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

The Interreg IVB North Sea Region Programme

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TIDE pilot 1: large scale execution “plaatrandstortingen” (2010), relocation of dredged sediment to shallow water areas near sandbars

Monitoring results

The implementation of the sediment relocation at the three sandbars is still in progress, but we will here describe the predicted effects and some preliminary results for the first two years (2010-2012).

Expectations based on prior research studies

The expected results as discussed here are based on the environmental impact assessment, appropriate assessment, in-situ pilot projects Walsoorden 2004 and 2006, and an extensive research program. The global objective of those studies was to define a relocation strategy that maximises the ecological benefit. First of all, all four of the projects are positive in the light of future dredging activities because they offer new (semi) permanent relocation sites. But even more important, the results suggest that the relocation of sediment at the sandbars could improve the presence of low dynamic intertidal mudflats and shallow areas in the Western Scheldt (Figure 4 - Figure 6).

Based on the specific location on the sandbar where the sediment is relocated, two types can be distinguished. On one hand both the relocation sites near the Walsoorden sandbar and the Hooge Platen West are located near the seaward tip of the sandbar, attacked by flood currents. It is expected that sediment relocated at these locations, will be gradually transported towards the sandbar. On the other hand the relocation sites near Rug van Baarland and Hooge Platen North are located along the sandbar, guiding the currents. It is expected that sediment relocated at these locations, will be rather transported along the sandbar.

Another difference was found in the flow dynamics: the 2 locations near Hooge Platen (North and West) are characterized by far more dynamic conditions, both hydrodynamic (factor 1.5 for flow velocities) as morphodynamic (factor 2.5 for sediment transport) in comparison with the location Rug van Baarland and Walsoorden sandbar (Plancke et al. 2010).

The main objective of the extensive research program was to adapt the proposed relocation sites to maximise the creation of low dynamic shallow and intertidal areas. In total, the aim is to create 114 ha of new low dynamic areas of which 85% littoral and 15% sublittoral (Plancke et al. 2008). Different strategies were used in the four measures to realise these ecologically valuable areas. The relocation at Walsoorden and Hooge Platen West is similar: a subtidal macro-dune is constructed to slow down the current velocity. At the Hooge Platen North, the material was disposed in the shape of two ‘arms’ enclosing a low dynamic area. Finally, at the Rug van Baarland, a shallow area was built to slow down the current and create an ecological valuable low dynamic area.

From the numerical calculations was concluded that the realisation of low dynamic areas at the Walsoorden sandbar will be limited because of strong current velocity gradient along the sandbar. However, also a reduction in current velocity is predicted by which the opportunity that low dynamic areas will develop increases.
Because the current velocity gradient over the Rug van Baarland is already small, a limited current velocity decrease caused by the construction of extra shallow area can provide a strong increase in low dynamic area.
Although the area around the Hooge Platen is highly dynamic, a positive effect on the development of low dynamic areas is expected for the Northern part. On the Western part, however, only a very limited effect is expected. The reconstruction of the sandbar tip is indeed very difficult because of the high dynamics.

Preliminary results (2010-2012) regarding “Ecological benefits from the relocation activities” (IMDC 2012)

In 2012, two years after the start of the relocation activities, the stability of the sediment is positive at three of the four locations (Figure 7). Hooge Platen Noord and Rug van Baarland show stability rates of >100%, this means that the relocation goes along with natural sedimentation. At Hooge Platen West the stability after two years is 70%, however the stability varies between 60% and 80%. Only at the Walsoorden sandbar the stability was too low (62%). The main problem at the Walsoorden sandbar was the sediment instability during the relocation activities. After the relocation, the sediment was mainly stable.

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