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During the last century, major engineering works have been carried out in the Dutch coastal system, for flood defence and / or land reclamation purposes. Among these engineering works, two of the tidal inlets in the Dutch Wadden Sea and three of the estuaries in the south-west delta were entirely closed off, partly reclaimed or semi-closed (i.e. separated from the sea by a movable storm surge barrier). These works have impacted the development of the (semi-) enclosed basins themselves, but also the adjacent coast and tidal basins. On this knowledge page, we focus on the development of the south-west delta.

By influencing the large-scale morphological development and sediment budget, the closures have their effect on the need for coastal maintenance. From a coastal dynamics point of view, tidal basin, ebb-tidal delta and adjacent coast together form a sediment sharing system. The combined sediment deficits and/or surpluses of the elements determine if after the intervention the tidal inlet or estuary will be a sediment source or sink.

On a smaller scale, closures influence the development of channels, intertidal flats, saltmarshes and other morphological elements inside the basin. Especially the effect on the morphological development of the ecologically highly productive intertidal flats affects the ecosystem. With their relatively large and food-abundant intertidal area, estuaries like the Eastern Scheldt play a crucial role in the East-Atlantic Flyway for bird migration.

Understanding the morphological processes in the estuary is therefore of crucial importance. With that understanding, we can better comprehend and predict the effects of environmental change (Climate change, accelerated sea level rise) and human activities. This will help us safeguarding accessibility, naturalness, productivity and safety of the estuary for the future.

    Introduction

     

    During the last century, major engineering works have been carried out in the Dutch coastal system, among which closure, partial enclosure or semi-closure with a movable barrier of two tidal basins in the Dutch Wadden Sea and three of the estuaries in the south-west delta. These works have had important effects on the morphological and ecological development, not only of the basins themselves, but also the adjacent coast and tidal basins. On a smaller scale they have influenced the development of channels, intertidal flats, saltmarshes and other morphological elements. Especially the effects on the intertidal flats have had significant ecological effects.

    On this knowledge page, we focus on the development of the Rhine-Meuse-Scheldt delta, also called the south-western delta of the Netherlands (Figure 1). Since in the nineteen sixties the Deltaworks started in order to guarantee flood safety in this area, which was hit hard by the 1953 flood, this delta has undergone major changes. Dams and barriers have been built, on the coast to separate the tidal basins from the sea and further inland to separate saline from fresh water and to create a safe fairway between the harbours of Rotterdam and Antwerp.

    Special attention is paid to the Eastern Scheldt, which is still open, but protected from storm surges by a movable barrier. With its relatively large intertidal area rich of food the Eastern Scheldt plays a crucial role as a stop-over in the East-Atlantic Flyway, which is used by some by 90 million migratory birds each year. The Eastern Scheldt is protected by the Natura2000 nature conservancy law. The construction of the storm surge barrier and the inner dams, however, have led to a decrease in tidal amplitudes, tidal volumes, and average flow velocities. This means that the channels suddenly had to convey a smaller volume of water each ebb and flood tide and were too large for that purpose. It also means that the tidal currents building up the flats were no longer in balance with the wave action eroding them. Moreover, there is hardly any sediment exchange through the barrier. As a result, the sediment inside the basin is redistributed, with the channels filling up and the ecologically important tidal flats eroding.

    Figure 2 shows the Eastern Scheldt, with the names of the relevant channels (in white), shoals (in black) and dams and barriers (in yellow).

    Also the Western Scheldt, which is still completely open, plays an important role in The Netherlands. Firstly, the estuary gives access to the ports of Antwerp, Flushing and Terneuzen and is therefore vital to shipping. Secondly, the Dutch part of the estuary is of great ecological importance due to the presence of its flats, saltmarshes and dynamic channel system. And thirdly, since the Western Scheldt is an open estuary, safety against flooding is an important aspect in the management of the estuary. Furthermore, comparison of the Western and the Eastern Scheldt, with their vastly different morphological developments over the last few decades, will help understanding the underlying mechanisms.

    Understanding sediment transport and morphological processes in these estuaries is of crucial importance. With that understanding, we can better comprehend and predict the effects of environmental changes (accelerated sea level rise, changes in tidal conditions) as well as human activities (engineering works, channel deepening and maintenance, dumping of dredged material, sand mining). This will help us to safeguard the essential functions of these estuaries for the future. Moreover, understanding these processes enables application of the knowledge and experience gathered to estuaries elsewhere in the world.

    Tidal basins

     

    Historical development of the Eastern Scheldt.

