Sediment dynamical and morphological study of a marine shoal: the Paardenmarkt, Belgian Continental Shelf: contribution of grain-size analysis, acoustic instruments, current modelisation (poster)

The Paardenmarkt is a marine shoal, located two kilometers east of the Zeebrugge harbour (51°20’N-03°16’E), in water depths varying between 3-7m. This site is characterized by the presence of shallow biogenic gas. Moreover, this site forms a dangerous area for shipping and fishing because it was used as a dumpsite for WWI bombs. For this reason, the understanding of the sedimentary transport is important to estimate the future evolution of this site. This study combines acoustic data (side-scan sonar, single- and multibeam, acoustic doppler current profiler), current modeling and grain-size analysis of surficial sediment samples to evaluate the relative contribution of the hydrodynamic and morphological effects on the sedimentary transport in the area. The hydrodynamic results show a dominance of the flood tidal current for the entire study area. However, locally near the Scheldt Estuary, the ebb current dominates as an effect of the Scheldt outflow. Based on the grain-size analysis, a sediment map is made in combination with the seabed morphology and permits a classification of 5 sediment types. The residual sediment transport, based on the grain-size data, is estimated by a numerical model (Gao and Collins, 1991) and is related to seabed features observed on the acoustic data (side-scan sonar and multibeam). Four different sedimentary transport zones are distinguished with this method: Ä in the western part of the study area, the flood currents dominate and influence the sedimentary transport;Ä in the southern part, the gyre created behind the Zeebrugge harbour influences the sedimentary transport;Ä in the central part, no significant residual transport is observed;Ä to the east, near the Scheldt Estuary, the sedimentary transport is influenced by the amplification of the ebb currents. This amplification is due to a minimal addition of freshwater and the contrasted morphology of the estuary, which canalizes and accelerates the ebb current.This case study was carried out in the framework of an evaluation study of the anthropogenic influence of the harbour construction between 1954-2000. Due to the construction of the harbour, a gyre is generated resulting in a sedimentary accretion south and erosion north of the Paardenmarkt site

Monitoring of the impact of the extraction of marine aggregates, in casu sand, in the zone of the Hinder Banks

The far offshore Hinder Banks are targeted for exploitation of huge quantities of sand, mainly for coastal defence works. Here, up to 2.9 million m³ can be taken over 3 months, with a maximum of 35 million m³ over a period of 10 years. Large vessels can be used extracting 12500 m³ per run. South of the Hinder Banks concession, a Habitat Directive area is present, hosting ecologically valuable gravel beds. For these, it is critical to assess the effect of multiple and frequent depositions of fine material from dredging-induced sediment plumes.A monitoring strategy was set-up, tailored for assessing the importance and extent of physical perturbations that are created by the extraction activities. The monitoring design focuses on hydrodynamics and sediment transport with feedback loops between both modelling and field studies. Main targets are assessing changes in seafloor integrity and hydrographic conditions, two key descriptors of marine environmental status within Europe’sMarine Strategy Framework Directive. Seafloor integrity relates to the functions that the seabed provides to the ecosystem (e.g., structure; oxygen and nutrient supply), whilst hydrographic conditions refer to currents and/orother oceanographic parameters of which changes could adversely impact on benthic ecosystems.State-of-the-art instrumentation (from RV Belgica) has been used, to measure the 3D current structure, turbidity, depth, backscatter and particle size of the material in the water column, both in-situ and whilst sailing transectsover the sandbanks. In the Habitat Directive Area, gravel bed integrity (i.e., epifauna; sand/gravel ratio; patchiness) was measured as well. Most innovatively, an autonomous underwater vehicle was deployed (Wave Glider®, Liquid Robotics Inc.), resulting in quasi 22 days of current, turbidity and other oceanographic data.From a first data-model integration, and analyses against hydrometeorological databases, main results showed: (1) high spatial and temporal variability of turbidity, both current- and wave-induced; (2) important topography-induced resuspension over the sandbanks, especially under wave agitation; (3) spreading and deposition of dredging-induced sediment plumes; and (4) competitiveness of ebb and flood, meaning that deposition of fine sediments on the gravel beds is realistic. Field data on currents were used for the validation of a three-imensional hydrodynamic model. Results confirmed good model predictions of current magnitude and current directions in zone 4, critical for future impact assessment.Data will be integrated with results from the morphological and biological monitoring, respectively carried out by the Continental Shelf Service of FPS Economy and the Institute for Agricultural and Fisheries Research

Sediment plume monitoring in the Clarion-Clipperton Zone

OD Nature has a vast experience in monitoring and modelling Suspended Particulate Matter concentration in shelf areas. In the framework of the JPI-Oceans cruise with the RV Sonne in the Belgian, French and German concession zones for deep-sea mining in the Clarion-Clipperton Zone, this experience will be used to monitor sediments plumes, caused by deep-sea mning exploration activities