Interaction between tidal basins and ebb-tidal deltas

Ebb-tidal delta

The evolution of the ebb-tidal delta of the Eastern Scheldt between the 16th and 20th century is mostly governed by the continuous increase in tidal prism after the 1530 storm event. This increase must not only have led to a general increase of the sediment volume of the delta, but also to the development of the various channels running through this area. The orientation of these channels does not seem to be influenced much by the developments within the basin in this period. The historic evolution of the Easter Scheldt outer delta has been described and analysed in detail in Eelkema (2013).

By the end of the 20th century both back-barrier dams and the storm surge barrier were constructed, both resulting in changing processes in the inlet area and on the ebb-tidal delta. Observed behaviour and model results have shown that the ebb-tidal delta is still far from any kind of morphological equilibrium and that it is yet unclear what the new equilibrium state of the ebb-tidal delta will be like , or how long it will take for this new state to be reached.

Detailed information about the influences on the ebb-tidal delta due to the construction of the back-barrier dams and the storm surge barrier is given in the section below.

Influence of the back-barrier dams

The closure of the Volkerak Channel by one of the back-barrier dams amplified the existing trend of scour and export of sediment in the inlet area and on the ebb-tidal delta. It caused an increase in tidal flow, and consequently an increase in sediment transport. Of the main inlet channels, the Roompot, the southernmost channel, conveyed most of the tidal volume and transported most of the sediment. Seaward of the inlet, parts of the proximal ebb-tidal delta gave way to the ebb channels coming from the inlet, which were growing deeper and longer as the tidal volume passing through the inlet increased.

A considerable part of the deepening inside the Eastern Scheldt since 1960 resulted from dredging. The total natural erosion in the period 1960-1986 is estimated at 40 million m3 (Louters et al. 1998, Eelkema et al. 2012b). The average export rate in this period must therefore have been about 1.5 million m3 per year. The export is likely to have shown a peak around 1970 and to have decreased from that time on. Model results show that the export in 1968 may well have been twice that in 1983, with the same hydrodynamic forcing and a closed Volkerak. This decrease is the result of the basin’s bathymetry adapting itself to the closure of the Volkerak.

On the ebb-tidal delta, the increased flow from the inlet amplified the morphological activity. The fact that this increased activity persisted even after the sediment supply from the basin had dropped indicates that this activity is primarily driven by the stronger flow. The main ebb channels straightened and grew longer, making them more efficient in depositing sediment further away on the main ebb shields and the terminal lobe. The ebb shields located on the northern edge of the ebb-tidal delta were pushed into a region with stronger residual flow. This could explain why the growth and migration speed of these elements hardly decreased over time.

Influence of the storm surge barrier

In response to the construction of the storm surge barrier in 1986, the morphology of the Eastern Scheldt has been changing. Bathymetric surveys show multiple effects of the barrier on the morphodynamics of the ebb-tidal delta:

  1. an overall decrease in sediment volume,
  2. a decrease in morphological activity,
  3. erosion of the shoals and sedimentation in most channels,
  4. northward reorientation of channel-shoal pattern, and
  5. an increase of wave-related features.

Process-based numerical modelling, meant to gain insight into the mechanisms behind the observed behaviour, shows that the erosion is related to wave action, and that the reorientation is related to the interaction between cross-shore and alongshore tidal flow. Because the cross-shore current out of the inlet decreased in strength, the alongshore component gained in importance. As a result, the channels rotated clockwise, and the shoals expanded northwards.The combined results of data analysis and modelling are illustrated in the top figure.

Generally speaking, the shallow parts of the ebb-tidal delta are eroding, while the deeper parts gain sediment. Yet, this trend is not seen everywhere on the ebb-tidal delta. The channels close to the shore have scoured since the barrier’s construction, even though the tidal current through these channels has decreased. Probable causes of this erosion are stronger residual flows and tidal asymmetries.

The driver of this redistribution of sediment is the weakened tidal current in and out of the estuary, due to which longshore tidal currents and wave action became relatively more important. The overall weakening of the tidal currents also caused a strong decrease in morphological activity. Nonetheless, the sediment budget still shows a distinct erosive trend, meaning that the bed-level changes, though being smaller in magnitude, have become more negative

With the process-based model we have gained insight into the processes governing the sediment redistribution and the associated transport paths. The simulations indicate that wave forcing is important in the reduction of the sediment volume of the ebb-tidal delta. In general, waves on Dutch ebb-tidal deltas are dominant on the shoal areas and tend to transport sediment onshore. They also cause higher sediment concentrations through enhanced bed shear stress, wave breaking, and stirring. The model needs wave forcing to adequately reproduce the observed trend in the hypsometry, with the deeper parts gaining sediment and the shallow parts eroding. This trend is only reproduced when tidal forcing and wave forcing are both taken into account.

Import of sediment onto the ebb-tidal delta still occurs in the southwestern part, with the flood-dominated current pushing sediment through Southern Roompot channel and up the Hompels shoal, see the bottom figure. Sediment transport over the Banjaard shoal is mainly directed northward. As a result of the barrier, the ebb currents coming out of the inlet have less transport capacity, but still transport sediment away from the inlet and onto the distal parts of the ebb-tidal delta, where it is further reworked by a combination of wave and tidal action. Two main deposition areas for this sediment where this mechanism is most visible are the northern ends of the Krabbengat and Banjaard Channels.

The ebb-tidal delta of the Eastern Scheldt behaves differently from the ebb-tidal deltas of closed inlets such as the Grevelingen and Haringvliet. On these ebb-tidal deltas, the shoals are pushed shoreward into large intertidal shore-parallel bars by the waves. The development of these bars is absent at the Eastern Scheldt, probably because the tidal current is still strong enough to prohibit the cross-shore wave-driven sediment transport from building up these bars.

With the knowledge gained from observed behaviour and model results, a view emerges of an ebb-tidal delta which is still far from any kind of morphological equilibrium, and which is steadfastly adapting itself to the new hydraulic forcing regime, even though sediment transport capacities have decreased. It is yet unclear what the new equilibrium state of the ebb-tidal delta will be like, or how long it will take for this new state to be reached. So far, the measured trends showed no sign of levelling out. The future ebb-tidal delta will become smoother, with smaller depth differences between shoals and channels. Preliminary model results show that the adaptation may well take more than 200 years, which is beyond the design lifetime of the barrier.