Disposal strategy to create ecological valuable habitats in the Western Scheldt estuary

To guarantee optimal accessibility to the port of Antwerp, a deepening of the navigation channel was proposed within the Long Term Vision for the Scheldt estuary. An environmental impact assessment and an appropriate assessment were carried out for this project. From these studies it was concluded that the most preferred scenario for the enlargement of the navigation channel includes a strategy aiming at disposing dredged material near 3 sandbars in the Western Scheldt, creating opportunities for nature. To determine the optimal disposal strategy on a detailed and more practical level, research was performed by Flanders Hydraulics Research. The main objective of this study was to determine a detailed disposal strategy in order to maximize the ecological benefits, i.e. maximizing the potential creation of low dynamic intertidal and shallow water area. This paper describes the studies undertaken by Flanders Hydraulics Research to determine the most optimal disposal strategy. In a next part the most optimal disposal strategy resulting from the research will also be presented, as well as the strategy being used to evaluate this new disposal strategy

A new approach for managing the morphology and ecology of a coastal plain estuary

An expert team appointed by the Port of Antwerp proposed the idea of morphological dredging, aiming at steering the estuarine morphology. A pilot project was implemented, modifying the shape of a sandbar by disposing dredged material on the eroded tip of the Walsoorden sandbar. The new shape would help restoring the degraded ecology and morphology, but it also aims at modifying the flow on a crossing in the navigation channel, reducing the dredging effort if the self-dredging capacity of the flow could be increased on it.A research programme about the feasibility of the idea was conducted in 2002 and 2003, combining several tools: desk studies on historical changes with maps, field measurements, physical scale model tests and numerical simulations. The expert team concluded in 2003 that none of the results denied the feasibility of the new disposal strategy, although final judgement would only be possible after the execution of an in situ disposal test.During one month at the end of 2004, 500.000 m³ of sand was disposed with a diffuser in relatively shallow water at the seaward end of the sandbar. The experiment was well monitored with frequent multi-beam bathymetric surveys, LIDAR-flights, marked sediment tracing, in-situ sediment measurements and ecological monitoring. After one year, it was concluded that the test was a success from morphological viewpoint. Also the ecological monitoring did not reveal any significant negative changes in trends due to the disposal test.In 2006 a new disposal test was executed, using the traditional technique with hopper dredgers. Due to practical limitations, the disposal (again 500.000 m³) was spread over a 3 months period. The new experiment was again thoroughly monitored for morphology as well as for ecology. Up to now, the morphological results of the second test are satisfying. The ecological results will become available during the first half of 2007, with the preliminary results not showing any negative impact

A synthesis on the assessment of an alternative disposal strategy to serve sustainability in the Scheldt estuary

The Scheldt estuary is characterised by a valuable multiple-channel system consisting of sandbars in between primary and secondary channels. Within the framework of the Long Term Vision (LTV) for the Scheldt estuary the conservation of this multiple-channel system is defined as the key goal to achieve morphological sustainability.Dredging is one of the human activities having an impact on morphology, hence dredging and disposal should be optimised by minimising the negative impact on morphology. The current disposal strategy for maintenance dredging consists mainly of disposing sediment into the secondary channels. As a consequence of the sustainability goal a maximum dumping capacity is assigned to each secondary channel in order not to exceed the natural bearing capacity of the multiple-channel Scheldt estuary.The assigned yearly disposal capacity for all secondary channels together is sufficient to counter the yearly sediment volume arising from maintenance dredging operations. However, if the total sediment volume that is produced by the deepening of the navigation channel towards Antwerp would be disposed into the secondary channels, the conservation of the multiple-channel system would not be guaranteed. To anticipate on this expectation, shallow water sandbar slopes have been proposed as an alternative disposal location. The use of these alternative disposal locations provides an increase in the estuary’s disposal capacity without increasing the disposal intensity in the secondary channels and is expected to enlarge the area of shallow water habitats, creating opportunities for ecosystem development.The feasibility of this proposal has been investigated by desk studies on historical maps, by field measurements, by physical scale model tests and by numerical simulations and has been confirmed by two in situ experiments. Consequently, the alternative disposal locations will be used during the deepening of the navigation channel towards the port of Antwerp. As a result of the adaptation of the disposal strategy, the conservation of the valuable multiple-channel system in the Scheldt estuary is not endangered by dredging and disposal operations