3.7 Seiches
103
approximately l.8 m was reported in Nemuro, Japan, about 90 minutes after
the earthquake (Yeh et al., 1995). In the South Kuril Islands, 11 people were
killed and 242 were injured. In Hokkaido, Japan, one person was killed and
140 injured. None of the casualties were due directly to the tsunami, in spite
of its significant run-up of approximately 10 m on Shikotan Island.
In order to minimize the number of casualties and reduce damage due to
tsunamis, three types of tsunami warning systems exist: the Pacific Tsunamis
Warning Center, five regional systems (two in the United States and one each
in Japan, Russia, and French Polynesia), and local systems in Chile and Japan
(Bernard, 1998). The Pacific-wide System can issue a warning about 1 hour
prior to arrival of the tsunami (useful for populations located more than about
750 km from the source), regional systems warn in about 10 minutes prior to
arrival of the tsunami (useful for 100-750 km from the source) and local systems
warn in about 5 minutes prior to arrival of the tsunami (useful for areas situated
less than 100 km from the source). Potential earthquake zones and locations
of tsunami detectors in the North-East Pacific are shown in Fig. 3.14.
3.7 Seiches
Under specific conditions, very long waves can also be generated in closed
water basins such as lakes, bays, estuaries and harbours, and they are known as
seiches. A seiche is a standing wave, which can be considered as the sum of two
progressive waves, travelling in opposite directions. Everybody has experienced
a seiche in their domestic bath by setting the water into oscillatory motion by
moving a hand to and fro in the water. Figure 3.15 shows an idealized vertical
profile of a seiche. At either end of the container, the water level is alternatively
high and low, whereas in the middle, the water level remains constant.
For the case illustrated in Fig. 3.15, the length of the container, I, corresponds
to half the wavelength, L, of the seiche. At the point where the water level is
constant (the node), the horizontal flow of water from one end of the container
to the other is greatest. Where the fluctuations of water level are greatest (the
antinodes), there is minimal horizontal movement of the water. The seiche
period is the time from peak to peak of the oscillation; it varies with the basin
length and depth.
Seiches in natural conditions are much more complicated than that in a simple
closed container. The most probable sources of seiche forcing mechanisms are
(Masse!, 1989):
- passage of barometric fluctuations,
- impact of wind gusts on the water surface,
- release of water forced up on a leeward shore through the lapse of strong
onshore winds, and
- seismic oscillations of the earth during earthquakes.
However, the principal source of excitation remains meteorological. After
being excited by meteorological disturbances, water surges back and forth until
103
approximately l.8 m was reported in Nemuro, Japan, about 90 minutes after
the earthquake (Yeh et al., 1995). In the South Kuril Islands, 11 people were
killed and 242 were injured. In Hokkaido, Japan, one person was killed and
140 injured. None of the casualties were due directly to the tsunami, in spite
of its significant run-up of approximately 10 m on Shikotan Island.
In order to minimize the number of casualties and reduce damage due to
tsunamis, three types of tsunami warning systems exist: the Pacific Tsunamis
Warning Center, five regional systems (two in the United States and one each
in Japan, Russia, and French Polynesia), and local systems in Chile and Japan
(Bernard, 1998). The Pacific-wide System can issue a warning about 1 hour
prior to arrival of the tsunami (useful for populations located more than about
750 km from the source), regional systems warn in about 10 minutes prior to
arrival of the tsunami (useful for 100-750 km from the source) and local systems
warn in about 5 minutes prior to arrival of the tsunami (useful for areas situated
less than 100 km from the source). Potential earthquake zones and locations
of tsunami detectors in the North-East Pacific are shown in Fig. 3.14.
3.7 Seiches
Under specific conditions, very long waves can also be generated in closed
water basins such as lakes, bays, estuaries and harbours, and they are known as
seiches. A seiche is a standing wave, which can be considered as the sum of two
progressive waves, travelling in opposite directions. Everybody has experienced
a seiche in their domestic bath by setting the water into oscillatory motion by
moving a hand to and fro in the water. Figure 3.15 shows an idealized vertical
profile of a seiche. At either end of the container, the water level is alternatively
high and low, whereas in the middle, the water level remains constant.
For the case illustrated in Fig. 3.15, the length of the container, I, corresponds
to half the wavelength, L, of the seiche. At the point where the water level is
constant (the node), the horizontal flow of water from one end of the container
to the other is greatest. Where the fluctuations of water level are greatest (the
antinodes), there is minimal horizontal movement of the water. The seiche
period is the time from peak to peak of the oscillation; it varies with the basin
length and depth.
Seiches in natural conditions are much more complicated than that in a simple
closed container. The most probable sources of seiche forcing mechanisms are
(Masse!, 1989):
- passage of barometric fluctuations,
- impact of wind gusts on the water surface,
- release of water forced up on a leeward shore through the lapse of strong
onshore winds, and
- seismic oscillations of the earth during earthquakes.
However, the principal source of excitation remains meteorological. After
being excited by meteorological disturbances, water surges back and forth until