Morphological evolution and time-varying bedrock control of main channel at a mixed energy tidal inlet: Maumusson Inlet, France

Abstract

Maumusson Inlet, located on the French Atlantic coast, connects the Atlantic Ocean with the Marennes–Oléron tidal bay. The tidal range (2–6 m) and wave climate (mean height 1.5 m) place this tidal inlet in the mixed energy, tide dominant, category of Hayes [(1979) Barrier island morphology. In: Leatherman, S.P. (Ed.), Barrier Island, Academic Press, New York, pp. 1–28]. An innovative method, combining high quality bathymetric data (nine accurate Digital Elevation Models since 1824) with a very high seismic resolution, demonstrates a major tidal inlet evolution from 1824 onwards and its dramatic acceleration since 1970. The chronology of those morphological changes suggests strong coupling between the location of the tidal channel and the behaviour of the adjacent shorelines. The recent shoaling and migration of the inlet channel can be attributed to a decrease in tidal prism due to the filling in sediment of Marennes–Oléron Bay. Seismic data give evidence that the inlet was located on a major incision of the bedrock. It can be inferred that the bedrock exerts control of channel location, this control varying in time as a function of channel depth. A conceptual model is proposed, including the inlet, its adjacent shorelines, the tidal bay and the time-varying bedrock control of main channel location. Such a model could be considered valid in similar cases along other coastlines, i.e. coastlines with a fine unconsolidated sediment sheet.