Morphosédimentologie de la partie occidentale de la rade de Brest

Abstract

In the western part of the Brest Roadstead, tidal streams are responsible for the sorting of sediments on the sea floor. In the deepest part where the flood is stronger than the ebb, the superficial layer is a mixture of gravels and shingles. The finer grains tend to move toward the remote edges of the roadstead. In the South and the West however, sands do not reach the bays where th&fillingin is therefore delayed. There, flood and ebb, run in the same direction ; they have built a 4 km long linear underwater dune heading to the entrance of the roadstead. This proves that the sand moves towards this direction. In the Bottleneck sand is liable to be carried back in the roadstead by the flood and to enter a new vortex ; this reflects the pattern of surface currents. An other linear underwater dune constructed by the flood at the outled of the bottleneck shows that sands coming from outside enter the roadstead. Up to now, the action of the waves in the roadstead has been considered as insignificant away from the shoreline. Although very short in length, theses waves are responsible for the good sorting of sediments and the low mud rate on top of the slopes surrounding the central part of the roadstead, sometimes, more than one kilometer away from the coast.