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4 - Wave-dominated coasts
- Edited by R. W. G. Carter, University of Ulster, C. D. Woodroffe, University of Wollongong, New South Wales
- Foreword by Orson van de Plassche
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- Book:
- Coastal Evolution
- Published online:
- 06 July 2010
- Print publication:
- 05 January 1995, pp 121-186
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Summary
Introduction
Wave-dominated sedimentary coasts comprise accumulations of detrital sand and gravel-sized material which undergo high levels of physical reworking, interspersed with periods of burial before finally being deposited as the coastal deposits we see today (Davis & Hayes, 1984). Quite commonly sediments tend to be of clean sand and gravel, often quite well sorted and abraded, containing relatively high proportions of more resistant minerals and rock types such as quartz, chert and heavy minerals. Waves and wave-induced currents are the dominant mechanisms for moving and depositing sand on shorefaces and beaches of the open coast, although winds, river discharge, tidal currents and Ekman flows variously act as transporting agents landward of the beach, in estuaries and seaward of the shoreface. In relation to the shoreface and beach, open coastal types are determined by four factors: (i) substrate gradient, (ii) wave energy versus tidal range; (iii) sediment supply versus accommodation volume (Swift & Thome, 1991); and (iv) rates of sea-level change. At one extreme are steep, high-energy, sediment-deficient coasts that have bedrock cropping out as headlands, with negligible sand at their base and relatively deep water offshore (autochthonous, accommodation-dominated coast of Swift & Thorne, 1991). At the other extreme are low-gradient, low-energy coasts that are typically muddy with a coastal fringe of wetland vegetation. Here, incident wave action is dissipated over very shallow offshore gradients such as those associated with deltaic environments at river mouths (autochthonous, sediment-supply dominated coast of Swift & Thorne, 1991; see Chapter 3). But even here, rare high-energy events such as cyclones can cause episodes of wave reworking leading to the formation of cheniers.
2 - Morphodynamics of coastal evolution
- Edited by R. W. G. Carter, University of Ulster, C. D. Woodroffe, University of Wollongong, New South Wales
- Foreword by Orson van de Plassche
-
- Book:
- Coastal Evolution
- Published online:
- 06 July 2010
- Print publication:
- 05 January 1995, pp 33-86
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- Chapter
- Export citation
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Summary
Introduction
Coastal evolution is the product of morphodynamic processes that occur in response to changes in external conditions (Wright & Thorn, 1977). Coastal morphodynamics is defined as the ‘mutual adjustment of topography and fluid dynamics involving sediment transport’ (Wright & Thorn, 1977) or, alternatively, the ‘dynamic behaviour of alluvial boundaries’ of fluid motions (de Vriend, 1991b). Sediment transport provides the time-dependent coupling mechanism by which this adjustment occurs (Fig. 2.1). Fluid dynamics drive sediment transport resulting in morphological change over time. Progressive modification of topography in turn alters boundary conditions for the fluid dynamics, which evolve to produce further changes in sediment-transport patterns and their depositional products. Sediment properties and abundance affect the process through their influence upon sediment transport and sediment budgets respectively.
The essential properties of coastal morphodynamic processes are attributable to the feedback loop between topography and the fluid dynamics that drive sediment transport producing morphological change (Fig. 2.1). The feedback can be either negative or positive. Negative feedback confers properties of self regulation in response to minor perturbations (Wright & Thorn, 1977). Positive feedback signifies growth of an instability and confers properties of self organisation, which results in new modes of operation (Waldrop, 1992; Phillips, 1992). Feedback reversal marks thresholds in morphodynamic behaviour.
A fuller appreciation now exists of the complexity inherent in these morphodynamic processes (de Vriend, 1991b) following recent developments in non-linear dynamics (Gleick, 1988; Waldrop, 1992; Phillips, 1992). The complexity derives from the morphodynamic feedback that is responsible for state-determining behaviour or, to use the new language of chaos theory, ‘sensitive dependence upon initial conditions’.