Morphodynamics of coastal evolution

Abstract

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’.