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 Building with Nature Guideline > Knowledge - Interaction between tidal basins and ebb-tidal deltas

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Knowledge - Interaction between tidal basins and ebb-tidal deltas

Abstract: Morphological processes in the estuary with the Eastern Scheldt as an example.

Environment: Estuaries

Technology Readiness Level: 9 (understanding of natural system)

Keywords: Ebb-tidal deltas, Tidal basins, Channel-shoal interaction, Sediment exchange, Eastern Scheldt

 

Understanding the morphological processes in the estuary is of crucial importance to better comprehend and predict the effects of environmental change and human activities. For instance, what is the influence on the processes when barriers (partial enclosure of semi-closure with a movable barrier) are constructed in a tidal basin or estuary. These works have 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.


Understanding of these processes will help us safeguarding accessibility, naturalness, productivity and safety of the estuary for the future. Moreover, these enables application of the knowledge and experience gathered to estuaries elsewhere in the world.

 

On this page the influences on the Eastern Scheldt as an example are described. Based on the historical development combined with specific information of the tidal inlet, ebb-tidal delta, channel-shoal interaction, sediment exchange and the lessons learnt.

 

A short description of the South-western Delta of the Netherlands is given in the read more.

 Read more

 

Tidal basin

The Eastern Scheldt tidal basin has changed drastically in the past five centuries under the influence of both human interventions and extreme events. The trend of continuously increasing of the tidal prism and continuously deepening of the channels has been set in motion by the inundations in the 16th century (large storm surge in 1530) and the land reclamation since then. These have not been able to reverse the trend, but have probably only amplified the exporting trend. Due to the construction of back-barriers (1965 & 1969) the tidal prism increased even more, resulting in pushing the basin even more out of equilibrium than it already was. After construction of the storm surge barrier (1983-1986) a decrease of tidal currents occur, resulting in a 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 developments and effects on the tidal basin due to the interventions are described below.

 Read more

 

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.

 Read more

 

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.


The process governing the degradation of the Galgeplaat (in the eastern part of the Eastern Scheldt, see figure) was found to be the combination of breaking waves and wave-induced currents. Due to the storm surge barrier the tidal flow has no longer enough shoal building capacity to counteract erosion caused by waves.


More detailed explanation of these governing processes and the influence of the storm surge barrier is described in this section, just as the possible developments based on simulations.

 Read more

 

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.

 Read more

 

Lessons learned

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. General effects of these are explained in the section below, just as the specific lessons from the Eastern Scheldt case.

 Read more

 

References

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