Double bar beach dynamics on the high-energy meso-macrotidal French Aquitanian Coast: A review

The French Aquitanian Coast is an approximately 250 km long straight low coast exposed to high energy conditions in a mesomacrotidal setting. Offshore wave conditions are seasonally modulated, predominantly with a WNW incidence, with offshore significant wave height likely to reach 10 m during winter. Truc Vert Beach, representative of most of the Aquitanian Coast beaches, commonly exhibitstwodistinctsandbarsystems.Theinnerbarcangothroughallthestateswithintheintermediateclassificationandusuallyexhibitsa TansverseBarandRipmorphology.Afterafewweeksoflowerenergyconditionsduringsummer,theinnerbarcommonlybecomesaLow TideTerracewithameanwavelengthof400mandameansoutherlymigrationrateofabout2–3m/day.Crescenticbarshavebeenreported in the literature in nontidal to microtidal settings. Long term persistent crescentic patterns are, however, exhibited at a narrow range of wavelength (mean of 700m) by the outer bar at the meso-macrotidal Truc Vert Beach. Most of the time, the outer bar is inactive and stagnates as offshore waves ofHsN3 m are required to induce a significant morphological change. The crescent shape varies from a symmetric shape to a strongly asymmetric shape, likely to be the result of a long period of NW wave conditions. A strong, and rarely observed elsewhere, morphological coupling between the inner and outer bars can sometimes be observed, and may be the result of the combined effectsof the initial presence of a well-developed outer crescentic bar and a long period of shore–normal low energy conditions. A synthesis of all the data available on the area combined with observations on other environments leads to a Truc Vert Beach state model ranging from a modal double bar configuration to an occasional triple bar configuration. This work also identifies knowledge gaps to be explored by further numerical and field studies in tidal double sandbar environments. © 2007 Elsevier B.V. All rights reserved.