4.3.3: Coastal impact of different wave conditions
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The world-wide distribution of wave characteristics has strong implications for coastal engineering. The wave climate at a particular site is dependent on regional and local factors such as basin characteristics and local geometry. It is therefore almost impossible to classify coasts on the basis of the global wave climates. Nevertheless, the following broad generalisations can be made for a wave-dominated coastal system (see also Mangor, 2004):
- On open coasts, a storm wave climate is characterised by waves which are highly variable in height, period and direction. The waves are continuously reshaping the coastal profile, which results in a dynamic sandy coastal profile with bars and wide sandy beaches backed by dunes. The profile often has an offshore storm bar, which is formed by offshore transport by the larger waves associated with storms, which in the NH mostly occur in winter. These larger waves break at relatively deep water, so the littoral zone (the active coastal zone, Fig. 1.14) extends to relatively large water depths. Since the slope of the profiles tends to be flatter for steeper waves, the result is a wide littoral zone. Since the wave influence decreases offshore, there is an offshore fining of sediments. The breaking waves tend to be of the spilling type (see Sect. 5.2.5);
- Since swell waves are relatively low and long (low steepness) and have a more or less constant wave height year-round, a swell climate gives a relatively narrow sandy littoral zone. The breaking waves tend to be of the plunging type rather than of the spilling type (see Sect. 5.2.5). The transition from coarser sandy sediment in shallower waters to finer sediments in deeper waters is quite abrupt. The gently sloping outer part of the littoral zone is dominated by finer sediments. The low and long waves tend to move sand onshore;
- The monsoon climate of Southeast Asia gives a seasonal wave climate with the highest waves in summer under the influence of the SW-monsoon. The summer waves are moderate in height and relatively constant in direction and height. The corresponding profiles therefore are comparable to the swell climate profiles: a fairly narrow sandy inner littoral zone, shifting to a gently sloping outer part of the littoral zone dominated by finer sediments;
- Cyclones give rise to very high waves and storm surge. When they hit they greatly impact the coastal profile causing erosion and storm bars. But because of the relatively low frequency of occurrence (for instance one or two per year that make landfall at a particular location), the coastal morphology will first and foremost be determined by the normal wave climate (either a monsoon or swell climate).