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|Dispersion and deposition of sediment plumes, resulting from intensive marine aggregate extraction|
Evangelinos, D. (2014). Dispersion and deposition of sediment plumes, resulting from intensive marine aggregate extraction. MSc Thesis. VUB/Universiteit Gent/Universiteit Antwerpen: Brussel, Gent, Antwerpen. 15, 42 pp.
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To cope with increasing demands on marine aggregates, new extraction practices have been implemented in the Belgian Part of the North Sea. Since 2012, intensive extractions take place, using both small (2.500 m3) and large (>10.000 m3) trailer suction hopper dredgers. Investigations were needed on the near and far field impact; the latter with respect to deposition of fines from sediment plumes. This could impact on a nearby Habitat Directive Area hosting ecologically valuable gravel beds, adapted to a clear water regime. Near the dredge tracks, sediments were more heterogeneous and some fining trend was observed in the top surface of the seabed. Acoustic data, in combination with water samples, showed distinct sediment plumes constrained in space and time. Under disturbed conditions, multimodal particle-size distributions tended to show higher class-weight percentages of the 6-10 µm fractions. In the far field, seabed samples were enriched with mud, evidenced by an extra particle-size mode around 10 µm. Mud percentages were up to 22 %, unusual for an area rich in gravel and coarse sediments. Plume modelling, considering both dredging activities and ruling hydro-meteorological conditions did not show significant deposition in the Habitat Directive Area, though simulations were restricted to winter conditions. Future simulations should consider calm weather conditions and simultaneous activities, in combination with trapping of fines in permeable beds. Within the context of the Marine Strategy Framework Directive consequences of habitat changes are discussed and recommendations are given on minimizing environmental impacts.