalascañas, Huelva.Numerosos estudios en playas disipativas con barras en entornos de régimen meso y macromareal se hancentrado en la movilidad de las barras tanto lateral como longitudinalmente. Por menos que el impacto de las estructurasde defensa costera (rompeolas, espigones, malecones) en la morfodinámica de estas barras ha recibido mucho menosatención y los estudios experimentales en el campo de estos ambientes son escasos. El objetivo de este trabajo escontrastar bajo varias condiciones hidrodinámicas el comportamiento de una playa natural y otra protegida por peinesde espigones transversales. Este trabajo presenta datos hidrodinámicos y morfológicos de un experimento de campo queabarca 10 días en la playa de Matalascañas, una playa mesomareal con un tramo protegido y otro natural en la costasuroeste de España. Bajo condiciones de baja energía, la migración hacia tierra de la barra en el área protegida fue másimportante que el área no protegida. Al mismo tiempo, una clara migración longitudinal de la barra también fueobservada en la zona no protegida con la ausencia de este proceso en la zona protegida durante algunas mareasenergéticas. Los resultados sugieren que la movilidad de barra longitudinal puede ser mitigada por la presencia deespigones, que favorecen la migración de las barras hacia tierra a la vez que minimizan su movilidad en el sentidolongitudinal

The influence of rocky outcrops in the morphodynamics of an embayed beach: example of Suscinio Bay, Morbihan, France

International audienceRock outcrops impact beach and nearshore dynamics of coastal embayments, inducing boundary effects that constrain the cross-shore and alongshore beach morphodynamics. The northwestern coast of France is characterized mainly by seacliffs and headland embayed and pocket beaches with a "low-tide-terrace" active morphology. They also possess several rocky platforms and outcrops between the inner foreshore and shoreface zones which play an important morphological role by generating wave refraction and diffraction and gyre currents. Sediment transport and hydrodynamic circulation are, however, perturbed and complex on these areas. This study analyses the impact of a large rocky outcrops on the morphodynamics of Suscinio embayed beach (Morbihan-South Brittany, France) by comparing beach profiles evolution and hydrodynamic conditions between beach sectors fronted and non-fronted by a large rocky outcrops. Field work focused on a 5 day period from 13 to 18 November 2016. Eight beach profiles were surveyed along the study area, The sheltered profiles were central (4 profiles) and directly fronted to seaward by submerged large granitic rocky outcrops. The exposed profiles were situated in the both extremities of the study area (2 profiles at each extremity). Two currentemeters and six wave gauges were deployed between the central sheltered and extremely (each limit) non-sheltered profiles. The mean wind speed and directions averaged every three hours highlight closely-spaced high-energy events during the experiment, with long phases of significant lateral wind stress (W to NNW). The measured waves and currents showed two different stages. During low to moderate energy conditions, the non-sheltered zones highlights tidal modulated currents (bidirectional current) when the sheltered zone presents a multi-directional current which correspond to a locally gyre generated current. The significant wave heights were similar in both sheltered and non-sheltered zone. During the high-energy event, a rapid and strong response to both the changes in wind speed and direction were observed. In both sheltered and non-sheltered zones, an unidirectional wind drived current (SE directed current) was measured. Significant wave height values were higher in the sheltered area than the non-sheltered area. The topographic survey of the profiles during low energy conditions shows a global stability of the study area with no significant morphological changes. However, during high-energy conditions, significant changes were observed along the beach, the non-sheltered zones shows erosion in the upper intertidal zone and a weak accretion in the lower intertidal zone (cross-shore sediment mobility) when the sheltered area highlights a significant accretion (on the four sheltered profiles). The net beach volume over the experiment was negative thus showing that sediment may be preserved in the sheltered zone but not in the non-sheltered zone under high-energy conditions. The findings of the present study may suggest that sheltered zones can be preserved against erosion processes during high-energy but the atypical generated currents generated in the some zone during low to moderate conditions may enhance erosion processes

TI'FOND (dynamique hydro-sédimentaire des petits-fonds aux abords du GPM de Guadeloupe)

National audienc

AlgoBox ® : Un outil écologique pour la régénération des pieds de dunes grâce aux échouages de macro-algues

International audienceLe littoral Morbihannais en Bretagne-Sud, (France), est victime d'une érosion chronique qui est principalement affrontée dans la région par des aménagements lourds. L'utilisation d'aménagements "doux" pour la gestion du littoral ne concerne majoritairement que l'implantation de ganivelles en pied de dune pour restreindre le piétinement. Or, le rôle du transport sédimentaire éolien est souvent négligé et peu exploité dans la régénération de la dune. Le littoral morbihannais connait également une récurrence des échouages d'algues (principalement Solieria chordalis) qui forment des accumulations massives qui impactent la dynamique morphologique de la plage et son écosystème. Afin de tirer un maximum de profit de ce potentiel éolien et maintenir les nutriments de la dégradation des algues sur la plage (qui permettront la colonisation des plantes responsables de la fixation des sédiments), nous avons mis en place un système expérimental, les AlgoBox ® , qui consiste en l'installation de casiers de ganivelles remplis d'algues placés en pied de dune. L'objectif des AlgoBox® est d'accélérer la dynamique naturelle de création de l'avant-dune afin de renforcer l'amortissement des vagues et éviter la rupture de la dune. L'expérimentation AlgoBox® est en cours depuis juillet 2014 sur deux plages de la presqu'île de Rhuys. Cet article présentera le principe de mise en place des AlgoBox ® , les différents suivis qui accompagnent cette expérimentation (topo-morphologique, dégradation des algues, vitesse de colonisation par la végétation,...) afin d'estimer l'efficacité du système face aux différentes conditions météo-marines et d'apporter des améliorations à ce système écologique de régénération de l'avant-dune