Stage and rate of formation of a complex spit in a megatidal environment Zustand und Formungsrate eines komplexen Nehrungshakensystems in einem Makrogezeiten-Ästuar Etapes et rythmes de formation d'une flèche sédimentaire à crochets multiples en environnement mégatidal
Résumé
The Pointe d'Agon is a complex spit located on the west coast of the Cotentin (Normandy, France), evolving across the megatidal Regnéville inlet. This spit has on its distal part several well defined hooks. Using aerial photographs covering half a century makes it possible to monitor the formation of a new hook in a two stage process. The influence on the shoreline of the swash bar located on the ebb delta can be highlighted. Longshore transport is disturbed by the bar and its onshore migration. Updrift of the bar, this disturbance leads to the advance of the shoreline. On the downdrift side, an erosion zone results from the decrease of sediment supplied by longshore transport as the bar and the tombolo that formed prior to its attachment to the beach act as natural breakwater. The northward migration of the outlet channel leads to the formation of a sand platform which in turn changes the orientation of the shoreline axis from SE to SSE, initialising the formation of a new spit. The welding of the bar and the resumption of the north-south longshore transport regulate the shoreline and spit axis towards SE. The lengthening and shape of the Agon spit seems to be controlled by the migration of swash bars on the ebb delta of Regnéville inlet. These bars influence the sediment flux and shoreline mobility over a timescale of several years. Bar migration takes approximately 12 years but formation of hook spit has taken 26 years during the recent period. In addition, this study underlines the specific characteristics of the morphology of a megatidal sandy coast and its influence on swash bar migration. An increase of the migration rate is observed near the upper beach in accordance with the reflective profile of this part of beach which is typical for these megatidal environments.