Interaction between tidal basins and ebb-tidal deltas

Sediment exchange

As described before, large sediment exchanges were governing processes in the Eastern Scheldt between the 16th and 20th century, due to the large tidal prism. After the construction of the back-barriers Philipsdam and Oesterdam in 1986 and the storm surge barrier, the tidal prism decreased, leaving the system in demand of sand. It is estimated that an amount of 400 to 600 million m3 of sand is necessary to restore the morphodynamic situation of before 1986. In a ‘natural’ situation and starting from an equilibrium situation, a decrease in tidal prism would generally cause import of sediment from the outer delta. Due to the presence of the barrier, however, there is almost no sediment exchange between the basin and the delta. More about the sediment transport through the barrier is described in this section (read more).

As the system is in demand of sand, possible measures for stimulating sediment import are investigated. Conclusions are summarised (read more), but none of the considered measures will be effective, given the large amount of sediment needed for the basin to re-establish an acceptable morphological equilibrium state. Most of the considered measures will cause no or just a limited increase of sediment import to the basin. The erosion problem of the intertidal flats due to the sediment demand in the channels cannot be solved by the suggested measures to stimulate sediment import. This means that probably only direct interventions on the tidal flats, such as nourishment and shoal edge protection, can solve the problem.

Sediment transport through the barrier

Observations suggest that the sediment exchange between the basin and the ebb-tidal delta is very limited since the construction of the barrier. This means that the sediment demand behind the barrier cannot be met and morphological equilibrium cannot be re-established. A possible cause for the limited sediment transport through the barrier is the presence of scour holes at either side of bottom protection next to the barrier. These scour holes have developed after construction of the barrier and have reached depths up to 60 m below mean sea level. In combination with the asymmetric flow velocities near the barrier, each scour holes acts as a sediment trap when the flow is directed to the barrier, thus blocking the sediment exchange between the basin and the delta.

A very important aspect of the sediment transport near the barrier is the asymmetry of the flow velocities. Because of the large water mass that is forced through the storm surge barrier, a tidal jet develops at the downstream. Flow velocities in these jets are much larger than those upstream of the barrier. Moreover, when leaving the area of the bed protection, the turbulence level is high and sediment content low. As a result, the jet picks up sediment beyond the edge of the bed protection and creates a scour holes.

  • During ebb, the tidal jet will develop at the seaward side of the barrier. The sediment that was deposited in the seaward scour holes during the preceding flood phase will be transported back to the ebb-tidal delta. On the landward side, however, flow velocities are not strong enough to carry sediment transport over the scour hole and through the barrier. The sediment will be deposited in the scour holes and stay there until it is picked up by the next flood jet.
  • During flood, this phenomenon reverses. The tidal jet develops on the landward side, scouring away the ebb-deposited sediment from the landward scour hole and carrying it back into the basin. On the seaward side, flow velocities remain low. The sediment originating from the delta will deposit in the seaward scour holes.

As a result, exchange of sediment through the barrier is hardly possible. This conceptual model is confirmed by the results of a 2D depth-averaged model for Roompot. Another hypothesis is that vertical eddies developing in the scour holes would block the sediment transport. Theory suggests that flow separation and reversal is to be expected for slopes of 1:5 and steeper. However, analysis of the bathymetry shows that the slopes at the Hammen and Schaar van Roggenplaat inlets are not steeper than 1:8. The slope at the Roompot inlet is slightly steeper, approaching 1:6. Based on theory, no large flow recirculation is expected here either. Also a 2D vertical non-hydrostatic numerical model of the Schaar van Roggenplaat scour hole shows no indication that flow separation takes place.

Possible measures for stimulating sediment import

Various measures to stimulate sediment import to the basin have been investigated. The conclusions are summarised below.

Adaptations to the scour holes

Three different adaptations to the scour holes have been investigated for their effectiveness to increase the sediment transport towards the basin (Hoogduin, 2009: filling the landward scour hole, filling the seaward scour hole and extending the bottom protection over the (filled) seaward scour hole.

All scenarios show an effect on the seaward and the landward directed net sediment transport. But in all cases, the added sand is mainly transported in the direction of the ebb-tidal delta rather into the basin. The adaptations will not result in sediment import rates large enough to restore an acceptable morphodynamic equilibrium state (i.e. with enough intertidal area) of the tidal basin.

A new inlet channel

Another study (De Bruijn, 2012) was aimed at finding a structural solution for the sand demand by creating an opening in the storm surge barrier. In this study, a scenario with a new inlet channel at Neeltje Jans was evaluated for its influence on the hydrodynamics and sediment transport.

With the new inlet channel, the tidal prism would increase, together with the flow velocities in the channels. The discharge through the already existing inlet channels will decrease, except for the channels connected to the new inlet. The connection between the old channels and the new one will therefore have a large influence on which areas will experience an increase or a decrease in flow velocities. The overall increase in tidal prism and flow velocities will bring the Eastern Scheldt closer to an acceptable morphodynamic equilibrium state. In some parts, shoal building may occur again. However, according to the empirical relations between the cross-sectional channel area and the tidal prism, the channels in most parts of the basin will still be too large for the tidal volume they have to convey.

A disadvantage of a new inlet channel could be that the increase in tidal prism will enhance the ebb-dominance in the basin, hence a net export of sediment. But a tidal jet in the new inlet channel will hinder the export of sediment, just as in the present situation. The tidal amplitude also increases with a new inlet channel. This will enlarge the intertidal area, but will not make the emerging time of shoals longer.

The study showed that the sand demand of the Eastern Scheldt cannot be structurally changed or optimized with a new inlet channel alone. Another option would be to combine a new inlet channel with artificial filling of the channels. This would further increase the flow velocities and bring the basin even closer to the desired equilibrium state. Model results show that an amount of approximately 200 million m³ of sediment would be needed to create a flood-dominant basin.

Removal of the storm surge barrier

Although complete removal of the barrier does not seem a realistic option at this moment, investigating this scenario could still give valuable insights. This scenario was investigated with an analytical and a numerical (Delft3D) model (De Pater, 2012).

Results from both models indicate that removing the barrier will cause an increase in the tidal range by 10 to 20%. The tidal prism will also increase. But the tidal range and prism will still be smaller than in the situation before the Delta Works. Nonetheless, the stronger tidal currents will enable shoal build-up.

Evaluation of the asymmetry of the water level and discharge signal indicates that removal of the barrier will also strengthen the ebb-dominance of the basin. This means that the basin will start exporting sediment. This is in contrast to what the empirical relations for morphological equilibrium suggest. According to the empirical relations the basin will need sediment for re-establishing an equilibrium state.

General realignment of basin

Large-scale realignment of the Eastern Scheldt is simulated by adding intertidal area without increasing the channel volume (De Pater, 2012). These simulations show increased ebb-dominance, leading to export of sediment. The set-back of part of the dikes will increase the flow velocities inside the basin, but not enough to enforce shoal build-up. This will only occur when the barrier is removed.