Changes of sedimentological patterns and morphological features due to the disposal of dredge spoil and the regeneration after cessation of the disposal activities

Abstract

The interaction of dumped sediments with the morphology and sedimentology of a coastal environment and the recovery of the seafloor after cessation of dumping activities, is still poorly known, especially from a longer term perspective. This paper presents a quantitative impact study of the dumping of dredged material on two overlapping disposal sites, situated on different morphological entities: an old disposal site located on a sandy shoal, was closed in 1999; afterwards a new disposal site was designated, mainly located in a tidal gully. The approach included the interpretation of chrono-sequential single-beam echosoundings, high-resolution multibeam bathymetrical and backscatter data, ground truthed with boxcores. Dumping has caused the formation of a large-scale dump mound on both the old and new disposal site. On the new disposal site, the dump mound is superposed by individual disposal mounds and depressions that are probably formed by scouring events. On the old disposal site, also small depressions occur; which are likely associated with the former dumping process. The disposal has led to the accumulation of sand, both on the old and new disposal site, regardless of the high mud content of the dredge spoil. For the new disposal site, dumping efficiency has been estimated as 30–40%, which would lead to a yearly growth of 380,000 m 3. A different sedimentation pattern was revealed for the old and new disposal site, which is ascribed to the difference in morphological setting and differences in dredge spoil nature. After cessation of the disposal of dredged sediments on the old disposal site, a morphodynamic equilibrium was fairly quickly restored. Bedforms were formed within less than one year and their distribution and surficial sediments are again representative for the Belgian coastal zone. The consistent ebb-dominated asymmetry of the very large dunes, their SW migration and sediment accretion extending beyond the SW limit of the new disposal site, suggest an important bedload transport in the direction of the navigation channels. These findings contrast to the overall flood dominated residual current transport direction.