The dynamical behaviour of shallow-marine dunes

In the period 1995-1999, a detailed morpho- and sediment dynamical investigation was carried out in the western Belgian near coastal area, in water depths of -3 to -15 m MLLWS. Through chronosequential bathymetrical and digital side-scan sonar registrations supplemented by intensive samplings, a quantitative monitoring of the most dynamic zones was accomplished. Although, the area is generally devoid of bedforms, well-constraint zones of very large dunes occur where the bed shear stresses are highest. Since, suspended load accounts for more than 80 %, the dimensions seem to be largely controlled by the amount of the total load actually taking part in bedform development. A clear differentiation exists between the morphology of the dunes and their surficial sediments, the troughs being up to 0.30 phi finer than the crests. During the observation period, the similarities in crestline positions were more striking than the differences and showed a maximum shift of only 20 m. The data set allowed to differentiate the vulnerability of the area to varying hydro-meteorological conditions and showed that the area recovers fairly quickly from stormy periods

Decifering mega-ripple variability in an anthropogenically steered environment: implications for mine burial studies

In 2007 the Ministery of Defence, in collaboration with Ghent University, developed a project on the understanding of mega-ripple variability in view of improving mine burial prediction models in sandbank areas. Results will assist in the monitoring of sea-mines, heritage of two World Wars, nowadays partially or totally buried by sandy bedforms

Spatio-temporal variation of surface suspended particulate matter concentration in the Belgian-Dutch coastal zone

sensing (MODIS-Aqua) data, were evaluated for their use in the assessment of coastal turbidity maximum (CTM) dynamics in Belgian coastal waters. The CTM is a dynamic coastal feature of which the geographic position and extent varies under different meteorological, astronomical and climatological conditions. Analyses were based on grouping-averaging of SPM concentration maps, using different classification schemes. To better spatially depict the CTM, entropy grouping was introduced. This technique analyses, per pixel, the total information contained within the probability distribution of SPM concentration. Results revealed wind-induced variations in position and extent of the CTM, with southwesterly winds inducing a largest CTM extent, in contrast to a strong reduction under northeasterly winds. Climate-induced variations were assessed contrasting 2 winters with opposing indices of the North Atlantic Oscillation (NAO). In a winter with a positive NAO index, hence stronger-than-average southwesterly winds, the CTM was extended to the Dutch waters, whereas the opposite occurred in winters with a negative NAO index, hence less-than-average southwesterly winds. To evaluate astronomical forcing (tides) grouping-averaging was performed of SPM concentration maps over a tidal cycle, and spring-neap conditions. Although, only part of the tidal cycle can be analysed, due to the sun-synchronicity of the MODIS-Aqua satellite, comparison of the results with in-situ data from a single observatory station showed good resemblance. It is concluded that MODIS-Aqua satellite data can be used to assess SPM concentration variability related to tides, neap-spring cycles, meteorological and climatological events

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

Mine burial in the seabed of high-turbidity area (Belgian coastal zone): findings from a first experiment

Suspended particulate matter; particle size distribution; statistical handling; coastal turbidity maximum; wind impact; seabed variations The seabed of the North Sea is covered with ammunition dating back from World Wars I and II. With increasing human interference (e.g. fisheries, aggregate extraction, harbour related activities), it forms a threat to the safety at sea. In this study, test mines were deployed on a sandy seabed for three months to investigate mine burial processes as a function of hydrodynamic and meteorological conditions. The mine experiment was conducted in a shallow (9 m), macrotidal environment characterized by highly turbid waters (yearly and depth-averaged suspended particulate matter concentration of 100 mg l-1). Results showed some variability of the overall mine burial, which corresponded with scouring processes induced by a (sub-) tidal forcing mechanism. The main burial events however were linked to storm-related scouring processes, and subsequent mine roll into the resulting pit. Two storms affecting the mines during the 3-month experiment resulted in enduring increases in burial volume to 60% and 80%, respectively. More cyclic and ephemeral burial and exposure events appear to be linked to the local hydrodynamic regime. During slack tides, suspended sediment settles on the seabed, increasing the burial volume. In between slack tides, sediment is re-suspended, decreasing the burial volume. The temporal pattern of this never reported burial mechanism, as measured optically, mimics the cyclicity of the suspended sediment concentration as recorded by ultrasonic signals at a nearby benthic observatory. Given the similarity in response signals at the two sites, we hypothesize that the formation of high-concentrated mud suspensions (HCMS) is a mechanism causing short-term burial and exposure of mines. This short-term burial and exposure increase the chance that mines are ‘missed’ during tracking surveys. Test mines contribute to our understanding of the settling and erosion of HCMS, and thus shed a light on generic sedimentary processes

Monitoring of the impact of the extraction of marine aggregates, in casu sand, in the zone of the Hinder Banks

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