Coastal morphodynamics — Introduction
Coastal morphodynamics examines how seabed geometry and sediment distributions evolve through interaction with fluid forcings — principally waves, tides and wind‑driven currents — to create and modify coastal landforms and sedimentary patterns. Hydrodynamic forcing adjusts essentially instantaneously to changes in bed form, whereas morphological adjustment requires sediment transport over finite timescales; consequently sediment acts as a time‑dependent coupling agent that produces a lagged morphological response to varying hydrodynamic conditions.
Because boundary forcings vary frequently and sediment transport is rate‑limited, beaches rarely achieve a static equilibrium. Instead morphodynamic systems commonly display nonlinear behaviour, threshold responses and both positive and negative feedbacks, so that coastal shores may behave as self‑organising systems across different temporal and spatial scales. The systems perspective and a practical classification of beach states were first proposed by Wright and Thom (1977) and later formalised by Wright and Short (1984), with subsequent synthesis by Short (1996).
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Exposed sandy shores are usefully represented along a continuous morphodynamic spectrum from dissipative to reflective end‑members, with intermediate states characterised by bar–trough topographies. Dissipative beaches have wide, low‑gradient profiles composed of fine sediment, broad shoaling and surf zones where waves break well offshore and lose energy gradually (spilling breakers); infragravity energy grows shoreward within the surf zone and commonly controls inner‑surf near‑bed flows and sediment transport. Reflective beaches are steep and coarse‑grained, lack appreciable shoaling zones, and experience abrupt breaking on the foreshore (surging breakers); high permeability enhances percolation during swash, weakens backwash and favours sediment deposition on the foreshore.
Near‑bed current regimes vary systematically along the morphodynamic spectrum. Dominant motions include incident wave orbital velocities, subharmonic oscillations (e.g. edge waves), infragravity standing waves and mean currents (longshore and rip currents). Reflective systems are typically driven by incident orbital motions and subharmonic edge waves, whereas in highly dissipative surf zones infragravity standing waves often dominate inner‑surf currents. Intermediate bar–trough states are principally governed by incident wave orbital velocities but also show significant contributions from subharmonic and infragravity oscillations together with mean longshore and rip flows; the strongest rip currents and their feeder flows are frequently associated with transverse bar and rip configurations.
Shifts between morphodynamic states are commonly forced by changes in wave energy: for example, storm‑generated high‑energy waves can steepen nearshore forcing, mobilise and export sediment offshore, flatten the profile and move a reflective beach toward a more dissipative condition. The same coupled sediment–hydrodynamic feedbacks apply beyond sandy shores, controlling features such as spur‑and‑groove reef morphology and the dynamics of tidal flats in infilling estuaries, demonstrating the broad applicability of morphodynamic principles to coastal depositional and erosional systems.