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

Abstract

The French Aquitanian Coast is an approximately 250 km long straight low coast exposed to high energy conditions in a meso-macrotidal 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 exhibits two distinct sandbar systems. The inner bar can go through all the states within the intermediate classification and usually exhibits a Tansverse Bar and Rip morphology. After a few weeks of lower energy conditions during summer, the inner bar commonly becomes a Low Tide Terrace with a mean wavelength of 400 m and a mean southerly migration rate of about 2–3 m/day. Crescentic bars have been reported 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 of H s > 3 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 effects of 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.