Environmental impacts resulting from the disposal of dredged material at the S1 dumping site, Belgian Continental Shelf

To guarantee a safe access to harbours, dredging of the maritime access channels is needed. Moreover, shipping channels have to be adapted to the continuously increasing size of vessels. In some cases, the dredged material is reused for beach nourishments or infrastructure projects, but mostly, it is dumped on offshore dumping sites. The efficiency of a dumping place is determined by economic, physical and ecological criteria: (1) dumping places need to be situated close to the navigation channels; (2) a low recirculation of the dumped material towards the dredging places is aimed at; and (3) a negligible effect on the ecosystem should be ensured. The goal of this research is providing baseline information on the impact of dumping on the morphology, sedimentology and biology and the adaptation after cessation of dumping. Both the dumping site as its environment have been considered. The investigated dumping site S1, the largest on the Belgian Continental Shelf, and its surroundings were surveyed with a multibeam echosounder (RV Belgica) providing bathymetrical and backscatter data. Additionally, seabed samples were taken for sedimentological/biological analyses with a box corer, a Reineck corer or Van Veen grab. Finally, chrono-sequential single-beam echosounding was used to investigate the morphodynamic evolution from 1995 until 2002. Generally, the results reveal that on a short- and medium-term, the impact of the dumping of dredged material on the morphology, sedimentology and ecology is drastic, but localised. After the cessation of the disposal of dredged sediments, it seems that the site has restored a morphodynamic equilibrium, both from a morphological, sedimentological point of view, however this is hard to say for the biology (cf. nine stations are devoid of macrobenthos)

Mapping the seabed of the Belgian part of the North Sea to assist in a science-based approach to management

Knowledge of the spatial distribution of sediments, the impact of human activities on the seabed and the natural evolution of the seabed is important to assist in a more sustainable management of the Belgian Part of the North Sea (BPNS). Sediments are an essential part of any aquatic system and are the key for a better understanding of the marine ecosystem; moreover, they are a valuable socio-economic resource for construction material and beach nourishment; therefore, they are extracted increasingly. Research on sediment nature and processes are important also for the optimisation of dredging and dumping activities, the implantation of windmill farms, seabed constructions, cables and pipelines, the designation of marine nature reserves and, generally, any spatial planning initiative.Related to the BPNS research, the Renard Centre of Marine Geology (RCMG) of Ghent University has been/is actively involved in various inter- and multidisciplinary projects. Throughout these projects, a research strategy is followed that balances between process/modelling studies, and the optimization of the use of various acoustical and sampling techniques.Results from three research projects, related to the mapping of the seabed of the BPNS, will be presented. During the Marebasse project (Belspo, SPSDII; http://users.ugent.be/~vvlancke/ Marebasse/), significant new sampling and acoustical data was acquired in typically mud-, sand- and gravel-dominated areas (Van Lancker et al., 2007). On the scale of the BPNS, new thematic maps, related to the bathymetry, sedimentology and morphology, were produced. Automated modelling of these physical data layers, based on advanced geostatistical methods, have resulted in physical habitat maps with ecological relevance. On a smaller scale, impact studies of aggregate extraction and dumping of dredged material were performed on selected sites, integrating results from geo-acoustical surveys and terrain verifications. Results have shown that, on a short- to medium-term, the effects seem relatively localised. All results are integrated into a Geographical Information System GIS@SEA (The physical seabed GIS to support Studies and Environmental Applications).As the Marebasse project focussed on the spatial variability of the seabed nature and the short- to medium-term impact of human activities, knowledge of the natural evolution of the seabed, its response to sea-level rise and the long-term anthropogenic impact remains poor. These issues are investigated now in the QUEST4D project (Belspo, SSD; http://www.vliz.be/projects/Quest4D/). Through quantification of erosion/sedimentation patterns, the natural evolution will be compared against the impact of human activities. Results will be presented of the area north of the Vlakte van de Raan, including the dumping ground Br&W S1. Acoustical maps of the seabed reveal different sedimentation and erosion patterns, originating from natural processes and human-induced activities (Van Lancker et al., 2008). The socio-economic importance of seabed resources is further dealt with in theRESOURCE-3D project (Belspo Targeted Action), in which a 3D reconstruction of the internal structure of sandbanks is aimed at. A case study on the Kwinte Bank will be presented

Acoustic habitat modelling for the mapping of biological communities

In the framework of the MAREBASSE project (‘‘Management, Research and Budgeting of Aggregates in Shelf Seas related to End-users’’, EV/02/18, Van Lancker et al. 2005) and the MESH project (“Mapping European Seabed Habitats, Interreg IIIb; www.searchmesh.net), biologically relevant habitat maps are produced, based on multibeam acoustic datasets, ground truthed with physical and biological samples. The habitat map production comprised of the following four key steps: (1) getting the best out of the ground truth data; (2) selecting and deriving the best available input and most appropriate data coverages; (3) using the most appropriate techniques for interpreting the data through integration and modelling, and; (4) designing the map layout to create a map fit for purpose. Results are presented where classes derived from acoustic seabed classification are translated into habitat maps. Cross tabulation is used to correlate biological ground truthing data with the acoustic classes. Finally, habitat maps are produced, representing the likely occurrences of the different macrobenthic communities occurring on the Belgian part of the North Sea (BPNS). For the study areas of Oostende, Hinder Banken, Sierra Ventana, habitat maps of macrobenthic communities have been created. The habitat maps are based on a cross tabulation of acoustic backscatter classification of the multibeam images, overlaid with biological samples worked out on a macrobenthic community level

