Factors influencing subaqueous dunes in the Scheldt Estuary

The size (wavelength and height) and the factors influencing the characteristics of subaqueous dunes in the river Scheldt between Antwerp and the Belgian-Dutch border were studied. More than 60 dune clusters (each cluster represented by a mean height and wavelength of mostly 10 to 20 or more observations) were analysed. A very good exponential correlation between dune height and wavelength was observed (r=0.90) which can be described by H-mean=0.0321L(0.918). Length/depth ratios vary between 0.2 and 9, height/depth ratios between 0.25 and 0.01. There was no clear relationship between current velocity and dune size, as dunes of various sizes co-exist at the same current velocities. Twenty-two dunes were sampled and show grain sizes (d50) ranging between 0.1 and 0.7 mm. Seven out of eight large scale dune fields were found in sandy and/or hard bottom (gravel, coarse sand and shell fragments) environments, whereas small and medium dunes were found in muddy sand and sandy mud environments

Suspended particulate matter dynamics and aggregate sizes in a high turbidity area

Measurements of aggregate size, suspended particulate matter (SPM) concentration and current velocity have been carried.out in the Belgian coastal zone (southern North Sea). Two stations were situated in the coastal turbidity maximum zone; another station was located more offshore at the edge of this turbidity maximum. The data have been collected using a LISST 100, OBS sensors, water samples and a bottom mounted ADCP. Turbulence (Kolmogorov microscale) has been modelled for the same period using a 3D numerical model. The results show that the size of the aggregates is significantly smaller in the coastal turbidity maximum area. The processes responsible for the occurrence of smaller aggregate size in the coastal zone compared with the more offshore location are: the higher turbulence; the smaller time available for the aggregates to grow up to an equilibrium size; the higher deposition of mud, resulting in a break-up of the floes and the lower availability of organic matter, which may limit the size of the floes. (c) 2006 Elsevier B.V. All rights reserved

Insights into the Short-Term Tidal Variability of Multibeam Backscatter from Field Experiments on Different Seafloor Types

Three experiments were conducted in the Belgian part of the North Sea to investigate short-term variation in seafloor backscatter strength (BS) obtained with multibeam echosounders (MBES). Measurements were acquired on predominantly gravelly (offshore) and sandy and muddy (nearshore) areas. Kongsberg EM3002 and EM2040 dual MBES were used to carry out repeated 300-kHz backscatter measurements over tidal cycles (similar to 13 h). Measurements were analysed in complement to an array of ground-truth variables on sediment and current nature and dynamics. Seafloor and water-column sampling was used, as well as benthic landers equipped with different oceanographic sensors. Both angular response (AR) and mosaicked BS were derived. Results point at the high stability of the seafloor BS in the gravelly area (2 dB and 4 dB at 45 degrees respectively. The high-frequency backscatter sensitivity and short-term variability are interpreted and discussed in the light of the available ground-truth data for the three experiments. The envelopes of variability differed considerably between areas and were driven either by external sources (not related to the seafloor sediment), or by intrinsic seafloor properties (typically for dynamic nearshore areas) or by a combination of both. More specifically, within the gravelly areas with a clear water mass, seafloor BS measurements where unambiguous and related directly to the water-sediment interface. Within the sandy nearshore area, the BS was shown to be strongly affected by roughness polarization processes, particularly due to along- and cross-shore current dynamics, which were responsible for the geometric reorganization of the morpho-sedimentary features. In the muddy nearshore area, the BS fluctuation was jointly driven by high-concentrated mud suspension dynamics, together with surficial substrate changes, as well as by water turbidity, increasing the transmission losses. Altogether, this shows that end-users and surveyors need to consider the complexity of the environment since its dynamics may have severe repercussions on the interpretation of BS maps and change-detection applications. Furthermore, the experimental observations revealed the sensitivity of high-frequency BS values to an array of specific configurations of the natural water-sediment interface which are of interest for monitoring applications elsewhere. This encourages the routine acquisition of different and concurrent environmental data together with MBES survey data. In view of promising advances in MBES absolute calibration allowing more straightforward data comparison, further investigations of the drivers of BS variability and sensitivity are required

Long-term influence of maritime access works on the distribution of cohesive sediments: analysis of historical and recent data from the Belgian nearshore area (southern North Sea)

Long-term changes in the cohesive sediment distribution of the Belgian-Dutch nearshore zone (southern North Sea) are related to human activities (port construction, deepening of navigation channels, disposal of dredged sediments) and to natural variability, due to tides and meteorological effects. Results are based on the combined analyses of recent and historic (100 years ago) sediment sample information and bathymetric maps. Data processing was based mainly on field descriptions of the samples (consolidation, thickness) and on bathymetric maps of 1866-1911. Results indicate that the distribution of fresh mud and suspended sediment has changed during the last 100 years, due mainly to maritime access works. Most of the present deposition of thick layers of fresh mud (> 30 cm) has anthropogenic causes. The results further indicate that erosion of older Holocene mud has increased in recent times and, as a consequence, higher amounts of fine-grained sediments are being released into the southern North Sea today

Building a 4D voxel-based decision support system for a sustainable management of marine geological resources

For sustainable management of marine geological resources, a geological knowledge base is being built for the Belgian and southern Netherlands part of the North Sea. Voxel models of the subsurface are used for predictions on sand and gravel quantities and qualities, to ensure long-term resource use. The voxels are filled with geological data from boreholes and seismic lines, but other information can be added also. The geology provides boundary conditions needed to run environmental impact models that calculate resource depletion and regeneration under various scenarios of aggregate extraction. Such analyses are important in monitoring progress towards good environmental status, as outlined in the Marine Strategy Framework Directive. By including uncertainty, data products can be generated with confidence limits, which is critical for assessing the significance of changes in the habitat or in any other resource-relevant parameter. All of the information is integrated into a cross-domain, multi-criteria decision support system optimised for user-friendliness and online visualisation