|Morphological management in estuaries conciliating nature preservation and port accessibility|
Plancke, Y.M.G.; Peters, J.J.; Ides, S. (2006). Morphological management in estuaries conciliating nature preservation and port accessibility, in: Peeters, Y. et al. (Ed.) Seminar: Flanders, a maritime region of knowledge (MAREDFlow), 24 March 2006, Flanders Marine Institute (VLIZ), Oostende, Belgium. VLIZ Special Publication, 29: pp. 35-54
In: VLIZ Special Publication. Vlaams Instituut voor de Zee (VLIZ): Oostende. ISSN 1377-0950, meer
Morphological dredging; Ecomorphology
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- Evaluation of the in situ disposal test near the Walsoorden sandbar 2004, meer
In 1999, Flanders and The Netherlands agreed to set up a common strategy for managing the Scheldt River in its estuarine reach. In 2002, both parties signed a memorandum of understanding in which was defined a ‘Long Term Vision’ strategy and its objectives. One of these is the preservation in the Western Scheldt of a dynamic and complex flood and ebb channel network, the so-called ‘multi-channel system’. The present trend, a continuation of past natural morphological evolutions combined with human interference (poldering, dredging and other river works) may jeopardise this objective. An expert team appointed by the Port of Antwerp proposed the idea of morphological dredging for curbing this negative trend, aiming at steering the estuarine morphology. In a first phase, sediment from dredging works could be used to reshape sandbars where needed. One case study is discussed in this paper, the aim being to reconstruct the eroded tip of a sandbar at a bifurcation so that the flood and ebb flows would be perseved, a condition to maintain the multi-channel system in the reach. The strategy would not only cut back on the ongoing degradation of the ecological and morphological values of the estuary, but it could also possibly help reducing the quantity of material to be dredged on the crossings by increasing the scouring or self-dredging capacity of the flow. A diffuser-type device was used to disperse the dredged material in a controlled way in shallow water along the sandbar edges. In 2002-2003, the new disposal strategy has been investigated by Flanders Hydraulics Research as a pilot project (Plaat van Walsoorden). The research programme combined three tools: field measurements, physical scale models and 3D numerical models. The results of the research work confirmed the feasibility of the idea. However, the Port of Antwerp Experts concluded that a real life (in situ) disposal test was required to give final proof of the feasibility of this new disposal strategy. At the end of 2004, 500,000m³ of sand was disposed at the seaward tip of the shoal of Walsoorden using a diffuser. The main idea was to modify the morphology of this sandbar by disposing dredged material very precisely. The amount of 500,000m³ was chosen because it is large enough to see an effect of the disposed sediment, while it is small enough to be reversible if something would go wrong. To evaluate the success of this in situ test, an extensive monitoring programme was set up, including bathymetric surveys, ecological monitoring, sediment tracing tests and sediment transport measurements. After one year of monitoring the disposed sediments, it can be concluded that the experiment is very successful. The morphological monitoring showed that almost 80% of the disposed sediments is still on the disposal location after one year. The ecological monitoring did not reveal any significant negativeimpact, neither in the intertidal areas, nor in the subtidal areas. This in situ test confirmed the feasibility of the proposed disposal strategy. An estimated volume of 4 to 5 million m³ could be disposed here to reach the proposed objectives, representing more than half of the volume dredged yearly in the Western Scheldt.