Interaction between tidal basins and ebb-tidal deltas

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.

General effects of (semi-) closures of tidal basins

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.