Basins are said to be flood-dominant or ebb-dominant, which refers to net sand import or sand export. In the case of neither flood nor ebb-dominance, there is no net sediment gain or loss and the system is said to be in equilibrium (on the scale of the basin). Morphological equilibrium is dependent on external conditions, but if we assume these to be neutral, different basin geometries may fulfil the condition for equilibrium:
- Shallow channels and large intertidal (storage) flat area (e.g. Wadden Sea);
- Deep channels and small intertidal (storage) flat area (e.g. Oosterschelde);
- Intermediate geometries.
Figure 9.34: Relation between basin-averaged channel depth and flat area for Dutch tidal basins (after Dronkers, 2005). The ratio of water depths at HW and LW (\(y\)-axis) gets larger for shallower channels. The wet surface area ratio on the 𝑥-axis increases for larger intertidal storage areas.
In the equilibrium condition, the geometrical characteristics of the basin are such that the durations of ebb and flood are approximately equal. (Dronkers, 1998) derived an expression for this equilibrium condition based on the assumption that the tidal asymmetry at sea is not strong. This condition is shown in Fig. 9.34. It indicates that for Dutch tidal basins the ratio between the average channel depths and flat surface area is such that net ebb and flood transports are approximately equal, but have a slight flood-dominant tendency.
The tidal basins in the Wadden Sea (especially in the eastern Wadden Sea) are generally considered to be in morphological equilibrium. The flood-dominance found in Fig. 9.34 could be the result of the neglect of residual currents in the derivation. As a result of Stokes’ drift a compensating Eulerian current will enhance the ebb flow (see Sect. 5.7.6). It could also be that the flood-dominance is the result of a sediment demand of the basin as a result of sea-level rise (Sect. 9.8.3).
What happens if we disturb this equilibrium slightly? An increase of the channel depth will enhance the ebb-dominance and cause a net sediment export. The result is a further deepening of the channels; this is a case of an unstable morphological equilibrium (Sect. 1.5.2). An increase of the channel width is equivalent to a relative decrease of the intertidal areas. In this case a net sediment import occurs, annihilating the initial disturbance (stable equilibrium, see Sect. 1.5.2).
Some tidal basins are unstable and silt up (with sand and/or finer fractions) in a relatively short period, such as the Zwin that gave navigational access to the town of Bruges. In other basins there is a dynamic equilibrium between import and export of sediment. This is for instance the case for the eastern Wadden Sea that has had the same topography for nearly a thousand years. The locations of flats and channels fluctuate over timescales of tens to hundreds of years.
The fact that tidal basins exist implies that they display a large degree of morphological stability. This stability requires that channel width, channel depth, flat width and flat depth are dynamically coupled. Especially the transversal transports (between flats and channels) in tidal basins are responsible for this dynamic coupling. This internal dynamic coupling can restore the morphological stability of the basin after for instance the channel depth is disturbed by deepening or filling.