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Further analysis is needed on the applicability of this measure to several other locations in the Western Scheldt (Westerschelde). Commonly known knowledge gaps are on the understanding of sediment transport pathways and resulting sedimentation and erosion patterns and on the inhabitation of benthic macrofauna of new shallow areas.
The second relocation test (2006) proved that also the traditional clapping technique is successful. This technique has the advantage to execute the relocation much quicker compared to the diffuser technique used for the first relocation test in 2004. In this way the relocation along sandbars can be integrated in the continuous maintenance dredging works. The disadvantage of the clapping technique is however that a minimum depth is needed depending on the draft of the used hopper. The technique with the diffuser can, in contrast, be used at shallow water and is much more precise to relocate material at certain spots.
Both relocation tests were successful (all criteria were met), but both tests involved however only very limited relocation quantities which amount to just part (20%) of the relocation quantities that are necessary for full reconstruction of the sandbar point. Extensive monitoring is therefore still required to further investigate the impact of full-scale relocations on the local morpho- and hydrodynamic conditions (Vos et al. 2009). Initially, this would be to see whether the results remain positive with larger volumes of relocated material and, on the other, to evaluate a number of desired effects that could not have been checked during the relocation tests due to their limited nature. Some examples (WL 2006, Vos et al. 2009):
As a result of the success of the relocation tests in 2004 and 2006, it was decided that this new relocation strategy (whereby dredged material is relocated along sandbar edges) would be expanded to multiple locations in the Western Scheldt (Westerschelde) (Vos et al. 2009). This is analysed as a separate measure: sandbar relocations 2010 (‘plaatrandstortingen’).
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Walsoorden pilot part B (2006): relocation of dredged sediment to a shallow water area at the edge of the Walsoorden sandbar
Table of content
- 1. Description of measure
- 1a. Measure description
- 1b. Monitoring
- 1c. Monitoring results
- 2. Execution of main effectiveness criteria
- 2a. Effectiveness according to development targets of measure
- 2b. Impact on ecosystem services
- 2c. Degree of synergistic effects and conflicts according to uses
- 3. Additional evaluation criteria in view of EU environmental law
- 3a. Degree of synergistic effects and conflicts according to WFD aims
- 3b. Degree of synergistic effects and conflicts according to Natura 2000 aims
- 4. Crux of the matter
- 5. References
Additional information
for this measure:
No further information available.
for this measure:
No further information available.
Crux of the matter
This second relocation test also proved that the new relocation strategy is feasible. When relocating sediment near the Walsoorden sandbar, the sediment is stable. Due to larger currents in the relocation area, a higher percentage of the material was transported towards the Walsoorden sandbar. This morphological evolution was seen as positive within the objectives of the relocation strategy. From ecological viewpoint no significant negative changes in trends have been identified.Further analysis is needed on the applicability of this measure to several other locations in the Western Scheldt (Westerschelde). Commonly known knowledge gaps are on the understanding of sediment transport pathways and resulting sedimentation and erosion patterns and on the inhabitation of benthic macrofauna of new shallow areas.
The second relocation test (2006) proved that also the traditional clapping technique is successful. This technique has the advantage to execute the relocation much quicker compared to the diffuser technique used for the first relocation test in 2004. In this way the relocation along sandbars can be integrated in the continuous maintenance dredging works. The disadvantage of the clapping technique is however that a minimum depth is needed depending on the draft of the used hopper. The technique with the diffuser can, in contrast, be used at shallow water and is much more precise to relocate material at certain spots.
Both relocation tests were successful (all criteria were met), but both tests involved however only very limited relocation quantities which amount to just part (20%) of the relocation quantities that are necessary for full reconstruction of the sandbar point. Extensive monitoring is therefore still required to further investigate the impact of full-scale relocations on the local morpho- and hydrodynamic conditions (Vos et al. 2009). Initially, this would be to see whether the results remain positive with larger volumes of relocated material and, on the other, to evaluate a number of desired effects that could not have been checked during the relocation tests due to their limited nature. Some examples (WL 2006, Vos et al. 2009):
- Improved distribution of the tidal flow between the ebb and flood channel;
- An increase of the velocities in secondary channels adjacent to the Walsoorden sandbar, particularly above the Hansweert sill. This will allow the self-eroding capacity to increase and dredging efforts to be reduced;
- Enrichment of the shallow water and inter-tidal areas with finer granular fractions as a consequence of a reduction of the flow speeds in these areas.
As a result of the success of the relocation tests in 2004 and 2006, it was decided that this new relocation strategy (whereby dredged material is relocated along sandbar edges) would be expanded to multiple locations in the Western Scheldt (Westerschelde) (Vos et al. 2009). This is analysed as a separate measure: sandbar relocations 2010 (‘plaatrandstortingen’).