Erosion and recycling of aeolian dunes in a rapidly infilling macrotidal estuary: the Authie, Picardy, northern France

The Authie is a rapidly infilling macrotidal (mean estuary-mouth spring tidal range = 8.5 m) estuary in Picardy, northern France, whose mouth is affected by strong tidal currents and wind waves generated in the English Channel. The estuary cuts across a major sand dune barrier and has been sourced by sand derived from offshore and alongshore, as well as from recycling of the aeolian dunes lining its north bank. Sand released by the severe erosion of these north bank dunes is temporarily stored on the beaches. A small fraction of the sand is back-cycled into the dunes via blowouts. The rest is transported towards the inner estuary where it forms longitudinal aeolian dune ridges and sand sheets that are ultimately recycled into sandy-muddy intertidal flats that develop into salt marshes. The erosion of the dunes lining the north bank of the estuary represents a morphodynamic adjustment to concentration of the tidal flux against this north bank by massive accretion and progradation of a south-bank sand platform. This erosion contributes in giving a funnel-shaped estuary mouth and probably in accommodating the tidal prism following large-scale reclamation of the inner estuary. The estuary-ward recycling of aeolian dune sand enhances overall accretion of the estuary whose ultimate fate is complete silting up in the decades to come.