The influence of changes in tidal asymmetry on residual sediment transport in the Western Scheldt
Bolle, A.; Wang, Z.B.; Amos, C.; De Ronde, J. (2010). The influence of changes in tidal asymmetry on residual sediment transport in the Western Scheldt. Cont. Shelf Res. 30(8): 871-882. http://dx.doi.org/10.1016/j.csr.2010.03.001
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| Keyword |
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| Author keywords |
Tidal constituents; Tidal propagation; Sediment transport; Estuaries; |
| Authors | | Top |
- Bolle, A., more
- Wang, Z.B., more
- Amos, C.
- De Ronde, J.
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| Abstract |
This study fits into a wider research program from RWS RIKZ concerning the exchange of sediment between the coast and the tidal basins, Western Scheldt and Wadden Sea, which are the largest basins along the Dutch Coast, over different time-scales. For both basins, questions about the evolution of the import/export at the mouth recently arose. In case of the Western Scheldt, which is the subject of this study, mainly the uncertainty about the future developments after the change from import to export at the mouth was noticed in the 1990s, which necessitated a more detailed study of this area.
To understand the sediment exchange between the Dutch coast and the tidal basin Western Scheldt, the mechanism responsible for this transport must be identified. This study focuses on only one possibly important mechanism: the interaction between the tides and bathymetry.
An existing numerical, depth-averaged Delft3D model of the Western Scheldt has been applied. The bathymetry of the years 1970, 1983 and 2002 was selected based on previous sand balance studies. Thus three different situations at the mouth are represented: strong import, import and export of sediments. Each situation was forced by the same morphological tide, where a tide is selected in such a way that during ebb and during flood the computed sediment transport through a cross-section near Vlissingen is similar to the average ebb and flood transport for a complete spring-neap tidal cycle.
A comparison was made of the vertical tidal asymmetry in the model with field data for calibration purposes. Then, both the vertical and horizontal tides were analysed. Time series of water levels, velocities and discharges were harmonically analysed to derive the M2, M4 and M6 tidal constituents. The tidal asymmetry is expressed by the amplitude ratios and the phase differences. Furthermore the tide-driven sediment transport ascribed to a given bathymetry was determined. The residual transport patterns were analysed and a sand balance was derived from the modelled transports. This led to the evolution in time of the sediment exchange between, and the erosion/sedimentation within, different macro cells (Winterwerp et al., 2000) of the Western Scheldt.
The human interventions, the changes in tidal asymmetry and the evolution in the sediment transport interact. The increased landward transport inside the river is reproduced in the model; however, the change from sediment import to sediment export at the mouth could not be explained by tidal asymmetries only. Other processes such as winds and waves should also be studied to determine the mechanisms, which influence most of the observed sediment exchanges. |
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