|Monitoring of the impact of the extraction of marine aggregates, in casu sand, in the zone of the Hinder Banks|
Van Lancker, V.; Baeye, M.; Fettweis, M.; Francken, F.; Van den Eynde, D. (2014). Monitoring of the impact of the extraction of marine aggregates, in casu sand, in the zone of the Hinder Banks. Report MOZ4-ZAGRI/X/VVL/201401/EN/SR01. RBINS-OD Nature: Brussels. 384 + annex pp.
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- Francken, F., more
- Van den Eynde, D., more
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.
- Wave Glider data on surface and water-column currents and turbidity. 22-day trajectory around Oosthinder sandbank, Belgian part of the North Sea, more