The morphodynamic behaviour of tidal inlet systems is highly dynamic and strongly determined by the tide. As an illustration of the complex tidal dynamics, Fig. 9.5 and Table 9.2 give an overview of the Wadden Sea. Many of the Wadden Sea inlets have distinct catchment areas that in calm conditions are separated by a watershed (or tidal divide or wantide – from the Dutch wantij). The locations of the tidal divides in the Wadden Sea are not fixed, but can move due to human interference.
At watersheds the inflowing flood currents through different inlets meet and tidal currents are practically zero. The watersheds are conducive to fine-grain deposition. This is because the low current velocities permit the fines to settle. Large tidal eddies are common along the back barrier flanks, sometimes superimposed on a unidirectional stream creating a spiralling current. Each catchment area exchanges large volumes of water with the outside area (the tidal prism – dependent on the tidal range and the surface area of the catchment area). This results in large inflowing flood and outflowing ebb currents that keep the inlets from closing naturally. The basin area consists of deeper areas where the flow is concentrated during lower tidal water levels and of tidal flats that are covered during higher water and exposed during lower water. In the Wadden Sea, the tidal flats are covered by salt marshes, but may be covered with mangroves in other climatic regions (Sect. 2.6.6).
Although the Wadden Sea can be conceptually divided into separate basins during calm conditions, wind-driven currents and storm surges may lead to inter-basin flow and transport over the tidal watersheds (Duran-Matute et al., 2016).