Geoscience Reference
In-Depth Information
Wave energy and power for typical ocean waves (based on Wiegel, 1964; Dalrymple, 2000).
Table 8.1
Energy m
-1
of wave
Power m
-1
crest in one wave
of wave crest
Power relative
Wave period (s)
Length (m)
Height (m)
(103 joules)
(103 watts)
to 1 m high wave
10.0
156
1.0
0.196
9.79
1.0
1.4
0.384
19.20
2.0
2.0
0.784
39.18
4.0
3.0
1.763
88.15
9.0
7.0
9.598
479.92
49.0
14.0
306
1.0
0.384
13.71
1.0
2.0
1.536
54.85
4.0
3.0
3.455
123.41
9.0
reduced 2.4 per cent in fresh water because of its lower
density.) A doubling in wavelength leads to a doubling
in wave energy and power, whereas a doubling in wave
height leads to a quadrupling in wave energy and
power. In the coastal zone under strong winds, wave
height can change more rapidly than wavelength. The
amount of wave energy in a storm wave can become
abnormally large (Figure 8.4). In swell environments,
wave heights of 1.0 m and periods of 10 seconds are
common; however, if a storm wave increases in height
to 7 m - a value typical of the storm waves off the
eastern coasts of the United States or Australia - then
wave energy will increase almost fiftyfold. This is the
reason why storm waves can be so damaging in such a
short period of time.
resolution of imaging is too low, satellites have
effectively eliminated the necessity for hindcasting to
determine wave height. Altimeters in satellites now
measure wave heights with an accuracy of 2.5-4.0 cm.
Instruments were first used on SEASAT in 1978, then
on GEOSAT from 1985 to 1988, and ERS-1 and 2 from
1991, TOPEX/POSEIDON from 1992, and Jason-1
from 2001. CNES, the French space agency, and
NASA, the United States space agency, jointly operate
the latter two satellites. Waves measured using satellite
altimeters range from 1 to 5 m at daily and monthly
levels. Figure 8.4 illustrates these patterns for January
and July 1995, which are typical of these months.
Maximum wave heights correspond to belts of maxi-
mum winds generated over oceans by mid-latitude
low-pressure cells (Figure 2.3). Wave climate in the
extra-tropical north Pacific and north Atlantic is
especially seasonal, with much rougher conditions in
the northern (
boreal
) winter than in the summer. The
Southern Ocean has high waves throughout the year,
but is roughest in the southern (austral) winter. Near
the equator, larger waves are produced either by swell
propagating from higher latitudes, or by seasonal winds
(for example, monsoons and tropical storms). The
Arabian Sea is particularly rough from June to August,
coinciding with the south-west monsoon.
Wor
ld distribution of high waves
(PO.DAAC, 2003)
The generation of waves represents transference of
energy from wind to the water body. The height to
which a wave will grow depends upon four factors:
wind speed, the duration of the wind, the length of
water over which wind blows - termed fetch - and the
processes leading to decay. Generally, wave height is
dependent first upon fetch, second upon wind speed
and last upon duration. Before the advent of
wave-
rider buoys
to measure waves along a coastline, or
altimeters in satellites to measure wave period and
heights over large areas, data on wave characteristics
depended upon ship observations and hindcasting
procedures. Hindcasting theoretically estimates wave
height and period based on wind duration, speed, and
fetch derived from synoptic pressure maps. Except
on small bodies of water or near shore where the
Wav
es as a hazard at sea
(Beer, 1983; Bruun, 1985; Kushnir et al., 1997; Zebrowski,
1997; Lawson, 2001)
The effect of waves as a hazard really depends upon
where the wave is experienced. In the open ocean,
freak, giant, or rogue waves pose a hazard to naviga-
tion. This does not involve a single wave. Instead, it
