As a part of the 'Study on density flows in the Lower Sea Scheldt in the framework of the Long-Term Vision for the Scheldt estuary', a physical scale model was constructed to acquire knowledge and data on the effect of training walls, artificial sills and other current deflecting devices on the exchange and density flows in the Lower Sea Scheldt.
The physical model study focuses firstly on the exchange now between a new tidal dock, Deurganckdok (DGD), and the Scheldt river, and secondly on the influence of a current deflecting wall (CDW) on this exchange flow.
The exchange of water between river and tidal dock is the result of the combined effect of I) tidal filling and emptying, 2) entrainment in the mixing layer between river and harbour and 3) density driven flow induced by salinity difference between river and harbour. Near high water and low water slack, the third effect dominates, whereas during the remainder of the tide all three factors are important. The flow pattern in the harbour entrance has a strong three-dimensional character.
The total amount of water exchange is estimated at 1.6 m3/tide (22x 106 m3/tide in prototype). With a total harbour volume of 2.0 m3 (27x106 m3 in prototype), the average residence time in the harbour is 1.2 tide. The maximum velocity in the harbour induced by eddies is about 4 cm/s (0.5 m/s in prototype). The maximum velocity because of density-driven exchange is 2.5 cm/s (0.3 m/s in prototype).
After the installation of a CDW, it is noticed that helical now, such as observed in the LIP-experiments, is not generated in the current configuration. Instead, downwelling in the harbour entrance is observed. Based on the data from the physical scale model, it is concluded that approximately half the near-bed inflow occurs in a period of the tide that the CDW cannot not be effective because of insufficient now energy at the river. The remainder of the near bed-inflow is reduced with about half as a result of the combined effect of CDW, sill and optimised river embankments.
Apart from the reduction by a CDW on siltation, it also reduces the strength of eddies in the harbour, which is favourable for navigation.
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