Sediment dynamical and morphological study of a marine shoal: the Paardenmarkt, Belgian Continental Shelf: contribution of grain-size analysis, acoustic instruments, current modelisation (poster)

The Paardenmarkt is a marine shoal, located two kilometers east of the Zeebrugge harbour (51°20’N-03°16’E), in water depths varying between 3-7m. This site is characterized by the presence of shallow biogenic gas. Moreover, this site forms a dangerous area for shipping and fishing because it was used as a dumpsite for WWI bombs. For this reason, the understanding of the sedimentary transport is important to estimate the future evolution of this site. This study combines acoustic data (side-scan sonar, single- and multibeam, acoustic doppler current profiler), current modeling and grain-size analysis of surficial sediment samples to evaluate the relative contribution of the hydrodynamic and morphological effects on the sedimentary transport in the area. The hydrodynamic results show a dominance of the flood tidal current for the entire study area. However, locally near the Scheldt Estuary, the ebb current dominates as an effect of the Scheldt outflow. Based on the grain-size analysis, a sediment map is made in combination with the seabed morphology and permits a classification of 5 sediment types. The residual sediment transport, based on the grain-size data, is estimated by a numerical model (Gao and Collins, 1991) and is related to seabed features observed on the acoustic data (side-scan sonar and multibeam). Four different sedimentary transport zones are distinguished with this method: Ä in the western part of the study area, the flood currents dominate and influence the sedimentary transport;Ä in the southern part, the gyre created behind the Zeebrugge harbour influences the sedimentary transport;Ä in the central part, no significant residual transport is observed;Ä to the east, near the Scheldt Estuary, the sedimentary transport is influenced by the amplification of the ebb currents. This amplification is due to a minimal addition of freshwater and the contrasted morphology of the estuary, which canalizes and accelerates the ebb current.This case study was carried out in the framework of an evaluation study of the anthropogenic influence of the harbour construction between 1954-2000. Due to the construction of the harbour, a gyre is generated resulting in a sedimentary accretion south and erosion north of the Paardenmarkt site

QUantification of Erosion/Sedimentation patterns to Trace the natural versus anthropogenic sediment dynamics - QUEST4D

Sustainable development requires the quantification of human impacts, against the seafloor’s ecological value. Recent impact studies have shown only localised effects, though indications of a longer-term and broader-scale degradation of the seafloor exist. This is due possibly to cumulative anthropogenically-induced effects, but the natural evolution and the response of the seafloor due to sea-level rise are poorly known. Nonetheless, it is likely that changing wave climate and an increased storminess induce different erosion/sedimentation patterns. Such evolution needs to be evaluated against the impact of dredging, aggregate extraction, fisheries and beach replenishment on the ecosystem’s physical functioning. The research proposal will focus on the sand and mud balance of the Belgian shelf, coupled to the Schelde Estuary and the coastal zone. The methodology will consist of: advanced modelling, validated with experiments; targeted observations/samplings, within the space, depth and time domain (4D); and various long-term datasets. Predictions will be made using different sea-level rise scenarios. The quantification of erosion/sedimentation processes is important for: the development of criteria for a sustainable exploitation/management and sustainable coastal protection schemes; allocating efficient dumping grounds; understanding coastal habitat change; and the prediction of sources/sinks of pollutants. QUEST4D is a Belspo SPSDIII funded strategic research network

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