Page 334 - Fundamentals of Geomorphology

COASTAL LANDSCAPES 317

COASTAL ENVIRONMENTS
The motion is called ‘free orbital’ because the chief
movement of the water is roughly circular in the direction
Waves
of flow, moving forwards on the crest, upwards on the
Waves are undulations formed by wind blowing over front, backwards in the trough, and downwards on the
a water surface. They are caused by turbulence in air- back(Figure13.1).Watermovesslowlyinthedirectionof
flow generating pressure variations on the water. Once wave propagation because water moves faster on the crest
formed, waves help to disturb the airflow and are partly than in the troughs. Oscillatory waves form wave trains.
self-sustaining. Energy is transferred from the wind to the Solitary waves or waves of translation, in contrast,
water within the wave-generation area. The amount of involve water moving in the direction of propagation
energy transfer depends upon the wind speed, the wind without any compensatory backward motion. They are
duration (how long the wind blows), and the fetch (the single, independent units and not associated with wave
extent of water over which the wind blows). Sea waves trains. They lack the distinct crests and troughs of oscil-
are formed by the wind within the generation area. They latory waves and appear as weals separated by almost flat
often have short crests and steep cross-sections, and are water surfaces and are effective transporters and eroders
irregular. In mid-ocean, prolonged strong winds associ- of sediments and rocks. They are often generated by the
ated with severe storms and blowing over hundreds of breaking of oscillatory waves.
kilometres produce waves more than 20 m high that Once waves approaching a coastline ‘feel bottom’, they
travel up to 80 km/hr. On passing out of the gener- slow down. The waves crowd together, and their fronts
ation area, sea waves become swell waves (or simply steepen.Wave refraction occurs because the inshore part
swell) and they are more regular with longer periods and of a wave crest moves more slowly than the offshore
longer crests. They may travel thousands of kilometres part, owing to the shallow water depth, and the off-
across oceans. shorepartswingsforwardsandthewavecreststendtorun
Waves formed in water deep enough for free orbital paralleltothedepthcontours.Waverefractionnearasub-
motion to occur are called waves of oscillation. marine canyon and a headland is shown in Figure 13.2.

Figure 13.1 Terms associated with waves, including the orbital motion of waves in deep, intermediate, and shallow water.
Source: Adapted from Komar (1998, 166)