Monitoring the impact of sand extraction on the bathy-morphology and the seabed sediment in the Belgian part of the North Sea: Lessons of fifteen years of MBES measurements and current innovation.

For several decades the Belgian Continental Shelf (BCS) has been exploited for its sand. As this exploitation intensified over the years, according with the environmental regulation both on national and European levels, evaluating the extraction impact on the seabed bathymetry and morphology as well as on the sediment nature itself is a legal obligation.

The impact assessment of sand extraction is based on multiple types of data: statistics derived from the extraction registers, data from the Electronic Monitoring Systems (EMS = “black-boxes”) on board the dredging vessels, bathymetric and backscatter time series derived from regular MBES surveys, and ground truth data. In this framework, the most extracted areas have been surveyed several times each year for more than 15 years, enabling the development of well-controlled highly valuable and informative time series. In addition to this local approach, regular but less frequent surveys along straight lines, parallel and perpendicular to the sandbanks and the gullies, provide valuable information on the global evolution of the bathymetry and the sediment allowing a comparison between extracted and non-extracted areas. This multi-scale approach combining various types of data provides a 4D (space and time) overview of the evolution of the extraction and leads to robust and pragmatic conclusions about the impact of the sand extraction on the bathymetry, the morphology and the seabed habitats.

The long MBES bathymetric and BS time series on several monitoring areas inside the extraction sectors demonstrate the direct and non cumulative impact of the extraction on the bathymetry of the sandbanks and the unsustainable character of the sand resource. However, several questions remain regarding how the extraction impacts the seabed morphology and sedimentology in real time. Dredging by suction generates fine sediment plumes which could, after transport and sedimentation, modify the habitats within a certain radius around the extraction sites. Although the plumes generated during dredging operations have been the subject of numerous publications, few projects have been attempted to visualize these plumes and quantify the volume of fine sediment by using MBES water column amplitude data.

Specific series of acoustic measurements using the Kongsberg EM2040 MBES installed on the RV Simon Stevin were carried out following dredging vessels on the Belgian Continental Shelf. The resulting high quality dataset allows the evaluation of the real time impact of the extraction on the seabed and the water column in a 4D visualization. Another goal of this research is to evaluate the feasibility to use the MBES water column amplitude data to characterize and quantify the sediment plumes generated by the dredging operation. In a second experiment the scope was extended to plumes from fishing vessels and an evaluation of the impact of tidal currents on the water column measurements. The results of the simultaneous measurements with several acoustic and optical instruments and water sampling will contribute to the establishment of a methodology which will extend the present monitoring program to include the important impact on the seabed from the extraction technique itself.