The future of the Oosterschelde with a new inlet channel

Abstract

After the storm surge of 1953, the Dutch Delta project was initiated in order to protect the southwestern part of The Netherlands. A storm surge barrier in front of the Oosterschelde and various dams at the back of the estuary were constructed. These interventions led to a large change of the hydrodynamics of the Oosterschelde: a large decrease in tidal volume and flow velocities. This decrease in flow velocities caused a decrease in sediment transport from the channels with about 75%. It is estimated that an amount of 400-600 million m3 of sediment is necessary to increase the flow velocities, restore the sediment transport from the channels and to obtain a new dynamic equilibrium (Kohsiek, 1987). This need for sand is called the ‘sand demand’. At present, the shoal height inside the estuary decreases by wave erosion. This decrease in shoal height mainly has a negative influence on the protected nature in the Oosterschelde. The Oosterschelde was ebb dominant and exporting sediment for centuries. All the events and interventions from 1530 up to the construction of Volkerakdam and Grevelingendam in 1969, caused an increase in tidal prism and export of sediment towards the ebb tidal delta. By the construction of the storm surge barrier, Philipsdam and Oesterdam in 1986, the situation changed, the tidal prism decreased and the ‘sand demand’ started. This research is aimed at finding a structural solution for the ‘sand demand’ by opening the storm surge barrier. The present situation of the Oosterschelde and a future situation with a new inlet channel at Neeltje Jans are analyzed in order to determine if a new inlet channel could influence the hydrodynamics and sediment transport in order to structurally solve the ‘sand demand’. A process based hydrodynamic and morphological model (Delft3D) is used to analyze the present and possible future situations with a new inlet channel. The new model and the methods of Van de Kreeke (1993) and Groen (1967) applied to the present situation of the basin, show that the Oosterschelde is still ebb dominant and would be exporting fine and coarse sediment if the inlet would not block the sediment transport. This ebb dominance follows from the large intertidal area and deep channels. Notwithstanding the ebb dominance, there is no sediment export possible through the inlet. The inlet blocks the sediment transport in both directions mainly because of a ‘tidal jet’, caused by the small inlet and large tidal prism. The tidal prism increases with a new inlet channel and thus increases the flow velocities in the channels. The increase in tidal prism and thus flow velocities brings the Oosterschelde closer to the old situation. The higher flow velocities increase the sediment transport from the channels and thus increase the shoal building. It is not known how much the shoal building is exactly restored. Some channels have such an increase in flow velocities that shoal building occurs again. However, parts of the basin are still not in equilibrium, which can be seen from comparing the old with the new flow velocities and by comparing the tidal prism and the cross-sectional areas of the channels with the empirical relations of Louters (1998) and Haring (1976). An important disadvantage of an increase in tidal prism is the enhancement of the ebb dominance that causes more sediment transport in ebb direction. However there is no export possible through the new inlet channel, because also the new inlet channel has a ‘tidal jet’ that blocks all sediment transport through the inlet. The large-scale effects of the Oosterschelde, like the ebb dominance and ‘sand demand’ cannot be structurally changed a new inlet channel. However the shoal degradation rate will probably be slowed down with an increase in tidal prism.