    The Eastern Scheldt tidal basin has changed drastically in the past five centuries under the influence of both human interventions and extreme events. In 1530 A.D. a storm surge inundated large areas in the landward part of the basin, and in the following four centuries local inhabitants reclaimed about as much land as was lost during that storm (see Figure 3). Modelling of physical processes for schematized tidal basins representing the various historical situations showed that the large-scale inundations caused the basin to scour to greater depth. 
    Analysis of these observations, supported by hydrodynamic and sediment transport models, led to the conclusion that the morphological evolution of the Eastern Scheldt basin was dominated by a sediment exporting trend, with a continuously increasing tidal prism and continuously deepening channels. This trend has been set in motion by the inundations in the 16th century and the land reclamations since then have not been able to reverse these trends, but have probably only amplified the exporting trend. Tidal asymmetry seems to be much more sensitive to the depth of the channels than to the size of the intertidal area in this particular case. As a result of the scouring, also the tidal watershed between the Eastern Scheldt and the Grevelingen, the next basin to the north, has disappeared. This happened when the residual longshore tidal current across this tidal watershed increased and scoured the connection between the estuaries to a greater depth.

    Lessons learned: 
    1) Extreme events can have significant influence.
    2) Developments can be irreversible.

    Human interventions: completion of the Delta Works.

    TThe Eastern Scheldt tidal basin has undergone substantial changes due to human intervention in the past few decades. The construction of two back-barrier dams in 1965 and 1969 had a significant impact on the tidal hydrodynamics and sediment transport. The response to these interventions was still ongoing when the hydrodynamic regime was altered again by the construction of the storm surge barrier between 1983 and 1986. 
    In the decades before 1965, the Eastern Scheldt already exported large quantities of sediment to the sea. The implementation of the back-barrier dams caused a significant increase in tidal prism, which yielded a response of the bathymetry. The rates of sediment export, channel deepening and ebb-tidal delta growth further increased. Analyses of tidal flow measurements and model output show a persistent trend to export sediment. The construction of the back-barrier dams amplified this export by increasing the tidal prism, thus pushing the basin even more out of equilibrium than it already was. 
    The Eastern Scheldt storm surge barrier was the first of its kind ever implemented in a tidal inlet. An inevitable effect of such a structure is constriction of the flow-conveying cross-sectional area of the inlet. This constriction caused a decrease of tidal current velocities inside and outside the basin, hence an even sharper decrease in sediment transport capacity. This in turn causes erosion of shoals and flats and sedimentation in the channels (see 'Channel - shoal interaction'), and probably also the vanishing of sediment exchange (see 'Sediment exchange') between the basin and the ebb-tidal delta. These effects have to be kept in mind whenever the construction of a storm surge barrier is considered anywhere else in the world.

    Ebb-tidal deltas

    Historical development of the Eastern Scheldt 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).

    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.

    The morphology of the Eastern Scheldt has been changing during the past 25 years in response to the construction of the storm surge barrier in 1986. 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 Figure 4.

    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 Figure 5. 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.

    Channel-shoal interaction

    The change in hydrodynamic conditions caused about by the storm surge barrier forced the estuary out of equilibrium and brought the channels in demand of sediment. This sediment is supplied by the adjacent intertidal areas inside the basin. As a result, these intertidal areas are eroding, which has unfavourable consequences for ecology, safety, shipping, recreation and fisheries.

    Governing processes

    PProcess-based modelling showed that shoal accretion above mean low water (MLW) was possible as long as tidal characteristics in the basin were like before closure. In that situation, velocities on top of the shoal were 30 - 40 % larger than at present. Under the present conditions, the model indicates that shoal accretion does not occur. This confirms that tidal flow is responsible for the build-up of shoals and mudflats: their accretion only occurs when the tidal flow velocities are strong enough.

    The process governing the degradation of the Galgeplaat was found to be the combination of breaking waves and wave-induced currents.

    • During calm weather, waves break on the higher parts of the shoal (depending on wave height, local bathymetry and water level). The breaking waves lead to erosion there. When the tide falls, waves can also break on lower parts of the shoal, closer to the edges. Under these conditions, there is some redistribution of sediment on the shoal and sediment transport from the shoal into the channel.
    • During storm events, wave breaking is not limited to the higher areas. The breaking waves stir up sediment and bring it into suspension. Wave-induced currents then transport this sediment from the shoal into the channels. Because breaking can occur everywhere on the shoal and no areas are sheltered, erosion is ubiquitous on the intertidal areas and can be severe. This holds for north-westerly as well as south-westerly wind conditions.

