Beach morphodynamics

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Beach morphodynamics refers to the study of the interaction and adjustment of the seafloor topography and fluid hydrodynamic processes, seafloor morphologies and sequences of change dynamics involving the motion of sediment. Hydrodynamic processes include those of waves, tides and wind-induced currents.

While hydrodynamic processes respond instantaneously to morphological change, morphological change requires the redistribution of sediment. As sediment takes a finite time to move, there is a lag in the morphological response to hydrodynamic forcing. Sediment can therefore be considered to be a time-dependent coupling mechanism. Since the boundary conditions of hydrodynamic forcing change regularly, this may mean that the beach never attains equilibrium.

This systems approach to the coast was developed by Wright and Thom in 1977.

Examples of beach morphodynamic processes include the formation of beach cusps (on a very small scale), intertidal bars and log-spiral (Yasso 1965)/crenulate (Silvester 1980) embayments. Morphodynamic processes exhibit positive and negative feedbacks (such that beaches can, over different timescales, be considered to be both self-forcing and self-organised systems), nonlinearities and threshold behaviour.

According to their dynamic and morphological characteristics, exposed sandy beaches can be classified into several morphodynamic types (Short & Wright 1983, Short 1996). There is a large scale of morphodynamic states, this scale ranges from the dissipative state to the reflective extremes. Dissipative beaches are flat, have fine sand, incorporating waves that tend to break far from the intertidal zone and dissipate force progressively along wide surf zones. Reflective beaches are steep, and are known for their coarse sand; they have no surf zone, and the waves break brusquely on the intertidal zone.

Depending on beach state, near bottom currents show variations in the relative dominance of motions due to: incident waves, subharmonic oscillations, infragravity oscillations, and mean longshore and rip currents. On reflective beaches, incident waves and subharmonic edge waves are dominant. In highly dissipative surf zones, shoreward decay of incident waves is accompanied by shoreward growth of infragravity energy; in the inner surf zone, currents associated with infragravity standing waves dominate. On intermediate states with pronounced bar-trough (straight or crescentic) topographies, incident wave orbital velocities are generally dominant but significant roles are also played by subharmonic and infragravity standing waves, longshore currents, and rips. The strongest rips and associated feeder currents occur in association with intermediate transverse bar and rip topographies.(Virginia Institute of Marine Science- Scopus 460).

With respect to beach morphodynamics, a spectrum of morphodynamic states has been developed, ranging from reflective beaches at one end to dissipative beaches at the other. Reflective beaches are typically steep in profile with a narrow shoaling and surf zone, composed of coarse sediment, and characterised by surging breakers. Coarser sediment allows percolation during the swash part of the wave cycle, thus reducing the strength of backwash and allowing material to be deposited in the swash zone. At the other end of the continuum of beach states, dissipative beaches are wide and flat in profile, with a wide shoaling and surf zone, composed of finer sediment, and characterised by spilling breakers. Transitions between beach states are often caused by changes in wave energy, with storms causing reflective beach profiles to flatten (offshore movement of sediment under steeper waves), thus adopting a more dissipative profile.

Morphodynamic processes are also associated with other coastal landforms, for example spur and groove topography on coral reefs and tidal flats in infilling estuaries.