Physical impact of dredged material disposal sites and their recovery after cessation of dumping: a case study in the Belgian coastal zone

The interaction of dumped sediments with the existing morphological and sedimentological conditions and the recovery after cessation of dumping activities is still poorly known, especially on the long-term. We present a quantitative impact study of the dumping of dredged material on two overlapping dumping sites, situated on different morphological entities; an old dumping site closed in 1999 and located on a sandy shoal, and a new one located in a tidal swale. Chronosequential single-beam echosoundings, high-resolution multibeam bathymetrical and backscatter data, ground-truthed with boxcores and vibrocores, were used. Dumping has caused a clear depth reduction between 1995 and 2004. Different yearly growth values and sedimentation patterns were found for the old and new dumping sites, due to the difference in morphological setting. The present morphology of the old dumping site -an irregular-shaped dump mound, with sand piles on the edges and a depression in the centre- reflects the way of disposal. On the new dumping site, a dump mound is found, which is characterised by impact features such as depressions and topographic highs. After cessation of the disposal of dredged sediments on the old dumping site, the site has restored quickly its morphodynamic equilibrium. Bedforms were formed within less than one year and their distribution and corresponding sedimentology is representative for the Belgian coastal zone. Between the large dunes, small depressions occur, which are thought to be remnants of single dumping events.