1.4.5: Tidal inlets and basins
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Tidal inlets are openings in the shoreline, such as between two barrier islands that connect bays or lagoons to the open ocean. They are maintained (viz. kept from closing naturally) by tidal currents. Figure 1.8 shows various tidal inlets in the Wadden Sea, the Netherlands. Essential for a tidal inlet is the tidal variation in the open sea; the tide is the engine that determines most of the occurring features of the inlet and the basin it connects to.
The tidal range – the difference between High Water Level (HWL) and Low Water Level (LWL) – and the surface area of the tidal basin together determine in principle the volumes of water that have to flow in and out through the inlet during a tide. This tidal prism is in some cases in the order of magnitude of one billion (\(1 \times 10^9) m^3\). For instance, during one tidal cycle about \(10^9\ m^3\) of water enters the Texel Inlet or Marsdiep (between Texel and Den Helder, see Fig. 1.8) and leaves the Texel Inlet again.
A typical tidal inlet system consists of several morphological units (see Ch. 9 for more details):
- The actual entrance or tidal inlet, often dominated by a main channel;
- A shallow ebb-tidal delta seaward of the inlet that often folds around a deep channel.
- The flood basin with possibly a distinct flood-tidal delta just landward of the inlet and an inner tidal basin consisting of the channels that are followed by the tidal currents and of the lower and higher tidal flats that alternately inundated and exposed by the tides and possibly covered by salt marshes or mangroves.
The position of the different elements of the tidal inlet system (e.g. ebb-tidal delta, flood-tidal delta, flood channels, ebb channels, shoals, tidal flats and gorge) is changing with time. Figure 1.9 shows a schematic plan view of a tidal inlet with some characteristic notions.
From a morphological point of view, tidal inlets form highly dynamical systems, which are interlinked with the adjacent coast and the tidal basin or backbarrier area to which they give access. Often, the natural morphodynamic behaviour interferes with unnat- ural constraints (e.g. coastal defence works) and with the effects of human utilisation (e.g. sand mining). The dynamic behaviour may complicate navigation. Sometimes measures are taken to restrict the dynamic behaviour of the morphological system.