Geomorphological time scales and processes
A coastal system comprises components like processes, sediment transport, morphology and stratigraphy, that are linked by energy and material flows. In coastal areas the processes include winds, waves, tides and currents, which provide the energy that shapes and modifies a coastline by eroding, transporting and depositing sediments. Although waves, tides and currents interact, one process may be augmenting or diminishing the effects of another.
The turbulent flow of the wind blowing over the water produces stress and pressure variations on the surface, initiating waves which grow as the result of the pressure contrast between their driven (upwind) and advancing (downwind) slopes. Orbital motion in waves is not quite complete, so that water particles move forward as each wave passes, producing a slight drift of water in the direction of wave advance. Along the most coastlines, waves represent the dominant source of energy in the nearshore zone.
Wave characteristics and terminology:
Wave Height (H) is the vertical distance between successive crests and troughs. Wave steepness the ratio between height and the length (H/L). Wave velocity ( C ) the rate of movement of a wave crest. Wave period (T) is the time interval between the passage of successive wave crests
Simple equations indicate the relationships between wave parameters. In deep water , wave velocity (C0) is the ratio (L0/T0) of wavelength (in metres) to wave period (in seconds). Wave length (L0) in deep water (where d>L/2) can be used to calculate wave velocity (C0) from the following formula, in which g is the gravitational acceleration (980.62 cm/sec2 at latitude 45º):
C02=gL0/2π, from which, since L0=C0T,
C0=gT/2π or 1.56T in m/sec.
So that L0=1.56T 2 , providing a means of calculating wave length from measurements in deep water.
Equations for the shallow water are Ls=T(gh)1/2 and Cs=(gh)1/2, where h is the water depth in metres.
During storms strong winds generate irregular patterns of waves, varying in height, length and direction, which radiate from the generating area. The longest waves move most rapidly , and are most durable, so that as waves move across the ocean they became sorted into swell of more regular height and length, which eventually arrives to break on a distant shore. Ocean swell consists typically of long, low waves with periods of 10 to 16 seconds.
Storm waves, generated by strong wind action, arrive frequently on west-facing coasts in latitudes subject to frequently westerly gales, as on the Atlantic coasts of north-west Europe.
When ocean swell arrives from different sources there are variations in wave height as interacting sequences of waves break upon the shore. The idea that every seventh wave is larger is legendary, but occasional higher waves occur as the result of the merging of two or more groups of waves. Sometimes there is a steady increase in the height of successive waves to a combined phase maximum, followed by a diminution as the waves move out of phase. Known as surf beat, this interaction can produce maximum waves breaking at intervals of several minutes, accompanied by pulsations of current flow alongshore and onshore-offshore. Wave set-up is the raising of sea level close to the shore as the result of waves driving water in. It is roughly proportional to incident wave height, but also depends on shore gradient and beach texture, shingle absorbing more wave energy than sand.
As waves move into shallow water (d<L/2) they are modified in several ways . Their velocity Cs dimiminishes according the formula.
Cs2=gL/2π tan h2πd/L
The shallowing water also diminishes their length and period, and as they approach the shore their height increases, they steepen, the crests becoming narrower and sharper, and the intervening troughs wider a flatter. Orbital movements within each wave become increasingly elliptical, and shoreward velocity in the wave crest increases until exceeds the wave velocity. When the orbital motion can no longer be completed, the oversteepened wave front collapses, forming a breaking wave (breaker).
The energy of a breaking wave (E) depends largely on breaker height (Hb) and the density of the water (p), together with gravitational acceleration (g) according to the formula: E=1/8pgH2
Tides are movements of the oceans set up by the gravitational effects of the moon and the sun in relation to the earth. They are very long waves that travel across the oceans and are transmitted into bays, inlets, estuaries or lagoons around the world's coastline. The ebb and flow of tides produces regular changes in the level of the sea along the coast, and genera tidal currents. The lunar cycle produces semidiurnal tides (two high and two low tides in approximately 25 hours), well displayed around the Atlantic Ocean. Solar cycle produces diurnal tides (one high and one 1 tide every 24 hours), as registered on the Antarctic coast. Elsewhere the two are mixed, yielding unequal high and low tides (e.g. high high, low lo low high and high low), as around much of t Pacific and Indian Ocean coasts. Where the effects lunar gravity are stronger, high and low tides o about 50 minutes later each day.
