Geology Reference
In-Depth Information
COASTAL ENVIRONMENTS
The motion is called 'free orbital' because the chief
movement of the water is roughly circular in the direction
of flow, moving forwards on the crest, upwards on the
front, backwards in the trough, and downwards on the
back (Figure 13.1). Water moves slowly in the direction of
wave propagation because water moves faster on the crest
than in the troughs. Oscillatory waves form
wave trains
.
Solitary waves
or
waves of translation
, in contrast,
involve water moving in the direction of propagation
without any compensatory backward motion. They are
single, independent units and not associated with wave
trains. They lack the distinct crests and troughs of oscil-
latory waves and appear as weals separated by almost flat
water surfaces and are effective transporters and eroders
of sediments and rocks. They are often generated by the
breaking of oscillatory waves.
Once waves approaching a coastline 'feel bottom', they
slow down. The waves crowd together, and their fronts
steepen.
Wave refraction
occurs because the inshore part
of a wave crest moves more slowly than the offshore
part, owing to the shallow water depth, and the off-
shore part swings forwards and the wave crests tend to run
parallel to the depth contours. Wave refraction near a sub-
marine canyon and a headland is shown in Figure 13.2.
Waves
Wave s
are undulations formed by wind blowing over
a water surface. They are caused by turbulence in air-
flow generating pressure variations on the water. Once
formed, waves help to disturb the airflow and are partly
self-sustaining. Energy is transferred from the wind to the
water within the wave-generation area. The amount of
energy transfer depends upon the wind speed, the wind
duration (how long the wind blows), and the fetch (the
extent of water over which the wind blows).
Sea waves
are formed by the wind within the generation area. They
often have short crests and steep cross-sections, and are
irregular. In mid-ocean, prolonged strong winds associ-
ated with severe storms and blowing over hundreds of
kilometres produce waves more than 20 m high that
travel up to 80 km/hr. On passing out of the gener-
ation area, sea waves become
swell waves
(or simply
swell
) and they are more regular with longer periods and
longer crests. They may travel thousands of kilometres
across oceans.
Waves formed in water deep enough for
free orbital
motion
to
occur
are
called
waves of oscillation
.
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)
