|A new approach for managing the Western Scheldt's morphology and ecology|
Plancke, Y.M.G.; Peters, J.J.; Ides, S.J. (2008). A new approach for managing the Western Scheldt's morphology and ecology, in: 32nd Congress of IAHR, the International Association of Hydraulic Engineering and Research. Abstracts: Harmonizing the Demands of Art and Nature in Hydraulics. Engineering and Management of Fresh-water Systems. Data Acquisition and Processing for Scientific Knowledge and Public Awareness. Fluid Mechanics and Hydraulics. Maritime and Coastal Research and Engineering, July 1-6, 2007, Venice, Italy. pp. 233
ANE, Nederland, Westerschelde, Walsoorden [Marine Regions]
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- Evaluation of the in situ disposal test near the Walsoorden sandbar 2004, meer
An expert team appointed by the Port of Antwerp proposed the idea of morphological dredging, aiming at steering the estuarine morphology. The idea was based on positive experience with morphological dredging in the maritime reach of the Congo River in Africa. As a first step of morphological management of the estuary, the experts proposed for the Western Scheldt a new disposal strategy, using the sediment dredged in the fairway to modify the shape of the sandbars and channels system. As a pilot project the reshaping of the eroded tip of the Walsoorden sandbar was proposed. The sediment stored in the hopper would be pumped to a pontoon equipped with a diffuser, in order to dispose it as quietly as possible close to the bottom. In this way, almost all the material would remain in place where deposited. If successful, this strategy could be applied on other locations and would not only make the estuary ecologically and morphologically healthier, but also possibly reduce the quantity of material to be dredged on the crossings if the self-dredging capacity of the flow could be increased.
A research programme for investigating the feasibility of the idea was conducted by Flanders Hydraulics Research in 2002 and 2003, combining several tools: desk studies on historical changes with maps, field measurements including flow and sediment transport, physical scale model tests and numerical models. The expert team concluded in 2003 that none of the results contradicted 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 the diffuser in relatively shallow water at the seaward end of the Walsoorden sandbar. The experiment was thoroughly monitored with frequent multi-beam bathymetric surveys, several LIDAR-flights, marked sediment tracing, in-situ flow and sediment measurements and an extensive ecological monitoring. One year after the execution of the in situ disposal test, it was concluded that from morphological viewpoint the test was a success. Also the ecological monitoring revealed no significant negative changes in trends due to the disposal test.
In 2006 a new disposal test was executed, using the traditional releasing technique from hopper dredgers in deeper water. 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. Since this second disposal test, a total of 700.000 m3 was used for successfully reshaping the sandbar. Up to now, the morphological outcome is according to the expectations. The report on the ecological monitoring was issued in May 2007 and is under review. The preliminary results did not show any negative effect on nature values of the site. The experiment goes on and it is