Short-term Morphological Expression of Dune Sand Recycling on a Macrotidal, Wave-Exposed Estuarine Shoreline

Abstract

A topographic survey of the north bank shoreline of the Authie estuary, in northern France, was carried out between 1997 and 1999 in order to determine the short-term morphological organisation, within the estuary, of sand eroded from this chronically retreating north bank. The data complement meso-scale analyses of aerial photographs that highlighted a recycling of this sand, eroded from coastal dunes lining the estuary, into the intertidal inner estuarine sink. The beach at the foot of these dunes serves as an important temporary storage zone for this sand and may show substantial short-term accretion. Much of this sand is, however, transported into the inner estuary by the westerly winds that build up longitudinal dune ridges over a shallow intertidal inner estuarine sand platform that forms the long-term sand sink on this northern shoreline. Sand transported from these ridges ends up as diffuse sand sheets that impinge on muddy sediments derived from upland. Over the longer-term, these dune ridges are recycled into this inner estuarine sediment sink by winds, wave erosion and changes in the position of the Authie channel. In this highly accreted estuary, where minor transport of sand towards the inner estuary by weak flood currents is balanced by seaward evacuation of sand in the main channel, this beach and dune sand transport pathway constitutes the major infill pathway of the estuary. The accretion of sand in the intertidal estuarine sink engendered through this transport pathway is also important in encouraging fine-grained deposition and salt marsh development by providing a shallow substrate. This example shows that accelerated silting of a sandy wave-exposed estuary may occur without the intervention of meso-scale changes in tidal transport.