    Influence of the storm surge barrier

    Figure 6 (Louters et al., 1998) shows measured tidal currents and sediment transport on the Galgeplaat, during calm weather and spring tide. Before construction of the Eastern Scheldt barrier, the tidal flow was strong enough to have a significant shoal building capacity, and shoal accretion could take place under calm conditions. This shoal building effect is visible in the left-hand panel: the velocities and sediment transports on the shoal are significantly stronger during flood than during ebb (Das, 2010). This means that more sediment is brought onto the shoal during flood than is taken away from it during ebb.

    The right-hand panel shows that, due to the barrier, particularly these flood velocities and transports have reduced significantly. At present, the tidal flow in the Eastern Scheldt no longer has enough shoal building capacity to counteract the erosion caused by waves. This explanation is also supported by the observation that in the same period of time (from 1986 till present the intertidal flats in the Western Scheldt have become higher instead of lower).

    Possible future developments

    Simulations with different bathymetries dating from 1983 and 2008 indicate that the loss of sediment from the shoals to the channels leads to a less ebb-dominant system. Ongoing loss of sediment from the intertidal area can lead to a scenario without intertidal flats around 2100. To investigate the implications of such a scenario on the hydrodynamics, a representative situation for 2100 was simulated. In this scenario, all sediment above NAP - 2 m was removed and redistributed in the channels. This caused a stronger distortion of the vertical tide, compared to the situation in 2008, and made the system slightly flood-dominant. This flood dominance is also visible in the tidal velocities, be it that the distortion is weaker.

    Figure 7 shows profiles of a shoal in 1986 and 2001, and an expected profile for 2015. Erosion occurs mainly on the top of the shoal. Sedimentation occurs on the edges of the shoal, below the low-water line. The result is a net loss in intertidal area.

    Figure 8 shows two bathymetrical maps of the basin. The map on the left shows the measured bathymetry, at the time of completion of the barrier in 1986. The map on the right shows the expected bathymetry for 2045, if the present-day trends would continue. Also here, the loss in intertidal area (above MSL -1.5) is clearly visible. The eroded sediment has contributed to the area below low-water and the infilling of deeper channels.

    Sediment exchange

    Import / export in the Eastern Scheldt

    Historical development

    The evolution of the Eastern Scheldt between the 16th and 20th century is mostly governed by the continuous increase in tidal prism, initiated by the large-scale inundations in the 16th century. This situation changed with the construction of the storm surge barrier and the back-barriers Philipsdam and Oesterdam in 1986. Especially the storm surge barrier had a large impact on the hydrodynamics inside the basin. 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.

    Figure 2 shows the Eastern Scheldt, with the names of the relevant channels (in white), shoals (in black) and dams and barriers (in yellow).

    Model results and the methods of Van de Kreeke and Robaczewska (1993) and Groen (1967) applied to the present situation of the basin show that the Eastern Scheldt is still ebb-dominant and would be exporting both fine and coarse sediment if the storm surge barrier would not block the sediment transport (De Bruijn, 2012). This ebb dominance follows from the large intertidal area and deep channels. The residual flow velocities are in most parts of the basin in ebb direction. It should be noted, however, that the tidal asymmetry in the present situation is negatively ‘skewed’ and very close to flood-dominance in some parts of the basin.

    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.

    Conclusion

    Given the large amount of sediment needed for the basin to re-establish an acceptable morphological equilibrium state, none of the considered measures will be effective. Most of them 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.

    Lessons learned

    General effects of (semi-) closures of tidal basins

    An analysis on the three (semi-) closures in the Delta area with the two closures in the Wadden Sea as reference (Wang et al., 2009) learned that these (semi-) closures have long-term effects on both sides: the (semi-) closed basin and the coastal water seawards.

    Seawards a (semi-) closure of (part of) a tidal basin can make the ebb-tidal delta a sink or source of sediment for the adjacent coasts. The position of the closure structure with respect to the coastline is important for this, for the coastal maintenance very important, aspect. When almost the entire basin is closed as in the cases of the closures in the Delta area, the area outside the closure becomes a sink of sediment when the closure is relatively landwards located and vice versa.

    The type of environmental problems in the remaining and/or closed basin caused by the closure mainly depends on the type of the closure. If the closed basin becomes a stagnant freshwater lake/reservoir, influx of nutrients from point of diffuse sources may cause algal blooms (like in the case of the Volkerak/Zoommeer and the Grevelingen) and accumulation of fluvial sediment may cause bottom pollution (like in the case of the Haringvliet). If the tidal flow is weakened, like in the case of the Eastern Scheldt, serious erosion of the inter-tidal flats can take place.

