3.81 ASTRQNOMICAL TI DES
Tide is a periodic rising and falling of sea level caused by the
gravitational attraction of the moon, sun, and other astronomical bodies
acting on the rotating earth. Tides follow the moon more closely than they
do the sun. There are usually two high and and two low waters in a tidal
or lunar day. As the lunar day is about 50 minutes longer than the solar
day, tides occur about 50 minutes later each day. Typical tide curves for
various locations along the Atlantic, Gulf, and Pacific coasts of the
United States are shown in Figures 3-36 and 3-37. Along the Atlantic
coast, the two tides each day are of nearly the same height. On the Gulf
coast, the tides are low but in some instances hâve a pronounced diumal
inequality. Pacific coast tides compare in height with those on the
Atlantic coast but in most cases hâve a decided diurnal inequality.
(See Appendix A, Figure A-10.)
The dynamic theory of tides was formulated by Laplace (1775) and
spécial solutions hâve been obtained by Doodson and Warburg (1941) among
others. The use of simplified théories for the analysis and prédiction
of tides has been described by Schureman (1941), Defant (1961) and Ippen
(1966). The computer program for tide prédiction, currently being used
for official tide prédiction in the United States is described by Pore
and Cummings (1967).
Data concerning tidal ranges along the seacoasts of the United States
are given to the nearest foot in Table 3-5. Spring ranges are shown for
areas having approximately equal daily tides; diumal ranges are shown
for areas having either a diurnal tide or a pronounced diumal inequality.
Detailed data concerning tidal ranges are published annually in Tide Tables,
U.S. Department of Commerce, National Océan Survey.
3.82 TSUNAMIS
Long period gravity waves generated by such disturbances as earthquakes, landslides, volcano éruptions and explosions near the sea surface
are called tsunamis. The Japanese word tsunami has been adopted to replace
the expression tidal wave to avoid confusion with the astronomical tides.
Most tsunamis are caused by earthquakes that extend at least partly
under the sea, although not ail submarine earthquakes produce tsunamis.
Severe tsunamis are rare events.
Tsunamis may be compared to the wave generated by dropping a roc in
a pond. Waves (ripples) move outward from the source région in every
direction. In general, the tsunami wave amplitudes decrease but the
number of individual waves increases with distance from the source région.
Tsunami waves may be reflected, refracted, or diffracted by islands, seamounts, submarine ridges or shores. The longest waves travel across the
deepest part of the sea as shallow-water waves, and may obtain speeds of
several hundred knots. The travel time required for the first tsunami
disturbance to arrive at any location can be determined within a few
Percent of the actual travel time by the use of suitable tsunami traveltime charts.
3-71
Tide is a periodic rising and falling of sea level caused by the
gravitational attraction of the moon, sun, and other astronomical bodies
acting on the rotating earth. Tides follow the moon more closely than they
do the sun. There are usually two high and and two low waters in a tidal
or lunar day. As the lunar day is about 50 minutes longer than the solar
day, tides occur about 50 minutes later each day. Typical tide curves for
various locations along the Atlantic, Gulf, and Pacific coasts of the
United States are shown in Figures 3-36 and 3-37. Along the Atlantic
coast, the two tides each day are of nearly the same height. On the Gulf
coast, the tides are low but in some instances hâve a pronounced diumal
inequality. Pacific coast tides compare in height with those on the
Atlantic coast but in most cases hâve a decided diurnal inequality.
(See Appendix A, Figure A-10.)
The dynamic theory of tides was formulated by Laplace (1775) and
spécial solutions hâve been obtained by Doodson and Warburg (1941) among
others. The use of simplified théories for the analysis and prédiction
of tides has been described by Schureman (1941), Defant (1961) and Ippen
(1966). The computer program for tide prédiction, currently being used
for official tide prédiction in the United States is described by Pore
and Cummings (1967).
Data concerning tidal ranges along the seacoasts of the United States
are given to the nearest foot in Table 3-5. Spring ranges are shown for
areas having approximately equal daily tides; diumal ranges are shown
for areas having either a diurnal tide or a pronounced diumal inequality.
Detailed data concerning tidal ranges are published annually in Tide Tables,
U.S. Department of Commerce, National Océan Survey.
3.82 TSUNAMIS
Long period gravity waves generated by such disturbances as earthquakes, landslides, volcano éruptions and explosions near the sea surface
are called tsunamis. The Japanese word tsunami has been adopted to replace
the expression tidal wave to avoid confusion with the astronomical tides.
Most tsunamis are caused by earthquakes that extend at least partly
under the sea, although not ail submarine earthquakes produce tsunamis.
Severe tsunamis are rare events.
Tsunamis may be compared to the wave generated by dropping a roc in
a pond. Waves (ripples) move outward from the source région in every
direction. In general, the tsunami wave amplitudes decrease but the
number of individual waves increases with distance from the source région.
Tsunami waves may be reflected, refracted, or diffracted by islands, seamounts, submarine ridges or shores. The longest waves travel across the
deepest part of the sea as shallow-water waves, and may obtain speeds of
several hundred knots. The travel time required for the first tsunami
disturbance to arrive at any location can be determined within a few
Percent of the actual travel time by the use of suitable tsunami traveltime charts.
3-71