9.3.3: Basin characteristics

The tides fill and empty the basin via channels that cut through (large) lower and higher tidal sand and mud flats. The intertidal flats serve to accommodate the tidal prism: at low water these flats fall dry, while at high water they are submerged. Intertidal flats are called platen, slikken or wadden in Dutch³. Only the most landward, higher parts remain dry at high water; these vegetated parts are named supra-tidal flats or salt marshes (in Dutch: schorren or kwelders3).

The combined action of amongst others centrifugal forces, earth rotation (Coriolis) and inertia, causes the existence of separate ebb and flood channels, channel sills and channel bifurcations (Sect. 9.6). In general, ebb-dominant channels follow a meandering course, whereas flood-dominant channels shoal landwards.

Figure 9.7: Sketch of ebb and flood channels in a typical Wadden Sea basin with an inlet between dune-islands, several flood channels coming in (indicated with V) and the tendency of the ebb channel (E) and delta to turn to the left due to the ebb tide leaving to the left, consistent with the propagation direction of the tidal wave along the Dutch coast (see Fig. 3.31). After Van Veen et al. (2005).

A very good description of the ebb and flood channel systems is found in Van Veen et al. (2005). It was originally published in Dutch by Van Veen (1950), translated annotated by Van Veen et al. (2002) and republished by Van Veen et al. (2005). Along barrier island coasts the basins are often rectangular or near square, and the channel structure is often more branched than braided (Fig. 9.7). The branching structure is found to show fractal characteristics, viz. if you zoom in on a part, you find subsets that look like the whole figure.

Figure 9.8: Sketch of ebb (E) and flood channels (V) in a wide estuary (Thames or Wash). Meander action may bring the ebb channel in connection with any of the flood channels (after Van Veen et al., 2005).

Figure 9.9: Sketch of an ideal system of ebb (E) and flood (V) channels (Scheldt estuary). It has a meandering main (ebb) channel and flood channels starting in every bend. The latter have a double function; viz. 1. filling the tidal sand flat in the inner bend of the main channel; 2. serving the cut-off current of the bend. After Van Veen et al. (2005).

In the case of larger rivers discharging in tidal basins, the tidal basin is often funnel shaped, and the channel structure is not as branched, but is potentially braided (Figs. 9.8 and 9.9). In areas without width restriction (very large estuaries, e.g. Thames estuary), the channels followed by the flood current are generally different from the channels followed by the ebb current. They may connect due to meander action. If the width is more restricted (e.g. Scheldt estuary) the flood and ebb currents partly follow the same pathways.

Section 9.6.1 describes the complex geometry of ebb and flood channels in a bit more detail.

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3. The terms ‘slikken’ and ‘schorren’ are mostly used for the southwestern delta area, whereas the terms ‘wadden’ and ‘kwelders’ are exclusively used for the Wadden Sea.