Geo-environmental characterization of the Kwinte Bank

International audience; A detailed geomorphological and sedimentological study has been performed at a tidal sandbank, which has been dredged during 30 years. Localised intensive aggregate extraction created a depression in the central part of the sandbank, upon which the Government decided to close this section of the bank for further exploitation. Multibeam and side-scan sonar technology was used to survey the bank, in combination with extensive ground-truthing. Automated seabed classification was performed, but showed no direct correlation with the mean grain-size; the primary drivers influencing the classification being the sorting of the sediments, the presence of shells and of fine sediments. Very-high resolution seismics revealed the internal architecture of the bank. In the central depression, the upper unit is locally severely dredged. The central depression is characterized by distinct morphosedimentary facies, compared to the western and eastern part of the bank and the Kwinte swale, adjacent of it. The difference between the western and the eastern part is essentially due to different tidal current characteristics, each having their particular sedimentation-erosion patterns. These processes seem to be rather stable, though the evolution of the sediments in the central depression shows similarities with the Kwinte swale sediment evolution. Since the depression is somewhat oblique to the normal crestline, it now forms an open transport pathway from the swale up to the crest of the sandbank. This led to a canalization of the flood current which is witnessed mostly by the northwards and faster progression of bedforms. Because of the difference in sediment characteristics between the dredged material and the present-day supply of sand, it is unlikely that natural processes will be able to counterbalance the severe dredging activities. Moreover, the presence of the central depression is located close to the kink of the sand bank, which is influenced by a high-energy hydrodynamic regime. Its presence could intensify the current action in this area and could enhance the evolution of the bank.