Geostatistical modelling of sedimentological parameters using multi-scale terrain variables: application along the Belgian part of the North Sea

In the nowadays highly pressurized marine environment, a science-based approach to management becomes increasingly important. In many cases, the sediment nature and processes are the key to the understanding of the marine ecosystem, and can explain particularly the presence of soft-substrata habitats. For predictions of the occurrence of species and habitats, detailed sedimentological information is required. This paper presents a methodology to create high quality sedimentological data grids of grain-size fractions and the percentage of silt-clay. Based on a multibeam bathymetry terrain model, multiple sources of secondary information (multi-scale terrain variables) were derived. Through the use of the geostatistical technique, Kriging with an external drift (KED), this secondary information was used to assist in the interpolation of the sedimentological data. For comparison purposes, the more commonly used Ordinary Kriging technique was also applied. Validation indices indicated that KED gave better results for all of the maps

Towards a high-resolution 3D-analysis of sand-bank architecture on the Belgian Continental Shelf (RESOURCE-3D): Final report

Revealing the internal structure of sand banks by means of high-resolution seismic (acoustic) methods remains one of the classic methodological challenges in shallow marine geophysical prospection. This is mostly due to the strong heterogeneity of the sand-bank body in combination with complex sea-floor morphology. This study has focussed on the optimisation of a methodological-technological approach through a comparison of various state-of-the-art high-resolution seismic source/receiver configurations for the investigation of the internal architecture of sand banks. On the basis of a dense network of seismic profiles, the 3D architecture of a test site on the Belgian Continental Shelf was studied in detail. Digital acquisition of the data enabled postacquisition processing and data enhancement. Specialised software was used to identify, trace and map the structuring sediment bodies. To translate the “acoustic information” in a most unbiased way, in terms of its lithological and sedimentological nature, UGent-RCMG’s knowledge database and available background information on the Quaternary geology of the Belgian part of the North Sea has been used intensively. Finally, the interpreted seismic data were integrated with other datasets, such as multibeam bathymetry. This enabled a highresolution 3D quantitative analysis and representation of the sand-bank architecture and its economical potential. After comparison of the acquired test data sets, a set of recommendations is formulated regarding the most optimal strategy for future 4D prospecting of marine aggregates on the Belgian Continental Shelf

Ecosystem engineers stabilize sand bank systems: <i> Owenia fusiformis</i> aggregations as ecologically important microhabitat

Ecosystem engineers modify the physical environment and have profound effects on ecosystem functioning and on local biodiversity. Yet, in soft bottom marine environments, they have rarely been included in hydrodynamic studies or in management strategies. The lack of quantified stabilization potential and ecological impact are, respectively, the main reasons for not including ecosystem engineers. The present study evaluates the ecosystem engineering capacity of the tube dwelling polychaete Owenia fusiformis. The ecological implications of aggregations of this species are investigated using a long term dataset (1994- 2006). Results show that its presence has significant implications for species richness and species density. These aggregations are further investigated with different remote sensing tools. Owenia fusiformis is able to stabilize sand dunes that normally migrate 12 m a year. Specific biological characteristics explain the stabilizing effects in a highly dynamic environment. Our results confirm the need to pay attention to biota in sediment transport modelling. We conclude that O. fusiformis creates an important marine system which can be investigated with sensing techniques. These techniques can therefore be used to visualize hotspots of biodiversity, to quantify their biogeomorphological impacts and to underpin ecosystems based management in the marine environment

