Factors controlling sediment dynamics of a recently deposited mud layer over a sheltered sandy beach following a drastic regression of Zostera meadows (Arcachon Bay, France)
|Ganthy Florian1, Cognat Mathis1, 2, Lanson Meline1, Rigouin Loic1
|1 : IFREMER, France
2 : Université de Bordeaux, UMR 5805 EPOC, Allée Geoffroy St Hilaire, CS50023, F-33615 Pessac cedex, France, France
|ISOBAY 16 - XVIth International Symposium of Oceanography of the Bay of Biscay. 5-7 June 2018, Anglet, France
Within the semi-enclosed mesotidal lagoon of Arcachon, most of intertidal mudflats are colonized by seagrass meadows of Zostera noltei. However, a drastic spatial regression of this species has occurred over the last 20 years. In the meantime, important modifications of hydrodynamics and sediment dynamics were observed: significant changes in tidal asymmetry, increase of suspended sediment concentration, massive accumulation of mud over eastern sandy beaches, increase in dredging requirements in uspstream shallow channels. This study investigates the dynamics of newly deposited decimetric mud layer over a sheltered sandy beach, in order to assess the origin and the future of these muddy deposits. To achieve these objectives, a four-month high frequency field survey was performed. It consists in high-frequency measurements of bed altimetry, suspended sediment concentration, water level and waves characteristics, coupled with weekly surficial sediment sampling for grain-size and erodibility parameters determination. Our results showed that tidal asymmetry, favouring flood dominance, increased during spring tides, leading to an increase of sediment fluxes toward the study area. Moreover, suspended sediment concentrations tend to strongly decrease between flooding and drying of the mudflat, suggesting a significant deposition, confirmed by measurements of sediment altimetry. Despite the sheltered position of the site, significant wind-waves were measured (Hs~ 0.3m), able to induce centimetric erosion during storms. An accretion phase was often observed at the end of the storm events, indicating deposition of previously re-suspended sediments. Because the study site is dominated by flood currents, the recently deposited mud over the sandy beach may come from far downstream tidal flats where the seagrasses disappeared, promoting bed erosion induced by both tidal current and wind-waves. During storm, local wind-waves lead to bed erosion followed by accretion when wave energy decreases. This indicates that locally eroded sediments tends be re-deposited in the same area after storms and cannot be exported.