Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme

The authors are solely responsible for the content of this report. Material included herein does not represent the opinion of the European Community, and the European Community is not responsible for any use that might be made of it.
Back to overview measures

Walsoorden pilot part A (2004): relocation of dredged sediment to a shallow water area at the edge of the Walsoorden sandbar

Measure description

This project fits in with "The Long Term Vision 2030 (LTV) for the Scheldt estuary" which presents a view on the preferred functioning of the system, accepted by both the Dutch and the Flemish government. One of the main questions considered in the LTV was where to relocate the large volumes needed for further deepening and widening of the navigation route, respecting the preservation of the estuary’s physical system characteristics. An international expert team proposed that strategic relocation of dredged sediment could fit in a proactive morphological management strategy as an instrument to improve the morphology of the Western Scheldt, for instance by steering the development of channels and shoals. As a pilot project to test this strategy, the expert team proposed to relocate sediment at the eroded tip of the Walsoorden sandbar.

Since 2002, this new relocation strategy is being investigated as a pilot project on the Walsoorden sandbar in the Western Scheldt. An extended research was conducted in 2002 and 2003, combining several tools: desk studies with maps on the historical morphological changes, field measurements, physical scale model tests and numerical modeling. As a result it was concluded that none of the results contradicted the feasibility of the new relocation strategy at the Walsoorden sandbar, although final judgment would only be possible after the execution of an in situ relocation test. This project would test the stability of the relocated sediments. Indeed, it was the fear of some morphologists and ecologists that the material would not be stable, possibly inducing negative effects in the estuary.

During one month (17 November – 20 December 2004), 500.000 m³ of dredged material (sand) was almost continuously relocated with a diffuser in relatively shallow water at the seaward end of the Walsoorden sandbar (Figure 1). The dredged material originated from regular maintenance dredging works in the navigation channel in the Western Scheldt (Westerschelde). The amount of 500.000 m³ for the in situ relocation test was chosen in order to be detectable from bathymetrical viewpoint. On the other hand this amount is small enough not to induce irreversible negative effects if negative effects would occur.

The dredging vessel (self-discharging hopper dredger) was connected to a floating pipeline through which the sand is transported to a pontoon “Bayard II” (Figure 3). On this pontoon the sand is pumped to a diffuser (Figure 3) that relocates the sediment in an accurate way on to the bottom, with minimal disturbance of the local environment. This methodology is much more accurate for relocation in shallow waters compared to the traditional method of relocation (so-called “clapping”), which involves the hopper of the dredging vessel being opened so that the material is released in the water column just under the keel of the ship, from where it will sink to the river bed (Vos et al. 2009).

Back to top

Important to know

Reports / Measures / Tools

Report: Management measures analysis and comparison