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In the foregoing two chapters, we have mainly focused on wave-dominated coastal systems. These coastal systems with well developed beaches are typically shaped by waves and wave-generated currents. The moderate to high wave energy (see Sect. 4.3.1) overwhelms any tidal energy present, for which reason these coasts are called wave-dominated. The shoreline in wave-dominated environments is characterised by elongated sediment (mainly sand) bodies. These include spits (Ch. 8), alongshore bars (Ch. 7) and beaches. Besides straight coastlines, whether or not interrupted by breakwaters, groynes, rivers and lagoon entrances, pocket beaches may also fall into the wave- dominated category.
So far we have only encountered tidal influence in deeper water outside the surf zone, around long structures and at low wave-energy coastlines. In general, tidal conditions dominate where wave energy is relatively low. The word ‘relative’ is crucial here, since it is the relative influence of waves and tides that determines the morphology (Fig. 4.13). Tide-dominated environments may occur due to restricted fetch or where incident wave energy is trapped or reflected. Such environments include tidal basins.
As discussed in Ch. 2, tidal basins are the result of breakthroughs and flooding of low- lying areas due to the global rise of the post-glacial sea-level. Processes contributing to basin formation include tectonic subsidence, fluvial erosion and glacial action. Bottom subsidence of the coastal plains due to human interferences (peat-harvesting, impoldering, water, oil and gas extraction) can be an additional cause for the creation of tidal basins. Besides through breaks and flooding, basins can also evolve due to the formation of barriers enclosing a body of water.
A few examples of tidal basins are: Chesapeake Bay, San Francisco Bay, Waddenzee and Baie d’Arcachon. All of these were created by flooding of low-lying coastal plains on coasts with a strong tidal energy and with little sediment discharge from rivers. The emphasis in this chapter is on physical insight and predictions about changes, which are expected to occur on the scale of the tidal basin itself. This does not only concern changes in water motion, but also changes in the physical structure of the basin, i.e. the morphology. Relevant practical questions are amongst others:
- What is the response of a tidal basin to sea-level rise (does the basin floor follow the sea-level rise by sedimentation)?
- What are the consequences of dredging for navigation or sand extraction (does sedimentation increase)?
- What is the effect of a dike or breaching of a beach barrier (does a new tidal basin form)?
- What are the impacts of land reclamation (does the inlet close, does sedimentation or erosion occur)?
- How do basin changes influence the adjacent coast (will the adjacent coast erode or accrete)?
The answers to a few of these practical questions are discussed in the final section of this chapter, Sect. 9.8, where the knowledge acquired in the preceding sections is applied. We start this chapter with a treatment of general basin and inlet types (Sect. 9.2) and a description of the main morphological units of tidal inlet systems (Sect. 9.3). The subsequent three sections focus on the three important morphological units, viz. the ebb-tidal delta (Sect. 9.4), the entrance or inlet channel (Sect. 9.5) and the inner basin (Sect. 9.6). Besides relevant processes, these sections discuss how the stability of these morphological units can be described with empirical relations relating the geometric properties to hydraulic boundary conditions. In Sect. 9.7 the mechanisms responsible for net sediment import into and export from basins are dealt with.