3.6 Tsunamis
101
trace off this coast. All except one of the eleven incidents involved vessels riding on the rapid Agulhas Current. This current, generated in the Passat region
of the Indian Ocean, flows in a southwest direction along the African coast.
The typical current speed is of the order of 2.0-2.5 mis, and its width is about
90-170 km. Energetic swell, generated at higher latitudes, propagates against
this current resulting in very high and steep waves. Interaction of waves propagating against Agulhas Current is discussed in Sect. 7.7.2. For more general
elaboration see also Massel (1989).
Very comprehensive atlas on wind and wave climate has recently been published by Young and Holland (1996). The atlas provides quantitative estimates
of the global wind and waves which are based on the data measured by the
GEOSAT satellite, for 3 years period. The resulting charts contain the contour
fields of wind speeds and wave heights for any region of the globe.
3.6 Tsunamis
Wind-induced waves are very common on the ocean surface. However, in
some circumstances waves of unusual character may result. They are the long
waves generated by seismic disturbances, or by slumping of submarine sediment masses due to gravitational instability. Such waves are called tsunamis.
Tsunami is a Japanese word being, in fact, a combination of two words: 'ami'
which means wave, and 'tsu' which denotes a particular point at the waterline.
Thus, a tsunami is a wave which approaches the shoreline.
In the open ocean, tsunamis often remain undetected because of their great
wavelength, of the order of hundreds of kilometers, and its small wave height,
usually in the order of one metre. However, the velocity of propagation of
tsunami in deep water is very high (exceeding 700 km/hour when the mean
ocean depth is used), which can be estimated approximately as:
c = Jgh,
(3.20)
in which h is the water depth (for more vigorous derivation of formula (3.20) see
Sect. 4.2). When tsunami reaches shallower water, the velocity decreases, but
the energy flux in the wave remains the same, resulting in an increase in wave
height, sometimes to catastrophic values. Tsunamis occur mostly in the Pacific
as that ocean experiences frequent seismic activity. Catastrophic tsunamis are
recorded in many historical Japanese, Chinese and American documents. In
1495, a high tsunami attacked the southern coast of the Japanese Honshu
Island. A huge hall, containing an 11.3 metres tall bronze statue of Buddha
was completely washed away, however the statue is still in place. It should be
noted that this statue has a total weight of 800 tonnes and is about 1.5 km
from the present shoreline and about 50 m above sea level.
Almost four hundred years later, in August 1883, a gigantic explosion of the
volcano Krakatoa, situated between Sumatra and Java in the heavy active
101
trace off this coast. All except one of the eleven incidents involved vessels riding on the rapid Agulhas Current. This current, generated in the Passat region
of the Indian Ocean, flows in a southwest direction along the African coast.
The typical current speed is of the order of 2.0-2.5 mis, and its width is about
90-170 km. Energetic swell, generated at higher latitudes, propagates against
this current resulting in very high and steep waves. Interaction of waves propagating against Agulhas Current is discussed in Sect. 7.7.2. For more general
elaboration see also Massel (1989).
Very comprehensive atlas on wind and wave climate has recently been published by Young and Holland (1996). The atlas provides quantitative estimates
of the global wind and waves which are based on the data measured by the
GEOSAT satellite, for 3 years period. The resulting charts contain the contour
fields of wind speeds and wave heights for any region of the globe.
3.6 Tsunamis
Wind-induced waves are very common on the ocean surface. However, in
some circumstances waves of unusual character may result. They are the long
waves generated by seismic disturbances, or by slumping of submarine sediment masses due to gravitational instability. Such waves are called tsunamis.
Tsunami is a Japanese word being, in fact, a combination of two words: 'ami'
which means wave, and 'tsu' which denotes a particular point at the waterline.
Thus, a tsunami is a wave which approaches the shoreline.
In the open ocean, tsunamis often remain undetected because of their great
wavelength, of the order of hundreds of kilometers, and its small wave height,
usually in the order of one metre. However, the velocity of propagation of
tsunami in deep water is very high (exceeding 700 km/hour when the mean
ocean depth is used), which can be estimated approximately as:
c = Jgh,
(3.20)
in which h is the water depth (for more vigorous derivation of formula (3.20) see
Sect. 4.2). When tsunami reaches shallower water, the velocity decreases, but
the energy flux in the wave remains the same, resulting in an increase in wave
height, sometimes to catastrophic values. Tsunamis occur mostly in the Pacific
as that ocean experiences frequent seismic activity. Catastrophic tsunamis are
recorded in many historical Japanese, Chinese and American documents. In
1495, a high tsunami attacked the southern coast of the Japanese Honshu
Island. A huge hall, containing an 11.3 metres tall bronze statue of Buddha
was completely washed away, however the statue is still in place. It should be
noted that this statue has a total weight of 800 tonnes and is about 1.5 km
from the present shoreline and about 50 m above sea level.
Almost four hundred years later, in August 1883, a gigantic explosion of the
volcano Krakatoa, situated between Sumatra and Java in the heavy active