    Specific lessons from the Eastern Scheldt Case

    • Extreme events like the 1530 storm surge in the Eastern Scheldt can play an important role on the morphological development of an estuary, also on the long-term. Such an event can cause irreversible changes to the system.
    • The phasing of a large-scale project like the Delta Works can have a substantial impact on the morphological development. The impact of intermediate phases may be essentially different from that of the completed project. The time scale of the morphological response to large-scale engineering works on a tidal basin can be very large. The adjustment of the ebb-tidal delta of the Eastern Scheldt to the construction of the storm surge barrier and the back-barrier dams is bound to take centuries.
    • The relative importance of wave action to the morphological development of the ebb-tidal delta increases after semi-closure of the tidal basin, due to the weakening of the in- and outgoing tidal flow.
    • The equilibrium height of a tidal flat (with respect to LW) should be related to the strength of the tidal flow in the adjacent channels, rather than to the tidal range, as suggested by empirical relations in literature (Eysink, 1990), which are derived from field data collected in e.g. the Wadden Sea where the channels are not far from morphological equilibrium.
    • Due to the strong interaction between the tidal motion and the morphological development in an estuary, the response of the estuary to human interference is not always straightforward. For example, sediment withdrawal from the Western Scheldt has caused a sediment surplus rather than a sediment deficit in the estuary.
    • Measures to stimulate sediment import through the storm surge barrier will not be effective to solve the problem of tidal flat erosion in the Eastern Scheldt.

    References

    • Bruijn, Robbert de. 2012 . The future of the Eastern Scheldt with a new inlet channel. Msc. thesis, Delft University of Technology, Faculty of Civiel Engineering and Geosciences
    • Das, Ingrid. 2010. Morphodynamic modelling of the Galgeplaat. Msc. thesis, Delft University of Technology, Faculty of Civiel Engineering and Geosciences
    • Eelkema, Menno, 2013. Eastern Scheldt Inlet Morphodynamics. PhD thesis, Delft University of Technology, 145 pp., ISBN 978-90-9027347-1
    • Eelkema, Menno; Zheng Bing Wang; Anneke Hibma; Marcel Stive. 2012. Morphological effects of the Eastern Scheldt Storm Surge Barrier on the ebb-tidal delta; Paper submitted to Coastal Engineering Journal
    • Eelkema, Menno; Zheng Bing Wang; Marcel Stive. 2012 . Impact of Back-Barrier Dams on the Development of the Ebb-tidal Delta of the Eastern Scheldt. Paper accepted for publication in Journal of Coastal Research
    • Eelkema, Menno; Zheng Bing Wang; Marcel Stive. 2009. Historical morphological development of the Eastern Scheldt tidal basin. In Mizuguchi, M. and S. Sato (Eds.), Proceedings of Coastal Dynamics 2009, World Scientific Publ Co PTE LTD, Singapore.
    • Eysink, W.D., 1990, Morphologic response of tidal basins to changes, Coastal Engineering, 37, 1948-1961.
    • Groen, P., 1967. On the residual transport of suspended matter by an alternating tidal current. Netherlands Journal of Sea Research, pp. 564-574.
    • Hoogduin, Lars. 2009 . Sediment transport through the Eastern Scheldt storm surge barrier.Msc. thesis, Delft University of Technology, Faculty of Civiel Engineering and Geosciences
    • Louters, T., van den Berg, J.H. and J.P.M. Mulder, 1998, Geomorphological Changes of the Oosterschelde Tidal system during and after the implementation of the Delta project. Journal of Coastal Research, Royal Palm Beach Florida, 14(3):1134-1151.
    • Ofori, Komla. 2009 . Investigating the long-term sediment import-export trends of the Western Scheldt estuary. Msc. thesis, UNESCO-IHE
    • Pater, Pim de. 2012 . Effect of removal of the storm surge barrier. Msc. thesis, Delft University of Technology, Faculty of Civiel Engineering and Geosciences
    • Van de Kreeke, J. and K. Robaczewska. Tide-induced residual transport of coarse sediment; Application to the Ems estuary. Netherlands Journal of Sea Research, 31(3):209-220, 1993.
    • Wang, Zheng Bing; John de Ronde; Ad van der Spek; Edwin Elias. 2009. Responses of the Dutch coastal system to the (semi-) closures of tidal basins. Proceedings of ICEC 2009, Sendai, Japan, Vol. 1, 203-210. Edited by Department of Civil Engineering, Tohoku University.

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