Variations in tidal type are significant in terms durations of low tide drying (less than 12 hours with semidiurnal tides, but more than 12 hours with diurnal and mixed tides) which affect shore weathering intertidal ecology. Tidal currents are also generally stronger with semidiurnal tides because of the greate rapidity of tidal fluctuations, but in general the most important aspect of tides in geomorphology is the vertical range.
Storm and surges
Storm surges occur when strong onshore winds build up coastal water to an exceptionally high level for a few hours or days, and are most pronounced when they coincide with high spring tides. Strong onshore winds also generate large waves accompanying the raised sea level, overwashing beaches, flooding low-lying coastal areas and causing extensive changes in a short period. Beach erosion is usually severe during a storm surge, and if the coast consists of soft formations it may be cut back substantially to a new morphology.
Other sea disturbances
Exceptional disturbances of sea level occur during and after earthquakes, landslides or volcanic eruption in and around the seas and oceans. These produce tsunamis, very large waves which may attain heights of more 30 metres by the time they reach the coast.
We need to indicate the time scale we are interested in coastal area processses. Cowell and Thom (1994) group the time scales at which coastal processes operate into four classes : ‘’’Instantaneous time scales’’’ - These involve the evolution of morphology during a single cycle of the forces that drive morphological change (waves, tides). The destruction of wave ripples under a group of high waves and the onshore migration of an intertidal bar over a single tidal cycle are examples of morphological change over instantaneous time scales. During few seconds to many days or weeks.
Event time scales - These are concerned with coastal evolution in response to processes operating across time spans ranging from that of an individual event, through to seasonal variation in driving forces. Examples of morphological change over event time scales are the scarp¬ing of coastal dunes in response to a major storm and the seasonal closure of an estuary by a sand bar. During few days to many years.
Engineering time scales - Coastal evolution resulting from many fluctu¬ations in the driving forces takes place over engineering time scales. It is the time scale that coastal engineers are most concerned with and ranges years to centuries. The migration of tidal inlets and the develop¬ment of a foredune ridge are examples of coastal development over engi¬time scales. During few months to decades.
Geological (Geomorphological) time scales - These time scales operate over decades to mil¬lennia. Whereas at the previous three time scales morphological change is the result of fluctuations in the driving forces, on geomorphological time coastal evolution occurs more in response to mean trends in the forces (sea level, climate). Examples of coastal evolution at geo¬morphological time scales are the infilling of a tidal basin or estuary, onshore of a barrier system and the switching of delta lobes.
Nearshore Morphodynamic Processes
Processes of coastal morphological evolution
Coastal morphological changes vary in time scale and in space, depending on the interaction between operating hydrodynamic and sediment dynamic processes and the morphology (topography) of the coastal strip under investigation. Morphodynamic processes with respect to time scale have been grouped into four classes (Cowell and Thom, 1997):
Processes of morphodynamical changes
Morphodynamic changes induced by nearshore hydrodynamics, sediment dynamics as well as local weather conditions and human interference (e.g dredging) can be also related to space and time scales as shown on Figure 2. In general, processes such as turbulence, wind, individual waves, individual grains, bed and/or shoreface scour and beach profile change occur within micro (sec to min) time periods covering a micro (mm to cm) to meso (m to km) spatial scale. Tides, sediment pathways, shoreline changes cover longer time scales from macro (months to years) up to mega (decade to centuries), whilst in term of space represent forms of 1 to 10 km. Finally, sub-regional and regional (mega spatial scale >10km) occur within macro to mega time scale (Kraus et al., 1991).
Cowell P.J. and Thom B.G., 1997. Morphodynamics of coastal evolution. In: R.W.G. Carter and C.D. Woodroffe, Coastal evolution, Late Quaternary shoreline morphodynamics, Cambrich University Press, 33-86.
Kraus N.C., Larson M., Kriebel D.L, 1991. Evaluation of beach erosion and accretion predictors. Proc. Co. Sediments’91, ASCE, Seattle, pp.527-587.
Examples of Morphological Changes can be found in the case studies section
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