Recognizing the seafloor’s characteristics using habitat signatures

Seafloor images become increasingly available, both derived from video or photographs and from acoustic remote sensing. Very-high resolution acoustic imagery has indeed the potential of depicting a recognisable sign on an image that relates to a physical and biological nature, i.e. its habitat signature. Still, most of this information is stored at institutes or universities and no up-to-date comprehensive compilation is yet available. Moreover, the acoustic imagery often remains hard to interpret; this is mainly because of the multitude of factors influencing the image and the lack of reference material. When an interpreter studies a remote sensing image, he indeed needs to refer to particular textures and patterns that are recognisable on the image and relate that to reality. Ground truthing remains crucial; still comparison with a large number of similar cases is a necessity. In the framework of marine environmental issues this becomes increasingly important and the need for sound interpretations is real. To anticipate on this need, a web-based catalogue of seabed habitat signatures is being built in the framework of the MESH project (Mapping European Seabed Habitats), for both scientists and non-scientists. The catalogue contains a collection of images produced by different remote sensing techniques (acoustic and optically derived images, photographs and video). As such, the results of the different techniques can be compared and can strengthen interpretations in view of seabed assessments. The catalogue has a comprehensive list of metadata per habitat signature, both in terms of its physical and biological environment and the conditions under which the signatures were generated. The web catalogue is easy manageable. Habitats can be searched using their own name or by typing a key word or choosing a EUNIS code or making a query on physical factors. For every habitat one or more significant locations in the Mesh area are chosen and every location displays all the signatures available. Every location is identified by its coordinates (lat., long.) to be easily positioned on the MESH webGIS (http://www.searchmesh.net/webGIS). The signatures are presented as little thumbnails to let the web user have an easy overview. These link to a page where a description of the image, an enlarged image and all technical data referred to it can be found. The catalogue will largely increase the visibility of how the seafloor looks like, but above all it is hoped that it will assist in the interpretation of newly acquired data in view of - 48 - environmental assessments. Any potential contributor to this catalogue is invited to share their images to a wider European community. The web-catalogue is developed at Ifremer (http://www.ifremer.fr/meshmalo/ essai_signatures). RCMG is responsible for the input of imagery related to the Belgian part of the North Sea

Mud Origin, Characterisation and Human Activities (MOCHA): Final report

The cohesive sediments, which are frequently found in the Belgian nearshore zone (southern North Sea), are of different age such as tertiary clays and Holocene, modern and recently deposited muds. The area is characterised by a turbidity maximum. The source areas of the recently deposited muds and the effect of human impact vs. natural processes on the distribution and/or erosion of these sediments have been investigated using historic and recent bottom samples, in situ and remote sensing (satellite images) SPM concentration measurements, numerical modelling, GIS and clay mineral and microfossil analysis. The Schelde estuary, the potential erosion areas of cohesive sediments on the BCS and adjacent areas and the SPM transport through the Dover Strait have been considered as possible source areas.The historic bottom samples have been collected in the beginning of the 20th century, the quality of these samples and the meta-information is very high and they have proven to be a major reference to understand the evolution of the cohesive sediment distribution. The recent bottom samples consist of box core, Reineck core and Van Veen grab samples collected during the last 10 years. The processing of the historic and recent data on cohesive sediments was mainly based on field descriptions of the samples (consolidation, thickness) and morphological evolution. On some of the recent samples radioactive and gamma densitometric measurements have been carried out. During the processing the emphasis was put on the occurrence of thick layers (>30 cm) of freshly deposited to very soft consolidated mud and of clay and mud pebbles, because these sediments are witnesses of changes.Satellite images, in situ measurements and a 2D hydrodynamic numerical model have been combined to calculate the long term SPM transport through the Dover Strait and in the southern North Sea. The satellite images (SeaWiFS) provide synoptic views of SPM concentration. The representativness of SPM concentration maps derived from satellites for calculating long term transports has been investigated by comparing the SPM concentration variability from the in situ measurements with those of the satellite data. It is underlined that SPM concentration measurements should be carried out during at least one tidal cycle in high turbidity areas to obtain representative values of SPM concentration.Areas where the thickness of the Quaternary cover is less than 2.5 m were defined as potential erosion areas of Palaeogene clay containing deposits. In the framework of this project, the geological data related to the BCS have been reviewed and the relevant information was compiled into a GIS. This also included a small part of the French continental shelf. Additionally information was added from vibrocores analysis and Dutch geological data. Approximately 20 % of the BCS, 6 % of the small part of the French area and only 3% of the Dutch study area could possibly serve as a source for fine suspended sediments. Quaternary muds are mostly presented in the 2 eastern nearshore area; on the Dutch part they occur more offshore. Their occurrence represents 11% of the BCS and approximately 35% of the Rabsbank area Cretaceous microfossils are present in all samples and have been transported into the area with the residual water transport. Material from the east, in particular from the Eocene-Oligocene transitional strata, has been found in the eastern nearshore area up to about Oostende. This zone coincides with the extension of the Holocene mud and could indicate an erosion of these sediments and/or a transport of clay minerals from the Schelde estuary.Clay mineral analysis has been carried by two approaches in order to determine source areas. The results of the second approach show that no systematic differences in the clay mineralogy depending on geographic location could have been found within the samples. The results clearly prove the necessity of using more elaborate sample preparation procedures in examining the provenance of the mud deposits.Thick layers of fresh mud were deposited in the beginning of the 20th century mainly in a narrow band along the coast from about Nieuwpoort up to the mouth of the Westerschelde. These deposits were mainly the result of natural morphological changes. Today, most of the depositions of thick layers of fresh mud have been induced by anthropogenic operations, such as dumping, deepening of the navigation channels and construction and extension of the port of Zeebrugge. Comparing the actual situation with the situation 100 years ago reveals that the area around Zeebrugge where fresh mud is deposited extends more offshore today