Environmental Engineering Reference
In-Depth Information
Fig. 1.20
Waves of the sea showing relevant terminologies
Table 1.5
Classi
cation of ocean surface waves
Classi
cation
Period range
Depth classi
cation
Restoring force
Generating force
Capillary
waves
<0.1 s
Deep-water wave
Surface tension
Winds
Ultra-gravity
waves
0.1
-
1 s
Deep-water wave
Surface tension
and gravity
Winds
Gravity
waves
1
30 s
Deep-water wave to
shallow water wave
Gravity
Winds
-
Infra-gravity
waves
30 s to 5 min
Deep-water wave to
shallow water wave
Gravity and
coriolis force
Winds and gravity
Long-period
waves
5 min to 12 h
Transitional to shallow
water wave
Gravity and
coriolis force
Storms and earthquake
Ordinary
tidal waves
(semi-diurnal and
diurnal) 12
Shallow water wave
Gravity and
coriolis force
Gravitational attraction
of sun and moon
24 h
-
Trans-tidal
waves
>24 h
Shallow water wave
Gravity and
coriolis force
Storms, sun and moon
In addition to the above categories of waves,
several researchers have stated about coastal
trapped waves in the Sydney region of Australia.
These are not waves in true sense, since they have
little observable effects at the sea surface. These
are characterized by timescales of 7
Very little information is available on the
classi
cation of waves in the Bay of Bengal
region adjacent to the Indian part of Sundarbans.
Surface waves in the Bay of Bengal are caused
due to wind system whose direction and velocity
are mainly controlled by northeast and southwest
monsoons. The wind from north and northeast
starts blowing from the beginning of October to
the end of March. The months of January and
February are relatively calm with an average
speed of around 3.5 km/h. It again commences to
blow violently from southwest around the middle
of March and continues till September. During
this period, several low-pressure systems are
formed in this region, among which few take the
shape of depressions and cyclonic storms of
varying intensity (Table
1.6
). The wind speed
20 days and
scales of long-shore variability of many hundreds
of kilometres (Church et al.
1986
). They may be
thought of as a current oscillation which moves
equatorward from its generation source in Bass
Strait affecting Sydney waters some two days after
being generated by strong winds in the Bass Strait
(Grif
-
n and Middleton
1991
). They are, therefore,
predictable to some extent and can have the effect
of producing strong currents in the near-shore
region which may have either northward
fl
owing
or southward
fl
owing.
