Evolution of a Coastal Beach/Barrier/Marsh System in Response to Sea Level Rise, Storm Events and Human Impacts: A Case Study of Trunvel Marsh, Western Brittany
Résumé
The evolution of coastal sites such as beach/barrier/marsh systems is known to be strongly forced by sea level rise and controlled by storms, sediment input and human impacts. The relative weight of each may vary in time. However, it is difficult to determine the relative importance of these forcing controls and, therefore, how coastal systems evolve through time. In order to study this evolution we have selected the case study of Trunvel marsh, western Brittany, France, which is directly exposed to the most violent storms and has been extensively depleted of sediment during and since WW2. The relative balance of anthropogenic and meteorological controls and relative sea level rise is compared. Sediment cores have been obtained from within the marsh, cross sections of the barrier have been studied and air photos and old maps have been analysed. From 4000 BP to recent times the system has behaved in a simple way: the beach and the barrier accumulated sand and gravel, seeming to migrate inland with relative sea level rise and the marsh was alternatively eroded by the local river or fed by aeolian drifted sands. Very occasional storms may have breached the barrier and temporarily invaded (flooded) the marsh. Conversely, large events of river discharge may have breached the barrier, although there appears to be some natural resilience and the barrier rebuilds itself after each storm and the marsh is, once again isolated from the sea. At the beginning of the Roman period land use change appears to have modified the river discharge, following which the marsh seems to have been in its natural condition again until WW2, although some dykes were built and channels excavated. During WW2 the gravel was almost totally removed and used for concrete to build fortifications along the coast. After WW2, the system was totally controlled by management practices, the aim of which was to recreate a “natural” environment so that today this is a “human made natural landscape” and is now classified as a nature reserve. The barrier is no longer able to withstand storms and the river discharge does not always reach the sea. Therefore human management of water level in the marsh is today the main morpho-dynamic control for the whole system.